VEMP: Vestibular-evoked myogenic potential

What are VEMPs?

Vestibular Evoked Myogenic Potentials, or VEMPs for short, are special tests that measure tiny electrical responses in your muscles when we stimulate your inner ear, and this response gives us an understanding of how well the vestibular part of the inner ear is working (the part responsible for for balance).  These responses happen because of a reflex that connects your inner ear to certain muscles in your body. There are two main types of VEMPs:

1. Cervical VEMPs (cVEMPs): These test the saccule (a part of your vestibular system that detects changes in vertical acceleration) and the inferior vestibular nerve. We record cVEMPs from the muscles in your neck.
   
   
2. Ocular VEMPs (oVEMPs): These mainly test the utricle (another part of your vestibular system that detects changes in horizontal acceleration) and the superior vestibular nerve. We record oVEMPs from the muscles around your eyes.

The Reflex Arc: How VEMPs Work

Imagine a game of telephone, but instead of words, your body is passing along signals. Here's how it works:

1. For cVEMPs:
   • We send a sound to your ear
   • Your saccule picks up this sound
   • It sends a signal through your inferior vestibular nerve then back through your accessory nerve (CN XI) that supplies a muscle in the neck
   • This causes a brief relaxation in your neck muscle.
     
   • This is called the "sacculocolic reflex".
     
     
2. For oVEMPs:
   • We send a sound or vibration to your ear
   • Your utricle mainly responds to this
   • It sends a signal through your superior vestibular nerve
   • This signal crosses to the other side of your brain to reach the nerve that controls eye movements
   • Finally, it causes a tiny contraction in the muscle under your eye (inferior oblique muscle)
   • This is called the "utriculo-ocular reflex".

When Do We Use VEMPs?

VEMPs are very helpful in evaluating the health of the vestibular part of the inner ear. For that reason, it is used to diagnose certain conditions of the ear that affect the vestibular system. Here are some situations where we might use them:

1. Superior Canal Dehiscence Syndrome (SCDS): VEMPs can be very accurate in diagnosing this condition
    
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2. Vestibular Neuritis: While we're still learning about how useful VEMPs are for this condition, they can provide valuable information.
   
   
3. Menière's Disease: VEMPs might help us understand how this disease is affecting different parts of your inner ear.
   
   
4. General Balance Problems: VEMPs give us a unique look at parts of your balance system that other tests can't assess. It is often used before certain surgeries of the ear (such as cochlear implantation).

When VEMPs Might Not Work

Sometimes, we can't perform VEMPs or the results might not be reliable. This can happen if:

• You have significant hearing loss, especially in the lower frequencies.

• You have trouble contracting the necessary muscles, like the neck muscles for cVEMPs.
  

• There are issues with the conductive part of your ear (the part that carries sound waves to your inner ear)

You can read more about the different types of hearing loss here.

Understanding VEMP Results

When we look at VEMP results, we're like detectives solving a mystery. Here's what we consider:

1. Is there a response? Sometimes, no response can indicate a problem with the tested ear
   
   
2. How big is the response? We measure the size (amplitude) of the waves we see. In some conditions, like SCDS, these waves can be much larger than normal.
   
   
3. How quickly does the response happen? We look at how long it takes for the response to appear (latency).
   
   
4. Are the responses similar on both sides? We compare the results from your left and right ears. Big differences can suggest a problem on one side.
   
   
5. What's the threshold? This is the lowest level of sound that can produce a response. In some conditions, this threshold can be much lower than normal.

Conclusion

Remember, VEMPs are just one piece of the puzzle. We always consider them along with your symptoms, other test results, and the overall clinical picture.VEMPs have revolutionized how we diagnose and understand certain ear disorders. They give us a window into parts of your balance system that we couldn't easily test before. As we continue to learn more about VEMPs, they're becoming an increasingly valuable tool in our diagnostic toolkit.