Overview of Acoustic Neuroma
The acoustic neuroma, or vestibular neuroma, is a common tumor of the skull base. Even so, only about 2-3000 new cases are diagnosed each year in the United States. Although much is known about the acoustic neuroma its cause has not been identified. Over the years, many surgeons have developed different approaches to its treatment. More and more, these approaches have been modified with a common goal in mind: tumor control with preservation of facial and hearing function, and minimal side effects. Reaching that goal requires expert surgical teamwork, sophisticated nerve monitoring, skilled neuro-anesthesia, intensive nursing, and rehabilitation therapy. Because tumors are now being detected at earlier and smaller stages of development, thanks to MRI scanning, it is possible to achieve that goal in the majority of cases.
What is an acoustic neuroma?
The acoustic neuroma is a benign growth that arises in the balance portion of the eighth cranial nerve. This nerve carries both hearing (cochlear) and balance (vestibular) senses from the ear to the brain. The eighth cranial nerve runs next to the facial nerve at the base of the skull in a bony canal -- the internal auditory canal, which is approximately one centimeter in length. Acoustic neuromas begin in this internal auditory canal, slowly enlarge to fill the canal, gradually grow toward the brainstem, and may begin to compress it. These tumors are very slow growing, about 2-3 mm per year and cause symptoms as they press on the surrounding brain tissue. In the early stages, patients have no symptoms. In the advanced stages, the tumor may cause paralysis of the face, inability to walk, severe headaches from increased pressure, blindness, and even death.
Acoustic neuromas are non-malignant and do not spread to other parts of the body. In very rare cases, acoustic neuromas occur on both sides. This happens only as part of the uncommon disorder, neurofibromatosis, type II.
Symptoms of Acoustic Neuroma
Because the tumor is slow growing the initial symptoms may be few and are easily overlooked. In most cases, the first symptom is hearing loss which may be accompanied by ringing in the ear (tinnitus). The onset of the hearing loss may be insidious, and usually worsens as the tumor grows. On the other hand, hearing loss may be sudden or may even fluctuate. Under these circumstances, the diagnosis may initially be confused with other inner ear disorders or degenerative diseases. The tumor enlargement is always painless and headache is unusual. As the tumor grows, the balance nerve is slowly destroyed and an intermittent sense of mild disequilibrium may be experienced. However, the other uninvolved balance organ is generally able to compensate.
Despite its close proximity to the balance nerve, facial nerve function is almost always intact regardless of the size of the tumor. This nerve is very robust, and while it may be compressed and thinned by the tumor, facial paralysis is very unusual except in later stages. If the tumor is very large, other neurological symptoms occur as the tumor compresses brain tissue or other cranial nerves; in particular, compression of the fifth cranial nerve may result in facial numbness on the side with the tumor.
Diagnosis of Acoustic Neuroma
The first objective evidence of an acoustic neuroma is detection by a conventional hearing test that identifies hearing loss in one ear. Often, there is reduced ability to understand speech. These findings will generally alert the physician to the possible presence of a tumor and further tests will be performed.
A common screening test is the auditory brainstem response test (ABR). This test measures the impulses along the hearing nerve in response to sound delivered to the suspect ear. This is a non-invasive study and will be abnormal in 80-90% of patients with acoustic neuromas depending on the tumor size. An abnormal ABR indicates pressure on the cochlear nerve by the tumor.
When a tumor is suspected, imaging studies are the optimal method to establish the diagnosis. Advances in imaging techniques now allow us to identify very small tumors. The magnetic resonance imaging (MRI) has become the gold standard for diagnostic studies and can identify tumors 1 mm and larger when enhanced with gadolinium.