Cholesterol Granulomas and Congenital Epidermoid Tumors of the Temporal Bone

Introduction

Benign lesions of the temporal bone present treatment challenges due to their central location among critical neurovascular structures. These lesions may remain undetected for years due to their insidious growth pattern and are often diagnosed incidentally by radiologic imaging. Larger lesions can encroach upon adjacent neurovascular structures, presenting with hearing loss, tinnitus, vertigo, cranial neuropathies, or headaches. Accurate diagnosis of petrous apex lesions is made possible by computed tomography (CT) and magnetic resonance imaging (MRI) characteristics. The surgical approach to lesions of the petrous apex varies based on the type and location of the lesion ( Fig. 143.1 ).

The petrous apex is a pyramidal bony structure at the most medial aspect of the temporal bone ( Fig. 143.2 ). At its base is the otic capsule, the canal of the tensor tympani muscle, and the petrous carotid artery. The superior boundary is the floor of the middle cranial fossa, extending from the arcuate eminence of the superior semicircular canal to the Meckel cave. The jugular bulb, vertical petrous carotid canal, and inferior petrosal sinus make up the inferior border. Posteriorly, the petrous apex is bounded by the posterior cranial fossa, extending from the posterior semicircular canal and endolymphatic sac to the petroclinoid ligament. Anteriorly, the petrous bone articulates with the greater wing of the sphenoid bone, extending medially to the foramen lacerum. The petrous bone articulates with the squamous portion of the temporal bone at its lateral edge.

The petrous apex may be divided into anterior and posterior portions by a vertical line drawn in the coronal plane through the internal auditory canal. Use of this plane as a dividing line can help determine the appropriate approach to lesions of the petrous apex. Most petrous bones are filled with bone marrow or alternatively comprised of dense bone. However, 30% of petrous apices are pneumatized. Pneumatization of the petrous apex may be highly variable and can differ from side to side in the same patient.

Although rare, there are a wide variety of lesions of the petrous apex. Asymmetric fatty marrow and unilateral opacification of the petrous apex air cells are the most commonly identified radiologic “abnormalities” of the petrous apex. It is important to correctly diagnose these entities, as they may be confused with dangerous pathologies, resulting in misdiagnosis and unnecessary evaluation or intervention. These lesions typically do not require surgical intervention and can be managed effectively by observation alone. Unilateral effusions may be noted in pneumatized petrous apexes. These almost uniformly cause consternation in radiologists, who often describe these as “petrous apicitis” or as tumors of the petrous apex. If asymptomatic, these do not require further treatment. If effusions cause troublesome symptoms, initial treatment includes a trial of antibiotic and steroids, with surgical drainage reserved for recalcitrant cases.

True lesions of the petrous apex can be categorized as cystic or solid. Cystic lesions include cholesterol granulomas, congenital epidermoid tumors (cholesteatomas), and mucoceles. Solid lesions of the petrous apex include chondromas, chondrosarcomas, and metastatic carcinomas. All of these pathologies can be more rarely identified within the other portions of the temporal bone. Lesions of the temporal bone are often asymptomatic and increasingly identified serendipitously on imaging for other complaints. When symptomatic, these lesions cause complaints of headaches or signs due to impingement on neurovascular structures. Within the petrous apex, hearing loss is the most common presenting symptom (64%), followed by dizziness (49%), headache (43%), tinnitus (40%), and facial twitching (14%). Trigeminal paresthesia may also occur. Symptoms more rarely reported include cranial neuropathies, facial paralysis, and diplopia. Eustachian tube involvement may present as recurrent otitis media or otorrhea.

Cholesterol granulomas are the most common cystic lesion of the petrous apex (0.6 cases per million per year), occurring 10 times more frequently than cholesteatomas and 40 times more frequently than mucoceles. A cholesterol granuloma is an intraosseous cyst with a fibrous lining, filled with a brown-yellow fluid composed of cholesterol crystals and inflammatory cells ( Fig. 143.3 ). Cholesterol granulomas form as a result of anaerobic catabolism of blood and blood products. The blood products incite a foreign-body giant cell inflammatory response. They can form anywhere within pneumatized spaces of the temporal bone. Cholesterol granulomas lack a true epithelial lining and therefore do not require complete excision.

