Acoustic neuromas are benign tumors that arise from the cochleovestibular (hearing and balance) nerve. Three to four thousand of these tumors are diagnosed in the United States per year. These tumors are slow growing and begin within the temporal bone which contains the inner ear. As these tumors expand, they grow into the intracranial cavity and lead to compression of cranial nerves that regulate facial movement, facial sensation, hearing, balance, and speech and swallowing. In addition, the portion of the brain that regulates coordination and motion (the cerebellum) and the brainstem itself may be severely compressed by these tumors. Early in tumor growth the acoustic neuroma is limited to the internal auditory canal. Patients with a stage one or intracanalicular stage acoustic neuroma (confined entirely inside the internal auditory canal) often complain of difficulty with hearing in one ear, which may begin suddenly or insidiously. Hearing loss may be accompanied by noise inside the ear (tinnitus), dizziness and vertigo.
Figure 1 is a view of the head from above. The posterior fossa with the intracranial contents of the cerebellum, brain stem and cranial nerves, as well as the temporal bone can be seen. The temporal bone is a part of the skull base. In the temporal bone is the internal auditory canal that allows the cochleovestibular (hearing and balance) nerve and the facial nerve to pass from their intracranial site of origin to either the cochlea (snail like hearing organ), vestibular apparatus (balance organ with the 3 semicircular canals), or the muscles that move the face. Acoustic neuromas most commonly arise in this canal. In Figure 1, a small intracanlicular tumor (colored brown in this illustration) can be seen.
In Figure 2, a larger acoustic neuroma is present. This tumor has grown out of the internal auditory canal into the region known as the cerebellopontine angle, or CPA. The tumor has a characteristic shape, with a root like extension in the internal auditory canal and a globular portion in the intracranial cavity, just touching the brainstem, cerebellum and some of the cranial nerves. Patients may have the same symptoms as when the tumor is in the internal auditory canal or patients may have additional symptoms such as headache.
The brainstem compressive stage can be seen in Figure 3. In this figure the brainstem, cerebellum and cranial nerves are being compressed by the tumor. Acoustic neuromas are almost universally benign (not cancer), so they do not erode into, or replace brain tissue. Rather acoustic neuromas cause damage by taking up space in the intracranial cavity where no extra space exists. Compression and attentuation of vital structures in this stage can lead to increased headaches and numbness of the face.
The final stage of acoustic neuroma growth, seen in Figure 4, causes hydrocephalus or a blockage of drainage of the cerebrospinal fluid in the head that bathes the brain. This increased pressure and further compression of the brainstem, cerebellum and cranial nerves results in severe symptoms such as double vision , difficulty with speech and swallowing, and even difficulty with breathing, and eventually death. It is very unusual for patients to have tumors this large without seeking medical help when the tumors are in a smaller stage. Also acoustic neuromas characteristically grow slowly, usually over years, so that patients with smaller size tumors very rarely grow to the point seen in the figure above.
The treatment of acoustic neuromas is complex and requires a sophisticated and well coordinated team. Our Consortium on Cranial Base Surgery cares for patients with these and other complicated tumors in this region. The cranial base surgery team is composed of a neurotologist-skull base surgeon and neurosurgeons who perform microsurgical resections of these lesions. During the microsurgery a third physician, a neurophysiologist, performs electrophysiologic monitoring of many of the nerves compressed by the tumor, so that these nerves can be both electrically and visually identified, and to help preserve them.