Understanding VEMPs: A Guide for Vestibular Therapists

As vestibular therapists, we often focus on assessing the function of the semicircular canals in the inner ear. However, the otolithic organs—the utricle and saccule—play a crucial role in maintaining balance and spatial orientation and are not routinely assessed. Two electrophysiological tests, Cervical Vestibular Evoked Myogenic Potentials (cVEMPs) and Ocular Vestibular Evoked Myogenic Potentials (oVEMPs), provide valuable insights into otolithic function. I often forget the difference between the two VEMPs tests, so hopefully writing this blog will make it stick. If not, at least I have a quick resource to refer back to!

If you are like me, these tests may not be readily available in your area. It is, however, good to know what they are, especially if you can access them—or at least understand what they are when you come across them in the literature. In this blog post, we’ll explore the physiology behind these tests, compare and contrast them, and discuss their clinical indications, limitations, and an alternative test.

The Physiology Behind VEMPs

The utricle and saccule, while primarily responsible for detecting linear acceleration, linear head motion, and static head positioning, also exhibit a rudimentary response to vibration. This sensitivity to vibration is due to their otolithic membrane, which contains otoconia that also respond to vibratory stimuli. When exposed to vibrations, the otoconia shift and stimulate the hair cells within these organs. This leads to transient signals being sent to the brain, resulting in a motor response that can be measured as VEMPs. As there are two otolithic organs there are two VEMP responses and as you will see both VEMP test actually assess more than otolithic function.

cVEMPs: Evaluates the function of the saccule and the inferior vestibular nerve. During a cVEMP test, a loud click or tone burst is delivered to the ear. This stimulates the saccule causing an involuntary contraction of the sternocleidomastoid (SCM) muscle. The response is recorded via surface electrodes placed over the SCM. This test primarily evaluates the saccule's response to acoustic stimulation, reflecting the function of the vestibulocollic reflex.

oVEMPs: Evaluates the function of the utricle and the superior vestibular nerve. In an oVEMP test, a similar acoustic stimulus is used which stimulates the utricle. The response is, however, recorded from electrodes placed near the eyes, typically at the lower eyelid. The oVEMP evaluates the utricular response and the vestibulo-ocular reflex.

Comparing and Contrasting cVEMPs and oVEMPs

Similarities:
- Both tests involve acoustic stimuli and measure myogenic potentials.
- They provide insights into otolithic function and the integrity of vestibular pathways.

Differences:
- Anatomical Targets: cVEMPs assess the saccule and inferior vestibular nerve, while oVEMPs target the utricle and superior vestibular nerve.
- Response Location: cVEMPs are recorded from the SCM muscle, whereas oVEMPs are recorded from electrodes near the eyes.
- Reflex Pathways: cVEMPs evaluate the vestibulocollic reflex, while oVEMPs assess the vestibulo-ocular reflex.

Performing VEMPs

To see a video of how to set up cVEMPs click HERE.

To see a video of how to set up oVEMPs click HERE.

Clinical Indications

cVEMPs assists in diagnosing impaired functioning anywhere along the vestibulocollic reflex loop to include the:

• Saccule
• Inferior vestibular nerve
• Vestibular nucleus
• Vestibular spinal tract
• Sternocleidomastoid (SCM) muscle

oVEMPs assists in diagnosing impaired functioning anywhere along the vestibulo-ocular reflex loop to include the:

• Utricule
• Superior vestibular nerve
• Vestibular nucleus
• Medial longitudinal fasciculus
• Oculomotor muscle nerve pathway
• Inferior oblique muscle of the eye

Both VEMPs tests can also be used to detect conditions like a third window, Meniere's Disease, and can also be used to tease out if the superior or inferior branch of the vestibular nerve, or both branches are affected with a vestibular neuritis.

Limitations

cVEMPs:

- The test may not be suitable for patients with neck muscle weakness or those unable to contract the SCM muscle effectively.
- Results can be influenced by muscle artifacts and patient movement.

oVEMPs:

- The test requires precise electrode placement, and recordings can be affected by eye movement artifacts.
- It may be challenging to perform in patients with significant eye or eyelid movement disorders.

Note: the results of VEMPs should be used in combination with other tests to come up with a diagnosis.

 Alternative: The Bucket Test

For those, like most of us, who do not have access to VEMPs, the Bucket Test can be a useful alternative to assess otolithic function. This test evaluates the subjective visual vertical (SVV), which is influenced by the otolithic organs, specifically the utricle which is orientated more horizontally, as well as other neural pathways. It involves having the patient view a tilted bucket or container and report their perceived vertical orientation. While not as precise as VEMPs, it provides a crude measure of otolithic function and can be useful in clinical practice. According to a 2017 study, the bucket test is easy and quick to perform, with a recommended range of -1.0° to +3.0° as normal values in the healthy population

Conclusion

Understanding the principles behind VEMPs enhances our ability to diagnose and manage vestibular disorders. While not all vestibular therapists have access to these tests, knowing their strengths and limitations helps us make informed decisions and explore alternative methods like the Bucket Test. Whether you have access to these advanced tests or rely on clinical observations, a comprehensive understanding of otolithic function is essential for effective vestibular rehabilitation.