OVEMP to air-conducted tones reflects functions of different vestibular populations from cVEMP?

Abstract

Vestibular evoked myogenic potentials recorded on the sternocleidomastoid muscle (SCM) have been called cVEMP (cervical VEMP) and have drawn the attention of clinicians, because cVEMPs can reflect functions of the eighth cranial nerve that differ from those shown by caloric tests and auditory brainstem responses (ABRs), predominantly saccular functions [1, 2]. Recently, myogenic potentials around the eyes have been studied, having also been regarded as of vestibular origin [3, 4]. These are called ocular vestibular evoked myogenic potentials (oVEMPs). While Iwasaki et al. [5] have proposed that oVEMPs to bone conducted vibration could be of utricular origin, the origin of oVEMPs to air-conducted sound remains unknown. Herein, we present a patient in whom oVEMPs in response to air-conducted sound might provide suggestions concerning the origin. A 52-year-old woman consulted our clinic with a complaint of right tinnitus. The tympanic membranes were normal and pure tone hearing was also normal on both sides. ABRs (air-conducted 80 dB nHL clicks) showed prolongation of the interpeak interval between waves 1 and 5 to the right ear stimulation. She did not show gaze nystagmus or positional nystagmus. Caloric tests to cold water were normal bilaterally. Vestibular evoked myogenic potentials on the SCM (cVEMPs) in response to air-conducted 500 Hz short tone bursts (STB, rise/fall time = 1 ms, plateau time = 2 ms, 95 dB nHL) were normal (Fig. 1). Then, ocular VEMPs (oVEMPs) were measured. The methods of recording of oVEMPs were almost the same as those in previously reported except for bandpass-filter [6]. The electrodes were placed just beneath the lower eye lids (active) and 2 cm below the active electrodes (indifferent). Stimuli were presented through the headphone. The stimulation rate was 5 Hz. During recording, the subject was asked to maintain an upward-gaze. The signals were bandpass-filtered (20–2,000 Hz) and 100 responses were averaged. In this patient, recording conditions for oVEMP were the same as those for cVEMP except for the sites of electrodes. According to the previous report [4], recordings under the eye contralateral to the stimulated ear were assessed. Then, oVEMPs in response to right ear stimulation were absent (Fig. 1). Finally, MRI demonstrated right vestibular schwannoma (VS) (20 mm in diameter) (Fig. 1). In this particular patient, oVEMPs in response to airconducted 500 Hz STB were absent on the VS affected side, while cVEMPs using the same stimuli were normal. How can these findings be explained? One of the simplest explanations may be a difference in the contribution rate of utricular afferents between vestibulo-collic reflexes and vestibulo-ocular reflexes. Concerning the vestibulo-ocular reflexes, utricular afferents could provide a greater contribution than saccular afferents [7]. Then, dysfunctions of utricular afferents affected oVEMPs while they did not affect cVEMP because of the rich contribution of saccular afferents to cVEMPs. To justify this hypothesis, the responsiveness of utricular afferents to air-conducted 500 Hz STB must be proven. Currently, there is no clear evidence. However, at the early stage of VEMP study, T. Murofushi (&) K. Wakayama Department of Otolaryngology, Teikyo University School of Medicine Mizonokuchi Hospital, 3-8-3 Mizonokuchi, Takatsu-ku, Kawasaki 213-8507, Japan e-mail: murofush@med.teikyo-u.ac.jp