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 sof t tissue signal characteristics. The employment of these two imaging m odalities is often successful in differentiating between cholesterol g ranuloma, cholesteatoma (epidermoid), and asymmetric pneumatization, r elatively common entities encountered in the petrous apex. The finding of a smoothly marginated, expansile, bone eroding lesion on CT, coupl ed with hypointensity on T1-weighted and hyperintensity on T2-weighted images on MRI, is highly suggestive of cholesteatoma. The authors rec ently encountered two cases of arachnoid cyst involving the petrous ap ex that shared the same imaging features seen with the more common cho lesteatoma. One patient presented with tic douloureux, whereas the oth er had a spontaneous transotic cerebrospinal fluid leak. The contempor ary skull base surgeon should include arachnoid cyst as a rare possibi lity in the evaluation and treatment of petrous apex lesions.