Vestibular Evoked Myogenic Potentials (VEMPs)

𝗢𝗿𝗶𝗴𝗶𝗻𝗮𝗹 𝗗𝗲𝘀𝗰𝗿𝗶𝗽𝘁𝗶𝗼𝗻: Vestibular-evoked myogenic potentials (VEMP) are electromyographic potential reflex tests that reflect the function of the saccule in cervical VEMP and the utricle in ocular VEMP.1 In the cervical VEMP an inhibitory reflex is evoked from the saccule to the sternocleidomastoid ipsilateral to the stimulus. The most common stimulus is a sound (either a 500 Hz tone burst or a broad-band click) played to the ear on the side being tested. The patient is instructed to contract their sternocleidomastoid muscle while the stimulus is played, and an average of a series of responses is recorded using surface electrodes on the neck (Figure 1). A characteristic biphasic waveform is assessed with a latency of the peaks at 13 and 23 milliseconds (Figure 2a). Typically a threshold of sound intensity is obtained at which the characteristic waveform is observed. The saccule functions similar to a hearing organ in fish, responding to lower frequency stimuli and the cervical VEMP may be a vestigial reflex. An absent response at the loudest stimulus level can reflect impairment of the saccule or the inferior vestibular nerve; however, an absent response can also reflect inadequate activity of the sternocleidomastoid muscle, middle ear disease, or a lesion along the pathway from the saccule to the central vestibulospinal pathways. An especially large response at a low threshold (i.e., a present waveform at a much weaker stimulus intensity) is typical of superior semicircular canal dehiscence syndrome, in which a hole in the bony superior semicircular canal creates a new low impedance pathway in the ear. 𝗡𝗲𝘂𝗿𝗼-𝗼𝗽𝗵𝘁𝗵𝗮𝗹𝗺𝗼𝗹𝗼𝗴𝘆 𝗮𝗻𝗱 𝗡𝗲𝘂𝗿𝗼-𝗼𝘁𝗼𝗹𝗼𝗴𝘆 𝗧𝗲𝘅𝘁𝗯𝗼𝗼𝗸 𝗟𝗲𝗴𝗲𝗻𝗱: An introduction to vestibular evoked myogenic potentials