What is an Acoustic Neuroma?
An Acoustic neuroma is the most common skull base tumor that involves the ear. It is a benign (not cancerous) tumor that typically begins in the balance nerve, the eighth cranial nerve. The eighth cranial nerve carries both the hearing and balance sensation from the ear to the brain. The eighth cranial nerve runs next to the facial nerve (7th cranial nerve) and the blood supply to the inner ear. The acoustic neuroma usually starts in the internal ear canal, but occasionally arises on the eighth nerve closer to the brainstem. As it gradually increases in size, the tumor fills the internal ear canal. If the tumor is not managed early, growth in size may compress the brainstem, causing major neurological problems.
How common are acoustic neuromas?
Acoustic neuromas make up about 6% of all tumors inside the head and about 80% of tumors that involve the part of the skull that surrounds the inner ear, the temporal bone. Annual incidence rates are about eleven per million population, slightly more common in women than in men. While many acoustic neuromas occur in persons aged 30-70, some are found at other ages. The average age at diagnosis is 50. Cause for the majority of these tumors is thought to be a genetic defect isolated to the balance nerve. For reasons unknown, DNA changes affecting the tumor suppressor gene in both related chromosomes result in over growth of the insulation cells that surround balance nerve fibers.
Acoustic neuroma causes a wide variety of symptoms. Hearing loss in the affected ear one of the earliest symptoms. Tumor growth compresses and may invade the hearing portion of the 8th cranial nerve (the hearing and balance nerve). Blood supply to the inner ear may be involved. Typically, high frequency hearing loss on the tumor side and ear noise (tinnitus) are the earliest symptoms. Hearing loss is gradual about 85-90% of the time, but may be sudden in 10-15%. Rarely, acoustic neuroma is found without a hearing loss.
Tinnitus or ringing only in the tumor ear is a common but often ignored symptom of acoustic neuromas.
Imbalance or dizziness are common, but usually mild. Tumor growth invades and destroys the balance nerve but it happens so slowly that the brain adapts for the imbalance. Commonly, only vague imbalance is noticed.
As the tumor gets larger (> 1 inch), about half of the patients experience numbness on one side of the face. The numbness starts in the cheek region then spreads to the upper and lower parts of the face.
Rarely, facial pain occurs, usually as a sharp stabbing pain. Numbness and pain are caused by pressure and irritation of the 5th cranial nerve (trigeminal nerve). The trigeminal nerve detects soft touch, warmth-coolness, and pain in the face. It is also the nerve supply for the muscles of chewing. The 7th cranial nerve, also called the facial nerve, is closely associated with acoustic neuromas. The 7th nerve controls facial strength. It is normally is somewhat resistant to tumor growth, but in far advanced tumors, facial weakness or paralysis may develop. Headaches are quite rare in small tumors, may occur as the tumor gets larger. Large tumors may obstruct flow of spinal fluid increasing pressure, causing headaches, and repetitive vomiting. Huge tumors may affect blood pressure, heart rate, and suppress ability to breathe. With large tumors and chronically high intracranial pressure, other symptoms that may develop include visual problems, hoarseness, dizziness, swallowing, and choking on liquids and solids.
Diagnosing Acoustic Neuromas
For the vast majority of tumor patients, hearing loss in one ear is the initial symptom.
An audiogram (basic hearing test) finds hearing loss, normally in the higher frequencies. Ability to understand speech is also impaired. The most sensitive home test is to compare ability to understand on the telephone. If one ear has perceptibly poorer speech understanding on the phone, acoustic neuroma should be considered unless an obvious other explanation exists. Thus, any one-sided hearing loss should prompt further investigation. When a hearing test raises a mild suspicion of acoustic neuroma, other screening tests may be done. When suspicion is significant, a Magnetic Resonance Imaging scan is definitive way to discover the tumor.
Sometimes, an MRI for a problem unrelated to hearing and balance concerns, reveals an incidental acoustic neuroma. In elderly persons, an incidental small to medium sized tumor seldom requires any treatment. For reasons unknown, small tumors are often minimally symptomatic and quite unlikely to grow in elderly persons. Observation without tumor removal is the recommendation for such circumstances. Yearly MRI is the treatment of choice.
Definitive Tumor Management
Surgical removal of tumor is the treatment of choice for most persons who have reasonably good health. Several surgical approaches exist that depend on a variety of factors. The surgical team considers the entire health and tumor evaluation in recommending a specific remedy. Among the options are the translabyrinthine, retrosigmoid, middle fossa, transpetrosal, and extended middle fossa approaches. Each employs a surgical microscope. The surgical team consists of a neurotologist (Dr. Bartels & Dr. Danner) plus other physicians as needed including a neurosurgeon consultant, a critical care medicine specialist, and others. Facial nerve function, and when appropriate, hearing nerve function are monitored intra-operatively.
Radiotherapy is an option, as well. While surgery removes the tumor, radiotherapy does not, but radiotherapy may stop the tumor from growing any larger. After radiotherapy, possibly 20% of acoustic neuromas shrink to varying degrees. Because about 5% of radiated tumors grow again, surgery may eventually be necessary, anyway. Both radiation and surgery have complication risks. Each person should weigh the options and make a treatment selection appropriate for their individual circumstance. Among the several surgical approaches possible, the surgeon recommends a specific selection after considering patient preference, the exact location and size of the tumor and the possibility of saving residual hearing.