Ocular vestibular evoked myogenic potentials to head tap and cervical vestibular evoked myogenic potentials to air-conducted sounds in isolated internuclear ophthalmoplegia

Abstract

Objective: The central pathways responsible for ocular vestibular evoked myogenic potentials (VEMPs) to forehead tapping remain to be determined. This study aimed to determine whether the medial longitudinal fasciculus (MLF) carries the signals for ocular VEMPs (oVEMPs) in response to this mode of stimulation. Methods: Twelve patients with isolated unilateral internuclear ophthalmoplegia (INO) due to brainstem infarction underwent evaluation of the ocular tilt reaction (ocular torsion and skew deviation), tilt of the subjective visual vertical (SVV), cervical VEMPs (cVEMPs) in response to tone burst sound, and oVEMPs induced by tapping the forehead. Results: Eight (67%) patients showed abnormal oVEMPs that included no wave formation (n = 4) and decreased amplitude (n = 3) in the lesion side, and bilaterally absent responses in the remaining patient. Furthermore, the patients showed diminished oVEMPs responses in the lesion side compared with normal side (6.0 +/- 5.6 vs. 11.7 +/- 5.5 mu V, paired t-test, p = 0.001) and increased IAD(amp)(%) of the oVEMPs compared with normal controls (43.6 +/- 41.2 vs. 9.1 +/- 6.2, t-test, p = 0.018). In contrast, cVEMPs were abnormal in only three (25%) patients, decreased (n = 2) or no response in the lesion side. Eleven (92%) patients showed contraversive ocular tilt reaction or SVV tilt. Conclusion: Patients with INO frequently show impaired formation of ipsilesional oVEMPs in response to forehead tapping. The occasional abnormality and decreased amplitude of ipsilesional cVEMPs also suggest a modulatory pathway for the inhibitory sacculocollic reflex descending in the MLF. Significance: This study suggests that the MLF contains the fibers for the otolith-ocular reflex from the contralateral ear.