Description – Acoustic Neuroma
Acoustic Neuroma is also known as vestibular schwannoma. An acoustic neuroma is a benign tumor that develops when the specialized (Schwann) cells surrounding the vestibular division of the auditory nerve, grow at an abnormal rate in the internal auditory canal. The tumor if left untreated, can grow into the auditory canal and all the way through to the brain.
Acoustic neuromas generally grow slowly, so symptoms develop gradually. The main ones – dizziness, hearing loss and ringing in the ears (tinnitus) – are due to the effects of the tumor pressing on the auditory nerve. If the tumor grows large enough, it also may press on the nearby facial nerve and cause facial paralysis or tingling. Although the tumors are not cancerous, they can become life-threatening if they grow so large that they press on brain structures that control vital body functions.
People with a hereditary disease called neurofibromatosis have a higher risk of developing acoustic neuromas and can develop tumors on both sides of the head.
Types of Acoustic Neuroma
There are two main types of acoustic neuroma:
A tumor affects only one ear. This variant is by far the more common, accounting for 95% of all instances of acoustic neuroma. It is also known as the ‘sporadic’ type and the causes behind its appearance are not well understood.
Tumors arise on both sides affecting both ears. Acoustic neuroma of this kind accounts for only 5% of reported cases, is clearly linked with a rare genetic disorder known as neurofibromatosis type II (NF2).
Pathophysiology of acoustic neuroma
As the acoustic neuroma grows, it compresses the hearing and balance nerves, usually causing unilateral (one-sided) hearing loss, tinnitus (ringing in the ear), and dizziness or loss of balance. As it grows, it can also interfere with the facial sensation nerve (the trigeminal nerve), causing facial numbness.
It can also exert pressure on nerves controlling the muscles of the face, causing facial weakness or paralysis on the side of the tumor.
Vital life-sustaining functions can be threatened when large tumors cause severe pressure on the brainstem and cerebellum.
What causes acoustic neuroma?
The cause of acoustic neuromas is largely unknown. No environmental factor (such as cell phones or diet) has been scientifically proven to cause these tumors. Acoustic neuromas can be sporadic or caused by an inherited condition called neurofibromatosis type 2 (NF-2). Sporadic tumors occur 95% of the time, while 5% of acoustic neuromas occur with NF-2.
Neurofibromatosis is a rare disease that occurs in two forms. Type 1 causes tumors to grow on nerves throughout the body, especially the skin. Type 2 can cause acoustic neuromas on both the left and right sides, creating the possibility of complete deafness if the tumors grow unchecked. The presence of bilateral acoustic tumors affects the choice of treatment, as hearing preservation is a prime objective.
Who is at risk?
The only known risk factor for acoustic neuroma is having a parent with the genetic disorder neurofibromatosis 2 (NF2). Most of these tumors appear spontaneously. They occur in people with no family history of the disease.
Scientists still don’t understand why some people get these tumors. Some risk factors might include:
- Loud noises
- A parathyroid neuroma, which is a benign tumor of the thyroid
- Exposure to low levels of radiation during childhood
How to find if you have acoustic neuroma?
Signs and symptoms of acoustic neuroma are often subtle and may take many years to develop. They usually arise from the tumor’s effects on the hearing and balance nerves. Pressure from the tumor on adjacent nerves controlling facial muscles and sensation (facial and trigeminal nerves), nearby blood vessels, or brain structures may also cause problems.
As the tumor grows, it may be more likely to cause more noticeable or severe signs and symptoms.
Common signs and symptoms of acoustic neuroma include:
- Hearing loss, usually gradual – although in some cases sudden – and occurring on only one side or more pronounced on one side
- Ringing (tinnitus) in the affected ear
- Unsteadiness, loss of balance
- Dizziness (vertigo)
- Facial numbness and very rarely, weakness or loss of muscle movement
In rare cases, an acoustic neuroma may grow large enough to compress the brainstem and become life-threatening.
Complications of acoustic neuroma
Several complications can arise, including:
- Hearing loss: This may persist even after treatment.
- Dizziness and loss of balance: If this occurs, it can make daily activities difficult to do.