Congenital epidermoid tumors and acquired cholesteatomas are the second-most common lesions of the petrous apex. These slow-growing tumors comprise 4% to 9% of petrous apex lesions. Epidermoids and cholesteatomas are sac-like cysts, lined by stratified squamous epithelium that contains keratin debris. The microscopic appearance of congenital epidermoids and acquired cholesteatomas is identical. These lesions behave aggressively with the potential for progressive expansion and destruction of surrounding structures. Congenital epidermoid tumors are thought to arise from epithelial rests deposited within the temporal bone in early embryonic development. Acquired cholesteatoma occurs from medial spread of cholesteatoma originating within the middle ear or mastoid. These patients are more likely to present with symptoms of chronic otitis media.

Like most lesions of the petrous apex, epidermoids often present when they become large. They produce symptoms by compression of the Eustachian tube, internal auditory canal, and cranial nerves or by putting tension on the surrounding dura; by the time most epidermoids are detected, cranial neuropathies are present (85%). In a series of 25 patients with cholesteatomas of the petrous apex, hearing loss was the most common presenting symptom (64%), followed by facial weakness (56%), dizziness (44%), otorrhea (32%), trigeminal paresthesia/neuralgia (12%), and cranial nerve VI palsy (4%). These tumors often involve vascular structures as well, including the carotid artery. For these reasons, complete surgical resection of epidermoid tumors in this location can be difficult and recurrence can occur even with adequate exposure due to inability to remove all of the epidermoid from critical structures. The most common site for recurrent petrous epidermoids is in the region of the carotid artery.

The common benign lesions of the petrous apex, cholesterol granulomas and petrous apex epidermoids, are often indistinguishable by history and physical examination alone, but typically can be diagnosed definitively by appropriate imaging ( Table 143.1 ). Increasingly, benign lesions of the petrous apex are detected serendipitously on imaging ordered for other indications. If this occurs, appropriate imaging must still be performed to determine the type of lesion and its extent. Accurate diagnosis is required prior to surgical management, because the approach for these lesions differs radically.

High-resolution temporal bone CT is valuable in both diagnosis and treatment planning. Cholesterol granulomas and petrous apex epidermoids are nonenhancing iso- or hypodense masses on brain-weighted algorithms. Irregularities and erosion of the temporal bone around the mass on CT indicates a more aggressive lesion (tumor or cholesteatoma; Fig. 143.4A ), whereas smooth, thickened borders around the mass indicate benign, less-aggressive lesions (cholesterol granulomas) that have slowly increased in size over time (see Fig. 143.4B ). High-resolution temporal bone CT is invaluable in delineating the precise anatomic relationship of the lesion to structures including the otic capsule, middle ear, carotid artery, and middle and posterior fossa. This detail is essential in determining the feasibility of different surgical approaches.

MRI of the brain and temporal bones is also a critical component of preoperative imaging. T1 signal hyperintensity indicates the presence of adipose tissue, blood, or proteinaceous fluid. T2 signal hyperintensity indicates the presence of water. Cholesterol granulomas have a very distinct appearance on MRI imaging ( Fig. 143.5 ). Because cholesterol granulomas are composed of the breakdown product of blood and inflammatory products, they produce hyperintensity on both T1- and T2-weighted MRIs. In addition, cholesterol granulomas do not enhance with gadolinium. Other lesions of the petrous apex, including epidermoids, mucoceles, unilateral benign effusions, arachnoid cysts, and neoplastic lesions, tend to be iso- or hypointense compared with brain on T1 imaging. Epidermoids and acquired cholesteatomas are characteristically hyperintense on T2-weighted MRI and often do not enhance with gadolinium (if they do, the pattern of enhancement is irregular; Fig. 143.6A ). On diffusion-weighted imaging (DWI), cholesteatomas are hyperintense (see Fig. 143.6B ), whereas mucoceles, effusions, and arachnoid cysts are hypointense. Neoplasms are isointense on DWI. MRI also provides soft tissue detail and information about intracranial extension or involvement of neurovascular structures.

Postoperative imaging is a crucial component of the care of lesions of the petrous apex. Following drainage of a cholesterol granuloma, the drained cavity may fill with fluid and thus make detection of a recurrence by CT difficult. However, serous fluid and recurrent cholesterol granuloma will have different appearances on T1-weighted images. Serous fluid-filled cavities will have decreased signal intensity, whereas recurrence will remain hyperintense on T1-weighted imaging. After resection of a petrous apex epidermoid, either MRI or CT can be used to monitor for recurrence.

Preoperative Period

Operative Period