Disclaimer: The information contained within the Grand Rounds Archive is intended for use by doctors and other health care professionals. These documents were prepared by resident physicians for presentation and discussion at a conference held at Baylor College of Medicine in Houston, Texas. No guarantees are made with respect to accuracy or timeliness of this material. This material should not be used as a basis for treatment decisions, and is not a substitute for professional consultation and/or peer-reviewed medical literature. Lesions of the Petrous Apex The petrous apex is the most inaccessible portion of the temporal bone. The petrous apex can be involved in congenital, infectious, inflammatory, and neoplastic processes. Since direct examination is not possible, careful attention to the often subtle signs and symptoms, as well as keeping a high index of suspicion, is necessary to diagnose these entities. The true incidence of these rare lesions is difficult to estimate, but the House group has reported that acoustic neuromas are about 30 times more common in their practice. The petrous apex is a truncated pyramid forming the medial portion of the temporal bone. The base is bounded by the bony labyrinth and the carotid artery anteriorly. The superior surface is the middle cranial fossa, Meckel's Cave, and the ascending carotid artery. The posterior surface is the posterior cranial fossa and Dorello's canal transmitting CNVI. The inferior surface is the jugular bulb and the inferior petrosal sinus. The petrous apex is marrow filled in 84%, pneumatized in 9%, and sclerotic in 7%. Clinical presentations can range from subtle to dramatic. Involvement of the Eustachian tube can result in aural fullness or otitis media. Compression at the foramen ovale can cause V3 paresthesia or numbness. Stretching of the dura can cause headache of eye pain. In addition, ophthalmoplegia, tinnitus, vertigo, hearing loss, and facial paralysis can occur. CT and MRI have revolutionized the preoperative work-up of petrous apex lesions. An accurate differential diagnosis is critical in that it may influence the surgical approach for specific lesions. The two modalities may provide complimentary information based on density, bone erosion, contrast enhancement, and imaging characteristics with T1 and T2 weighting. Several benign anatomic variants may produce radiographic abnormalities that can mimic pathologic conditions. Thirty to thirty-five percent of petrous apices are pneumatized and 6.8% may have asymmetric pneumatization that can be demonstrated by CT. On MRI, the less pneumatized, marrow-filled apex would appear bright on T1 and T2, mimicking a lesion. A giant petrous air cell can cause distortion of the IAC appearing similar to an expansile lesion. Lastly, simple effusions, mucoceles, and mucus retention cysts of the petrous apex appear as soft tissue density with preservation of the air cell septae. Expansile mucoceles may have eroded margins. On MRI, these appear low on T1 and bright on T2. Cholesterol granuloma is the most common lesion of the petrous apex. It is approximately 10 times more common than cholesteatoma and 40 times more common than mucocele. The cyst wall lacks the keratinizing squamous epithelium of cholesteatoma. On CT, the lesion appears as a homogeneous soft tissue ovoid lesion posterior to the horizontal carotid canal. On MRI, it appears bright on T1 and T2 without enhancement with gadolinium. The surgical treatment is re-establishment of aeration of the apex by using a silicone drain into the middle ear or mastoid. Cholesteatoma of the petrous apex can be congenital or acquired. Acquired cholesteatomas are rare and occur by extension from the middle ear space along the supralabyrinthian air cell system along the anterior epitympanic space. Facial nerve dysfunction occurs in 20% - 50% of cases. On CT, cholesteatoma appears as a smooth, expansile lesion hypodense to brain without contrast enhancement. On MRI, it appears as low intensity on T1 and hyperintense on T2, similar to CSF. The translabyrinthian-transcochlear and the middle fossa approaches are used most often for extirpation of these lesions. Metastatic lesions of the temporal bone are uncommon. However, the