Characteristics of vestibular evoked myogenic potentials as an otolith function test

In humans, a positive followed by a negative waveform with short latency is reproducibly evoked in the sternocleidomastoid (SCM) muscle in response to intense sound. This sound-evoked vestibulocollic reflex is termed the vestibular evoked myogenic potential (VEMP). We investigated the characteristics of the VEMP as an otolith function test. The human VEMP has its best frequency located between 500 Hz and 1 kHz that resembles a V-shaped tuning curve of the saccular nerve in cats, recorded in the inferior vestibular nerve. The amplitude of the human VEMP was enhanced under microgravity during parabolic flight and was attenuated by the rostro-caudal linear acceleration on Earth. These findings suggest that the human VEMP is an otolith function test and originates from the saccule. After vestibular deafferentiation, unilateral VEMP dysfunction in the affected side appears and lasts for a long time. In patients with inner ear and retrolabyrinthine disorders, abnormality of the VEMP was not always coincident with that of the caloric test. These findings suggested that the VEMP, like the caloric test, is another important tool for the diagnosis of the affected side, and that a test battery of the VEMP and caloric test may help to increase the rate of abnormal findings among patients with the peripheral vestibular disorders. In the human VEMP, activation of acoustically responsive saccular afferents triggers an acoustic reflex in the SCM muscle. Future studies are needed to clarify in detail, the clinical usefulness of the VEMP as an otolith function test.