Petrous apex arachnoid cyst: radiographic confusion with primary cholesteatoma

The advent of combined computed tomography (CT) and magnetic resonance imaging (MRI) for the evaluation of petrous apex lesions has improved the otologist's ability to differentiate among many disease processes. Temporal bone CT details osseous anatomy, whereas MRI delineates soft tissue signal characteristics. The employment of these two imaging modalities is often successful in differentiating between cholesterol common entities encountered in the petrous apex. The finding of a smoothly marginated, expansile, bone eroding lesion on CT, coupled with hypointensity on T1-weighted and hyperintensity on T2-weighted images on MRI, is highly suggestive of cholesteatoma. The authors recently encountered two cases of arachnoid cyst involving the petrous apex that shared the same imaging features seen with the more common cholesteatoma. One patient presented with tic douloureux, whereas the other had a spontaneous transotic cerebrospinal fluid leak. The contemporary skull base surgeon should include arachnoid cyst as a rare possibility in the evaluation and treatment of petrous apex lesions.