An acoustic neuroma is a non-cancerous tumor that grows on a tiny nerve between the inner ear and the brainstem. This eighth cranial nerve is made up of several nerves that connect the ear drum to the brain. The cochlear nerve carries hearing information, and there are two others that carry information to the brain to control a person’s balance.
These nerves are protected by what are called Schwann cells, and with an acoustic neuroma, a tumor begins to grow on these cells. If the tumor continues unchecked, it can eventually compress the brainstem and become life threatening.
Hearing loss is the most frequent symptom of AN, occurring in more than 95% of patients. About 90% present with a one-sided, slowly progressive hearing impairment. A high-frequency sensorineural pattern is the most common type, occurring in approximately two-thirds of patients. In the remaining third, the next most common observation is hearing loss at low frequency, which is more typical of Meniere’s disease. A sudden hearing loss occurs in about 25% of patients with acoustic neuroma. Hearing can be completely normal in about 11% of patients. Tinnitus is also very common in acoustic neuroma, and is usually unilateral and confined to the affected ear.
Because of the origin of acoustics in the vestibular nerve, only about twenty percent of the patients experience vertigo. Vertigo is more common with smaller tumors than larger ones. Lack of balance is common with about seventy per cent of patients experiencing it.
Although there may be facial nerve paralysis prior to surgery, this usually only occurs where a large tumor is present. Prior to operating this damage may be controlled with carbamazepine medication for neuralgia, at least in some cases. Some patients may experience headaches if there is a large tumor present.
The risk of facial paralysis with acoustic neuroma surgery is between 4-15%. Tumor size, surgeon experience and approach for surgery are important factors. The treatment approach to facial paralysis in this patient population depends on the intraoperative surgical findings. Conservative approach is prudent if the surgeon who performs the acoustic neuroma feels that the nerve was saved during the operation. Patients will often require eye care and temporary measures such as suture suspensions to prevent unwanted complications and improve oral issues.
Conventional audiometry is the most useful diagnostic test for acoustic neuroma. The most common abnormality is an asymmetrical high-frequency sensorineural hearing loss. It has been estimated that 5% of persons with sensorineural hearing loss have acoustics.
Testing such as auditory gadolinium enhanced magnetic resonance imaging, can establish the diagnosis. A new technique called “Auditory Brainstem Response” testing is less sensitive than enhanced magnetic resonance imaging, but it is considerably less expensive.
Removal of acoustic neuromaThere are four distinct treatment options for an acoustic neuroma:
- Medical treatment or “wait and see” (conservative management)
- Gamma-knife procedure
- Cochlear implantation
About 25% of all acoustic neuromas are treated with medical management. Medical management consists of periodic monitoring of the patient’s neurological status, use of hearing aids when appropriate, and periodic imaging studies. It is felt to be an appropriate method of management in some patients (Source: Hoistad et al, 2001).
There are no medications known to effect the growth of acoustic neuroma tumors. The tumors may grow very slowly. A patient may want to follow a tumor with serial audiometry and/or MRI scans. In individuals of advanced age, a serious threat to life or bodily function from tumor growth may be judged unlikely in the remainder of a patient’s expected lifespan, and for this reason, medications may be elected. Once a tumor is diagnosed, a repeat scan is obtained at six months and then at yearly intervals.
About half of all acoustic neuromas are presently treated with surgery. In most instances, surgical removal of the tumor is the preferred option because it prevents potentially fatal complications of tumor growth. Surgery may enable preservation of hearing. Usually the surgery is done at an academic center by a team of surgeons including a neurotologist (a specialized otolaryngologist) and a neurosurgeon. There are several operations, as follows:
- Retrosigmoid or suboccipital (through the skull, more posterior approach)
- Translabyrinthine (through the inner ear — hearing loss is expected)
- Middle fossa (through the skull — hearing preservation is the goal)Each of these approaches has advantages and disadvantages that must be considered in selecting an optimal approach. Most patients are admitted to the hospital a day before the operation. After surgery, they spend a night in a monitored unit. Most are discharged from the hospital within four to six days after surgery, and return to work is usually possible in six weeks. MRIs are usually obtained at 1 and 5 years to detect residual or recurrent tumor. (Reference: http://www.american-hearing.org )
Rarely, acoustic neuromas are inherited. Acoustic neuroma caused by neurofibromatosis type II (NF 2) should be suspected in young patients and those with a family history of neural tumors.