[Subacute subdural hematoma: findings in CT, MRI and operations and review of onset mechanism].

Subacute subdural hematoma was investigated in terms of findings in CT, MRI and operations and of onset mechanism. The subjects were 7 cases of subacute subdural hematoma in which CT and MRI were performed during the past 5 year period. Subacute subdural hematoma here was defined as a nonoperated case with acute subdural hematoma, accompanied by subacute exacerbation 1-3 weeks after head trauma. The time from the injury to the onset averaged 13.7 days. CT revealed mixed density in 4 cases, low density in 3 cases and cerebral atrophy in all cases, with increasing mass sign due to the enlarged low density area. MRI revealed mixed intensity in 5 cases and high intensity in 2 cases, with increasing mass sign due to the enlarged high intensity area. Operation disclosed the outer membrane of the hematoma only in 1 case, but the inner membrane could not be identified in any case. Analysis of the hematoma contents showed a low hemoglobin concentrations and a high level of methemoglobin. Lack of outer membrane in cases with subacute subdural hematoma suggests that this is a different disease entity from chronic subdural hematoma. It is surmised that subacute subdural hematoma is the result of subdural effusion in the subacute stage, because, judging from the findings of CT and MRI each performed over time, cerebrospinal fluid is considered accountable for the increase in the mass sign.