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What Is an Acoustic Neuroma?

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If you or a loved one have been diagnosed with an acoustic neuroma, you may have many questions about what this tumor is, where it develops, what symptoms it can cause, and how prevalent this tumor is in the general population. In this article, we’ll discuss the basics of acoustic neuromas to give you a foundation that we will build on in subsequent articles. 

What Is an Acoustic Neuroma?

An acoustic neuroma, also known as a vestibular schwannoma, is a non-cancerous growth that develops along the eighth cranial (brain nerve) nerve, also called the vestibulocochlear nerve. This nerve is responsible for hearing and balance. As such, pressure on this nerve from an acoustic neuroma can lead to hearing difficulties, instability in your balance, as well as other symptoms.

                                            Figure 1: Anatomic depiction of an acoustic neuroma in relation to the brainstem and cranial nerves.

Figure 1: Anatomic depiction of an acoustic neuroma in relation to the brainstem and cranial nerves.

Though they do not grow directly in the brain, an acoustic neuroma is considered a type of brain/skull base tumor that grows outside of the brain and can compress the brainstem. If one continues to grow, it could potentially become large enough to press on the brainstem, leading to significant imbalance and other neurological symptoms. Nerves responsible for movement of facial muscles, sensation of the face, and even swallowing are also nearby and could become affected if an acoustic neuroma is left untreated and grows to become very large.

Types of Acoustic Neuroma

Most acoustic neuromas are sporadic tumors occurring on one side (unilateral). They occur without a known cause in 95% of people. Sporadic unilateral acoustic neuromas may develop at any age, but they most commonly occur in adults 30 and 60 years of age. 

Roughly 5% of patients develop acoustic neuromas in both vestibulocochlear nerves (bilateral acoustic neuromas). These are associated with a rare genetic disorder known as neurofibromatosis type 2 (NF2). NF2 is inherited in an autosomal dominant fashion, meaning it is passed down from at least one parent to the children.

What Size Is an Acoustic Neuroma?

Acoustic neuromas can be classified according to their size. The size of the tumor is a critical factor in its ability to cause symptoms and the selection of the treatment:

  • Small acoustic neuromas measure smaller than a centimeter and are typically confined to the internal auditory canal (ear canal). They don't extend into or near the brain cavity itself.
    • Radiation is a reasonable option for the treatment of small growing tumors although surgery may be able to preserve hearing.
  • Medium acoustic neuromas measure between 1 and 2 cm near the brain. They may be large enough to extend from the internal auditory canal to abut the brain.
    • These tumors remain amendable to radiosurgery. Younger patients may benefit from surgical resection.
  • Large acoustic neuromas are between 2 and 3 cm in size. A large acoustic neuroma may press on the soft tissue of the brain, and cause pressure on the brainstem.
    • These tumors are often managed via either surgery or radiosurgery. Most 3 cm tumors may be best handled via surgical resection.
  • Giant acoustic neuromas are those that measure larger than 3 cm in size. Tumors of this size compress the brainstem and may create excess pressure within the brain cavity.
    • The only option here is surgical resection to relieve the pressure on the brainstem.

                                            Figure 2: Acoustic neuromas can grow to variable sizes including medium (left), small (middle), and giant (right).            

Figure 2: Acoustic neuromas can grow to variable sizes including medium (left), small (middle), and giant (right).            

How Common Are Acoustic Neuromas?

Acoustic neuromas develop in approximately ten people per million per year in the United States. In addition, acoustic neuromas make up 6% of all diagnosed brain tumors and are slightly more common in women than men.

What Are Common Symptoms of an Acoustic Neuroma?

Acoustic neuromas grow very slowly once they develop; they may even stop growing for some time before starting to grow again. The ones that do grow larger usually do so slowly (1-2mm per year). This slow growth makes signs and symptoms subtle early on. This makes them difficult to notice at first. It may even be years before a problem is recognized.

Common symptoms of acoustic neuroma include:

  • Unilateral (one-sided) hearing loss that typically develops over several years. However, the sudden and rapid onset of hearing loss can occur in rare cases. 
  • The sensation of fullness within the ear, as if there is water in the ear canal. The presence of the tumor within the ear canal causes this feeling, and if the acoustic neuroma grows, the sensation may worsen.
  • Development of tinnitus, usually described as ringing in the ear. It typically sounds like ringing, but it can present as hissing, buzzing, or roaring in some cases.
  • Development of unsteadiness and balance issues. If an acoustic neuroma grows to compress the brainstem, these balance issues can lead to falls. Patients with acoustic neuromas fall toward the side of the ear where the tumor is located.

Many of these symptoms, such as unilateral hearing loss, are also caused by other conditions within the inner ear. Therefore, it’s important to consult a Ear Nose Throat (ENT) or otolaryngologist as well as a neurosurgeon to ensure a proper diagnosis.

Are Acoustic Neuromas Dangerous?

As mentioned before, acoustic neuromas are typically benign, meaning that they do not usually invade and destroy surrounding tissues and cells. In addition, acoustic neuromas grow very slowly over years. These tumors do not spread to other parts of the body.

If you've developed a small acoustic neuroma, your neurosurgeon may recommend that it be monitored and that you undergo regular hearing and/or imaging tests to ensure that the tumor isn't growing out of control. If an acoustic neuroma is growing enough to cause progressively worsening symptoms, your doctor will likely recommend a more aggressive (surgery or radiation) treatment plan.

The key to dealing with an acoustic neuroma is to consult with an experienced neurosurgeon as soon as possible. The sooner a tumor is detected and treated, the more of its adverse effects can be prevented. If an acoustic neuroma grows larger and goes untreated, permanent issues can develop, including:

  • Complete hearing loss in one ear
  • Headaches
  • Severe vertigo
  • Facial numbness or tingling or weakness
  • Difficulty swallowing
  • Hydrocephalus (life-threatening buildup of excess fluid in the brain)

It's very unlikely that an acoustic neuroma will cause life-threatening problems, especially if it is detected and treated early. However, if left alone, a growing acoustic neuroma will eventually cause symptoms that can significantly impact a patient’s quality of life.

Key Takeaways

  • Acoustic neuromas are a type of non-cancerous brain tumor that grow on the eighth cranial nerve (vestibulocochlear nerve).
  • Acoustic neuromas vary in size and laterality (left, right, or both sides).
  • Rare genetic disorders (neurofibromatosis type 2) can cause acoustic neuromas to occur on both sides.
  • If they grow, they can place pressure on vital structures, affecting your hearing and balance.
  • Acoustic neuromas tend to grow slowly. Evaluation by an experienced physician or surgeon is important to differentiate symptoms from other conditions.