Articles on Medical Diseases and Conditions


In porphyric diseases, the main similarity is the abnormal secretion of substances that are precursors of the porphyrin compound heme (of hemoglobin). The known pathways of porphyrin synthesis begin with glycine and succinate, which are combined to eventually form a compound known as d-aminolevulinic acid (ALA). This goes on to produce a substance known as “porphobilinogen,” composed of a single pyrrole ring. Four of these rings are joined to form the tetrapyrrole compound proporphyrinogen; this is the precursor of protoporphyrin, which, in turn, is the precursor of heme. The tetrapyrrole compounds exist in eight isomers, depending on where certain side groups are located. The only isomeric forms that are clinically important are I and III. Normally, very small amounts of porphyrin degradation products appear in the feces or in the urine; these are called “coproporphyrins” or “uroporphyrins” (their names refer to where they were first discovered, but both may appear in either urine or feces).
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The porphyrias have been classified in several ways, none of which is entirely satisfactory. The most common system includes erythropoietic porphyria (EP), hepatic porphyria, mixed porphyria, porphyria cutanea tarda (PCT), and acquired (toxic) porphyria. EP is a small group of rare congenital diseases characterized clinically by skin photosensitivity without vesicle formation, pink discoloration of the teeth that fluoresces under ultraviolet light, and sometimes mild hemolytic anemia. If erythropoietic porphyria is suspected, the best diagnostic test is measurement of erythrocyte porphyrin.

Hereditary hepatic porphyria may be subdivided into three types: acute intermittent porphyria (AIP; Swedish genetic porphyria), variegate porphyria (VP; South African genetic porphyria), and hereditary coproporphyria (HC). All three are inherited as autosomal dominants, and all three may be associated with episodes of acute porphyric attacks, although such attacks are more widely publicized in association with AIP. All three subdivisions manifest increases in the enzyme ALA-synthetase, which catalyzes formation of ALA from its precursors. AIP is characterized by a decrease of 50% or more in the enzyme uroporphyrinogen-I-synthetase (abbreviated URO-I-S and also known as “porphobilinogen deaminase”), which catalyzes the formation of uroporphyrinogen I from porphobilinogen. Levels of URO-I-S are said to be normal in VP and HC. Acute intermittent porphyria is not associated with photosensitivity, whereas skin lesions due to photosensitivity are common in VP and also occur in HC. Parenthetically, these skin lesions resemble those of PCT, and some of these patients were probably included in the PCT group in some early classifications. In VP and HC, increased amounts of protoporphyrin are excreted in the feces, whereas this does not happen in AIP. Although AIP, VP, and HC have increased amounts of coproporphyrin in the feces, HC patients excrete much larger amounts of fecal coproporphyrin III than does AIP or VP.

The porphyrias can also be classified usefully according to clinical symptoms:

1. Neurologic only: AIP
2. Cutaneous only: PCT, EP, EPP
3. Both neurologic and cutaneous: VP, HC

Acute intermittent porphyria. URO-I-S is said to be decreased in all patients with AIP. However, about 5%-10% of AIP patients have values within the reference range, so that some overlap occurs. URO-I-S is also said to be decreased in relatives of patients with AIP, again with some overlap at the borderline areas of the reference range. At least one kindred with a condition closely resembling AIP has been reported with normal URO-I-S levels, but the significance of this is not clear. There may be some laboratory variation in results, and equivocal results may have to be repeated. Blood samples should be stored frozen and kept frozen during transit to the laboratory to avoid artifactual decrease in enzyme activity. Therefore, falsely low URO-I-S values may be obtained through improper specimen handling. Hemolytic anemia or reticulocytosis greater than 5% may produce an increase in URO-I-S activity. Assay for URO-I-S is available mostly in university medical centers and large reference laboratories.

The acute porphyric attacks consist of colicky abdominal pain, vomiting, and constipation (= 80% of patients); and mental symptoms (10%-30% of patients) such as confusion, psychotic behavior, and occasionally even convulsions. About one half of the patients display hypertension and some type of muscle motor weakness. The attacks are frequently accompanied by leukocytosis. These attacks may be precipitated by certain medications (especially by barbiturates;), by estrogens, and by carbohydrate deprivation (dieting or starvation). The attacks usually do not occur until adolescence or adulthood. Porphobilinogen is nearly always present in the urine during the clinical attacks and is an almost pathognomonic finding, but the duration of excretion is highly variable. It may occasionally disappear if not searched for initially. Between attacks, some patients excrete detectable porphobilinogen and others do not. Urine ALA levels are usually increased during acute attacks but not as markedly as porphobilinogen. During remission, ALA levels also may become normal. Patients with AIP may also have hyponatremia and sometimes have falsely elevated thyroxine (T4) results due to elevated thyroxine-binding protein levels.

Porphobilinogen is usually detected by color reaction with Ehrlich’s reagent and confirmed by demonstrating that the color is not removed by chloroform (Watson-Schwartz test). Since false positive results may occur, it is essential to confirm a positive test by butanol (butyl alcohol) extraction. Porphobilinogen will not be extracted by butanol, whereas butanol will remove most of the other Ehrlich-positive, chloroform-negative substances. Therefore, porphobilinogen is not removed by either chloroform or butanol. A positive result on the porphobilinogen test is the key to diagnosis of symptomatic acute porphyria; some investigators believe that analysis and quantitation of urinary porphyrins or ALA are useful only if the Watson-Schwartz test results are equivocal. However, the majority believe that a positive qualitative test result for porphobilinogen should be confirmed by quantitative chemical techniques (available in reference laboratories) due to experience with false positive Watson-Schwartz test results in various laboratories. They also advise quantitative analysis of porphyrins in urine and feces to differentiate the various types of porphyria. Glucose administration may considerably decrease porphobilinogen excretion.

Some investigators prefer the Hoesch test to the modified Watson-Schwartz procedure. The Hoesch test also uses Ehrlich’s reagent but is less complicated and does not react with urobilinogen. The possibility of drug-induced false reactions has not been adequately investigated. Neither test has optimal sensitivity. In one study the Watson-Schwartz test could detect porphobilinogen only about 50% of the time when the concentration was 5 times normal. Quantitative biochemical methods available in reference laboratories are more sensitive than these screening tests.

Porphyria cutanea tarda is a chronic type of porphyria. There usually is some degree of photosensitivity, but it does not develop until after puberty. There often is some degree of liver disease. No porphobilinogen is excreted and acute porphyric attacks do not occur.

Toxic porphyria may be produced by a variety of chemicals, but the most common is lead. Lead poisoning produces abnormal excretion of coproporphyrin III but not of uroporphyrin III. ALA excretion is also increased.

Familial dysautonomia (Riley-Day syndrome). Riley-Day syndrome is a familial disorder characterized by a variety of signs and symptoms, including defective lacrimation, relative indifference to pain, postural hypotension, excessive sweating, emotional lability, and absence of the fungiform papilli on the anterior portion of the tongue. Most of those affected are Jewish. Helpful laboratory tests include increased urine homovanillic acid value and decreased serum dopamine-beta-hydroxylase (DBH) value, an enzyme that helps convert dopamine to norepinephrine. Besides the Riley-Day syndrome, the DBH value may also be decreased in mongolism (Down’s syndrome) and Parkinson’s disease. It has been reported to be elevated in about 50% of patients with neuroblastoma, in stress, and in certain congenital disorders (results in the congenital disorders have not been adequately confirmed). There is disagreement as to values in patients with hypertension.

Enzyme Deficiency Diseases

Congenital cholinesterase deficiency (succinylcholine apnea). Cholinesterase is an enzyme best known for its role in regulation of nerve impulse transmission via breakdown of acetylcholine at the nerve synapse and neuromuscular junction. There are two categories of cholinesterase: acetylcholinesterase (“true cholinesterase”), found in RBCs and nerve tissue; and serum cholinesterase (“pseudocholinesterase”). Cholinesterase deficiency became important when it was noted that such patients were predisposed to prolonged periods of apnea after administration of succinylcholine, a competitor to acetylcholine. Serum cholinesterase inactivates succinylcholine, but acetylcholinesterase does not. Serum cholinesterase deficiency may be congenital or acquired; the congenital type is uncommon but is responsible for most of the cases of prolonged apnea. The patient with congenital deficiency seems to have an abnormal (“atypical”) cholinesterase, of which several genetic variants have been reported.

Laboratory diagnosis. Serum cholinesterase assay is currently the best screening test for cholinesterase deficiency. If abnormally low values are found, it is necessary to perform inhibition procedures with dibucaine and fluoride to distinguish congenital deficiency (atypical cholinesterase) from acquired deficiency of the normal enzyme. Deficiency of normal enzyme may cause prolonged succinylcholine apnea but not predictably and usually only in very severe deficiency. Acute or chronic liver disease is the most frequent etiology for acquired deficiency. Hypoalbuminemia is frequently associated in hepatic or nonhepatic etiologies. A considerable number of drugs lower serum cholinesterase levels and thus might potentiate the action of succinylcholine.

Cholinesterase levels are also decreased in organic phosphate poisoning; this affects both RBC and plasma enzyme levels. Screening tests have been devised using “dip-and-read” paper strips. These are probably satisfactory for ruling out phosphate insecticide poisoning but are not accurate in diagnosis of potential for succinylcholine apnea.

Alpha-1 antitrypsin deficiency. Alpha-1 antitrypsin (AAT) is a serine protease inhibitor that inactivates trypsin but whose primary importance is inactivation of neutrophil elastase that breaks down elastic fibers and collagen. AAT is produced by the liver and comprises about 90% of the globulins that migrate on electrophoresis in the alpha-1 region. AAT deficiency has been associated with two different diseases: pulmonary emphysema in adults (relatively common) and cirrhosis in children (rare). This type of emphysema is characteristically, although not invariably, more severe in the lower lobes. A substantial number of those with homozygous antitrypsin deficiency are affected; reports differ on whether heterozygotes have an increased predisposition to emphysema or to pulmonary disease.

