Arachnoid cysts in the middle cranial fossa: cause and treatment of progressive and non-progressive symptoms.

Ten consecutive patients with arachnoid cysts in the middle cranial fossa, diagnosed by CT, were studied. They were divided into three clinical groups: (1) four patients with progressive symptoms caused by secondary bleeding, (2) five patients with non-progressive symptoms of headache or epilepsy, and (3) one asymptomatic patient. The nine symptomatic patients were operated upon and eight showed clinical improvement. Measurements of the CT scans revealed equal brain volumes on the affected and normal sides, refuting the hypothesis that the cyst is caused by agenesis of the temporal lobe. The hemicranium on the affected side was larger, accommodating both the cyst and a normal brain volume. The progressive symptoms after bleeding were caused by brain shift and increased CSF pressure. The cause of the non-progressive symptoms has not yet been established: two mechanisms are suggested: (1) expansion of the cyst accompanied by compression of surrounding structures, and (2) disturbed CSF dynamics without increased intracystic pressure. Removal of the membranes of the cyst with an opening to the basal cisterns should resolve both problems. After surgery a residual cyst may remain.