An acoustic neuroma (also known as a vestibular schwannoma) is a benign tumor originating from the balance nerves, the nerves that carry balance signals from the inner ear to the brain. Acoustic neuromas are located deep inside the skull and are adjacent to vital structures such as the brain stem. The first symptoms a patient with an acoustic neuroma usually notices are ringing in the ear, hearing loss, and sometimes dizziness. The hearing loss will usually be much more severe on the side affected by the tumor and it is this “asymmetry” in the hearing that usually alerts the doctor to the possibility of an acoustic neuroma, as the hearing in the opposite ear will not be affected by the tumor. Generally, the tumor affects the hearing in one ear only, except in the case of a rare condition called neurofibromatosis type II, in which acoustic neuromas may involve both left and right balance nerves. As the acoustic neuroma enlarges, it may put pressure on the brain stem and can involve other surrounding nerves and structures. If allowed to continue, this pressure on the brain is can be fatal, although this is extremely rare. In most patients, acoustic neuromas grow slowly over a period of years. In some, the rate of growth is more rapid, however. Thus the symptoms may develop slowly or more rapidly.
Diagnosis of Acoustic Neuroma
An acoustic neuroma is diagnosed with an MRI scan of the brain. This is generally ordered when a patient has hearing loss that is significantly poorer in one ear than the other on a hearing test or when the patient has had a sudden decrease in the hearing in one ear. Other causes for the hearing loss, such as an ear infection, should be ruled out first. Your doctor may order an MRI of your brain if he believes there is a possibility of an acoustic neuroma. If you are diagnosed with an acoustic neuroma, your doctor will likely refer you to a specialist, a neurotologist (hearing and balance specialist) like Paul F. Shea, M.D., at the Shea Ear Clinic, who will discuss the options for treatment with you.
Risks and complications of acoustic neuroma treatment vary with the size and rate of growth of the tumor. In general, the larger the tumor, the more difficult to remove and the more serious the complications. The removal of an acoustic neuroma, whether large or small, is a major surgical procedure, with the possibility of serious complications, including death. Acoustic neuromas are classified as small, medium, or large.
Small (less than 1 centimeter) – A small acoustic neuroma is still confined within the bony canal that extends from the inner ear to the brain. The hearing, balance, and facial nerve, as well as the blood vessels that supply the inner ear, pass through this canal.
Medium (1 centimeter to 2.5 centimeter) – A medium-sized acoustic neuroma extends from the bony canal into the brain cavity, but is not putting pressure on the brain stem.
Large (more than 2.5 centimeter) – A large acoustic neuroma extends out of the bony canal into the brain cavity and is large enough to begin to put pressure on the brain stem.
Options for Treatment
If you are diagnosed with an acoustic neuroma, you and your surgeon will make the decision of which treatment is best for you. This decision will be based on a number of factors, including your age, general health, and how much hearing you still have on the affected side. There are three treatment options for acoustic neuromas.
- Observation As stated before, acoustic neuromas are slow growing, benign tumors. They may go through periods where they do not seem to grow at all. In some patients, it may be appropriate to observe the tumor for a period of time before the decision about the necessity for resection has to be made. These are generally older patients who have already lost the hearing in the affected ear but are experiencing no other symptoms.
- Radiation (Gamma Knife)
Acoustic neuromas may also be treated with a form of radiation called “stereotactic radiosurgery” or “gamma knife”. The patient’s head is placed into a machine that aims beams of radiation at the tumor in order to kill the tumor cells. There is no actual surgery or cutting involved. It is a one-day outpatient procedure and avoids the one-week hospital stay associated with surgical resection. Following treatment, MRI scans are performed on a periodic basis to watch for signs that the tumor has started growing again. However, gamma knife is not risk-free and does not result in disappearance of the tumor. Acoustic neuromas are benign tumors and thus their cells are not rapidly dividing like cancer cells. For this reason, acoustic neuromas are not as susceptible to radiation as cancerous tumors. In rare cases after gamma knife, the tumor can begin to grow again, necessitating surgical resection. The complication rates and likelihood for facial nerve weakness with surgical resection after gamma knife failure are higher than if the patient had not had gamma knife. Thus, gamma knife is best suited for older patients or patients who may not be able to tolerate 6-10 hours of general anesthesia for medical reasons.
