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Diagnosing Acoustic Neuroma

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An acoustic neuroma, also known as a vestibular schwannoma, is a non-cancerous growth that develops on the vestibulocochlear nerve that runs from your brainstem to your inner ear. This nerve is responsible for maintaining your balance and hearing. Therefore, a tumor that adversely affects the nerve will impair these essential functions.

Fortunately, most acoustic neuromas grow very slowly. Because of their slow growth rate, their symptoms can start subtly. Except for rare cases where an acoustic neuroma grows abnormally fast, any symptoms you experience will likely take a long time to develop in severity. As a result, some people go years without realizing that there is anything wrong.

The slow nature of this condition makes the diagnostic process more challenging and extensive to ensure that another more common medical issue isn't causing your symptoms. In this article, we will discuss considerations and tests used to diagnose acoustic neuromas.

                                            Figure 1: Anatomical depiction of an acoustic neuroma in relation to the inner ear (semicircular canals) and the vestibulocochlear nerve.

Figure 1: Anatomical depiction of an acoustic neuroma in relation to the inner ear (semicircular canals) and the vestibulocochlear nerve.

Symptoms of an Acoustic Neuroma

You're often the first to know when something is wrong with your body. However, the slow growing nature of acoustic neuromas may be difficult to differentiate from age-related changes. As a result, the symptoms may be very subtle or even hidden for years in some cases. Nevertheless, knowing what symptoms are caused by acoustic neuromas will help you determine whether the issue is something you should ask your doctor about.

The most common initial symptoms of an acoustic neuroma include:

  • Loss of hearing. This symptom is usually unilateral, meaning it only occurs on one side. The hearing loss may be partial or complete. Hearing loss usually develops slowly, with the person able to hear all but higher frequency sounds at first. However, partial or complete hearing loss can occur suddenly and intermittently in some cases.
  • Tinnitus. An acoustic neuroma can interfere with the hair-like cells in your inner ear that relay auditory signals to the brain. When pressure is placed on these cells, they release random electrical impulses. Your brain interprets these impulses as sound, which causes tinnitus. This condition presents as a ringing, buzzing, whining, humming, clicking, or low roaring in one or both ears with no external source. It can develop slowly, coming and going at first before becoming more constant.
  • Problems maintaining balance. The vestibular nerve is responsible for relaying signals to your brain that help your body maintain positional awareness. This function is what allows you to stand, walk, run, and jump without falling over. However, an acoustic neuroma puts pressure on this nerve, blocking the signals and preventing your body from balancing correctly.

If an acoustic neuroma grows larger than 2 cm, it can place pressure on the brainstem. Once an acoustic neuroma begins affecting facial nerves, it can cause problems such as twitching, numbness, face muscle weakness, and even facial paralysis in severe cases. Very large tumors that place significant pressure on the brainstem may lead to fluid buildup in the brain, which may become life-threatening.

How Do Doctors Diagnose an Acoustic Neuroma?

Because the symptoms of an acoustic neuroma are so subtle and often aren't noticed during the beginning stages of the condition, early diagnosis can be difficult. In addition, hearing loss, tinnitus, and balance issues are common symptoms of several other medical problems that occur with the inner ear. However, once symptoms appear, there are several tests that can be used to support the diagnosis and determine the extent of symptoms caused by the tumor.

Speech Audiometry

Speech audiometry is used to determine how loud sounds need to be before you can hear them and how well you can distinguish and understand sounds and individual words during speech. First, you will be seated in a booth where an audiologist will ask you to listen to various words spoken at lower volumes, then repeat them. Then, you will hear words spoken at an average volume and repeat them. Audiometry determines three things:

  • Pure tone average, which measures how loud a sound must be before you can hear it at multiple different pitches, also called frequencies.
  • Speech reception threshold, which measures how loud speech needs to be before you can hear it.
  • Speech discrimination, which measures how many individual words you can hear one ear at a time.

An audiogram is a graph of the audible threshold for standardized frequencies as measured by audiometry tests. Most people with acoustic neuromas tend to experience the most significant hearing loss at the mid-frequency range, resulting in a trough-shaped audiogram.

                                            Figure 2: Audiograms and MRI scans of 10 patients with acoustic neuromas who experienced sensorineural hearing loss. Credit: Song et. al 2022;

Figure 2: Audiograms and MRI scans of 10 patients with acoustic neuromas who experienced sensorineural hearing loss. Credit: Song et. al 2022;

Brainstem Auditory Evoked Response

This test measures brain wave activity in response to various tones and sounds. You will wear electrodes on the scalp and earlobes, then be given earphones that play clicks or tones. The electrodes will measure the frequency of electronic signals being sent back and forth between your vestibulocochlear nerves and your brain to evaluate the brain's response to these sounds.

                                            Figure 3: Patient preparing to undergo magnetic resonance imaging (MRI).

Figure 3: Patient preparing to undergo magnetic resonance imaging (MRI).

Magnetic Resonance Imaging (MRI)

An MRI is the imaging method of choice for diagnosing an acoustic neuroma. An MRI uses radio waves and magnetic fields to create a three-dimensional picture of the brain and surrounding structures. Contrast dye can be injected before imaging is conducted and can be used to enhance (make it easier to see) any mass lesion. With an MRI, it is easy to identify any mass lesion that could be causing your symptoms. This test is very specific. Over time, serial MRI scans can be used to track tumor growth.

                                            Figure 4: Magnetic resonance imaging (MRI) showing acoustic neuromas of various sizes are shown. Note the difference in small, medium, and giant sizes.

Figure 4: Magnetic resonance imaging (MRI) showing acoustic neuromas of various sizes are shown. Note the difference in small, medium, and giant sizes.

Computed Tomography (CT)

CT scans can be useful for some patients who are unable to undergo an MRI, specifically in patients harboring MRI non-compatible medical implants. CT scans utilize a series of x-ray images to provide a three-dimensional image. Compared to MRI, CT scans provide less tissue detail and can be distorted by artifacts created by nearby bony structures.

If a patient is undergoing surgery, a CT scan of the temporal bone may be needed to evaluate the bony anatomy and to plan the best trajectory for removing the tumor.

Can Another Condition Be Mistaken for an Acoustic Neuroma?

Acoustic neuromas share many of the same hearing and balance issues with a number of other medical conditions. Imaging and detailed auditory examinations can be used to differentiate between an acoustic neuroma and other pathologies.

However, it is possible for other tumors to arise in the cerebellopontine angle and cause similar symptoms to an acoustic neuroma. For instance, meningioma is another type of tumor which may show up in this area and cause similar symptoms.

Both meningiomas and acoustic neuromas are brain tumors that grow very slowly, often going for years without causing symptoms. A meningioma typically grows from the meninges, the membranes covering the brain and spinal cord. If one develops on or near the eighth cranial nerve, it has the potential to cause the same symptoms as an acoustic neuroma.

Fortunately, both meningioma and acoustic neuromas are treated similarly. It is possible for meningiomas to become malignant and invade nearby bony structures, however this is very uncommon. Nevertheless, it is important to distinguish these nuances as it may change the treatment strategy. Acoustic neuromas have favorable treatment outcomes and overall prognoses which may slightly differ from patients with meningioma.

Key Takeaways

  • The symptoms of an acoustic neuroma are subtle at the beginning stages of the disorder, which makes getting an early diagnosis difficult.
  • Audiometry is helpful to identify the source and extent of hearing loss.
  • MRI with and without contrast is the diagnostic imaging of choice for acoustic neuromas.