Skull base surgery refers to surgical techniques required to obtain access to the floor of the cranial cavity. The main indication for skull base surgery is the removal of various brain tumors. Although there are a number of tumors which can affect hearing and balance, one of the more common ones which neurotolgists at the Midwest Ear Institute treat is the acoustic neuroma (AN).
What is an Acoustic Neuroma?
An acoustic neuroma is a benign, usually slow-growing tumor that develops from the balance and hearing nerves supplying the inner ear. The tumor comes from an overproduction of Schwann cells–the cells that normally wrap around nerve fibers like onion skin to help support and insulate nerves.
Only 2,000 – 3,000 new cases are diagnosed each year. The incidence is approximately 1 in 100,000 per year. Most (95%) acoustic neuromas are unilateral, that is they occur on one side. About 5% are bilateral and are associated with an inherited syndrome known as neurofibromatosis type 2.
Acoustic neuromas commonly develop inside the internal auditory canal. This canal is a boney tunnel through which pass the balance, hearing and facial nerves. These tumors usually grow slowly over a period of many years. When acoustic neuromas grow in size, the tumor compresses the hearing nerve. Therefore, hearing loss is the first symptom in over 90% of patients with AN. Tinnitus or ear noise is also a common symptom. Loss of balance, dizziness, facial weakness or numbness or even difficulty walking may develop as the tumor continues to grow. If the tumor becomes large, it will eventually press against nearby brain structures (such as the brainstem and the cerebellum), and may becoming life-threatening.
Identifying the Tumor
With appropriate tests it is now possible to identify tumors as small as 1-3mm in size. The first step in identifying a possible tumor is a thorough history and physical examination by a physician. Hearing testing is performed to identify any loss of hearing or speech understanding. An auditory brainstem response test (ABR) is ordered if there are any asymmetries in hearing testing between the right and left ears. An ABR provides information on the passage of an electrical impulse along the hearing nerve from the ear to the brain. Abnormal ABR results suggest a poorly functioning hearing nerve. A detailed “imaging” is ordered if there is an abnormality of the ABR test.
The most accurate imaging technique for AN identification is a magnetic resonance imaging (MRI). A properly performed MRI can identify acoustic neuromas as small as 2-3mm.
Treatment of Acoustic Neuroma
Since an AN is a benign, often slow-growing tumor, careful observation over a period of time may be appropriate for some patients. Growth rates average 2mm per year (range 1 – 12 mm/yr). When a small tumor is discovered in an older patient, observation to study the growth rate of the tumor may be indicated if acute symptoms are not present. A MRI scan of the tumor and surrounding region must be performed periodically to determine if there is any significant change in the size of the tumor. If the tumor does not grow, observation is continued. If the tumor increases in size, treatment may be recommended.
The natural history of an AN is not known. Each tumor has its own biology and growth characteristics. In general, an AN is regarded to be slow growing. The critical question to be answered when observation is considered is: will this AN cause any problems in the natural course of the patient’s remaining life span? Careful observation by clinical visits and MRI scans assure the decision to observe an acoustic neuroma is done intelligently. The option is most often considered when the patient is a senior citizen although, in younger patients, this is not an unreasonable choice. More research is needed.
There are several surgical approaches that may be used to remove an AN. The earlier the AN is diagnosed and removed, the less likely the possibility of serious complications. The type of approach is individualized depending on the patient’s wishes, hearing level, other neurological symptoms, and location and size of the tumor.
* Middle Fossa Approach
In the middle fossa approach, the bone is opened above the ear and the bone overlying the tumor is removed. The inner ear is not entered. Therefore, hearing preservation is possible. The middle fossa approach is most suitable for small tumors with good hearing.
* Retrosigmoid Approach
In this approach, the bone is opened behind both the mastoid bone and the inner ear. In this way the tumor is removed from behind the inner ear. Retrosigmoid removal also allows the possibility of hearing preservation and may be used for both small and large tumors.
* Translabyrinthine Approach
In the translabyrinthine approach, the mastoid bone behind the ear is removed and the inner ear is opened. This exposes the internal auditory canal directly. All hearing is lost with this approach. The translabyrinthine approach is, therefore, used only for those cases where hearing loss is already severe or the tumor is so large that hearing conservation is not a realistic goal.
Hospital stay after any type of microsurgery ranges from 4-7 days. Approximately 4-6 weeks is suggested for recovery.
Stereotactic Radiation Therapy
Stereotactic radiation therapy, which is commonly known as “radiosurgery,” is a single session radiation treatment. Using computer imaging, a single, high dose of radiation is delivered to the acoustic tumor while minimizing injury to surrounding nerves and brain tissue. Treatment is often performed on an outpatient basis.
The goal of stereotactic radiation is to prevent further tumor growth and sometimes shrink a portion of the tumor. This type of treatment does not remove the tumor. Follow-up imaging studies are important because some tumors will continue to grow after stereotactic radiation. The long term growth control rates for stereotactic radiation therapy have not been established. Stereotactic radiation therapy tumor growth control has not been clearly shown to be statistically superior to the natural history of untreated tumors in a sizable group of patients: doing nothing. When stereotactic radiation therapy has failed and the tumor regrows, surgery is generally regarded as more difficult and, therefore, more complicated than for patients not previously irradiated.
Each form of acoustic neuroma treatment, either surgery or radiation, has advantages and disadvantages. The experienced neurotologists at the Midwest Ear Institute will discuss with you the most appropriate type of treatment.