Vestibular Evoked Myogenic Potential (VEMP) testing has become a well-established test to determine the integrity of the saccule in the inner ear. The purpose of this test is to determine if the saccule, a part of an inner ear organ called the otolith, and the VIIIth nerve are intact and functioning normally. The VEMP test has been gaining popularity for patients with symptoms of Meniere’s Disease. The VEMP is currently the only proven test available that can detect Meniere’s Disease when a patient is non-symptomatic.
The saccule, which is the lower of the two otolithic organs, the other one being the utricle, has a slight sound sensitivity which can be measured and recorded when tone bursts or forehead taps are administered. These sensory organs in the inner ear primarily respond to linear acceleration due to the orientation of gravity, but the saccule is also somewhat sensitive to sound which enables testers to isolate this organ and verify its integrity with the use of evoked potentials. The saccules sensitivity to sound is the basis of the VEMP test.
The VEMP testing procedure consists of placing electrodes to the middle third of the anterior (front) neck muscles (sternocleidomastoids) and the forehead. With the patient in a supine (laying back) position, the patient is instructed to hold their head up with no support surface using their anterior neck muscles. Tone bursts or loud clicks are then repetitively presented to each ear and generally 3 sessions of acoustic stimulation per each ear of sufficient for accurate data. Patients are instructed to tense their neck muscles during acoustic stimulation, and relax between bursts. If the patient does not activate their neck muscles during stimulation, no VEMP is produced.
The tone bursts stimulates the saccule, which then traverses the vestibular nerve (VIIIth) and the ganglion to reach the vestibular nucleus in the lower part of the brainstem. From there, impulses are sent to the neck muscles via the medial vestibulospinal tract (MVST) and the leg muscles via the lateral vestibulospinal tract (LVST). The electrode that is placed near the sternum is used as a reference during the test, and the forehead electrode serves as a ground. The myogenic potentials are amplified using the computer equipment and then averaged to represent the most accurate testing data. The response evoked in the neck electrode is averaged and presented as a VEMP.