Laboratory diagnosis. The most useful screening test at present is serum protein electrophoresis; the alpha-1 globulin peak is absent or nearly absent in homozygotes. More definitive diagnosis, as well as separation of severe from intermediate degrees of deficiency, may be accomplished by quantitation of AAT using immunoassay methods such as immunonephelometry or immunodiffusion. Estrogen therapy (birth control pills) may elevate AAT levels. Since this protein is one of the acute-phase reactants involving the alpha-1 and alpha-2 globulin group on electrophoresis, values are frequently elevated in acute or severe chronic infections, sarcoidosis, inflammation, active rheumatoid-collagen disease, steroid therapy, tissue destruction, and some cases of malignancy. In some cases, measurement of other acute-phase reactants, such as C-reactive protein or serum haptoglobin, might help decide whether AAT might be elevated for this reason. Conditions besides congenital deficiency that reduce AAT activity include severe protein loss, severe renal disease, malabsorption, and thyroiditis.

The gene for AAT is located on the long arm of chromosome 14 (14q). There are a considerable number of allelic variants of the AAT gene (often called protease inhibitor gene or Pi). Most normal persons have an MM phenotype; most carriers are MZ; and most symptomatic deficiency patients are ZZ. Definitive diagnosis can be made in most cases by DNA probe, either direct analysis with M and Z probes, or by restriction fragment linkage polymorphism (RFLP) methods.

Biotinidase deficiency. Biotin is a water-soluble vitamin that is present in most common foods and, in addition, can be synthesized by GI tract bacteria. Biotin is a cofactor for activity of several carboxylase enzymes that are found in the carboxylic acid cycle, leucine metabolism, and proprionic acid metabolism. Biotinidase converts the precursor substance biocytin to biotin. Biotinidase deficiency prevents conversion of biocytin from dietary sources and forces dependence on biotin produced by GI tract bacteria. Suppression of these bacteria or inactivation of biotin by certain substances such as the glycoprotein avidin in egg white (most commonly caused by eating large quantities of raw eggs) can precipitate biotin deficiency. Other possible causes of biotin deficiency include chronic hemodialysis, long-term total parenteral nutrition without biotin supplement, and occasionally long-term anticonvulsant therapy. Symptoms include retarded growth, weakness, ataxia, hair loss, skin rash, metabolic acidosis, and sometimes convulsions.

Laboratory diagnosis. Neonatal screening for biotinidase deficiency can be done on heelstick blood spotted on filter paper using a variety of assay methods. The same methods can be used on venous blood.

Abnormalities of Glandular Secretion

Cystic fibrosis. Cystic fibrosis (mucoviscidosis, or fibrocystic disease of the pancreas) is the most common eventually lethal autosomal recessive inherited disorder in Europeans (estimated gene frequency of 1 in 2,000 live births). Incidence in African Americans is 2% of that in Europeans; it is rare in Asians. About 90% of homozygotes have symptoms resulting predominately from damage to mucus-producing exocrine glands, although non-mucus-producing exocrine glands can also be affected. The mucus glands produce abnormally viscid secretions that may inspissate, plug the gland ducts, and generate obstructive complications. In the lungs, this may lead to recurrent bronchopneumonia, the most frequent and most dangerous complication of cystic fibrosis. Pseudomonas aeruginosa and Staphylococcus aureus are the most frequent pathogens. The next most common abnormality is complete or partial destruction of the exocrine portions of the pancreas, leading to various degrees of malabsorption, steatorrhea, digestive disturbances, and malnutrition. This manifestation varies in severity, and about 15% of patients have only a minimal disorder or even a normal pancreatic exocrine function. Less common findings are biliary cirrhosis, most often focal, due to obstruction of bile ductules; and intestinal obstruction by inspissated meconium (meconium ileus), found in 10%-15% of newborns with cystic fibrosis. Gamma-glutamyltransferase (GGT) enzyme is elevated in about one third of patients, predominantly those with some degree of active bile duct injury or cirrhosis.

Sweat test for screening. Non-mucus-producing exocrine glands such as the sweat glands do not ordinarily cause symptoms. However, they are also affected, because the sodium and chloride concentration in sweat is higher than normal in patients with cystic fibrosis, even though the volume of sweat is not abnormally increased. Therefore, unusually high quantities of sodium and chloride are lost in sweat, and this fact is utilized for diagnosis. Screening tests (silver nitrate or Schwachman test) that depend on the incorporation of silver nitrate into agar plates or special paper have been devised. The patient’s hand is carefully washed and dried, since previously dried sweat will leave a concentrated chloride residue on the skin and give a false positive result. After an extended period or after exercise to increase secretions, the palm or fingers are placed on the silver nitrate surface. Excess chlorides will combine with the silver nitrate to form visible silver chloride. However, this method is not accurate in patients less than 2 months old.

Sweat test for diagnosis. For definitive diagnosis, sweat is collected by plastic bag or by a technique known as iontophoresis. Iontophoresis using the Gibson-Cooke method is the current standard procedure. Sweat is induced by sweat stimulants such as pilocarpine. The iontophoresis apparatus consists of two small electrodes that create a tiny electric current to transport the stimulating drug into the sweat glands of the skin. The sweat is collected in small gauze pads. The procedure is painless. According to a report from a committee sponsored by the Cystic Fibrosis Foundation in 1983, the standard Gibson-Cooke method is difficult to perform in neonates, and it is better to wait until age 4 weeks if possible. Modifications of the equipment and collection system have been devised and are commercially available.

In children, a sweat chloride content greater than 60 mEq/L (60 mmol/L) or a sweat sodium content greater than 70 mEq/L (70 mmol/L) is considered definitely abnormal. Sodium and chloride may normally be higher (75-80 mEq/L) during the first 3 days of life, decreasing to childhood values by the fourth day. In children there is an equivocal zone (50-80 mEq/L chloride, possibly even up to 90 mEq/L) in which the diagnosis should be considered unproved. Repeated determinations using good technique and acquiring an adequate quantity of sweat are needed. Volumes of sweat weighing less than 50 mg are not considered reliable for analysis. For diagnostic sweat collection it is recommended that the hand not be used, because the concentration of electrolytes in the palm is significantly greater than elsewhere. A further caution pertains to reference values in persons over age 15 years. Whereas in one study there were only 5% of children with cystic fibrosis who had sweat chloride values less than 50 mEq/L and 3% of controls with values of 60-70 mEq/L, 34% of a group of normal adults were found to have sweat sodium concentration greater than 60 mEq/L and in 4% values were more than 90 mEq/L. Another report did not verify these data; therefore, the diagnosis may be more difficult in adults.

One might gather from this discussion that sweat electrolyte analysis is no more difficult than standard laboratory tests such as blood gas analysis or serum protein electrophoresis. Unfortunately, surveys have shown that the majority of laboratories have a relatively low level of accuracy for sweat testing. The newer commercially available iontophoresis modifications have not shown that they can consistently achieve the accuracy of a carefully performed Gibson-Cooke analysis, although some evaluations have been very favorable. Authorities in cystic fibrosis strongly recommend that the diagnosis of cystic fibrosis should not be made or excluded with certainty on the basis of a single sweat analysis. A minimum of two tests showing unequivocal results with adequate control results are necessary. It is preferable to refer a patient with symptoms suggestive of cystic fibrosis to a specialized cystic fibrosis center experienced in the Gibson-Cooke technique to obtain a definitive diagnosis.

Clinically normal heterozygotes and relatives of patients with cystic fibrosis have been reported to have abnormal sweat electrolytes in 5%-20% of instances, although some investigators dispute these findings.

Some investigators feel that sweat chloride provides better separation of normal persons from persons with cystic fibrosis than sweat sodium.

DNA linkage analysis. The gene causing cystic fibrosis is located on the long arm of chromosome 7 (7q). The most common variant of cystic fibrosis (70% of cases) results from deletion of the 3-nucleotide sequence of a phenylalanine molecule in amino acid position (codon) 508 (often called df508) of the cystic fibrosis gene. When combined with the four next most common gene abnormalities, current DNA probe techniques using the indirect linkage analysis method reportedly have a sensitivity of 85%. Some are using probes for more than five genetic defects and claim a sensitivity of 90% or more. This technique can be applied to prenatal diagnosis in the first trimester using a chorionic villus biopsy specimen.

Other screening tests. Other tests have been suggested to screen for cystic fibrosis. Trypsin is an enzyme produced only in the pancreas. Serum trypsin is elevated in the first months or years of the disease (as the pancreatic cells are destroyed) but eventually decreases below the reference range. However, this leads to considerable overlap with normal persons (in the transition stage between elevated values and decreased values), so that only unequivocally high or low values are significant. The precursor of trypsin is trypsinogen, and levels of this proenzyme are elevated in most neonates with cystic fibrosis and can be measured by immunoassay (immunoreactive trypsinogen, IRT). IRT assay can be performed on filter paper dried blood spots in a similar manner to other neonatal congenital disease screening. Several screening projects have been reported with good results. However, the Cystic Fibrosis Committee noted that various immunoassays have not been standardized, cutoff detection levels are not uniform, and there is a possibility that some of the 10% of infants with cystic fibrosis who have normal pancreatic function could be missed. IRT usually declines in infants with cystic fibrosis after a few weeks, so that repeat testing results would be difficult to interpret. Also, IRT may be normal in some infants with meconium ileus. A 1991 state screening program using IRT detected 95% of infants with cystic fibrosis and used a lower cutoff point on repeat testing to compensate for expected decrease in IRT.

