An acoustic neuroma, or more appropriately called a vestibular schwannoma, is a benign growth from the 8th cranial nerve. The 8th cranial nerve is comprised of two parts – a vestibular nerve, and a cochlear nerve. The cochlear nerve is the nerve that carries the sound signals from the inner ear to the brain. The vestibular nerve is the nerve that supplies the balance function of the ear. It is further subdivided into two parts – an inferior branch and a superior branch. These nerves course through the internal auditory canal from the inner ear towards the brainstem. The fourth nerve which courses through the internal auditory canal is the facial nerve, which supplies motor function to the muscles of the face (which lets you smile, close your eyes, etc.).
Most commonly, an acoustic neuroma arises off one of the vestibular nerve branches. The tumor grows from the sheath of the vestibular nerve and grows very slowly. The cause of acoustic neuromas is currently unknown. Neurofibromatosis type II is a condition where people have multiple nerve tumors and have bilateral acoustic neuromas.
The most common presenting complaint is that there is a change in the hearing in the affected ear. This can present suddenly or, more commonly, as a slowly progressive hearing loss in one ear. Many people notice that they have more difficulty talking on the telephone and have recently switched ears to talk with. Other presentations may include imbalance or tinnitus (ringing in the ear). As the tumor becomes much larger, other problems can arise such as numbness of the face, double-vision, and increased pressure in the head which can result in coma or death.
The gold standard for diagnosis of an acoustic neuroma is a MRI of the brain and internal auditory canals with contrast dye (gadolinium). Once the diagnosis of an acoustic neuroma is made, there are three treatments options: observation, radiation therapy (stereotactic radiosurgery, Gamma-knife), and surgery. Various factors influence the decision as to which treatment is best in each patient. Some factors include: size of the tumor, current hearing level in the ear, patient’s age and health status, and patient preference.
In general, small tumors or tumors in elderly patients can be observed, and a repeat MRI scan in about 6 months to 1 year can evaluate whether there has been an interval change in size. If the tumor is growing, then either radiation therapy or surgery would be recommended. If there is no change in size on the MRI, then yearly MRIs would be performed to continually evaluate whether the tumor grows. There can be periods of quiescence for many years, then growth later on, so continued follow-up is essential.
Stereotactic radiosurgery and gamma-knife are two forms of radiation therapy used to treat tumors such as acoustic neuromas. The premise of these treatments are to give the maximal amount of radiation to the tumor itself with minimizing radiation to the adjacent tissues. Stereotactic radiosurgery uses one external beam of radiation whose direction is changed on a daily basis over a few weeks to deliver 100% of the radiation to the tumor, while giving the surrounding tissues a mere fraction of the dose.
Gamma knife works by delivery of radiation through 201 separate cobalt-60 sources. This delivers the radiation in just one dose, but delivers it from multiple angles. The long-term results of hearing preservation and facial nerve weakness are comparable to surgery. However, in patients who were treated with radiation therapy and whose tumor subsequently grows, the complication rate of surgery is significantly increased due to scarring of the surrounding tissues. As a result, our philosophy on the use of radiation therapy to treat acoustic neuromas, is to use this only in elderly or infirmed patients whose tumor is growing.
There are three generally accepted surgical routes to remove an acoustic neuroma. Two routes are used to attempt hearing preservation in addition to tumor removal and one route destroys any residual hearing during access to the tumor. The first approach, called the translabyrinthine approach, is used when there is little to no residual hearing left in the ear, or in tumors over 2cm in size, as hearing preservation in this size tumor is near 0%. The two hearing preservation approaches are the retrosigmoid transmeatal approach and the middle cranial fossa approach. The middle cranial fossa approach is used for small tumors in patients under 60 years old with good hearing. This approach gives the best chance for hearing preservation, but does carry a slightly higher incidence of facial nerve weakness.
The retrosigmoid approach is used for larger tumors that still have good hearing. The hearing preservation rates are significantly lower than those for the middle fossa approach, however, facial nerve weakness rates are comparable to the translabyrinthine approach. Each case must be individually reviewed with your surgeon to decide which treatment option is best suited to you.
Content provided by the American Academy of Otolaryngology-Head and Neck Surgery