Arachnoid cyst of the middle fossa with paradoxical changes of the bony structures.

Two patients with an arachnoid cyst of the middle fossa showed paradoxical changes of the adjoining bony structures of the skull. There was a diminution of the middle fossa and hyperplasia of the sphenoid sinus (pneumosinus dilatans) as well as a marked bulging of the squamous part of the temporal bone. In one case in which scinticisternography was performed, communication between the cyst and the subarachnoidal space was proven as well as an extremely slow cerebrospinal fluid circulation in the cyst. The pathogenesis of the cyst is discussed, based upon the structural changes of the skull, the angiographic findings and the locally disturbed cerebrospinal fluid circulation. The primary disturbance seems to be a temporal lobe agenesis.