Other methods involve testing meconium in the first stools produced by newborns. One technique tests for increased protein levels (which are predominantly albumin) using a paper dipstick that detects elevated levels of albumin. Since 15%-25% of infants with cystic fibrosis have normal degrees of pancreatic enzyme activity, the test yields at least that number of false negative results. In addition, it yields a considerable number of false positive results. The greatest number of false positives (about 50% of all positive specimens in one study) comes from low-birth-weight infants. Other causes for false positive results include contamination by blood, protein from infant formula, and protein in baby cream. Another approach involves a test for glucose on the meconium stools, which is supposed to reflect the presence or absence of lactase activity. In one study, about one third of cystic fibrosis cases were missed by both the albumin and the glucose (lactose activity) tests.

Diseases of Mineral Metabolism

Wilson’s disease (hepatolenticular degeneration). Wilson’s disease is a familial disorder of copper metabolism transmitted as an autosomal recessive trait. It most often becomes manifest between ages 8 and 30 years; symptoms usually do not develop before age 6 years. About 30%-50% of patients initially develop hepatic symptoms, about 30%-40% begin with neurologic symptoms, and about 20%-30% initially are said to have psychiatric abnormalities such as schizophrenia. A few patients develop a Coombs’-negative hemolytic anemia. Children are more likely to be first seen with hepatic symptoms, although symptoms may occur at any age. In children, these most commonly take the form of chronic hepatitis, although in some patients the test results may resemble acute hepatitis virus hepatitis. A macronodular type of cirrhosis develops later and is usually present in patients with late-stage Wilson’s disease, whether or not there were symptoms of active liver disease. Some patients present with minimally active or with nonactive cirrhosis. Neurologic symptoms typically originate in the basal ganglia area (lentiform nucleus) of the brain and consist of varying degrees of incoordination, tremor, spasticity, rigidity, and dysarthria. There may also be a peculiar flapping tremor. Some young or middle-aged adults develop premature osteoarthritis, especially in the knees.

Wilson’s disease is characterized by inability of the liver to manufacture normal quantities of ceruloplasmin, an alpha-2 globulin that transports copper. For reasons not entirely understood, excessive copper is deposited in various tissues, eventually producing damage to the basal ganglia of the brain and to the liver. The kidney is also affected, leading to aminoaciduria, and copper is deposited in the cornea, producing a zone of discoloration called the Kayser-Fleischer ring.

Clinical diagnosis. The triad of typical basal ganglia symptoms, Kayser-Fleischer ring, and hepatic cirrhosis is virtually diagnostic. However, many patients do not have the textbook picture, especially in the early stages. The Kayser-Fleischer ring is often grossly visible but in many cases can be seen only by slit lamp examination. All patients with neurologic symptoms are said to have the Kayser-Fleischer ring as well as about 50% (range, 27%-93%) of those with hepatic symptoms. The Kayser-Fleischer ring is present in only about 20% (range, 0%-37%) of asymptomatic patients detected during family study investigation or at the beginning of symptoms from hepatic disease without neurologic findings. Overall, about 25% of patients (range, 22%-33%) do not have a demonstrable Kayser-Fleischer ring at the time of diagnosis. Patients with primary biliary cirrhosis or, occasionally, other types of chronic cholestatic liver disease may develop a corneal abnormality identical to the Kayser-Fleischer ring.

Plasma ceruloplasmin assay. Laboratory studies may be of value in diagnosis, especially in the preclinical or early stages. Normally, about 90%-95% of serum copper is bound to ceruloplasmin, one of the alpha-2 globulins. The primary excretion pathway for serum copper is through bile. The serum ceruloplasmin level is low from birth in 95% (range, 90%-96%) of homozygous patients, and is considered the best screening test for Wilson’s disease. About 10% (range, 6%-20%) of Wilson’s disease heterozygotes have decreased serum ceruloplasmin. However, normal newborn infants usually have decreased ceruloplasmin levels, and the test is not considered reliable until 3-6 months of age. Although a normal ceruloplasmin level (over 20 mg/100 ml; 200 mg/L) is usually interpreted as excluding Wilson’s disease, about 5% (range, 4%-10%) of homozygous Wilson’s disease patients have values greater than 20 mg/100 ml. This is more likely to be found in younger children and in those with hepatic disease. Estrogen therapy, pregnancy, active liver disease of various etiologies, malignant lymphoma, and occasionally various acute inflammatory conditions (since ceruloplasmin is one of the “acute reaction” proteins) can raise ceruloplasmin levels in variable numbers of cases. Smoking is reported to raise ceruloplasmin levels about 15%-30%. Although a decreased ceruloplasmin level is usually considered suggestive of Wilson’s disease, about 5% of normal persons may have values less than 20 mg/100 ml (200 mg/L), and values may be decreased in hereditary tyrosinemia, Menke’s kinky hair syndrome, the nephrotic syndrome, malabsorption syndromes such as sprue, and in various liver diseases (about 20% of cases in one study. However, it is possible that some patients with liver disease and decreased ceruloplasmin levels actually have Wilson’s disease).

Liver biopsy has also been used for diagnosis. The microscopic findings are not specific, and most often consist of either macronodular cirrhosis (often with some fatty change and occasionally with Mallory bodies) or chronic active hepatitis (10%-15% of patients with Wilson’s disease). The most typical finding is increased hepatic copper content by special stains (or tissue analysis, if available). For histologic staining of copper, fixation of the biopsy specimen in alcohol rather than the routine fixatives is recommended. Here again, it is advisable to wait 6-12 weeks after birth. Increased hepatic copper content is not specific for Wilson’s disease, since some degree of copper increase has been reported to occur in some patients with postnecrotic cirrhosis due to hepatitis virus hepatitis, in patients with primary biliary cirrhosis, and occasionally in patients with other chronic cholestatic syndromes. Also, increased hepatic copper content is not present in all patients with Wilson’s disease, especially in small-needle biopsy specimens.

Serum and urine copper. Total serum copper levels are decreased in 85%-90% of Wilson’s disease patients. However, serum copper not bound to serum ceruloplasmin is usually normal or increased. Twenty-four-hour urine copper excretion in symptomatic Wilson’s disease is increased in 90% of patients. However, 24-hour copper excretion is often normal in presymptomatic patients. Increased urine copper excretion is not specific for Wilson’s disease and may be found in various types of cirrhosis, especially those with some degree of cholestasis and in 10%-30% of chronic active hepatitis patients. However, these conditions usually have normal or elevated serum ceruloplasmin levels.

DNA probes. The gene affected in Wilson’s disease has been found on the long arm of chromosome 13, close to the gene responsible for retinoblastoma. DNA linkage probes for Wilson’s disease have been reported. In some cases, the retinoblastoma probe has been used.

Other laboratory abnormalities. Besides abnormalities in copper metabolism, over 50% of patients (78% in one study) have a low serum uric acid level, a finding that could arouse suspicion of Wilson’s disease if supporting evidence is present. Other laboratory findings that may be encountered in some patients are low serum phosphorus levels, thrombocytopenia (about 50%; range, 22%-82%, due to cirrhosis with secondary hypersplenism), aminoaciduria, glucosuria, and uricosuria. A Coombs’-negative hemolytic anemia occurs in a few patients.

Hemochromatosis. Hemochromatosis is an uncommon disease produced by idiopathic excess iron absorption from the GI tract, which leads to excess deposition of iron in various tissues, especially the liver. There still is dispute as to which iron storage diseases should be included within the term hemochromatosis. In this discussion, hemochromatosis refers to the hereditary iron storage disorder and hemosiderosis to nonhereditary (secondary) forms. Hemochromatosis is transmitted as an autosomal recessive trait with the gene being located on the short arm of chromosome 6 close to the class I histocompatibility antigen (HLA) locus. Males are affected more often than females (3:2 in one series), and males seem overall to have more severe disease than females. HLA-A3 antigen is present in 70%-80% of patients (vs. 20%-30% in the normal population).

Clinical onset of the disease is usually between ages 40 and 60 years. Signs, symptoms, and laboratory abnormalities depend on the stage of disease and (probably) whether there is also a significant degree of alcohol intake. Cirrhosis, diabetes mellitus, and bronze skin pigmentation form a classic triad diagnostic of hemochromatosis. However, this triad is a late manifestation, and in one study including more early cases it was present in less than 10% of the patients. The most frequent symptom is joint pain (47%-57% of patients; 50%-75% in patients with severe disease), which can be confused with rheumatoid arthritis. Hepatomegaly is present in 54%-93% of patients, cirrhosis on liver biopsy in 57%-94%, heart failure in 0%-35%, hypogonadism (in males) in 18%-61%, skin pigmentation in 51%-85% (not really noticeable in many patients), and clinically evident diabetes in 6%-72%. Alcoholism (15%-50%) or poor nutrition was frequent in some series. Hepatoma has been reported to develop in 15%-30% of patients.

Laboratory findings include the expected blood glucose abnormalities of diabetes (chapter 28) in those patients with overt diabetes, and decreased glucose tolerance in some of those without clinical diabetes. AST levels are elevated in 46%-54% of cases, reflecting active liver cell involvement. In one series, AST, alkaline phosphatase (ALP), and gamma-glutamyltransferase were normal or only mildly elevated unless the patient was alcoholic.

