acoustic neuroma

**Pathology/Pathophysiology:** Benign tumor of Schwann cells (neural crest-derived) arising from the vestibular portion of CN VIII, typically at the internal auditory canal/cerebellopontine (CP) angle. Histology: S-100 positive; Antoni A areas (dense, spindle cells with palisading nuclei and Verocay bodies) and Antoni B areas (hypocellular, loose, myxoid). Sporadic cases and NF2 involve inactivation of the NF2 tumor suppressor gene on chromosome 22q12, encoding merlin (schwannomin). Bilateral vestibular schwannomas = NF2 (autosomal dominant). Remember: NF1 is on chromosome 17 (neurofibromin); NF2 is on chromosome 22 (think "2 = 22", "bilateral acoustic schwannomas" = "2 ears, 2 sides").

**Clinical Presentation:** - Earliest/most common: progressive unilateral sensorineural hearing loss (especially high frequency, with poor speech discrimination) and tinnitus. - Disequilibrium or vertigo (less common than expected, because slow growth allows central compensation). - With enlargement at CP angle: CN VII involvement (facial weakness, loss of taste on anterior 2/3 of tongue, decreased corneal reflex as an early sign), CN V involvement (facial numbness), brainstem compression, cerebellar signs (ataxia, ipsilateral dysmetria), and obstructive hydrocephalus from 4th ventricle compression. - Associated findings in NF2: meningiomas, ependymomas, juvenile posterior subcapsular cataracts, and other schwannomas. Mnemonic: MISME (Multiple Inherited Schwannomas, Meningiomas, Ependymomas).

**Diagnosis:** - Audiometry first: asymmetric sensorineural hearing loss; Weber lateralizes to the unaffected ear, Rinne is normal (air > bone) bilaterally. Brainstem auditory evoked responses (ABR) show delayed wave latencies. - MRI of the brain/IAC with gadolinium is the gold standard: enhancing mass in the IAC extending into the CP angle ("ice cream cone" appearance).

**Management:** - Observation (serial MRI and audiometry) for small, asymptomatic tumors, particularly in older patients. - Stereotactic radiosurgery (Gamma Knife) for small to medium tumors or poor surgical candidates. - Microsurgical resection (translabyrinthine, retrosigmoid, or middle fossa approaches) for large tumors with brainstem compression. Risks include facial nerve injury and hearing loss. - Bevacizumab (anti-VEGF) can be used for NF2-related tumors.

**High-yield Boards Points:** - Most common tumor of the CP angle (~80-90%); next are meningioma and epidermoid cyst. - Most common presentation: unilateral hearing loss + tinnitus (the word "acoustic" is a misnomer, since it arises from the vestibular portion of the nerve and is a schwannoma, not a neuroma). - Unilateral SNHL in an adult requires MRI to rule out a retrocochlear lesion. - Bilateral vestibular schwannomas = NF2 (chromosome 22), whereas NF1 features neurofibromas, Lisch nodules, café-au-lait spots, and optic gliomas. - Differential for CP angle mass: schwannoma > meningioma > epidermoid cyst; also consider Meniere disease (episodic vertigo, fluctuating SNHL, aural fullness) for a non-tumor cause of similar symptoms.