The classic subdural hematoma develops after trauma, but in one series 20% did not have a history of trauma. In that same series, 60% of patients were alcoholics, 30% of patients had no lateralizing signs, and one third of the patients had multiple hematomas (with 21% having bilateral subdural hematomas and 14% having additional hematomas elsewhere within the brain or brain coverings).

CSF findings vary depending on whether the hematoma is recent or several days old. In recent subdurals (within 7 days of injury), the CSF is usually blood tinged or xanthochromic, due to concurrent cerebral contusion from the injury or leakage of RBC breakdown products. In late cases the CSF is most often clear. Protein levels are elevated in most of the acute cases, usually in the range of 50-150 mg/100 ml (0.5-1.5 g/L), and in about 50% of the chronic cases. After several weeks the CSF protein level is usually normal. Cell counts may be slightly elevated in acute or subacute cases (usually < 25 WBCs/mm3) and are usually normal in patients seen later.

The classic CSF triad of elevated protein level, relatively normal cell count, and xanthochromia is present in only about 50% of the subacute and chronic cases, with the percentage decreasing as the time from injury increases. CT or MRI is the best means of demonstrating the lesion. CT is probably better in the first week.