Laboratory iron studies. The body iron abnormality is manifested by actual or relative increase in serum iron levels and decrease in total iron-binding capacity (TIBC), producing increased saturation (% saturation) of the TIBC. In addition, hemosiderin very often can be demonstrated in the urine sediment by iron stains. The most sensitive laboratory test for hemochromatosis is percent saturation of TIBC (or of transferrin), which is greater than 60% (reference range, 16%-50%) in over 90% of male homozygotes and the 60% of females who have iron loading but which misses the 40% of females who do not have iron loading. Transferrin saturation of 50% detects most males or females with or without iron loading. Therefore, it has been proposed that the screening cutoff point should be 60% for males and 50% for females. Serum iron level is increased in more than 80% of patients and serum ferritin level is increased in more than 72% of patients; both of these tests are usually abnormal in affected males but much more variable in females. However, in one report about one third of patients with chronic hepatitis B or C also had elevated serum iron, ferritin, and percent saturation, and serum ferritin is often increased by various acute inflammatory conditions. Liver biopsy demonstrates marked deposition of iron in parenchymal cells and frequently reveals cirrhosis.

The most widely used screening test is serum iron. Elevated values raise the question of hemochromatosis. About 2.4% of normal persons are reported to have elevated serum iron values that spontaneously return to the reference range within 1-2 days. The effect of serum diurnal variation and day-to-day variation must be considered. Serum iron levels can also be increased in chronic hepatitis B or C infection (46% of cases in one study) and in hemosiderosis (nonhereditary iron overload) due to blood transfusion, chronic severe hemolytic anemias, sideroblastic anemias, alcoholic cirrhosis, parenteral iron therapy, and considerably increased iron intake. Several other conditions that may be associated with increased serum iron levels are listed in Table 37-2. Various conditions can lower the serum iron level (especially chronic iron deficiency and moderate or severe chronic disease without iron deficiency), and if one of these conditions is superimposed on hemochromatosis, the serum iron level might be decreased sufficiently to reach the reference range area.

As noted earlier, the best screening procedure is percent saturation of transferrin. This is calculated by dividing the serum iron value by the TIBC value. However, like serum iron, increase in percent transferrin saturation is not specific for hemochromatosis, since there are other conditions that decrease percent saturation, especially alcohol-related active cirrhosis. One study found that drawing specimens after an overnight fast considerably decreased false elevation of percent saturation. In addition, there is considerable variation in the literature as to the percent saturation cutoff point that should be used (50%-80%, with the majority using either 50% or 62%). The lower levels increase sensitivity in detecting hemochromatosis; the higher levels eliminate many patients who do not have hemochromatosis.

Definitive diagnosis is made by liver biopsy and measurement of hepatic iron content. Even liver biopsy iron may not differentiate hemochomatosis from hemosiderosis in some cases, and the liver cells of patients with cirrhosis but without demonstrable abnormality of iron metabolism may display some degree of increased iron deposition.

Family member screening. Hemochromatosis rarely becomes clinically evident before age 30, so that screening family members of patients has been advocated to detect unrecognized homozygotes to begin therapy before clinical symptoms develop. One study found that percent transferrin saturation detected about 90% of occult homozygotes, whereas assay of serum iron levels detected about 85% and assay of serum ferritin levels detected about 50%.

Congenital Diseases of Skeletal Muscle

Several well-known disorders affecting skeletal muscle either are not congenital or do not yet have any conspicuously useful laboratory test. Among these are disorders whose primary defect is located in the central nervous system or peripheral nervous system rather than in skeletal muscle itself. In this group are various neurologic diseases that secondarily result in symptoms of muscle weakness. The following discussion involves inherited muscle disorders. Some can be diagnosed in the first trimester of pregnancy by means of amniotic villus biopsy.

Muscular dystrophies. The muscular dystrophies can be divided into several subgroups. The most common is Duchenne’s (pseudohypertrophic) dystrophy. Duchenne’s muscular dystrophy and the closely related Becker’s muscular dystrophy is transmitted as a familial sex-linked recessive disorder in 60%-65% of cases and is said to be the most common lethal sex-linked genetic disease. As in all sex-linked genetic diseases, the X chromosome carriers the abnormal gene. In Duchenne’s dystrophy this gene controls production of dystrophin, a protein found in skeletal, cardiac, and smooth muscle at the muscle fiber outer membrane, where it apparently helps provide strength and elasticity to the muscle fiber. Although both males and females may have the defective gene, females rarely develop clinical symptoms. About one third of cases are sporadic gene mutations. The male patient is clinically normal for the first few months of life; symptoms develop most often between ages 1 and 6 years. The most frequent symptoms are lower extremity and pelvic muscle weakness. There is spotty but progressive muscle fiber dissolution, with excessive replacement by fat and fibrous tissue. The latter process leads to the most characteristic physical finding of the disease, pseudohypertrophy of the calf muscles.

Laboratory tests. Screening tests are based on the fact that certain enzymes are found in relatively high amounts in normal skeletal muscle. These include creatine phosphokinase, aldolase, aspartate aminotransferase (AST), and lactic dehydrogenase (LDH). Despite external pseudohypertrophy, the dystrophic muscles actually undergo individual fiber dissolution and loss of skeletal muscle substance, accompanied by release of muscle enzymes into the bloodstream. In many tissues AST, LDH, and aldolase are found together. Pulmonary infarction, myocardial infarction, and acute liver cell damage among other conditions cause elevated serum levels of these enzymes. Aldolase follows a pattern similar to AST in liver disease and to LDH otherwise. Creatine Kinase (previously creatine phosphokinase) or CK is found in significant concentration only in brain, heart muscle, and skeletal muscle.

The two most helpful tests in Duchenne’s muscular dystrophy are CK and aldolase assays. Aldolase and CK values are elevated very early in the disease, well before clinical symptoms become manifest, and the elevations usually are more than 10 times normal, at least for CK. This marked elevation persists as symptoms develop. Eventually, after replacement of muscle substance has become chronic and extensive, the aldolase level often becomes normal and the CK level may be either normal or only mildly elevated (less than 5 times normal). In the hereditary type of Duchenne’s dystrophy, most males with the abnormal gene have elevated CK values. In females with the abnormal gene, about 50%-60% have elevated CK. Aldolase values are much less frequently abnormal; AST and LDH values tend to parallel CK and aldolase values but at a much lower level. Therefore, other than CK, these enzymes are not of much use in detecting carriers. Even with CK, a normal result does not exclude carrier status.

The CK isoenzyme pattern in Duchenne’s dystrophy may show an increased MB isoenzyme as well as MM fraction, especially in the earlier phases of the illness.

In fascioscapulohumeral dystrophy and limbgirdle dystrophy, conditions that resemble Duchenne’s dystrophy in many respects, CK and aldolase levels are variable but frequently are normal.

Other muscular disorders in which the serum enzyme levels may be elevated are trauma, dermatomyositis, and polymyositis. The levels of elevation are said to be considerably below those seen in early cases of Duchenne’s dystrophy. Neurologic disease is usually not associated with elevated levels, even when there is marked secondary muscular atrophy.

Definitive diagnosis. Diagnosis of the muscular dystrophies may sometimes be made on the basis of the clinical picture and enzyme values. A more definitive diagnosis can be made with the addition of muscle biopsy. This becomes essential when the findings are not clear cut. The biceps or quadriceps muscles are the preferred biopsy location. The biopsy is best done at a pediatric or congenital disease research center where special studies (e.g., histochemical staining or electron microscopy) can be performed and the proper specimen secured for this purpose (these special studies provide additional information and are essential when biopsy results are atypical or yield unexpected findings). Biopsy specimens show greatly decreased dystrophin on assay or essentially absent dystrophin on tissue sections stained with antidystrophin antibody. In Becker’s dystrophy, dystrophin is present in tissue sections but considerably reduced. Another diagnostic method is DNA probe. About two thirds of Duchenne’s and Becker’s cases are due to partial deletion from the dystrophin gene. These cases can be diagnosed by standard DNA probe. The 35% without detectable deletion can be tested for with the restriction length polymorphism DNA probe method, which is less accurate than gene deletion DNA methods. Diagnosis in the first trimester of pregnancy can be done using DNA probe techniques on a chorionic villus biopsy specimen.

Malignant hyperpyrexia (MH) Malignant hyperpyrexia (MH) is a rare complication of anesthesia triggered by various conduction and inhalation agents (most commonly succinylcholine) that produces a marked increase in both aerobic and anaerobic skeletal muscle metabolism. This results in greatly increased production of carbon dioxide, lactic acid, and heat. Such overproduction, in turn, is clinically manifested by a marked increase in body temperature, tachycardia, muscle rigidity, tachypnea, and finally shock. The first clinical sign is said to be muscle rigidity, which occurs in about 70%-75% of patients. The next clinical evidence of developing MH is tachycardia or cardiac ventricular multifocal arrhythmias. A rise in temperature eventually occurs in nearly all patients (some cases have been reported without temperature elevation), first slowly and then rapidly. The characteristic temperature elevation may not be present in the early stages, or the initial elevation may be gradual. A defect in muscle cell membrane calcium release mechanism (“calcium channel”) has been postulated, leading to increased intracellular calcium. The majority of cases have been familial. There is frequent association with various hereditary muscle diseases, especially the muscular dystrophies.

Laboratory tests. Biochemical abnormalities include metabolic acidosis (markedly elevated lactic acid value) and respiratory acidosis (increased partial pressure of carbon dioxide [P CO2] due to muscle CO2 production). The anion gap is increased due to the lactic acid. In the early phases, venous P CO2 is markedly increased, whereas arterial P CO2 may be normal or only mildly increased (widening of the normal arteriovenous [AV] CO2 dissociation). The same accentuation of normal AV differences also occurs with the PO2 values. Later, arterial blood gas values also show increased PCO2, decreased pH, and decreased PO2. In addition to blood gas changes there typically is greatly increased CK levels (from muscle contraction), and myoglobin appears in the urine. In the later stages there may be hyperkalemia, hypernatremia, muscle edema, pulmonary edema, renal failure, disseminated intravascular coagulation, and shock. The serum calcium level may be normal or increased, but the ionized calcium level is increased.

