3.8 Cerebellopontine Angle Tumors
Key Features
The most common tumors of the cerebellopontine angle are vestibular schwannomas, followed by meningiomas and epidermoids.
Treatment options depend on the etiology but may include observation, stereotactic radiosurgery, and surgical excision.
Surgical approaches to the cerebellopontine angle are the middle fossa, retrosigmoid, and translabyrinthine approaches.
The cerebellopontine angle (CPA) is a cerebrospinal fluid (CSF)-filled space found at the ventral aspect of the junction between the cerebellum and the pons. Most neoplasms found in this location are benign. Vestibular schwannomas (also known as acoustic neuromas) are the most common, followed by meningioma, and epidermoids. Presenting symptoms usually include unilateral sensorineural hearing loss, tinnitus, and dizziness/imbalance.
Epidemiology
Vestibular schwannomas are the most common CPA neoplasm. These most commonly arise from the inferior vestibular division of CN VIII. Spontaneous yearly occurrence is ~1 in 100,000, or roughly 2,280 new cases annually in the United States. Most tumors grow slowly (average is 1–2 mm per year). Bilateral tumors can be seen in patients with neurofibromatosis 2 (NF2).
Clinical
Signs and Symptoms
Presenting symptoms can include unilateral hearing loss, unilateral tinnitus, or progressive imbalance or vertigo. Difficulty talking on the phone with one ear is a common complaint. Sudden hearing loss may be the presentation in ~10% of cases. Large tumors can cause brainstem compression and cranial nerve (CN) V, VI, VII, VIII, IX, X, and XI palsies. Compression of the fourth ventricle may cause hydrocephalus, typically with giant tumors measuring more than 4 cm. Ataxia, altered mental status, headache, nausea, vomiting, diplopia, respiratory depression, and coma can also occur.
Differential Diagnosis
The differential diagnosis may include vestibular schwannoma, meningioma, epidermoid, arachnoid cysts, facial nerve schwannoma, trigeminal schwannomas, endolymphatic sac tumors, chondrosarcoma, and metastatic tumor.
Evaluation
Physical Exam
A complete otoneurologic, head and neck, and full cranial nerve exam should be performed. Cranial nerve deficits should be documented. The corneal reflex (CN V1) and blink reflex (CN VII) can be assessed using a wisp of cotton. Hoarseness and dysphagia warrant fiberoptic laryngoscopy for evaluation. Spontaneous, gaze, and headshake nystagmus tests may indicate unilateral vestibular weakness (fast phase to the opposite side). Fall or sway may be seen with Romberg, Fukuda stepping, and tandem gait tests. Dysmetria may be seen on cerebellar tests such as the finger-to-nose test. Hitzelberger sign may also be elicited (reduced sensation of the posterior ear canal from tumor compression of the facial nerve).
Imaging
Magnetic resonance imaging (MRI) of the brain with and without contrast with thin cuts through the internal auditory canals is the gold standard for diagnosis. Vestibular schwannomas and meningiomas are slightly hypointense to isointense on T1-weighted images, are heterogeneously hyperintense with cystic areas on T2, and enhance with contrast. Meningiomas are more sessile, broad-based, and may display a dural tail. They are rarely centered within the internal auditory canal. Diffuse and extensive dural involvement can occur (“en plaque meningioma”). Computed tomography (CT) scan of a meningioma may demonstrate calcifications and adjacent hyperostosis. Epidermoids are usually hypointense on T1 and hyperintense on T2. They do not enhance with contrast but they may be hyperintense on fluid-attenuated inversion recovery (FLAIR).
Other Tests
Audiogram: Unilateral pure-tone sensorineural loss is usually seen. Decreases in speech discrimination are common and usually greater than expected considering pure tones. Decay in speech discrimination may be demonstrated at higher test stimuli (rollover).
Auditory brainstem response (ABR) test: The sensitivity of ABR is approximately 95%; however, this percentage drops to 67% for smaller, intracanalicular tumors. Interaural difference in wave V of > 0.2 ms is abnormal.
Vestibular testing: Tumors may cause weakness of the superior and/or inferior vestibular nerves. Peripheral vestibular weakness may be demonstrated on caloric testing (function of horizontal canal and its afferent pathway through the superior vestibular nerve) and vestibular evoked myogenic potentials (VEMP). Cervical VEMP may detect dysfunction of the saccule or inferior vestibular nerve; ocular VEMP may detect dysfunction of the utricle or superior vestibular nerve.
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