Acoustic Neuroma (Vestibular Schwannoma)

Acoustic neuroma, more accurately termed vestibular schwannoma (VS), is a benign tumor arising from Schwann cells of the vestibular branch of the eighth cranial nerve (vestibulocochlear nerve), most commonly originating at the junction between central and peripheral myelin (Obersteiner-Redlich zone) within the internal auditory canal (IAC). The tumor expands slowly (1-2 mm/year average), gradually filling the IAC and extending into the cerebellopontine angle (CPA), eventually compressing the cochlear nerve (hearing loss), facial nerve, trigeminal nerve, and brainstem.

Most cases (95%) are unilateral and sporadic, while bilateral acoustic neuromas pathognomonically suggest neurofibromatosis type 2 (NF2), a genetic disorder caused by mutations in the merlin/schwannomin gene on chromosome 22. Sporadic VS is more common in adults aged 30-60, with peak incidence in the fifth decade. Risk factors are largely unknown, with possible associations including childhood head/neck radiation and prolonged loud noise exposure.

Diagnosis is established with gadolinium-enhanced MRI of the IAC and CPA showing an enhancing mass, often described as having an 'ice cream cone' shape with intracanalicular and CPA components. Audiometry reveals asymmetric sensorineural hearing loss with poor word recognition. Auditory brainstem response (ABR) shows prolonged wave V latency and interaural latency difference. Vestibular testing (caloric, VEMP) often shows ipsilateral hypofunction. Tumor size classification: intracanalicular, small (<1.5 cm CPA), medium (1.5-3 cm), large (>3 cm).