Diagnosis. CK elevation without known cause has been proposed as a screening test; there is marked difference of opinion among investigators as to the usefulness of the procedure, either in screening for surgery or in family studies (literative reports vary from 0%-70% regarding the number of persons susceptible to develop MH that have elevated baseline total CK. About 30% may be a reasonable estimate. Also, elevated CK can be caused by a variety of conditions affecting muscle). Likewise, disagreement exists as to the CK isoenzyme associated with abnormality; BB has been reported by some, although the majority found MM to be responsible. However, CK assay is still the most widely used screening test. Muscle biopsy with special in vitro testing of muscle fiber sensitivity to such agents as caffeine and halothane (caffeine-halothane contractive test) is considered a definitive diagnostic procedure but is available only in a few research centers. Since the test should be performed less than 5 hours after the muscle biopsy, it is preferable (if possible) to have this biopsy performed at the institution that will do the test. Microscopic examination of muscle biopsy specimens shows only nonspecific abnormalities, most often described as compatible with myopathy.

Chromosomal Abnormalities

Chromosome analysis. There are several conditions, some relatively common and some rare, that result from either abnormal numbers of chromosomes, defects in size or configuration of certain single chromosomes, or abnormal composition of the chromosome group that determines sexual characteristics. Laboratory diagnosis, at present, takes three forms. First, chromosome charts may be prepared on any individual by culturing certain body cells, such as WBCs from peripheral blood or bone marrow, and by introducing a chemical such as colchicine, which kills the cells at a specific stage in mitosis when the chromosomes become organized and separated, and then photographing and separating the individual chromosomes into specific groups according to similarity in size and configuration. The most widely used system is the Denver classification. The 46 human chromosomes are composed of 22 chromosome pairs and, in addition, 2 unpaired chromosomes, the sex chromosomes (XX in the female and XY in the male). In a Denver chromosome chart (karyotype) the 22 paired chromosomes are separated into 7 groups, each containing 2 or more individually identified and numbered chromosomes. For example, the first group contains chromosomes 1 to 3, the seventh group contains chromosomes 21 to 22. In addition, there is an eighth group for the two unpaired sex chromosomes. Chromosome culture requires substantial experience and care in preparation and interpretation. Material for chromosome analysis can be obtained in the first trimester of pregnancy by means of chorionic villus biopsy.

Barr body test. The other, more widely used technique provides certain useful information about the composition of the sex chromosome group. Barr found that the nuclei of various body cells contain a certain stainable sex chromatin mass (Barr body) that appears for each X chromosome more than one that the cell possesses. Therefore, a normal male (XY) cell has no Barr body because there is only one X chromosome, a normal female (XX) cell has one Barr body, and a person with the abnormal configuration XXX has two Barr bodies. The most convenient method for Barr body detection at present is the buccal smear. This is obtained by scraping the oral mucosa, smearing the epithelial cells thus collected onto a glass slide in a monolayer, and, after immediate chemical fixation, staining with special stains. Comparison of the results, together with the secondary sex characteristics and genitalia of the patient, allows presumptive diagnosis of certain sex chromosome abnormalities. The results may be confirmed, if necessary, by chromosome karyotyping.

Specimens for buccal smear should not be obtained during the first week of life or during adrenocorticosteroid or estrogen therapy, because these situations falsely lower the incidence of sex chromatin Barr bodies. Certain artifacts may be confused with the nuclear Barr bodies. Poor slide preparations may obscure the sex chromatin mass and lead to false negative appearance. Only about 40%-60% of normal female cells contain an identifiable Barr body. The buccal smear by itself does not reveal the true genetic sex; it is only an indication of the number of female (X) chromosomes present. Many labs no longer do this test.

The third method is nucleic acid probe, more sensitive than either Barr body or standard chromosome analysis. However, the chromosome abnormality must be known and a probe must be available for that specific gene or chromosome area.

Klinefelter’s syndrome. In this condition the patient looks outwardly like a male, but the sex chromosome makeup is XXY instead of XY. The external genitalia are usually normal except for small testes. There is a tendency toward androgen deficiency and thus toward gynecomastia and decreased body hair, but these findings may be slight or not evident. There also is a tendency toward mental deficiency, but most affected persons have perfectly normal intelligence. Patients with Klinefelter’s syndrome are almost always sterile. Testicular biopsy used to be the main diagnostic method, with histologic specimens showing marked atrophy of the seminiferous tubules. A buccal smear can be done; it shows a “normal female” configuration with one Barr body (due to the two XX chromosomes). In the presence of unmistakably male genitalia, this usually is sufficient for clinical diagnosis. Since 10% of cases have a mosaic cell pattern, chromosome karyotyping is now the procedure of choice.

Turner’s syndrome (ovarian agenesis). Turner’s syndrome is the most frequent chromosomal sexual abnormality in females, just as Klinefelter’s syndrome is in males. In Turner’s syndrome there is a deletion of 1 female (X) chromosome so that the patient has only 45 chromosomes instead of 46 and only 1 female sex chromosome instead of 2. Typically the affected female has relatively short stature but normal body proportions. There is deficient development of secondary sex characteristics and small genitalia, although body hair usually is female in distribution. Some affected persons have associated anomalies such as webbing of the neck, coarctation of the aorta, and short fingers. They do not menstruate and actually lack ovaries. A buccal smear should be “sex-chromatin negative,” since Barr bodies appear only when the female sex chromosomes number more than one. If the buccal smear is “chromatin positive,” a chromosome karyotype should be ordered, because some patients with Turner’s syndrome have mixtures of normal cells and defective cells (mosaicism). Some investigators believe that in patients with short stature only, chromosome karyotyping should be done without a buccal smear, since most of the “nonphenotypic” Turner’s syndrome patients have mosaicism rather than XO genotype. Most geneticists karyotype without buccal smear due to smear interpretation problems.

Down’s syndrome. Down’s syndrome is a relatively frequent disorder associated with two different chromosome abnormalities. Most patients (about 92%) have an extra number 21 chromosome in the number 21-22 chromosome group (therefore having 3 chromosomes in this group instead of 2, a condition known as trisomy 21). These patients have a total of 47 chromosomes instead of 46. The chromosome abnormality has nothing to do with the sex chromosomes, which are normal. This type of Down’s syndrome apparently is spontaneous, not inherited (i.e., there is no family history of Down’s syndrome and there is very little risk the parents will produce another affected child). This nonfamilial (sporadic) type of Down’s syndrome occurs with increased frequency when the mother is over age 35. About 5% of patients have familial Down’s syndrome; the patient has an extra 21-type chromosome, but it is attached to one of the other chromosomes, most often in the 13-15 group (called the “D group” in some nomenclatures). This type of arrangement is called a “translocation.” The translocation attachment is most frequent on the number 14 chromosome, but it may attach elsewhere. The translocation abnormality can be inherited; it means that one parent has a normal total number of chromosomes, but one of the pair of number 21 chromosomes was attached to one of the number 14 chromosomes. The other number 21 and the other number 14 chromosome are normal. The two-chromosome (14 + 21) cluster behaves in meiosis as though it were a single number 14 chromosome. If the abnormal chromosome cluster is passed to a child, two situations could result: a child with clinical Down’s syndrome who received the translocated 14 + 21 chromosome plus the normal number 21 chromosome from one parent (and another number 21 chromosome from the other parent, making a total of three number 21 chromosomes), or a carrier who received the translocated 14 + 21 chromosome but did not receive the other (normal) number 21 chromosome from the same parent (the translocated 14 + 21 chromosome plus a number 21 chromosome from the other parent make a total of two number 21 chromosomes). The translocation Down’s syndrome patient has a total of 46 chromosomes (the two-chromosome unit counts as a single chromosome).

Clinically, an infant or child with Down’s syndrome usually has some combination of the following: prominent epicanthal folds at the medial aspect of the eyes, flattened facies, flat bridge of the nose, slanted lateral aspect of the eyes, mental retardation or deficiency, broad hands and feet, and a single long transverse crease on the palm instead of several shorter transverse creases. Other frequent but still less common associated abnormalities are umbilical hernia, webbing of the toes, and certain types of congenital heart disease. There also is an increased incidence of acute leukemia.

Diagnosis usually can be made clinically, but chromosome karyotyping is a valuable means of confirmation and of diagnosis in equivocal cases. It probably is advisable to do chromosome karyotyping in most children with Down’s syndrome, because the type of chromosome pattern gives an indication of the prognosis for future children.

Prenatal diagnosis can be made in the first trimester by chorionic villus biopsy with chromosome analysis. Screening for Down’s syndrome can be done using maternal serum during the 16th to 18th gestation week. If the maternal alpha-fetoprotein serum level is lower than normal, the unconjugated estriol (E3) lower than normal, and the beta human chorionic gonadotropin (beta-hCG) higher than normal, this suggests possible Down’s syndrome. This would have to be confirmed with fetal cells obtained by amniocentesis (chorionic villus biopsy is not done after the 12th week of pregnancy).

