An acoustic neuroma (also called a vestibular schwannoma, acoustic neurinoma, or neurilemmoma) is a benign (not cancer) growth and usually slow-growing tumor (only 1 millimeter per year) caused by an overproduction of Schwann cells, which normally insulate and support nerve cells by wrapping around them. The tumor develops on the outer cover of inner ear’s hearing and balance nerve (the vestibulocochlear nerve also called the eighth cranial nerve). This nerve transmits both balance and sound information to brain for processing. Acoustic neuromas are rare diagnosed among only about 2500–3000 new Americans each year. Symptoms may develop in individuals at any age, but are more common between the ages of 30 and 60 years. Although people with the genetic disorder neurofibromatosis have acoustic neuromas in both ears (bilateral), we usually discover the the majority of acoustic neuromas only on one-side (unilateral). Unilateral acoustic neuromas are not passed on genetically (non-hereditary) and their actual cause is not clear.
As an acoustic neuroma grows, it begins to press on the the vestibulo-cochlear nerve resulting in progressive hearing loss, ringing in the ears (tinnitus), and dizziness or lack of balance. Because the vestibulo-cochlear nerve is located near the trigeminal nerve, which controls facial sensation, large tumors may also present with facial numbness. An acoustic neuroma may also create pressure on neighboring nerves controlling the muscles of the face, causing facial weakness or paralysis on the side of the tumor. When acoustic neuromas become large enough they can begin to exert pressure on parts of the brainstem and cerebellum (the area of the back and bottom of the brain) and can become life threatening.
Diagnosis of acoustic neuromas may sometimes be elusive because symptoms may be nonexistent or subtle, sometimes mimicking other middle or inner ear problems. However, early detection of these benign growths is key to preventing the development of serious complications. Once symptoms appear, a thorough ear examination and evaluation is critical but is usually normal. The initial clue to the presence of an acoustic neuroma is a finding of assymetric hearing loss due to nerve damage. Very commonly during the part of the hearing test in which we test the ability of the brain to process sound, called the discrimination score, the result will indicate poor sound processing. A specialty test called auditory brainstem response test (also called ABR, BAER, or BSER) may then be used to evaluate whether sound information is being successfully passed along the nerve path leading from the ear to the brain. This test is painless and involves wearing an array of electrodes over the head and special insert ear phones in the ear that produce loud clicking sounds in the ear. A computer then records the electrical waves that are generated by the nerve activity along the hearing and balance nerve pathways. If the ABR result is abnormal, a detailed imaging test is typically performed. Magnetic resonance imaging (MRI) is considered the best method of evaluation because it can detect tumors of very small size and is designed to pick out soft tissue growths like acoustic neuromas. Sometimes MRI is not an option either because the patient is claustrophobic although open MRI machines do exist and in cases where metallic implants that are not safe to use with the powerful magent used during the MRI examination. In those cases, we use a computerized tomography scan (CT scan). The CT scan is a specialized X-ray device that is not an enclosed tunnel but rather a simple ring that goes around the head. These tests help determine a tumor’s location and size and in selecting a treatment plan.
The three treatment options are surgical removal, radiation, or watchful waiting. Typically, the tumor is surgically removed. The major risk with surgical removal is loss of hearing or paralysis of the muscles of facial expresion on the surgical side . The exact approach we take to remove the tumor depends on its size and the level of hearing and facial nerve function still present.
If the tumor is very small, hearing and facial nerve function is commonly preserved and accompanying symptoms may improve after surgery. As the tumor grows larger, its surgical removal can worsen imbalance symptoms because sections of the balance or vestibular nerves themselves may also need to be removed with the removal of tumor tissue. In this case, special balance treatment or vestibular rehabilitation may be required in order to help the central nervous system compensate for the injury of the balance system located in the inner ear.
An alternative to conventional surgical removal, radiosurgery with a gamma knife or linear accelerator may be employed to reduce the size or limit the growth of the tumor. Because this treatment may cause less injury, it is sometimes the preferred option for elderly patients , patients with underlying health conditions, those who have bilateral growths, or whose tumor is affecting their only hearing ear. In some cases, usually involving elderly or medically infirm patients, it may be preferable to monitor the tumor with repeated MRIs to monitor the tumor for any growth.