Clinical utility of the vestibular evoked myogenic potential (VEMP)

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The vestibular evoked myogenic potential (VEMP) is one of many sound-evoked muscle reflexes, or ''sonomotor'' responses that can be recorded using evoked potential techniques. This response is believed to be generated from acoustical stimulation of the saccule. This vestibular end organ that transduces linear accelerations and decelerations lies beneath the stapes footplate. There are a number of lines of evidence that suggest that the VEMP is generated by acoustical stimulation of the saccule. For instance, neurons from the saccule that respond to tilts also respond to click stimuli. The VEMP can be recorded from patients who have no hearing but have intact vestibular system function. Finally, in our own investigations we have recorded the VEMP using high intensity clicks. Then we have masked the acoustical perception of the click using bone-conducted white noise and still have been able to record the response.

The function of this ''vestibulocollic'' reflex is probably to stabilize the head in response to unpredictable displacements. The response can be recorded from a number of muscles including the trapezius muscle, the quadriceps muscle and the sternocleidomastoid muscle (i.e. SCM). This sound evoked myogenic potential is generated following high intensity transient acoustical stimulation. In subjects with normal auditory sensitivity this response is recorded to unfiltered click stimuli at intensities of approximately 95-105 dB nHL. Although an unfiltered click can be used to record this response, recent evidence suggests that this response is best elicited using a low frequency tone burst (e.g. a 2-4 msec rise time, 0-1 msec plateau, and 2-4 msec fall time). The best frequency appears to be 500 Hz. This frequency elicits responses that are characteristically high in amplitude (e.g. 10-50 uV) and accordingly the response can be obtained with as little as 100 samples. The acoustical stimulus is presented through an insert earphone at a rate of 3 Hz at an intensity that is 5 dB greater than the VEMP threshold of appearance. In our laboratory these intensity levels are usually between 95 and 100 dB HL. The patient is seated in a comfortable reclining chair. Alternately the patient may lay supine on an examination table. The non-inverting electrode is placed on the ipsilateral SCM between the insertion of the muscle at the mastoid and the sternum. The inverting electrode is placed on the thyroid notch or sternum. The ground electrode is placed at Fpz. The EMG activity is recorded using a band pass of 30-3000 Hz. The response is elicited by having the patient activate the muscle ipsilateral to the ear stimulated. This is done by asking the patient to turn their head sharply in the direction opposite the ear being stimulated (e.g. head turned toward the left for right ear stimulation). Patients who are lying supine are requested to lift their head up. The high intensity transient acoustical stimulus results in a transient inhibition of the spontaneous activity. This becomes the signal-averaged evoked potential. The prominent components occur at approximately 13 msec (positive-going potential) and 23 msec (negative-going potential). It is a comparison of the amplitude of the response following stimulation of the left ear with that following stimulation of the right ear that is used to determine whether the response is normal or abnormal. It has been suggested that a ratio of> 3:1 be used to determine whether there is an amplitude asymmetry. There have been offered a number of situations where the VEMP might have clinical usefulness. The obvious one is where there is abnormal coupling between the peripheral auditory system and the peripheral vestibular system. One such situation occurs in perilymphatic fistula where VEMP thresholds have been reported to be reduced (i.e. VEMP occurring at abnormally low sound intensities). Another such situation is where a patient demonstrates the Tullio effect. Again it has been reported that in this circumstance the VEMP can be recorded to auditory signals that are as low as 70 dB nHL. Another potential usefulness occurs when patients demonstrate normal function on conventional tests of the peripheral vestibular system (e.g. caloric testing, rotary chair testing) and complain of abnormal sensations of linear acceleration that would be referable to the otolith system.

All that glitters is not gold, however. A response from some patients is difficult or impossible to obtain. Elderly patients who have limited neck mobility, and/or who have reduced muscle mass due to the normal aging process and deconditioning may have difficulty accomplishing sufficient muscle activation. For these patients it often is necessary to signal average a ''block'' of data (e.g. 200 samples) in packets (e.g. 25-50 samples in each packet). Akin and Murnane (2001) have developed a novel method of attempting to standardize the level of muscle contraction in patients. The raw EMG activity is rectified and displayed on an oscilloscope screen that can be viewed by the patients. The patients are asked to attempt to maintain the continuous EMG activity at the same high level (e.g. 50 - 100 uV) throughout the recording session. In our laboratory we route the raw EMG through a loudspeaker and ask our patients to maintain a uniform sound intensity.

Reference:
Akin F, Murnane OD. 2001. Vestibular myogenic evoked potentials: Preliminary report. JAAA 12: 445-452.

Bio: Gary P. Jacobson, Ph.D. is the Director of the Division of Audiology for the Henry Ford Health System. He received his Ph.D. from Kent State University in 1978. He is a Fellow of ASHA, and the American Society of Neurophysiological Monitoring (ASNM). He is the Editor of the American Journal of Audiology (AJA) and an Associate Editor for the Journal of the American Academy of Audiology (JAAA). He serves on the editorial boards of Brain Topography and Seminars in Hearing and is an ad hoc reviewer for a number of medical journals. He is co-editor of the Handbook of Balance Function Testing and author of many articles and book chapters that deal with auditory and vestibular electrophysiology, outcomes measures, tinnitus, brain mapping and intraoperative Neurophysiological monitoring.