Late appearance of hydrocephalus associated with posttraumatic intradiploic arachnoid cyst.

CASE REPORT: We present a case of a posttraumatic intradiploic arachnoid cyst associated with hydrocephalus in a 16-year-old boy. The diagnosis was missed 2 years previously, when an MRI following a transient gait disturbance failed to show the intradiploic extension of the cyst. He presented to us with hydrocephalus and a CT scan clearly identified the intradiploic position of the cyst. OUTCOME: A fenestration between the lateral ventricle and the cyst with aqueductoplasty and ETV were performed with good results.