Abstract
Cholesterol granulomas are the most common lesions involving the petrous apex. However, they are still an uncommon finding overall, and they often remain undiagnosed until they have become extremely large and symptomatic. Many surgical approaches to the petrous apex exist. Factors that often influence the surgical approach include the surgeon's experience, the patient's anatomy, and the patient's hearing status. The purpose of this case report—which involved a 66-year-old woman who was referred to our clinic for evaluation of severe headaches, dizziness, and left sided pulsatile tinnitus—is to demonstrate the definitive need for an extended middle fossa approach when a bilobed petrous apex mass is encountered.
Introduction
Cholesterol granulomas are the most common pathologic entities in the petrous apex, accounting for 60% of all lesions in this area.1,2 Their pathogenesis remains controversial. The classic theory holds that a hemorrhage occurs as a result of an obstruction of ventilation in pneumatized bone.
Jackler and Cho have proposed that the mechanism for the formation of cholesterol granulomas involves a spontaneous hemorrhage into a well-pneumatized petrous apex air-cell system from adjacent bone marrow. 3 They contend that the petrous apex is pneumatized in patients with a petrous apex granuloma. In order for this to occur, the eustachian tube must be functional. Therefore, they believe it is unlikely that the hemorrhage occurs secondary to negative pressure within the middle ear or mastoid cavity.
While the cause of hemorrhage in these cases is unknown, it is known that hemorrhage into the petrous apex generates cholesterol crystals as red blood cells and serum degenerate. The cholesterol crystals then act as an irritant, inciting a foreign-body reaction and the subsequent development of an expansile cyst. As the cyst expands, it can cause a variety of symptoms, including facial numbness, diplopia, hearing loss, vertigo, facial paralysis, and headaches. However, these cysts often remain clinically unnoticed until they become extremely large. 4
Computed tomography (CT) and magnetic resonance imaging (MRI) are the modalities of choice for confirming the diagnosis. CT classically demonstrates a smooth bone erosion centered in the petrous apex. The septations of a pneumatized petrous apex are replaced by a cyst that has a margin that is attenuated, sclerotic, or both. MRI shows a classic hyperintense signal on both T1- and T2-weighted imaging, and this is pathognomonic for a cholesterol granuloma. 5 Occasionally, a cholesterol granuloma will demonstrate an area of hypointensity on MRI as a result of a solid mineral content within the cyst. 1
In this article, the author describes a new case of petrous apex cholesterol granuloma to illustrate the definitive need for an extended middle fossa approach when a bilobed petrous apex mass is encountered.
Case Report
The patient was 66-year-old woman who was referred to our clinic for evaluation of severe headaches, dizziness, and left-sided pulsatile tinnitus. Her symptoms had been gradually progressive over the previous 2 years until they reached the point where they had become unremitting. Their severity had recently caused the patient to miss several days of work as an instructor for a weight-loss center.
The patient's headaches were largely retro-orbital and at times very incapacitating. She described her dizziness as an ongoing sense of imbalance that constituted a mild impediment to her daily life. She experienced no episodes of true vertigo, and she could not specifically identify any specific triggers or factors that caused her symptoms to worsen or remit. She had no history of excessive noise exposure, ear surgeries, or head trauma, and she had no complaints of visual distortion, weight loss, facial twitching, or spasms.
Contrast-enhanced T1-weighted MRI shows the large, bilobed, hyperintense petrous apex mass on the left side.
Findings on the patient's comprehensive physical and neurotologic examinations were unremarkable, with the exception of a high-frequency sensorineural hearing loss and an absence of acoustic reflexes on the left side. MRI detected the presence of a large bilobed petrous apex mass on the left (figure). Based on the size and location of the mass, magnetic resonance angiography, magnetic resonance venography, and CT were obtained to determine its relationship to critical structures.
In view of the lesion's size and its proximity to critical structures, we offered the patient the option of an extended middle fossa approach to drain and aerate the mass. Because she considered her symptoms to be incapacitating, she elected to proceed with surgery.
