Chronic subdural hematoma may be preceded by persistent traumatic subdural effusion.

The incidence of traumatic subdural effusion (TSE) was analyzed to clarify the relationship with subsequent chronic subdural hematoma (CSH) in 500 patients with head injury evaluated over a 36-month period. TSE occurred in 108 patients (21.6%), and CSH developed in 29 (5.8%) of these. The incidence of TSE was high, although only hospitalized patients were included because of the necessity for serial computed tomography. TSE frequently developed into CSH, and all CSH followed TSE. Therefore, TSE is closely associated with CSH and subdural effusion is probably a preliminary stage necessary for the formation of CSH.