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An acoustic neuroma, also called a vestibular schwannoma, is a benign primary intracranial tumor of the myelin-forming cells of the vestibulocochlear nerve (CN VIII).[1] (Neuroma is derived from Greek, meaning “nerve tumor”.) The term “acoustic” is a misnomer, as the tumor rarely arises from the acoustic (or cochlear) division of the vestibulocochlear nerve. The correct medical term is vestibular schwannoma, because it involves the vestibular portion of the 8th cranial nerve[2] and it arises from Schwann cells, which are responsible for the myelin sheath in the peripheral nervous system. Approximately 3,000 cases are diagnosed each year in the United States with a prevalence of about 1 in 100,000 worldwide. It compromises 5-10% of all intracranial neoplasms in adults. Incidence peaks in the fifth and sixth decades and both sexes are affected equally.
Acoustic neuromas may occur sporadically, or in some cases occur as part of von Recklinhausen neurofibromatosis, in which case the neuroma may take on one of two forms.
The usual tumor in the adult presents as a solitary tumor, originating in the nerve. It usually arises from the vestibular portion of the 8th nerve, just within the internal auditory canal. As the tumor grows, it usually extends into the posterior fossa to occupy the angle between the cerebellum and the pons (cerebellopontine angle). Because of its position, it may also compress the 5th, 7th, and less often, the 9th and 10th cranial nerves. Later, it may compress the pons and lateral medulla, causing obstruction of the cerebrospinal fluid and increased intracranial pressure.
Schwannomas can occur in relation to other cranial nerves or spinal nerve roots, resulting in radiculopathy or spinal cord compression. Trigeminal neuromas are the second most common form of schwannomas involving cranial nerves. Schwannomas of other cranial nerves are very rare.
The earliest symptoms of acoustic neuromas include ipsilateral sensorineural hearing loss/deafness, disturbed sense of balance and altered gait, vertigo with associated nausea and vomiting, and pressure in the ear, all of which can be attributed to the disruption of normal vestibulocochlear nerve function. Additionally more than 80% of patients have reported tinnitus (most often a unilateral high-pitched ringing, sometimes a machinery-like roaring or hissing sound, like a steam kettle).
Large tumors that compress the adjacent brainstem may affect other local cranial nerves. Involvement of the nearby facial nerve (CN VII) may lead to ipsilateral facial weakness, sensory impairment, impairment of glandular secretions and loss of taste sensation in the anterior two-thirds of the tongue; involvement of the trigeminal nerve (CN V) may lead to loss of sensation in the involved side’s face and mouth. The glossopharyngeal and vagus nerves are uncommonly involved, but their involvement may lead to altered gag or swallowing reflexes.
Even larger tumors may lead to increased intracranial pressure, with its associated symptoms such as headache, vomiting, and altered consciousness.
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