Petrous Apex Mucocele

A 56-year-old woman presented with subjective right-sided numbness and paresthesias in the upper and lower extremities, along with fatigue and dizziness. 

She said that her symptoms had begun as numbness and tingling in the fingertips of her right hand. This sensation had progressively worsened, slowly migrating up the right arm to her right shoulder, with further progression to shooting sensations down her right leg. The symptoms had been associated with extreme fatigue and dizziness upon physical exertion. She stated that aside from walking to the restroom, she had not been able to get out of bed and perform her activities of daily living. She denied experiencing any changes in her ability to move and to use her limbs. She denied having had fevers, cough, chest pain, palpitations, or weight change, and she denied having recently traveled.

History. The patient had a history of type 2 diabetes with associated peripheral neuropathy, as well as hypertension, osteoarthritis, carpal tunnel syndrome, and obesity. She also had a history of decreased hearing, requiring the use of hearing aids bilaterally for the past year. Her surgical history included 3 cesarean deliveries and a laparoscopic cholecystectomy.

She had never smoked tobacco, was a social alcohol drinker, and denied illicit drug use. She had no known drug allergies.

Her current medications were aspirin, 81 mg once a day; duloxetine, 20 mg twice a day; gabapentin, 600 mg 3 times a day; insulin lispro injection, 5 U 3 times a day before meals; lisinopril-hydrochlorothiazide (20 mg lisinopril and 25 mg hydrochlorothiazide), 1 tablet once a day; loratadine, 10 mg once a day; lovastatin, 40 mg once a day at bedtime; metformin, 1000 mg twice a day; and insulin glargine injection, 85 U in the morning and 75 U in the evening.

Physical examination. The patient, who was obese, was in no acute distress but appeared to be very fatigued. Examination findings of the head, eyes, ears, nose, and throat were normal. The lungs were clear to auscultation bilaterally, with no wheezes, crackles, or rhonchi appreciated. Cardiovascular examination findings were normal. Her abdomen was soft and nondistended, with normal bowel sounds, and no tenderness, rigidity, or guarding. Examination of the skin and musculoskeletal system revealed no abnormalities.

Neurologic examination revealed no obvious focal deficits, and the patient was alert and oriented to person, place, and time. Cranial nerves II through XII were normal on examination. Motor strength in her upper and lower extremities was decreased at 3/5 due to fatigue. Biceps and patellar deep tendon reflexes were normal (2+) bilaterally. Sensations of pain, temperature, touch, and proprioception were normal bilaterally.

Diagnostic tests. Laboratory test results showed a glucose level of 232 mg/dL (reference range, 70-110 mg/dL), a creatinine level of 1.2 mg/dL (reference range, 0.6-1.2 mg/dL), and a lactate level of 4.2 mg/dL (reference range, 5.0-15.0 mg/dL). On urinalysis, the specific gravity was greater than 1.030 (reference range, 1.005-1.030).

Computed tomography (CT) scans of the head revealed a 1.2 × 2.0-cm low-attenuating mass within the right lateral aspect of the clivus (Figures 1 and 2). This mass extended to the right side of the skull base, just dorsal to the petrous portion of the right internal carotid artery, and demonstrated smooth, scalloped, sclerotic margins, without definite evidence of underlying bony erosion. 

Given the appearance of the margins of the mass, possible diagnostic considerations included a less-aggressive chondroid lesion or petrous apex cholesterol granuloma.

Magnetic resonance imaging (MRI) of the brain and skull base with and without gadolinium contrast was performed for further evaluation. Results showed a T1-hypointense, T2-hyperintense, nonenhancing, nondiffusion restricting, cystic lesion at the base of clivus/petrous apex on the right, measuring 1.3 × 1.6 cm. Thin internal septations were present within the lesion. The lack of enhancement and the signal characteristics of the lesion were most suggestive of a petrous apex mucocele.

Discussion. Petrous apex mucoceles are a rare finding that could be a cause of headache and dizziness. To the best of our knowledge, only 4 other cases have been reported in the literature. The pathophysiology of primary mucoceles is not completely understood. It is hypothesized that petrous apex air cells become obstructed, thereby leading to retention of their mucoid secretions, with ultimate formation of a mucocele and its possible related expansile effects.¹

The clinical presentation is a result of mass effect of the mucocele. Impingement on nearby cranial nerves (ie, IV, V, and VI) can lead to facial pain, numbness, or diplopia. Larger lesions that encroach on the temporal bone may cause hearing loss, vertigo, or facial weakness. More expansile lesions can erode intradurally and can present with cerebrospinal fluid (CSF) otorrhea or even chemical meningitis as cyst contents spill into the subarachnoid space.²

Our patient presented with many of these symptoms—specifically, a history of hearing loss of unknown etiology and headaches and vertigo upon physical exertion.

