Vergence deficits in cerebellar lesions

Abstract

The cerebellum is part of the cortico-ponto-cerebellar circuit for conjugate eye movements. Accordingly, cerebellar lesions cause smooth pursuit deficits and saccadic dysmetria. In contrast, the role of the cerebellum for disconjugate, i.e. vergence eye movements in humans is still unknown. The aim of this study was to investigate whether cerebellar lesions affect vergence eye movements. Similar to conjugate eye movements, vergence can be separated into two different components: vergence to step targets (fast vergence) and vergence to ramp and sinusoidal targets (slow vergence). Eye movements (fast and slow vergence, smooth pursuit, saccades) were binocularly recorded with the scleral search coil system in 20 patients with acute cerebellar lesions (ischemic strokes) and age-matched healthy controls. Small dim laser stimuli were presented on an earth horizontal platform at the level of the subject's nose. Smooth pursuit always had a constant vergence angle (iso-vergence). The majority of patients had unilateral lesions in the territory of the posterior inferior cerebellar artery (3 patients with infarctions in the superior cerebellar artery territory, one cavernoma patient). Group analysis revealed impairment of both conjugate and disconjugate eye movements as well as reduced saccadic gain (Table). The gain of slow vergence to sinusoidal targets was reduced in the patients. Patients with vermal lesions (subgroup, n=10) had a significant lower gain compared to controls while the other patients did not differ. In contrast, fast vergence to step targets remained unimpaired (acceleration). Smooth pursuit to step-ramp targets was impaired, with increased latency, decreased initial acceleration and reduced gain. Smooth pursuit eye movement gain to sinusoidal targets was significantly reduced. Similar to vergence eye movements to sinusoidal targets, patients with vermal lesions had a lower smooth pursuit gain while the other patients did not differ from controls. We propose that (1) the cerebellum is involved in the processing of vergence eye movements, (2) the cerebellar vermis plays an important role in executing vergence eye movements and (3) neural control of fast and slow vergence appears to be separated in the cerebellum.