- Facial palsy: If surgery, or rarely, the tumor itself, affects the facial nerve, which is close to the acoustic nerve, the face may droop on one side, and swallowing and speaking clearly may be difficult. This is facial palsy, also known as Bell’s palsy.
- Hydrocephalus: If a large tumor presses against the brainstem, this can affect the flow of fluid between the spinal cord and the brain. If fluid accumulates in the head, it can lead to hydrocephalus.
How is acoustic neuroma (vestibular schwannoma) diagnosed?
Because symptoms of these tumors resemble other middle and inner ear conditions, they may be difficult to diagnose. Preliminary diagnostic procedures include ear examination and hearing test. Computerized tomography (CT) and magnetic resonance imaging (MRI) scans help to determine the location and size of the tumor. Early diagnosis offers the best opportunity for successful treatment.
A hearing test (audiometry): A test of hearing function, which measures how well the patient hears sounds and speech, is usually the first test performed to diagnose acoustic neuroma. The patient listens to sounds and speech while wearing earphones attached to a machine that records responses and measures hearing function. The audiogram may show increased “pure tone average” (PTA), increased “speech reception threshold” (SRT) and decreased “speech discrimination” (SD).
Brainstem auditory evoked response (BAER): This test is performed in some patients to provide information on brain wave activity as a response to clicks or tones. The patient listens to these sounds while wearing electrodes on the scalp and earlobes and earphones. The electrodes pick up and record the brain’s response to these sounds.
Scans of the head: If other tests show that the patient may have an acoustic neuroma, magnetic resonance imaging (MRI) is used to confirm the diagnosis. MRI uses magnetic fields and radio waves, rather than x-rays, and computers to create detailed pictures of the brain. It shows visual “slices” of the brain that can be combined to create a three-dimensional picture of the tumor. A contrast dye is injected into the patient. If an acoustic neuroma is present, the tumor will soak up more dye than normal brain tissue and appear clearly on the scan. The MRI commonly shows a densely “enhancing” (bright) tumor in the internal auditory canal.
Acoustic Neuroma Treatments
There are three main courses of treatment for acoustic neuroma:
- Radiation therapy
Observation is also called watchful waiting. Because acoustic neuromas are not cancerous and grow slowly, immediate treatment may not be necessary. Often doctors monitor the tumor with periodic MRI scans and will suggest other treatment if the tumor grows a lot or causes serious symptoms.
Surgery for acoustic neuromas may involve removing all or part of the tumor.
There are three main surgical approaches for removing an acoustic neuroma:
Translabyrinthine, which involves making an incision behind the ear and removing the bone behind the ear and some of the middle ear. This procedure is used for tumors larger than 3 centimeters. The upside of this approach is that it allows the surgeon to see an important cranial nerve (the facial nerve) clearly before removing the tumor. The downside of this technique is that it results in permanent hearing loss.
Retrosigmoid/sub-occipital, which involves exposing the back of the tumor by opening the skull near the back of the head. This approach can be used for removing tumors of any size and offers the possibility of preserving hearing.
Middle fossa, which involves removing a small piece of bone above the ear canal to access and remove small tumors confined to the internal auditory canal, the narrow passageway from the brain to the middle and inner ear. Using this approach may enable surgeons to preserve a patient’s hearing.
Radiation therapy is recommended in some cases for acoustic neuromas. State-of-the-art delivery techniques make it possible to send high doses of radiation to the tumor while limiting expose and damage to surrounding tissue.
Radiation therapy for this condition is usually delivered in one of two ways:
Single fraction stereotactic radiosurgery (SRS), in which many hundreds of small beams of radiation are aimed at the tumor in a single session.
Multi-session fractionated stereotactic radiotherapy (FRS), which delivers smaller doses of radiation daily, generally over several weeks. Early studies suggest multi-session therapy may preserve hearing better than SRS.
Both of these are outpatient procedures, which means they don’t require a hospital stay. They work by causing tumor cells to die. The tumor’s growth may slow or stop or it may even shrink, but radiation doesn’t completely remove the tumor.
Early diagnosis of a vestibular schwannoma is key to preventing its serious consequences.