The petrous apex is the medial portion of the petrous ridge situated between the inner ear and the clivus. In its base lies foramen lacerum traversed by the internal carotid artery. Above lies Meckel’s cave and the fifth nerve as well as the sixth nerve in Dorello’s canal. In most patients, the petrous apex is filled with marrow with, at most, a small amount of pneumatization. In less than 10% of adults, the petrous apex is extensively pneumatized. A word to clarify the nomenclature of petrous apex surgical approaches is needed. The term “petrous apicectomy” which implies removal of the petrous apex is often inappropriately used to describe drainage procedures which more accurately should be referred to as “petrous apicotomy.”

The most common petrous apex lesion is fluid in a well-pneumatized petrous apex which is confined to the air cell tracts without osseous erosion. This entity should almost always be followed medically as one would for a maxillary antral inclusion cyst. The most common surgical petrous apex lesion is cholesterol granuloma, an expansile, destructive lesion driven by persistent blood seepage into mucosal spaces from adjacent richly vascularized bone marrow. Eroding laterally, cholesterol granulomas may invade the inner ear with resultant hearing loss and vertigo. Eroding superiorly, they cause diplopia due to pressure on the abducens nerve in Dorello’s canal and facial sensory disturbance due to pressure on the fifth nerve due to erosion of the floor of Meckel’s cave. The surgical treatment of petrous apex cholesterol granuloma is by establishing a drainage tract into the middle ear. The hypotympanic, infracochlear route is most direct when the anatomy is favorable. An alternative route, which is deeper, narrower, and more technically challenging, is the infralabyrinthine pathway between the posterior semicircular canal, descending facial nerve, and jugular bulb. When the anatomy is favorable, transsphenoidal drainage may also be feasible. When drainage laterally into the middle ear or medially into the sphenoid is not anatomically feasible, or drainage procedures have failed to adequately alleviate symptoms, resection of the granuloma via an extradural middle fossa craniotomy may be considered. In this procedure, care must be taken to avoid injury to the carotid artery, as its bony wall is often eroded.

Two forms of infection may involve the petrous apex. Most common in the antibiotic era is otogenic skull base osteomyelitis. This aggressive infection is most often caused by pseudomonas in elderly diabetics or fungus (e.g., Aspergillus fumigatus) in immunocompromised patients. Skull base osteomyelitis propagates through marrow rather than air cells. It is generally a nonsurgical disease except when sequestration has occurred or when tissue is needed to establish the causative organism. Petrous apicitis, by contrast, is infection within the air cells of a pneumatized apex. Apical infection may cause a syndrome of ear pain and discharge, diplopia, and retro-orbital pain often referred to eponymically as Gradenigo’s syndrome. Drainage of apicitis is via petrous apicotomy which may be conducted via numerous pathways including those above, below, or through (e.g., traversing the superior semicircular canal loop) the labyrinth or beneath the cochlea. The general rule is to “follow the pus” preferably using small curettes, which better respect the integrity of the otic capsule than high-speed drills.

The most common primary tumor of the petrous apex is chondrosarcoma arising from foramen lacerum at the petroclival junction. Secondary tumors include metastases, especially involving prostate and breast cancers. Arachnoid cyst descending from Meckel’s cave is an uncommon petrous apex lesion.

14.2 Petrous Apicotomy

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