Chronic Subdural Hygroma Caused by Rupture of Arachnoid Cyst

The patient was a 25-year-old male, who sustained a head injury and was found to have an asymptomatic arachnoid cyst of the middle cranial fossa and a frontal subdural space. Complete communication between the subdural space and the subarachnoid space was recognized on CT performed 5 days after the injury, but not on CT 35 days after the injury. No hemorrhage was found in the subdural space on follow-up CT. Operation was performed 42 days after the injury. It was found that the subdural space contained xanthochromic fluid and that the capsule of the hygroma was very similar histologically to that of chronic subdural hematoma in adults. In this case, tear of the arachnoid membrane and accumulation of CSF in the subdural space were considered to have been essential for the formation of chronic subdural hygroma. It was thought that adults with brain atrophy and infants with craniocerebral disproportion, like those with arachnoid cyst, tend to have their arachnoid membrane torn by minor head injury and that this tends to form chronic subdural hygroma. It was reported that chronic subdural hygroma might be transformed into chronic subdural hematoma. Tearing of the arachnoid membrane might be one important factor in the development of chronic subdural hematoma.