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Overview of Acoustic Neuroma Types

Last Updated: January 7, 2023

Your inner ear contains several structures vital to both your hearing and balance. They are also connected directly to your brain and several nerves within your face, so any condition that affects them may also have neurological consequences. One such condition is an acoustic neuroma.

When faced with a condition such as a brain tumor, you should have as much information as possible to make informed decisions about your health and treatment. This article aims to give a basic overview of acoustic neuromas, including their symptoms, diagnosis, and possible causes.

Figure 1: An acoustic neuroma or vestibular schwannoma is demonstrated in relation to the left ear (top image). Note the vicinity of the tumor to the brainstem. The left lower images demonstrate an axial MRI with the tumor in white.

Figure 1: An acoustic neuroma or vestibular schwannoma is demonstrated in relation to the left ear (top image). Note the vicinity of the tumor to the brainstem. The left lower images demonstrate an axial MRI with the tumor in white.

Basics of Acoustic Neuromas

For many people, the first question when they are diagnosed is, "What is an acoustic neuroma?" An acoustic neuroma, also called a vestibular schwannoma, is a non-cancerous or benign growth that forms on the vestibulocochlear nerve of your inner ear. This nerve branches off into your cochlear nerve, which is responsible for your hearing, and your vestibular nerve, which controls your sense of balance and spatial awareness. 

In addition, there are two types of acoustic neuroma. Sporadic unilateral acoustic neuromas only develop in one ear, affecting the hearing and balance on that side of the body. Genetic bilateral acoustic neuromas grow on both vestibulocochlear nerves. Unlike unilateral acoustic neuromas, which are typically the result of spontaneous non-hereditary mutations, bilateral neuromas occur due to hereditary genetic conditions. They are fortunately rare.

Acoustic Neuroma Symptoms

So, what are the symptoms of an acoustic neuroma? It is vital to understand the signs of one to identify its presence even without a diagnosis. Because acoustic neuromas start small and grow very slowly, symptoms start subtly, making it challenging to detect them timely during their development.

Growing on the vestibulocochlear nerve means that the primary symptoms of an acoustic neuroma involve your senses of hearing and balance. Though it is rare, an untreated acoustic neuroma can grow large and press against surrounding tissues to cause problems with your facial nerves and muscles. It can even become large enough to invade intracranial spaces to affect your brain adversely.

Common symptoms of acoustic neuromas include:

  • Total or partial hearing loss
  • Imbalance and dizziness
  • Tinnitus
  • Headaches
  • Facial numbness, tingling, or weakness

If you experience any of these signs or symptoms, consult your physician right away. Problems with hearing typically occur only on one side. Still, if they happen on both sides of your body, it may be a sign of a hereditary condition that led to the development of bilateral acoustic neuromas.

Diagnosing an Acoustic Neuroma

If the symptoms of this condition are so subtle, it's easy to wonder how an acoustic neuroma is diagnosed. Many acoustic neuromas develop slowly for years, and some don't ever cause symptoms. Unfortunately, these growths often go unnoticed until they have reached a significant size and begin to cause symptoms, or they're incidentally caught during testing for another medical issue. In addition, several other conditions of the inner ear can cause similar symptoms. 

Once they are informed of any symptoms or suspect the presence of an acoustic neuroma, physicians can perform several tests to determine the source of the symptoms to confirm their suspicion:

  • Speech Audiometry: A test used to measure how loud sounds must be before you can hear them or how well you can distinguish individual words during speech. It is used to confirm a level of hearing loss, which is one of the first symptoms of an acoustic neuroma.
  • Magnetic Resonance Imaging (MRI): Used to confirm the presence of an acoustic neuroma after other tests determine the presence of symptoms. MRI uses magnetic fields to create a three-dimensional image of your brain.
  • Brainstem Evoked Auditory Response: This test measures your brain's response to different sounds and tones by observing the electronic signals sent between your brainstem and your vestibulocochlear nerve.

Figure 2: Audiometry is an important part of evaluation for acoustic neuromas.

Figure 2: Audiometry is an important part of evaluation for acoustic neuromas.

Figure 3: Please note acoustic neuromas on MRI. Small and medium size tumors (left, in white) and a giant tumor (right).

Figure 3: Please note acoustic neuromas on MRI. Small and medium size tumors (left, in white) and a giant tumor (right).

What Can Cause an Acoustic Neuroma?

Understanding the risk factors and origin of acoustic neuromas can go a long way in figuring out how to treat the disorder. The most common source of the condition is a mutation of chromosome 22, a gene responsible for suppressing the formation of tumors by keeping cellular growth in check. Precisely, it controls the growth of Schwann cells, which form the myelin insulation that your nerves require to function. However, no known environmental or chemical factors are linked to the mutation that causes acoustic neuromas.

The only confirmed risk factor for a malfunction of chromosome 22 is a hereditary disorder called neurofibromatosis type 2 (NF2). NF2 prevents chromosome 22 from working correctly throughout the body, leading to the formation of tumors within the central nervous system, including the vestibulocochlear nerve. NF2 is also the only confirmed cause of genetic bilateral acoustic neuromas, which only occur in 5% of acoustic neuroma patients.

Can Acoustic Neuromas Be Malignant?

Because they are caused by a mutation that can lead to abnormal cellular growth, concerns about acoustic neuromas and cancer can go hand in hand. Therefore, being diagnosed with one can bring about worry in patients unfamiliar with the condition. However, acoustic neuromas are almost always benign. And while the possibility of an acoustic neuroma turning malignant does exist, the odds are extraordinarily low.

While these growths are not cancerous, their presence can still cause problems if allowed to go untreated. An acoustic neuroma has the potential to grow larger and press on surrounding tissues and nerves. This pressure on vital structures can affect hearing, facial movement and sensation, balance/gait, and other nervous system functions. If an acoustic neuroma grows large enough, it can press against the brain stem, causing a blockage of cerebrospinal fluid and leading to hydrocephalus, which is life-threatening.

Conclusions

While receiving a diagnosis of an acoustic neuroma can be an anxiety provoking event, the condition only rarely becomes life-threatening. Symptoms such as decreased hearing and imbalance will take a while to develop, and they may only be mild once they do. If you are experiencing such symptoms, please be sure to speak to your physician soon to make sure appropriate testing is undertaken.

Key Takeaways

  • Acoustic neuromas, or vestibular schwannomas, are growths on the eighth cranial nerve of the inner ear around the brainstem.
  • Over time, they can cause hearing loss and balance issues.
  • Because an acoustic neuroma grows very slowly, diagnosing one before symptoms occur or become significant can be challenging.
  • The most common methods of testing for the presence of an acoustic neuroma involve hearing and imaging tests.
  • Acoustic neuromas are almost always benign.

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