Petrous apex arachnoid cyst: a case report and review of the literature.

Cholesterol granuloma and cholesteatoma are the two most common destructive lesions of the petrous apex. Arachnoid cyst is much less common. These three expansile lesions are often indistinguishable on clinical grounds. Accurate pre-operative radiological diagnosis on computed tomography scan and magnetic resonance imaging is important in order to plan the appropriate treatment. Pre-operative radiological differential diagnosis between primary cholesteatoma of petrous apex and a intrapetrous arachnoid cyst remains a significant problem. The following aspects need evaluation for recognition of intrapetrous arachnoid cysts: 1) an awareness of their existence, 2) homogeneous signal on T1 and T2 weighted images, closely resembling cerebro-spinal fluid signal, 3) special heavily weighted T2 images on magnetic resonance imaging: fluid-attenuated inversion recovery imaging, 4) careful correlation of clinical-radiological data. Symptomatic arachnoid cysts are best treated with conservative drainage surgery through middle cranial fossa. A case of a petrous apex arachnoid cyst is reported which has been radiologically mistaken for a primary cholesteatoma and operated through an infratemporal fossa approach type B. The patient (40-year-old female) came to our attention with right trigeminal pain which had been present for one year and dizziness. Neurotologist and skull-base surgeons should include arachnoid cyst as a rare possibility in the evaluation and treatment of petrous apex cystic lesions.