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Acoustic neuromas (vestibular schwannomas) are benign Schwann cell tumors that typically arise from the vestibular portion of the eighth cranial nerve. The acoustic neuroma is the most common tumor of the cerebellopontine angle. The most common presenting symptoms are unilateral sensorineural hearing loss, tinnitus and imbalance. Ninety-five percent of AN are unilateral and occur as a random event. About 5 percent of patients have bilateral AN inherited as part of neurofibromatosis type II (NF II) (central von Recklinghausen's disease).
This dominant autosomal disorder of chromosome 22 is associated with acoustic neuromas, meningiomas and gliomas. Neurofibromatosis I is not usually associated with acoustic neuromas. The acoustic neuroma occurs equally between men and women and most frequently present in the fourth and fifth decades.
The diagnosis of an acoustic neuroma is often triggered by a patient’s symptoms. The most common presenting feature of acoustic neuromas, occurring in 90% of patients, is unilateral hearing loss. When "pure tone audiometry" is used, the most common finding is high frequency hearing loss. The hearing loss is progressive in most patients, but in approximately 12% of patients the hearing loss may occur suddenly. Other symptoms of the acoustic neuroma include asymmetric tinnitus (ringing in the ear), dizziness and disequilibrium (difficulty with balance).
An Acoustic Neuroma typically grows on one of the branches of the VIIIth cranial nerve—the nerve that serves as the conduit for information from the ear to support hearing and balance. The above CT scan image shows the VIIIth cranial nerve as it courses from the brain to the ear. The + denotes the site where an Acoustic Neuroma is most likely to form.
More than 80% of patients having acoustic neuromas have tinnitus. Tinnitus is usually described as hissing, ringing, buzzing or roaring. Tinnitus is often said to be high pitched. In some patients the tinnitus is a pure tone, and in others the tinnitus is a noise. Many patients with acoustic neuroma have combined tinnitus and hearing loss.
Acoustic neuromas typically begin in sites that are "transition zones" from the central to the peripheral nervous system along the VIIIth cranial nerve (the nerve that subserves hearing and balance function). The most common site of origin is thought to be that site along the nerve shown in the adjacent image (+). As the acoustic neuroma expands it fills the internal auditory meatus (opening of the internal auditory canal, thereby compressing the cochlear (hearing nerve) and facial (facial strength) nerves. Though hearing loss commonly occurs as a result of tumor compression of the hearing nerve, facial weakness often does not occur until acoustic tumors grow quite larger.
Symptoms occur as a result of compression of neural structures or their nutrient blood supply. Acoustic neuromas do not invade and destroy tissue as is common with cancerous tumors. Rather, the major clinical concerns related to acoustic neuroma growth center on the ability of these tumors to compress the soft, neural tissue that is confined to the tight quarters of the posterior fossa and internal auditory canal However, the greatest concern regarding acoustic neuroma growth, and the most compelling reason to seek treatment often relates to the risks posed to the brain. Ultimately if untreated, the acoustic neuroma can compress the cerebellar peduncles ("trunks" of the cerebellum that join the cerebellum to the pontine portion of the brain stem at the "cerebellopontine angle"), cerebellum, brainstem and cranial nerves IX-XI (IX: glossopharyngeal nerve, X: vagus nerve, XI: accessory nerve). This can result in increased pressure applied to key centers of the brain and neurologic impairment.