Fragile X chromosome. The fragile X chromosome refers to a narrowing in the X chromosome, at which point the chromosome breaks more easily than usual when cultured in a medium that is deficient in thymidine and folic acid. The syndrome is said to be second only to Down’s syndrome as a cause of hereditary mental retardation. The fragile X abnormality is reported to be associated with 30%-50% of cases of X-linked mental retardation as part of a syndrome which also includes certain mild facial changes. About 30%-35% of female carriers may have mild mental retardation, which is unusual for heterozygotic status in most genetic illnesses and very unusual for an X-linked inherited disorder (in which the carrier female seldom has clinical symptoms). In addition, about 20% of males with the chromosome defect are asymptomatic and not detectable by standard chromosome analysis. Male offspring of a (heterozygous) carrier female would have a 50% chance of developing the syndrome. Unfortunately, only about 30%-56% of heterozygotic females demonstrate the fragile X defect using current laboratory methods. Sensitivity of these methods is age dependent, and best detection rates occur testing women less than 30 year old. There have also been reports of some affected men with normal range IQ who would qualify as carriers. It has been estimated that as many as 20% of male offspring with normal IQS born to female carriers actually are themselves carriers. DNA probe methods are now available that can often detect fragile X presence when standard chromosome analysis is equivocal or negative.

Adult polycystic kidney disease (PKD-1). This autosomal dominant condition is reported to be present in 1 of 1,000 live births. Multiple cysts form in the kidney and eventually enlarge, destroying nearby renal parenchyma and in many cases eventually resulting in renal failure. The genetic abnormality is located on chromosome 16. DNA probes are used that bracket the gene area (gene linkage analysis using restriction fragment length polymorphism).

Other chromosomal disorders. A wide variety of syndromes, usually consisting of multiple congenital deformities and anomalies, are now found to be due to specific chromosomal abnormalities. The most common of these involve trisomy in the 13-15 (D) group and in the 16-18 (E) group. Some patients with repeated spontaneous abortions have abnormal karyotypes. Various tumors have yielded abnormal chromosome patterns, but no one type of tumor is associated with any consistent pattern (except for chronic myelogenous leukemia).

Commonly accepted indications for buccal smear. These include the following:

1. Ambiguous or abnormal genitalia
2. Male or female infertility without other known cause
3. Symptoms suggestive of Turner’s syndrome or Klinefelter’s syndrome, such as primary amenorrhea

Indications for chromosome karyotyping

These include the patients in the buccal smear groups just described for confirmation or initial diagnosis and the following:

1. Down’s syndrome infants or possible carriers
2. Mentally defective persons
3. Persons with multiple congenital anomalies

Defects in Amino Acid metabolism (Aminoacidopathies)

Primary (metabolic) aminoacidopathies

Phenylketonuria (PKU). Classic PKU is inherited as an autosomal recessive trait. It is uncommon in Asians and African Americans and is due to deficiency of a liver enzyme known as “phenylalanine hydroxylase,” which is needed to convert the amino acid phenylalanine to tyrosine. With its major utilization pathway blocked, phenylalanine accumulates in the blood and leads to early onset of progressive mental deficiency. This disease is one of the more common causes of hereditary mental deficiency and one of the few whose bad effects can be prevented by early treatment of the infant. At birth the infant usually has normal serum levels of phenylalanine (<2 mg/100 ml) due to maternal enzyme activity, although some instances of mental damage in utero occur. In the neonatal period, after beginning a diet containing phenylalanine (e.g., milk), serum phenylalanine levels begin to rise. When the phenylalanine to tyrosine pathway is blocked, some phenylalanine metabolism is shunted to ordinarily little-used systems such as transamination to phenylpyruvic acid.

Urine screening tests. When the serum phenylalanine level reaches 12-15 mg/100 ml, sufficient phenylpyruvic acid is produced that it begins to appear in the urine. This becomes detectable by urine screening tests (Phenistix or the older ferric chloride test) at some time between ages 3 and 6 weeks. Unfortunately, by then some degree of irreversible mental damage may have occurred. Therefore, it is highly desirable to make an earlier diagnosis to begin treatment as soon as possible after birth.

Blood screening tests. The most widely used screening test for elevated serum phenylalanine levels is the Guthrie test. This is a bacterial inhibition procedure. A certain substance that competes with phenylalanine in Bacillus subtilis metabolism is incorporated into culture medium; this essentially provides a phenylalanine-deficient culture medium. Bacillus subtilis spores are seeded into this medium; but to produce significant bacterial growth, a quantity of phenylalanine equivalent to more than normal blood levels must be furnished. Next, a sample of the patient’s blood is added, and the presence of abnormal quantities of serum phenylalanine is reflected by bacterial growth in the area where the specimen was applied. The Guthrie test, if properly done, is adequately sensitive and accurate and reliably detects definitely abnormal levels of serum phenylalanine (і4 mg/100 ml). It also fulfills the requirements for an acceptable screening method. However, there are two controversial aspects to this test. First, the standard practice is to obtain a blood specimen from the infant (usually as a filter paper blood spot using a heel puncture) before discharge from the hospital. In some cases this may result in the specimen being obtained 48 hours or less after birth, with an even shorter duration of phenylalanine intake if milk feeding is not begun soon after birth. There is some controversy in the literature as to whether a significant percentage (about 5%-10%) of infants with PKU will be missed if the specimen is obtained in the first 48 hours of life. A few studies indicate that this is unlikely, but the number of patients was not large enough to conclusively establish this point. Some screening programs recommend second testing of infants discharged before 48 hours of life, and most authorities recommend retesting if the initial test specimen was obtained before 24 hours of life.

Second, PKU is not the only cause of elevated serum phenylalanine levels in the neonate. In fact, more positive (abnormally elevated) Guthrie test results are reportedly caused by non-PKU etiologies than by PKU. A major etiology is temporary (“late enzyme development”) hypertyrosinemia associated with low birth weight, high-protein formulas, and vitamin C deficiency. Some infants with severe liver disease and some with galactosemia have been reported to have elevated serum phenylalanine levels. Since hyperphenylalaninemia is not diagnostic of PKU, and since a long-term low-phenylalanine diet could be harmful to some persons who do not have PKU, an abnormal Guthrie test result should be followed up by more detailed investigation, including, as a minimum, both the serum phenylalanine and tyrosine levels. The typical PKU patient has a serum phenylalanine level greater than 15 mg/100 ml, with a serum tyrosine level less than 5 mg/100 ml. The tests may have to be repeated in 1-2 weeks if values have not reached these levels. DNA probe diagnosis is also available for equivocal or problem cases or prebirth diagnosis.

Phenylketonuria variants. About 10% of infants with apparent PKU have been found to have a PKU variant. The enzyme system for alteration of phenylalanine to tyrosine is actually a group of at least four enzymes and coenzymes. Deficiency in the hydroxylase enzyme produces classic PKU. Deficiency in one of the other components of the system produces variant PKU. In particular, the variant caused by deficiency of the cofactor tetrahydrobiopterin (estimated to account for 0.5%-3.0% of persistent hyperphenylalaninemias) requires therapy in addition to a phenylalanine-deficient diet. Some patients with other variants of PKU do not require a phenylalanine-free diet.

Diagnosis of PKU variants was originally made with a tolerance test using oral phenylalanine (in milk or other materials). Persistence of elevated blood phenylalanine levels greater than 20 mg/100 ml for more than 72 hours was considered indicative of classic PKU, whereas a decrease below the 20-ml level before 72 hours was considered indicative of variant PKU. This association has been challenged by others, and sophisticated tests have been devised to determine the exact etiology of the different forms of persistent hyperphenylalaninemia. Some of these procedures are available only in pediatric research centers. It would seem reasonable to recommend that an abnormal Guthrie test result be immediately followed up with a blood specimen for assay of phenylalanine and tyrosine levels. Afterward, pending results, the infant should be placed on a low-phenylalanine diet. If both phenylalanine and tyrosine levels are high, the patient probably does not have PKU. If only the phenylalanine level is high, and especially if it is greater than 15 mg/100 ml, the infant should be referred to a specialized PKU center for additional studies while the low-phenylalanine diet is continued.

Alkaptonuria (ochronosis). The typical manifestations of this uncommon disease are the triad of arthritis, black pigmentation of cartilage (ochronosis), and excretion of homogentisic acid in the urine. Arthritis usually begins in middle age and typically involves the spine and the large joints. Black pigmentation of cartilage is most apparent in the ears but may be noticed in cartilage elsewhere or may even appear in tendons. The intervertebral disks often become heavily calcified and thus provide a characteristic x-ray picture. The disease is caused by abnormal accumulation of homogentisic acid, an intermediate metabolic product of tyrosine; this, in turn, is caused by a deficiency of the liver enzyme homogentisic acid oxidase, which mediates the further breakdown of the acid. Most of the hemogentisic acid is excreted in the urine, but enough slowly accumulates in cartilage and surrounding tissues to cause the characteristic changes previously described. Diagnosis is established by demonstration of homogentisic acid in the urine. Addition of 10% sodium hydroxide turns the urine black or gray-black. A false positive urine glucose test result is produced by copper reduction methods such as Benedict’s test or Clinitest, whereas glucose oxidase dipstick methods are negative.

Other primary aminoacidopathies. These are numerous, varied, and rare. They are mostly diagnosed by paper chromatography of urine (or serum), looking for abnormal quantities of the particular amino acid involved whose metabolic pathway has been blocked. The most widely known diseases (apart from PKU and alkaptonuria) are maple syrup disease and histidinemia. The most common is homocystinuria. Many of the primary aminoacidopathies can be diagnosed in the first trimester of pregnancy by means of chorionic villus biopsy.

Secondary aminoacidopathies. Secondary aminoacidopathies are associated with a renal defect, usually of reabsorption, rather than a primary defect in the metabolic pathway of the amino acid in question. The serum levels are normal. The most common cause is a systemic disease such as Wilson’s disease, lead poisoning, or Fanconi’s syndrome. In such cases, several amino acids usually are found in the urine. Aminoaciduria may occur normally in the first week of life, especially in premature infants. A much smaller number of patients have a more specific amino acid renal defect with one or more specific amino acids excreted; the most common of these diseases is cystinuria. Patients with cystinuria develop cystine renal calculi. Cystine crystals may be identified in acidified urine, proving the diagnosis. Otherwise, combined urine and serum paper chromatography is the diagnostic method of choice.

