[An infant with acute subdural hematoma after a minor head injury associated with arachnoid cyst].

A ten-month-old male infant fell onto the floor from a chair of 50 cm in height and hit his head on the day before hospitalization. He was admitted due to acute subdural hematoma, which was associated with arachnoid cyst. Head CT conducted on the 4th day confirmed that the hematoma had not enlarged. After discharge, enlargement of the hematoma was detected on MR imaging conducted on the 65th day after injury, followed by the diminution without surgical treatment. MR images were obtained on the 192nd day. In the case of head injury associated with arachnoid cyst, the risk of subdural hematoma as well as its ensuing enlargement in subacute or chronic phase needs to be considered.