Vestibular schwannoma

A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath on the nerve malfunction. Normally, Schwann cells function beneficially to protect the nerves which transmit balance and sound information to the brain. However, sometimes a mutation in the tumor suppressor gene, NF2, located on chromosome 22, results in abnormal production of the cell protein named Merlin, and Schwann cells multiply to form a tumor. The tumor originates mostly on the vestibular division of the nerve rather than the cochlear division, but hearing as well as balance will be affected as the tumor enlarges. The great majority of these VSs (95%) are unilateral, in one ear only. They are called "sporadic" (i.e., by-chance, non-hereditary). Although non-cancerous, they can do harm or even become life-threatening if they grow to press on other cranial nerves and vital structures such as the brainstem. Variations in the mutation determine the nature of the tumor's development. The only environmental exposure that has been definitely associated with the growth of a VS is therapeutic radiation exposure to the head. Sporadic VSs originate within the confining bony walls of the small (ca. 2 cm long) internal auditory canal. The most common early symptoms of these intracanalicular (IAC) VSs are gradual hearing loss and a feeling of fullness in the affected ear, some imbalance or dizziness, and tinnitus (ringing or other noise in the ear). Gradual single-sided hearing loss in the high frequencies is the first most obvious symptom for the great majority of patients. Headache as a presenting symptom of VS specifically is rare; facial symptoms (facial numbness, weakness) usually occur only as the tumor grows out of the canal and/or after therapeutic treatment. Delayed diagnosis and misdiagnosis are not unusual.