Mucopolysaccharidoses (Disorders of Connective Tissue and Bone)

The best known of this group are Hunter’s and Hurler’s syndromes. In these conditions there is inability to metabolize certain mucopolysaccharides, resulting in accumulation and storage of these substances in various body organs and tissues and excretion of some stored material in the urine.

Hurler’s syndrome. Hurler’s syndrome is caused by deficiency of the enzyme alpha-L-iduronase. Affected infants appear normal for the first 6-8 months of life but then develop skeletal abnormalities (short stature, kyphosis, broad hands, saddle-shaped deformity of the bridge of the nose), clouding of the cornea leading to severe loss of vision, a tendency toward umbilical and inguinal hernias, hepatosplenomegaly, thick tongue, and mental retardation. Diagnosis of Hurler’s syndrome (or any of the mucopolysaccharide disorders) can be made by chromatographic identification of the mucopolysaccharide excreted in the urine. A more conclusive diagnosis can be established by tissue culture of skin biopsy fibroblasts with assay for the specific enzyme involved.

Other connective tissue disorders. There is an important group of hereditary connective tissue disorders, including Marfan’s syndrome, Ehlers-Danlos syndrome, and osteogenesis imperfecta, for which no laboratory screening test or specific diagnostic biochemical test is available. Diagnosis is made by clinical criteria, in some cases supported by x-ray findings.

Lysosomal Storage Diseases

Lysosomal storage diseases are the result of genetic deficiency in certain enzymes found in tissue cell cytoplasmic lysosomes. These enzymes help metabolize certain glycoproteins, glycolipids, and mucopolysaccharides. The substance normally altered by the deficient enzyme accumulates in the cell lysosome area, and a storage disease results. The nonmetabolized substance either is stored in the tissue cell cytoplasm or is taken up by phagocytes. Most of these conditions can be diagnosed by rather sophisticated assays for the enzyme that is deficient. In some instances the assay can be carried out on plasma or serum; in other cases, on peripheral blood white blood cells (WBCs); and for some enzymes, it is necessary to use patient fibroblasts obtained by skin biopsy and grown in tissue culture. In some cases the diagnosis can be made from fetal cells obtained by amniocentesis and grown in tissue culture. In most cases the assays are available only at university medical centers or specialized clinics. A few are available at certain large reference laboratories. It is recommended that a university medical center specializing in such problems or the Neurological Diseases branch of the National Institutes of Health be contacted for details on how to proceed with any patient suspected of having a lipid storage disease. It is highly preferable that the patient be sent directly to the center for biopsy or required specimen collection to avoid unnecessary and costly delays and to prevent damage to the specimen in transport.

The lysosomal storage diseases can be divided into two major categories: the sphingolipidoses and the mucopolysaccharidoses. A short summary of these conditions is presented. Several are described below in more detail. Many of these disorders can be diagnosed in the first trimester of pregnancy by chorionic villus biopsy.

Sphingolipidoses (disordered lipid metabolism) The best known of this group are glycolipid storage diseases, including ganglioside storage (Tay-Sachs disease and metachromatic leukodystrophy) and ceramide storage diseases (Gaucher’s disease and Niemann-Pick disease).

Tay-Sachs disease. This condition is due to accumulation of the ganglioside GM2 in various tissues but most notably in the brain. The defective enzyme responsible is known as “hexaminidase.” There are two forms (similar to isoenzymes) of hexaminidase, commonly abbreviated as hex-A and hex-B. The ethnic groups most often affected by classic Tay-Sachs disease are Ashkenazic (Central or Eastern European) Jews and, to a lesser extent, other Jews and inhabitants of the Middle East country of Yemen. There are several other very similar disorders involving hexaminidase deficiency that are generally considered variants of Tay-Sachs disease and are not commoner in Jews. In the classic and commonest form of Tay-Sachs disease, the infant appears normal at birth and for the first 5-6 months but then fails to develop any further mentally, loses some motor ability, and develops a “cherry-red spot” on the macula of the eye. The disease proceeds to dementia, flaccid muscles followed by spastic muscles, blindness, and death by age 3 years. There are less common variants that proceed more swiftly or in a somewhat more prolonged fashion.

Diagnosis. The classic form of Tay-Sachs disease is due to deficiency in hex-A enzyme. The hex-A enzyme can be assayed in serum; peripheral blood WBCs and patient fibroblasts can also be used. The diagnosis can be established in the fetus by amniocentesis. The hex-A assay can be used to detect carriers of the Tay-Sachs gene. The disease is transmitted as an autosomal recessive trait, so that if one parent is tested and has normal hex-A levels, the infant will not be homozygous. Therefore, the infant will not be clinically affected, even if the other parent has the gene. DNA probe diagnosis is available in addition to hex-A enzyme measurement. Screening for Tay-Sachs disease can be performed on the fetus in utero by chorionic villus biopsy in the first trimester.

Gaucher’s disease. Gaucher’s disease is a disorder in which the glycolipid cerebroside compound kerasin is phagocytized by the reticuloendothelial system. There seem to be two subgroups of this disorder: a fatal disease of relatively short duration in infancy accompanied by mental retardation, and a more slowly progressive disease found in older children and young adults and not accompanied by mental retardation. Splenomegaly is the most characteristic finding, but the liver and occasionally the lymph nodes also may become enlarged. The most characteristic x-ray findings are aseptic necrosis of the femoral heads and widening of the femoral marrow cavities; although typical, these findings may be absent. Anemia is frequent, and there may be leukopenia and thrombocytopenia due to hypersplenism. The serum acid phosphatase level usually is elevated if the chemical method used is not reasonably specific for prostatic acid phosphatase. (There are several widely used chemical methods, and although none is completely specific for prostatic acid phosphatase, some are considerably more so than others.)

Before the late 1970s, diagnosis was made by bone marrow aspiration. Wright-stained bone marrow smears frequently would contain characteristic Gaucher’s cells, which are large mononuclear phagocytes whose cytoplasm is filled with a peculiar linear or fibrillar material. Splenic aspiration or liver biopsy was also done in problematic cases. The diagnosis is now made by assay of peripheral blood leukocytes for beta-glucosidase, the enzyme whose deficiency is the cause of the disease. Skin biopsy with tissue culture of skin fibroblasts followed by beta-glucosidase assay is also possible.

Niemann-Pick disease. Niemann-Pick disease is similar clinically and pathologically to the fatal early childhood form of Gaucher’s disease, except that the abnormal lipid involved is the phospholipid sphingomyelin. As in Gaucher’s disease, diagnosis used to be made by bone marrow aspiration, although the phagocytic cells are not as characteristic as those of Gaucher’s disease. Splenic biopsy with tissue lipid analysis was also done. The diagnosis is now made by skin biopsy with tissue culture of the fibroblasts and assay of the fibroblasts for sphingomyelinase, the enzyme whose deficiency is the cause of the disease.

Diseases of Carbohydrate Metabolism

Galactosemia. Galactosemia results from congenital inability to metabolize galactose to glucose. The most common source of galactose is milk, which contains lactose. Lactose is converted to glucose and galactose in the gastrointestinal (GI) tract by the enzyme lactase. There are three forms of galactosemia, each with autosomal recessive inheritance, and each caused by an enzyme defect in the galactose-glucose metabolic pathway. This enzyme system is located primarily in RBCs. The classic and most common type of abnormality is found in 1 in 62,000 infants and is due to deficiency of the enzyme galactose-1-phosphate uridyltransferase (Gal-1-PUT); the defect is called transferase deficiency galactosemia (TD-galactosemia). Normally, galactose is metabolized to galactose-1-phosphate, and Gal-1-PUT mediates conversion to the next step in the sequence toward glucose-1-phosphate.

TD-galactosemia is not clinically evident at birth, but symptoms commence within a few days after the infant starts a milk diet. Failure to thrive occurs in 50%-95% of patients. Vomiting (about 50% of cases), diarrhea (about 30%), or both occur in nearly 90% of patients. Evidence of liver dysfunction develops in about 90% of patients, consisting of hepatomegaly (about 70%), jaundice (about 60%), or both. Physiologic jaundice may seem to persist, or jaundice may develop later. Splenomegaly may appear in 10%-30% of patients. Individual signs and symptoms are sufficiently variable that a complete full-blown classic picture does not appear in a substantial number of affected infants. Cataracts develop in several weeks in about 50%, and mental retardation is a sequel in about one third of the patients. The disease is treatable with a lactose-free diet, if the diet is begun early enough.

Laboratory abnormalities. Transferase deficiency galactosemia has multiple laboratory test abnormalities. Urinalysis typically reveals protein and galactose, although in one series only two thirds of patients demonstrated urine galactose. Urine galactose excretion depends on lactose ingestion and may be absent if the infant refuses milk or vomits persistently. Galactose in urine can be detected by a positive copper sulfate-reducing substance test (e.g., Clinitest) plus a negative test specific for glucose (such as glucose oxidase test strips). A nonglucose reducing substance must be identified by more specific tests, because lactose and various other sugars can produce the same reaction as galactose on the screening procedure. There is also abnormal amino acid urine excretion that can be detected by chromatography, although such information adds little to the diagnosis. Positive urine galactose test results do not mean that the infant has galactosemia, since occasionally normal newborns have transient galactosuria. However, urine screening is important, because detection of galactose enables a tentative diagnosis to be made and treatment started, pending results of confirmatory tests.