- Surgical Removal
The decision of which surgical approach is best is based on the size of the tumor and the amount of residual hearing. In many cases it is not possible to save the hearing. If hearing is saved, it is no better than the preoperative level and may be worse. The larger the tumor, the less the chance of saving the hearing. With poor preoperative hearing or a large tumor, it is better to sacrifice the hearing in order to remove the entire tumor. All procedures are performed under general anesthesia in a hospital by a team including a neurotologist like Paul F. Shea, M.D. or Brian J. McKinnon, M.D. who provides access to the tumor, a neurosurgeon who actually removes the tumor, an anesthesiologist, a surgical nurse, a circulating nurse, and an audiologist who monitors function of the facial nerve during the operation. The neurotologists at the Shea Ear Clinic work as a team with the experienced neurosurgeons at the Semmes-Murphey clinic to provide the best possible care for patients electing to have surgical removal of their acoustic neuroma. There are three surgical approaches that are used to remove an acoustic neuroma. The most appropriate one for you will be determined by you and your surgeons.
- Translabyrinthine Approach – An incision is made behind the ear and the mastoid and inner ear structures are removed to expose the tumor. The tumor is delicately dissected from the facial nerve, and the opening is then closed with fat taken from a small incision in the abdomen. This method sacrifices any remaining hearing in the affected ear resulting in permanent and complete hearing loss in that ear but reduces the possibility of brain swelling and other complications and offers an easier recovery for the patient in most cases. It is the approach used in the majority of cases and is suitable for any size tumor. Even though the balance mechanism has been removed on the operated ear, the balance mechanism from the other ear takes over and provides stabilization for the patient in one to four months.
- Middle Cranial Fossa Approach – An incision is made above the ear, and the brain is elevated to expose the tumor. Every effort is made to preserve hearing and still remove the tumor. In about 50% of patients, the tumor involves the hearing nerve or the artery leading to the inner ear and total loss of hearing occurs in the operated ear. This method is used for smaller tumors when the hearing is still useful to the patient. It offers the possibility of preserving the hearing but carries with it the risk of other complications and is the most technically difficult for the surgeon.
- Retrosigmoid Approach – An incision is made behind the ear, and the brain is retracted back slightly to expose the tumor. Every effort is made to preserve hearing and still remove the tumor. In some patients, it is necessary to sacrifice the hearing to remove the entire tumor. This method is used for medium or larger tumors in cases where hearing in the affected ear is still worth trying to preserve. It also offers the possibility of saving the remaining hearing but has a significant risk of headaches postoperatively.
Complications of Treatment for Acoustic Neuroma
All treatments for acoustic neuroma involve the possibility of further loss of hearing, ringing in the ear, loss of balance, and facial nerve weakness (drooping of the face) or paralysis of the face. Complications from anesthesia, headaches, spinal fluid leakage, bleeding, infection, and death are much less likely complications of surgery. However, the most likely complication is usually facial nerve weakness or paralysis. For this reason, surgical approaches and techniques are designed to minimize this possibility. At the Shea Ear Clinic, this is accomplished through the use of a team in which a neurotologist like Paul F. Shea, M.D. or Brian J. McKinnon provides the surgical approach, an experienced neurosurgeon removes the tumor, and the neurotologist closes the wound at the end of the procedure. Facial nerve function is continuously monitored throughout the procedure by an audiologist and an alarm is heard if irritation of the nerve is detected while the surgeon is removing the tumor. This minimizes the likelihood of facial weakness following the procedure.
Goals of Treatment
The primary objective in surgery is to remove the acoustic neuroma to preserve life. A secondary objective of operation is to preserve as many vital structures as possible. For many, a completely normal life results following operation. Some patients experience physical disabilities, including permanent hearing loss in the affected ear, loss of balance and possibly weakness of the face on the affected side. For a few patients, maximum degrees of physical disability may persist. However, the single biggest factor that determines the success of surgical resection of acoustic neuroma remains the skill and experience of the surgical team. This skill and experience is responsible for the excellent results seen at the Shea Ear Clinic with surgical resection of acoustic neuroma.