Arachnoid cysts and pneumosinus dilatans.

The association of intracranial arachnoid cysts and pneumosinus dilatans is considered to be relatively rare, and has previously been described in only a limited number of cases. In this study the CT and/or MRI examinations of a total of 30 cases with middle cranial fossa arachnoid cysts were retrospectively evaluated in an attempt to note any association with pneumosinus dilatans of the paranasal sinuses. In four patients with surgically treated, large, middle cranial fossa arachnoid cysts, obvious paranasal sinus changes, consistent with pneumosinus dilatans, were noted. The frequency of the association of arachnoid cysts and pneumosinus dilatans was approximately 13% (n = 4 out of 30 patients) in this series.