Other nonspecific abnormalities in classic transferase-deficient galactosemia include hepatomegaly and jaundice, although jaundice may not be present. Liver function test results are variable, although the aspartate aminotransferase (AST, formerly serum glutamate oxaloacetate) and possibly alkaline phosphatase levels are frequently elevated. Liver function test results must be interpreted with knowledge that the reference ranges are different in the neonate than in adults. Liver biopsy has been used in certain problematic patients (mostly before Gal-1-PUT assays were available). The histologic changes are suggestive but not conclusive and consist of early fatty metamorphosis, with a type of cirrhosis pattern often developing after about 3 months of age.

Diagnosis. Diagnosis of classic TD-galactosemia depends on assay of Gal-1-PUT in the RBCs of the infant. Several methods have been described, all of which are sufficiently difficult that the test is available only in university centers and a few large reference laboratories. The specimen presents certain problems. RBCs from anticoagulated whole blood are tested, so the specimen cannot be frozen. On the other hand, one report indicates that 25% of the enzyme activity is lost after 24 hours at either room temperature or refrigerator temperature. For this and other reasons, many physicians rely on screening tests for transferase enzyme deficiency and, after starting therapy, refer the patient to a specialized center for definitive diagnosis.

The galactose tolerance test was once the most widely used method for confirmation of galactosemia. However, there is considerable danger of hypoglycemia and hypokalemia during the test, and it has been replaced by chromatography and RBC enzyme assay.

Screening tests. Several screening tests are available. The most popular are a bacterial inhibition method (Paigen assay, roughly similar to the PKU Guthrie test) and the Beutler fluorometric method. The Paigen assay measures elevated blood galactose (or galactose-6-phosphate) levels, and the Beutler assay measures Gal-1-PUT activity. Both can be performed on filter paper blood spots. The Paigen test depends on elevated blood galactose levels, and therefore milk feeding is necessary. Specimens from patients not receiving milk or specimens drawn several hours after a milk feeding may yield false negative (normal) results. If the Paigen test is adapted to detect galactose-1-phosphate rather than galactose, length of time after feeding is not a problem, and even most cases in patients on a galactose-free diet are reportedly detected. The Paigen test (either type) detects galactokinase deficiency as well as transferase deficiency. The Beutler test does not depend on milk feeding. However, the test does not detect galactokinase deficiency and is more subject to effects of heat and humidity on the filter paper specimen. The Beutler test also can detect certain nonpathologic variants of transferase enzyme such as the Duarte variant.

Variants. There are at least four variants of the transferase enzyme. The Duarte variant is the most frequent. Patients with this disorder exhibit about 50% of normal Gal-1-PUT activity on RBC assay and are clinically asymptomatic. In classic (homozygous) transferase deficiency there is almost no Gal-1-PUT activity on assay.

Other forms of galactosemia. There are two other forms of galactosemia. One consists of deficiency in the enzyme galactokinase. Development of cataracts is the only symptom. The incidence of galactokinase deficiency has been variously reported as equal to that of transferase deficiency or less. The third type is deficiency of the enzyme erythrocyte epimerase, of which very few cases have been reported. These patients seem to be completely asymptomatic.

Disaccharide malabsorption. Certain enzymes are present in the lumen border of small intestinal mucosal cells that aid in absorption of various complex sugars by preliminary hydrolyzation. Deficiency of one or more of these enzymes may impair absorption of the affected sugar, depending on the degree of enzyme deficiency. The most common deficiency affects the disaccharide sugar lactose. Lactose is present in milk or milk products such as ice cream, yogurt, and many types of cheese. Northern Europeans as a group usually have normal intestinal lactase throughout life and only about 10%-15% develop lactose intolerance. Many other ethnic populations have a high incidence of lactase deficiency. The highest incidences are reported in Asians (such as Chinese and Japanese) and Native Americans (over 90% are said to develop lactose intolerance). Eastern European (Ashkenazic) Jews, African Americans, and persons of Mediterranean or South American ancestry have a lesser but still high rate of deficiency (60%-70% incidence). Besides primary (genetic) lactase deficiency, secondary deficiency may be induced, more or less temporarily, by certain diseases affecting the small intestine. These include primary small intestine mucosal disease (sprue), short bowel syndrome, severe acute gastroenteritis, prolonged protein-calorie malnutrition, and certain antibiotics (e.g., neomycin and kanamycin). Lactase deficiency may also occur due to prematurity. Between 26 and 34 weeks of gestation there is only about one third of full-term lactase activity present. This increases to about 70% between 35 and 38 weeks. Full activity levels are attained by birth at 40 weeks.

Persons who inherit the trait for lactase deficiency usually have normal lactase activity levels at (full-term) birth. However, at about age 3-5 years there is a fall in lactase activity. After this time there is some individual variation in degree of clinical tolerance to milk, even when intestinal lactase activity measurement is low.

Symptoms of lactose intolerance include some combination of abdominal cramps, diarrhea, bloating, and flatulence, usually occurring at or becoming worse after meals. One study involving children from age 4 years to the teen-age years who had intermittent abdominal pain found their symptoms could be explained by lactose malabsorption in a high proportion of those from ethnic groups with a high incidence of lactase deficiency. Lactase deficiency in full-term newborns or young children is thought to be rare. Milk allergy may produce similar symptoms but is uncommon, usually includes allergy symptoms such as asthma with any GI symptoms, and the parents usually have a history of allergy.

Screening tests for lactase deficiency include testing of stool for pH and sugar at a time when the patient is symptomatic. Normal stool pH is 7.0-8.0. A stool pH below 6.0 raises the question of lactase deficiency. The stool can be tested for sugar by either a reducing substance method or a glucose oxidase paper strip method. The presence of glucose in the stool suggests lactase deficiency. However, in one study, parenteral antibiotics administered to neonates caused an increase in fecal reducing substances beginning within 48 hours after starting the antibiotics. Negative test results for pH and sugar do not exclude the diagnosis, and positive test results do not conclusively establish the diagnosis. Acidic stool pH can be found in certain other conditions associated with diarrhea, especially steatorrhea.

Diagnostic tests for lactase deficiency include the lactose tolerance test, hydrogen breath test, and small intestine biopsy with tissue assay for lactase. The lactose tolerance test is performed in a similar manner to the oral glucose tolerance test. After overnight fasting, 50 gm of oral lactose (in children, 1-2 gm of lactose/kg of body weight) is given in some type of flavored liquid. Serum glucose levels are assayed before lactose administration and 15, 30, 60, and 90 minutes afterward (some investigators use different time intervals; some beginning at 30 minutes instead of 15 and some ending at 120 minutes instead of 90 minutes). Normal lactase activity results in a postdose glucose elevation more than 20 mg/100 ml (1.1 mmol/L) over baseline. This assumes that malabsorption from some other cause is excluded. Some investigators measure galactose instead of glucose; 150 mg of ethanol/kg of body weight is administered with the lactose dose to inhibit liver conversion of galactose to glucose. A single blood or urine sample is obtained 40 minutes after the test dose and is assayed for galactose. The advantage of this procedure is need for only one test specimen. However, this method has not been evaluated as thoroughly as the standard lactose tolerance procedure. The hydrogen breath test is currently considered the most accurate of the tolerance tests. Briefly, it consists of analysis of expiratory breath for hydrogen, followed by administration of oral lactose and retesting of breath samples at either 30- or 60-minute intervals for 2-4 hours. Lactase deficiency results in deposition of excess lactose into the colon, where bacterial fermentation produces excess hydrogen, which is excreted through the lungs. The hydrogen breath test can be performed only by specialized medical centers. Small intestine biopsy with quantitation of tissue levels of intestinal lactase is performed during an endoscopy procedure. Tissue biopsy has the added advantage that some of the secondary causes of lactase deficiency (e.g., sprue) can be detected. However, intestinal lactase measurement is available only at specialized medical centers.

Lactosuria. Some patients with lactase deficiency absorb lactose from the GI tract after oral intake of lactose and excrete the lactose in the urine. However, other lactase-deficient persons do not exhibit lactosuria. Premature infants are said to be predisposed to temporary lactosuria (presumably due to their relatively lactase-deficient state). Lactosuria is apparently not uncommon in the last trimester of pregnancy and for several days following delivery.

Sucrase enzyme deficiency. The disaccharide sugar sucrose is commonly used as a carbohydrate supplement. Sucrose is hydrolyzed by the enzyme sucrase in small intestine mucosa cells. Congenital sucrase deficiency has been reported but is not common. Most cases became clinically manifest within the first year of life, with symptoms predominantly of failure to thrive, diarrhea, or both. Stool pH is usually acidic. Reducing substance sugar test methods are not accurate for stool testing, since sucrose is not a biochemical reducing substance. Fructose may also be malabsorbed by some persons. Diagnosis is most often made by the hydrogen breath test. Definitive diagnosis usually requires small intestine biopsy with tissue assay for sucrase (or fructose) activity.

Glycogen storage disease. Glycogen storage disease includes a spectrum of syndromes resulting from defective synthesis or use of glycogen. Clinical manifestations depend on the organ or tissue primarily affected and the specific enzyme involved. The disease in one or another of its clinical syndromes may affect the liver, heart, or skeletal muscle. The most common is von Gierke’s disease, whose clinical manifestations primarily involve the liver. Classic cases usually have elevated levels of triglyceride, cholesterol, and uric acid. Some have hypoglycemia. Patients typically have a diabetic type of oral glucose tolerance curve.

Diagnosis. Diagnosis of the various forms of glycogen storage disease requires biopsy of liver or muscle (depending on the particular disease variant) for glycogen and enzyme analysis. In some cases, enzyme assay can be performed on other tissues. These are specialized tests, and it is best to contact a pediatric research center rather than expose the patient to inappropriate or incomplete diagnostic procedures.