The differential diagnosis of petrous apex lesions includes cholesterol granuloma, cholesteatoma, mucocele, primary or metastatic neoplasm, vascular malformation, and aneurysm.³ Cholesterol granulomas are reported far more commonly than cholesteatomas, which in turn are more common than mucoceles.¹ These lesions in the petrous apex can be differentiated by their microscopic appearance, with primary mucoceles demonstrating a cyst wall lined by cuboidal epithelial cells.⁴ The appearance of the mucous contents within these cysts varies with degree of hydration.⁵ Cholesterol granulomas are characterized by a fibrous lining with inner contents comprising blood, hemosiderin, inflammatory cells, blood vessels, fibrous tissue, and cholesterol crystals, whereas cholesteatomas are a solid mass with a capsule composed of stratified squamous epithelium and containing desquamated keratin.⁵

Diagnosis is based on results of CT and MRI. CT scans will reveal a nonenhancing lesion within the petrous apex air cells.¹ MRI confirms the diagnosis of mucocele if the lesion is hypointense relative to brain tissue on T1-weighted imaging and hyperintense relative to brain tissue on T2-weighted imaging.¹ The imaging findings in our patient’s case were consistent with these findings.

Management of petrous apex mucoceles is complicated by the difficulty of the surgical approach given the proximity of the petroclival region to neurovascular structures.^2,6,7 No surgical intervention is necessary if the patient is asymptomatic and if discovery of the mucocele is incidental.^1,8 

In many cases, including our patient’s case, conservative management with serial neurologic examinations along with serial MRIs to monitor for expansion of the lesion and encroachment on nearby structures is appropriate.^1,8 Alternatively, significant neurologic impairment, large compressive lesions, and CSF otorrhea are appropriate clinical indications for surgical intervention.²

Many studies have been performed to investigate the safest and most effective surgical approach to petrous lesions other than mucoceles (eg, cholesterol granulomas, cholesteatomas). The traditional surgical approach has been drainage of the cyst contents via the transtemporal or transsphenoidal route, with subsequent placement of a silicon tube to reestablish aeration.² Research has demonstrated a mucocele recurrence rate as high as 60%² with a frequent need for subsequent revision surgery. However, the authors of one case series of 14 patients² demonstrated that complete removal of the cyst wall resulted in no recurrence of petrous apex lesions at a mean follow-up period of 3.8 years.

More recently, it has been proposed that an endoscopic endonasal approach may be preferred, given that it would reduce the risk of hearing impairment and facial nerve compromise that can be associated with a transtemporal approach.⁷ The utility of this approach depends upon the location of the petrous internal carotid artery. The possible benefits of this approach also include a decreased length of hospitalization and a lack of external scars.⁷

REFERENCES:

1. Arriaga MA, Brackmann DE. Differential diagnosis of primary petrous apex lesions. Am J Otol. 1991;12(6):470-474.
2. Eisenberg MB, Haddad G, Al-Mefty O. Petrous apex cholesterol granulomas: evolution and management. J Neurosurg. 1997;86(5):822-829.
3. Lin BM, Aygun N, Agrawal Y. Cystic lesions of the petrous apex: identification based on magnetic resonance imaging characteristics. Otol Neurotol. 2012;33(9):e75-e76.
4. Osborn AG, Parkin JL. Mucocele of the petrous temporal bone. AJR Am J Roentgenol. 1979;132(4):680-681.
5. Larson TL, Wong ML. Primary mucocele of the petrous apex: MR appearance. AJNR Am J Neuroradiol. 1992;13(1):203-204.
6. Jacquesson T, Simon E, Berhouma M, Jouanneau E. Anatomic comparison of anterior petrosectomy versus the expanded endoscopic endonasal approach: interest in petroclival tumors surgery. Surg Radiol Anat. 2015;​37(10):​1199-1207.
7. Zanation AM, Snyderman CH, Carrau RL, Gardner PA, Prevedello DM, Kassam AB. Endoscopic endonasal surgery for petrous apex lesions. Laryngoscope. 2009;119(1):19-25.
8. Muckle RP, De la Cruz A, Lo WM. Petrous apex lesions. Am J Otol. 1998;​19(2):​219-225.