petrous apex and clivus are two of the most common sites for metastasis in the head and neck. The most common primary sites are the breast, lung, kidney, stomach, prostate, and larynx. Metastases to the skull base in isolation are uncommon and infraclavicular metastases are usually present. A bone scan should be performed when metastasis is suspected. Most tumors are erosive, but some stimulate sclerosis from osteoblastic activity. The MRI appearance is variable depending on the type of lesion. Other lesions of the petrous apex that should be in the differential include: petrous apicitis usually due to pseudomonas and rarely TB; intrapetrous carotid artery aneurysm, chondroma/chondrosarcoma, meningioma, schwannoma, and chordoma. Case Presentation A 64-year-old white lady presented with a nine-month history of asymmetric left-sided hearing loss of sudden onset. The hearing loss was accompanied by tinnitus. She denied headache, vertigo, and otalgia. She denied any significant medical or surgical history and had no history of otologic disease, nor previous otologic surgery. She did have a history of left Bell's Palsy one year before presentation, which had resolved completely. She admitted having a 75 pack/year smoking history. Audiometry revealed an asymmetric, moderate to profound, left sensorineural hearing loss, absent acoustic reflexes and significantly depressed speech audiometry. CT scanning of the temporal bones showed a well defined, lobulated, expansile lesion of the left petrous apex. By MRI, the lesion was bright on T1 and T2. CT scanning of the chest and a screening mammogram were negative. The petrous apex was entered after complete mastoidectomy via the retrofacial air cell tract. A cyst filled with a brown exudate consistent with a cholesterol granuloma was drained into the mastoid using a pediatric feeding tube. Two months post operatively, she was asymptomatic, doing well, and had minimal change in her hearing. Bibliography Appling D, Jenkins HA, Patton GA. Eosinophilic granuloma in the temporal bone and skull. Otolaryngol Head Neck Surg 1983;91:358-364. Arriaga MA, Brackmann DE. Differential diagnosis of primary petrous apex lesions. Am J Otol 1991;12:470-474. 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Otolaryngol Head Neck Surg 1991;104:29-36. Glasscock ME 3d, Woods CI, Poe DS, Patterson AK, Welling DB. Petrous apex cholesteatoma. Otolaryngol Clin North Am 1989;22:981-999. Goin DW. Surgical management of petrous apex meningioma. Laryngoscope 1979;89:204-213. Goldsmith AJ, Myssiorek D, Valderrama E, Patel M. Unifocal Langerhans' cell histiocytosis (eosinophilic granuloma) of the petrous apex. Arch Otolaryngol Head Neck Surg 1993;119:113-116. Heinrich DE, Gantz BJ, Moore SA, Schubkegel AJ. Undifferentiated small-cell neoplasm of the petrous apex: a case report. Arch Otolaryngol Head Neck Surg 1992;118:767-770. Hendershot EL, Wood JW, Bennhoff D. The middle cranial fossa approach to the petrous apex. Laryngoscope 1976;86:658-663. Horn KL, Hankinson HL, Erasmus MD, Beauparalant PA. The modified transcochlear approach to the cerebellopontine angle. Otolaryngol Head Neck Surg 1991;104:37-41. Jackler RK, Parker DA. radiographic differential diagnosis of petrous apex lesions. 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Otolaryngol Head Neck Surg 1990;103:80-88. Samy LL, Girgis IH. Transzygomatic approach to the petrous apex with a record of three cases. J Laryngol Otol 1967;81:277-289. Smith PG, Leonetti JP, Kletzker GR. Differential clinical and radiographic features of cholesterol granulomas and cholesteatomas of the petrous apex. Ann Otol Rhinol Laryngol 1988;97:599-604. Solodnik P, Som PM, Shugar JM, Sachdev VP, Sacher M, Lanzieri CF, et al. Intraosseous petrous apex neuroma: CT findings. J Comput Assist Tomogr 1986;10:1027-1029. Thedinger BA, Nadol JB Jr, Montgomery WW, Thedinger BS, Greenberg JJ. Radiographic diagnosis, surgical treatment and long-term follow-up of cholesterol granulomas of the petrous apex. Laryngoscope 1989;99:896-906. Umansky F, Valarezo A, Elidan J. The microsurgical anatomy of the abducens nerve in its intracranial course. Laryngoscope 1992;102:1285-1292. 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