Asymmetry in Gaze-Holding Impairment in Acute Unilateral Ischemic Cerebellar Lesions Critically Depends on the Involvement of the Caudal Vermis and the Dentate Nucleus.

Stabilizing the eyes in space when looking at a target is provided by a brainstem/cerebellar gaze-holding network, including the flocculus/paraflocculus complex (non-human primate studies) and the caudal vermis, biventer, and inferior semilunar lobule (human studies). Previous research suggests that acute lateralized cerebellar lesions preferentially lead to gaze-evoked nystagmus (GEN) on ipsilesional gaze. Here, we further characterize the effect of unilateral cerebellar lesions on gaze-holding and hypothesize that the side-specific magnitude of gaze-holding impairment depends on the lesion location. Nine patients (age range = 31-62 years) with acute/subacute (≤ 10 days old) MRI-confirmed unilateral cerebellar stroke were included. Horizontal gaze holding was quantified while looking at a slowly moving (0.5°/s) flashing target (gaze angle = ±40°). Asymmetry in eye-drift velocity was calculated and compared with the different MRI patterns of cerebellar lesions. Individual peak eye-drift velocities (range = 1.7-8.8°/s) occurred at the most eccentric eye positions (gaze angle = 28-38°). We found significantly asymmetric eye-drift velocity (EDV) in eight out of nine patients. The four patients with MRI-confirmed involvement of the caudal vermis and the dentate nucleus all presented with ipsilesionally-predominant EDV, while in the five patients with lesions restricted to the cerebellar hemisphere, EDV was stronger on contralesional gaze in three out of four found with an asymmetric EDV. Involvement of the caudal vermis and the dentate nucleus is critical for determining the directional GEN asymmetry in unilateral cerebellar lesions. Thus, our findings support the occurrence of GEN without floccular/parafloccular lesions and suggest that the EDV asymmetry in relation to the side of the lesion provides information about the involvement of specific structures.