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Cystic Lesions of the Petrous Apex
Michael C. Byrd, Gordon B. Hughes, Paul M. Ruggieri, and Joung Lee
Anatomy of the Petrous Apex
The petrous bone can be divided into two main compartments: anterior and posterior. The internal auditory canal acts as the partition. The anterior compartment (the petrous apex) is the larger of the two; it lies anteromedial to the cochlea and is more frequently involved in disease. Pneumatization can vary, and is indirectly proportional to the amount of bone marrow that comprises the compartment. The more bone marrow found within the petrous apex, the less air, and vice versa. Approximately one third of adults have pneumatization, and pneumatization is usually symmetric bilaterally.
Many authors describe the petrous apex as a three-sided pyramid with an anterior, posterior, and inferior surface. It is located between the clivus in the anteromedial position and the otic capsule in the posterolateral position. The anterior surface forms the floor of the middle cranial fossa. The internal carotid artery passes through this area to the cavernous sinus; the tensor tympani muscle and eustachian tube are located just lateral to the artery. The facial hiatus can be found along the anterior border with the greater superficial petrosal nerve carrying parasympathetics to the sphenopalantine ganglion. Near the anterior apex, the Gasserian ganglion of the trigeminal nerve rests in Meckel’s cave. The posterior surface faces the posterior cranial fossa. Along the superior and inferior edges the superior and inferior petrosal sinuses can be found. The abducens nerve courses along with the superior petrosal sinus to enter the cavernous sinus through a tight fold of dura known as the petroclinoid ligament, or Dorello’s canal. Posterior to the apex is the internal auditory meatus. The inferior surface lies along the horizontal plane and contains the carotid artery canal meatus.
Pathology of Petrous Apex Lesions
Petrous apex lesions are rare. They can be categorized as primary or secondary. This chapter focuses on primary cystic lesions of the petrous apex, such as cholesterol granuloma, cholesteatoma, and mucocele, as well as petrous apicitis, retained fluid, and asymmetric petrous apex pneumatization.
Primary Cystic Lesions
Cholesterol granuloma is a foreign body giant cell reaction to cholesterol deposits, with chronic inflammation, fibrosis, and vascular proliferation all contained within a fibrous capsule. The first reported case of an otologic cholesterol granuloma was in the late nineteenth century.1 Cholesterol granulomas are 10 times more common than petrous apex cholesteatomas and 40 times more common than mucoceles.
These lesions tend to remain clinically silent until expansile growth produces headache, or encroachment on adjacent cranial nerves causes hearing loss, imbalance, facial weakness, or diplopia. Two theories have been proposed concerning the development of cholesterol granulomas. The classic hypothesis is known as the obstruction-vacuum theory, which entails a series of events. First, mucosal swelling occludes petrous air cell outflow tracts, resulting in gas trapping, which leads to vacuum formation. In this vacuum, pressure causes transudation of blood into the mucosal surfaces. Anaerobes then begin to break down the red blood cells, liberating cholesterol granules. The cholesterol granules initiate the inflammatory cascade, resulting in bony erosion and foreign body reaction.2–4
A second theory on cholesterol granuloma formation has recently been proposed.5 The new hypothesis is known as the exposed marrow theory. The theory states that during development there is aggressive pneumatization of the petrous apex resulting in the formation of a pathologic communication between mucosa-lined air cells and the marrow they gradually replace. The communication between the marrow and mucosa creates hemorrhage into the apical air cells. Once the blood becomes trapped in these cells, anaerobic bacteria digest the red cells, again releasing the cholesterol granules. The inflammatory cascade begins, resulting in a foreign body reaction and bony erosion.2–4,6
Primary cholesteatomas consist of stratified squamous epithelial lining surrounding desquamated keratin, and originate from epithelial rests within the petrous apex. Because these lesions expand into the area of least resistance, they have variable shapes. They can become quite large without producing symptoms; however, as they expand, compression and irritation of surrounding structures produce signs and symptoms similar to those of cholesterol granuloma.
Various theories on the development of petrous apex cholesteatomas have been proposed. Congenital onset is thought to be caused by the presence of epidermoid cells in the petrous apex during fetal development. A second hypothesis proposes migration of the external meatus ectoderm. In fetal development the internal and external meati are in close connection, which may result in trapping of epithelial remnants in the foramen lacerum.7–10
An exceedingly rare lesion of the petrous apex is the primary mucocele. The first reported case of petrous apex mucocele was in 1979.11 The etiology of the mucocele is uncertain; however, several theories have been postulated. Mucosal thickening, bony overgrowth, or fibrosis may obliterate the outflow tract resulting in mucocele formation, or nests of seromucinous tissue may be responsible for the development of mucoceles through mucus retention.10,12 It is logical to assume that, given the nature of ventilation and drainage of the petrous apex air cells, some individuals will develop a mucocele in these pneumatic spaces as a consequence of an upper respiratory tract infection. This situation is analogous to disease of the paranasal sinuses. As in the sinuses, the mucocele becomes expansile, resulting in bony erosion.
Other Lesions
Petrous apicitis can be classified as acute, subacute, or chronic. Acute apicitis usually involves an acute episode of otitis media or mastoiditis, potentially resulting in the formation of an apical abscess. More commonly, petrous apicitis results from chronic otitis media, with Pseudomonas aeruginosa being the predominant bacteria causing the infection. Long-standing infection can also potentially result in osteomyelitis. Prior to the era of antibiotics, suppurative disease of the petrous apex was often fatal. The classically described syndrome included a triad of signs and symptoms: (1) discharging ear, (2) deep retro-ocular pain, and (3) abducens paralysis.3,12–15 This syndrome was known as Gradenigo’s triad. Petrous apicitis should be suspected whenever a chronic suppurative ear is associated with deep pain. The pain is usually a result of either dural involvement over the apex or direct irritation of the Gasserian ganglion in Meckel’s cave. Apicitis should also be suspected when cranial nerve palsies occur.
Retained fluid is a serous effusion trapped in apical air cells. By definition, retained fluid does not destroy bone; however, headache and pressure symptoms may prompt surgery if serial computed tomography (CT) scanning does not show resolution. The clinical entity of apical retained fluid is poorly understood, but may be analogous to chronic serous mastoiditis or may be an initial step toward cholesterol granuloma formation.
Asymmetric fatty marrow in the petrous apex is usually noted as an incidental finding on radiographic imaging (Fig. 25–1). It is the residual fatty marrow in the nonpneumatized or less pneumatized petrous apex that causes concern. Correct identification of this normal variant is essential to prevent misdiagnosis or unnecessary workup and treatment.4,12 In a review of 500 CT scans, Roland et al16 found 34 patients with some asymmetry of pneumatization of the petrous apex.
Clinical Presentation and Assessment of Petrous Apex Lesions
Symptoms
Petrous apex lesions can be asymptomatic, and can be discovered only by coincidence on magnetic resonance imaging (MRI). Leonetti and colleagues17 performed a retrospective chart review to categorize a group of petrous apex lesions that were noted incidentally on MRI in 88 patients. These incidental findings included asymmetric fatty bone marrow, inflammation, cholesterol granuloma, and cholesteatomas. Asymmetric fatty bone marrow was the predominant finding in this review. Therefore, the clinician should remember that a petrous apex lesion noted on MRI may or may not be related to the initial presenting symptoms. The physician should not overreact.
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