Acoustic Neuromas

Acoustic neuromas arise from Schwann cells at the Schwann cell-glial junction, which is usually found in the internal auditory canal. Around 40% of patients with sporadic acoustic neuromas show loss of heterozygosity in the region of the tumor suppressor NF-2 gene on chromosome 22, suggesting that an underlying genetic predisposition exists in many patients [3]. Indeed the presence of bilateral acoustic neuromas is diagnostic for neurofibromatosis-2, further implicating dysfunction of this gene in the pathogenesis of these tumors.

Acoustic neuromas arise almost exclusively from the vestibular branches of the VIII nerve complex; hence, they are more correctly referred to as vestibular schwannomas. They expand the porus acousticus forming a cone-shaped mass with a canalicular and CPA component in 61% of patients. In 21% of cases, the canalicular component becomes sausage-shaped and "mushrooms" out of the porus into the CPA, leading to a dumbbell appearance. In the remainder (18%), the tumors appear to be largely confined to the CPA with no significant intracanalicular component [4]. The laterally arising tumors tend to present earlier with audiovestibular symptoms. Those arising within the CPA are more likely to present with signs of trigeminal compression, cerebellar dysfunction and raised intracranial pressure. A recent analysis of 473 patients with acoustic neuromas treated in Cambridge shows that 89.3% presented with typical audiovestibular symptoms. Of the 10.7% with an atypical presenting symptom, facial numbness (6.4%), headache (2.1%), otalgia (1%), visual changes (0.6%), taste disturbance (0.2%) and facial weakness (0.2%) were all recorded. A high degree of clinical acumen is required to avoid inadvertent delays in the diagnosis of the lesion in this group of patients.

Macroscopically, the superior cerebellopon-tine cistern is invaginated by the tumor, forming a double arachnoid plane around the lesion. Whilst many tumors are homogeneous in texture, macrocyst formation occurs.

Histological examination reveals regions characterized by spindle-shaped cells with hyperchromatic, elongated nuclei (Antoni A type) and other regions where vacuolated cells with pleomorphic nuclei are embedded in a loose eosinophilic matrix in which micro-cystic change may be prominent (Antoni B). Malignant change in acoustic neuromas is exceptional, with few cases reported in the world literature.

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