One-sided or unilateral hearing loss can be a warning sign for a brain tumor. Acoustic neuroma is a benign, slow growing tumor that most commonly begins on the balance portion of the eighth cranial nerve (cochleovestibular nerve). This nerve is encased in a very narrow bony canal as it approaches the inner ear structures. A tumor as small as 1 mm can decompress the nerve and cause ringing and/or hearing loss. Larger tumors can decompress the adjacent facial nerve (cranial nerve VII) which travels through the same bony canal.

Magnetic resonance imaging (MRI) is the most accurate way to diagnose an acoustic tumor and can be used to monitor its growth and response to treatment. In many cases, the tumors grow so slowly that they can be safely observed with periodic hearing tests (audiograms) and MRI scans. If the tumor is growing large or fast enough that it is causing compression of the vital structures, then surgery or radiation is indicated.

Acoustic tumor surgery is performed by neurosurgeons and by fellowship-trained neuro-otologists. The tumor may be approached through the bones of the ear, or it may be approached from above by gently lifting the temporal lobe of the brain. The approach depends upon the severity of hearing loss and to the perceived risk of injury to the facial nerve. Dizziness, permanent hearing loss, and permanent facial paralysis are possible complications of surgery.

A recent alternative to surgery is radiation treatment using a gamma knife. The gamma knife is a computerized radiotherapy device that exposes the tumor from dozens of angles, creating a high energy overlap precisely at the point of the tumor. While this nonsurgical technique can usually preserve facial nerve function, most patients experience continued progression of hearing loss despite treatment.