The surgery was generally uneventful. However, during the procedure, we discovered that the two cysts were separated by a very thick, bony septation, which required a curette to take down. Furthermore, we found that the anterior cyst was filled with material that had a peanut-butter–like consistency, and this required significant irrigation to remove. Electrocochleography and monitoring of auditory brainstem responses were performed throughout the operation, and no abnormality was noted. Likewise, facial nerve function was also monitored, and no incidents were recorded.
The patient recovered quickly, and she was discharged on postoperative day 3. Six months later, her symptoms had completely resolved, and her hearing was unchanged from preoperative levels.
Discussion
The differential diagnosis of a cholesterol granuloma includes cholesteatoma, mucocele, chondroma, chondrosarcoma, metastatic carcinoma, dermoid, giant-cell tumor, and cavernous hemangioma. 1
These lesions often become very large before they cause symptoms. 4 In the meantime, a petrous apex granuloma may erode into the jugular foramen, carotid canal, and the internal auditory canal prior to diagnosis. 1 A petrous apex lesion is classically identified by the presence of a hyperintense lesion on T1- and T2-weighted MRI. CT may provide complementary information and facilitate surgical planning if the lesion requires treatment. 5 CT venography has recently been offered as another modality that can help in treatment planning by highlighting the location of an expansile petrous apex granuloma in relation to the jugular bulb and petrous carotid artery. 1
Several surgical approaches to excision or drainage of petrous apex cholesterol granulomas have been described. A precise understanding of the relationship of the lesion to the jugular bulb and the internal auditory canal is a prerequisite to selecting an appropriate surgical approach.1,4,5 Since cholesterol granulomas can cause extensive petrous bone erosion into the jugular foramen or carotid canal, it is an essential part of preoperative planning to use an imaging modality that highlights vascular structures.
Surgical drainage of the cyst is the first-line, least morbid management strategy for these lesions. Neurosurgeons have typically advocated complete resection, while neurotologists recommend a more conservative treatment with drainage and the establishment of permanent aeration.4,6 The infracochlear and infra- labyrinthine approaches are frequently preferred, but the results of a comprehensive multicenter review by Castillo et al suggested that these approaches are best used only in certain circumstances. 7 Additionally, they pointed out that inadequate drainage with a limited approach may hamper the surgeon's ability to adequately aerate the cyst cavity, which could lead to a recurrence.
In the present case, neither an infracochlear nor an infralabyrinthine approach would have ensured access to both lobes of the cyst. The preoperative imaging in this case did not suggest the presence of solid material within the cyst, but that was indeed the case. Therefore, either of these approaches would have been even less desirable because full visualization of the cavity is limited with both options.
In conclusion, patients with a large petrous apex cholesterol granuloma may experience extensive bone erosion involving the jugular foramen, carotid canal, and internal auditory canal. In this subset of patients, inadequate drainage and aeration of the mass puts critical structures at risk because the mass will continue to expand.
If CT and MRI suggest that a petrous apex granuloma is lobulated and septated, an extended middle fossa approach for drainage and aeration is necessary to ensure complete drainage. This technique provides the best view of the area to be drained and ensures that septations are opened and that all contents of multilobulated cysts are opened. Finally, the superior visualization achieved with an extended middle fossa approach prevents confusion about adequate opening of the cyst if the contents are solid and proteinaceous and do not freely flow out.
Failure to adequately drain a petrous apex granuloma can lead to a retained cyst and continued growth. When approaching a petrous apex granuloma via an infralabyrinthine approach, it is the anterior aspect of the cyst that is inaccessible. Conversely, the posterior aspect of a cyst is more likely to be missed if an infracochlear tract is chosen; this is a particular risk if a patient has thick secretions like ours did.
Occasionally a cholesterol granuloma will demonstrate an area of hypointensity on MRI as a result of a solid mineral content within the cyst.3,6 However, relying solely on imaging to identify the contents of a cyst may lead to undertreatment and cyst retention. Failure to drain a cyst may lead to continued symptoms and subject the patient to revision surgery.