Brainstem representation of vestibular evoked muscle potentials

Vestibular evoked muscle potentials (VEMPs) are caused by a short-latency reflex recorded from averaged electromyography from the sternocleidomastoid muscle evoked by intense auditory clicks stimulating the sacculus. Besides peripheral vestibulopathy, abnormal VEMPs can be caused by lesions of the brainstem. 29 patients with brainstem infarcts were prospectively analyzed using VEMPs and MR imaging to evaluate the brainstem representation of the VEMP reflex. 12 patients showed unilaterally abnormal VEMPs, 10 patients had normal VEMPs. 7 patients with unspecific bilaterally absent VEMPs were not analyzed. Individual brainstem lesions were transferred to a MR dataset of a healthy brainstem for normalization. Right-sided lesions were projected onto the left side. Probabilistic lesion maps were calculated. A difference map between both groups was calculated representing positive counts for lesions associated to abnormal VEMPs and negative counts for lesions associated to normal VEMPs. A digitial brainstem atlas was fitted to the lesion maps. Most lesions were located in the lateral medulla oblongata involving the spinal accessory nerve (besides ventral and dorsal spinocerebellar tracts, spinothalamic tract, and the medullar trigeminal system). Most lesions in the pons were associated to anterolateral parts of pyramidal tract fibers. In less cases lesions were located in the tegmental area of the pons, including the vestibular nuclei. Neither the ventral nor the lateral vestibulospinal tract was included in the lesion profiles. VEMPs may be also influenced by effects caused by lesions located above the level of the vestibular nuclei.