Vestibular Evoked Myogenic Potentials
Vestibular evoked myogenic potentials are small electrical responses of muscles that are triggered by loud sound or vibration and which depend on the otolith organs of the inner ear.
They test the saccule and its nerve tract (cervical potential, recorded from the neck muscle) and the utricle and its nerve tract (ocular potential, recorded under the eyes). They complete the picture from caloric testing, which tests only the horizontal canals.
For the cervical test, loud clicks or short tone bursts (usually 500 Hz at 95 to 100 dB nHL) are delivered to one ear through insert earphones. The patient lies with the head raised, or sits and turns the head, to keep the sternocleidomastoid muscle tense.
Surface electrodes over the muscle record a short, biphasic wave (called p13 and n23) at about 13 and 23 milliseconds.
The saccule responds to the sound. The signal passes along the inferior vestibular nerve to the brainstem, and then along the descending tract to the ipsilateral neck muscle, causing a brief relaxation.
For the ocular test, electrodes are placed under the eyes, with the patient looking up. A crossed response (n10) from the contralateral inferior oblique muscle reflects the utricle and superior vestibular nerve.
The test takes 20 to 40 minutes. In normal people, responses are present in over 90% of ears, and the amplitude is compared between the two ears.
The test is useful in superior canal dehiscence, in which responses occur at abnormally low sound levels. It is also useful in Meniere disease and vestibular neuritis, in which patterns of loss help find the affected nerve branch.
Indications.
- Vertigo and imbalance, as part of a full vestibular test battery.
- Suspected superior semicircular canal dehiscence: sound-induced vertigo, pressure-induced vertigo, autophony, and low-frequency conductive hearing loss with normal middle ear.
- Meniere disease, to assess saccular involvement and as a marker for disease stage.
- Vestibular neuritis, to find whether the superior or inferior branch (or both) is affected.
- Vestibular schwannoma, to assess the involved nerve.
- Benign positional vertigo recurrence and otolith disease.
- Brainstem disease (multiple sclerosis, stroke), through the central tract.
- Perilymph fistula and enlarged vestibular aqueduct. Not a hearing test, and a normal response requires a working middle ear.
Contraindications and precautions.
- Conductive hearing loss or middle ear disease: the sound is reduced before it reaches the saccule, so the cervical response is absent or small, and bone-conducted stimuli are used instead.
- Neck problems that prevent turning the head or tensing the muscle: neck pain, cervical spine disease, or recent neck surgery. Eye problems affect ocular responses.
- Loud sound exposure of up to 100 dB nHL: the test is limited or avoided in patients with hyperacusis, severe tinnitus, recruitment, or recent ear surgery, and the duration of exposure is kept short.
- Perforated eardrum, ear discharge, or blocked canal.
- Patients who cannot cooperate or maintain muscle tension, such as small children and those who are weak.
- Neonates and some older adults have low or absent responses.
Preparation and technique.
The ears are examined and the skin is cleaned. For the cervical test, the active electrode is placed over the middle of the sternocleidomastoid muscle, the reference on the sternum or upper chest, and the ground on the forehead.
The patient lies with the head lifted, or sits turning the head to the opposite side, to keep the muscle contracted at a steady level, which is checked by a monitor.
Tone bursts at 500 Hz are delivered at 95 to 100 dB nHL at a rate of about 5 per second, and 100 to 200 sweeps are averaged. The test is repeated to show repeatability.
For the ocular test, the electrodes are placed under each eye, the patient looks upward about 30 degrees, and bone-conducted taps or vibration at the forehead are used. Thresholds are found by lowering the stimulus in 5 dB steps.
