Does the oculomotor vermis generate congenital nystagmus?

Introduction: Congenital nystagmus is a common eye movement disorder of unknown etiology, which consists of involuntary, invariably conjugate, and predominantly horizontal ocular movements with smaller torsional and vertical components. Congenital nystagmus may be present at birth and usually develops during the first months of life. A null zone can usually be identified, i.e., a particular orbital position, in which the nystagmus intensity is minimal. The aim of this study was to identify cerebellar and supratentorial brain areas that participate in the generation of congenital nystagmus.

Materials and Methods: Twelve subjects with idiopathic congenital nystagmus participated in a functional magnetic resonance imaging study. To analyze brain activations during nystagmus with maximal versus minimal intensity, two visual fixation targets were alternately presented at two differents positions. Nystagmus intensity depended on gaze direction in all subjects. During fixation of one target, nystagmus intensity was maximal, during fixation of the other target it was minimal. Eye movements were recorded using video-oculography.

Results: During nystagmus with maximal intensity, blood oxygen level dependent signal activations were seen in oculomotor cerebellar sites (vermis 6–8; lobuli 6 and crus 2 bilaterally), visual cortical areas including the visual motion-sensitive area MT+/V5 bilaterally and parieto-occipital sites, as well as in the frontal and supplementary eye fields. Deactivations were simultaneously seen in the left striate visual cortex.

Conclusion: Functionally, activations in the oculomotor vermis (vermis 6,7) may be associated with nystagmus generation. Activations in MT+/V5 may reflect excess visual motion stimulation due to nystagmus induced retinal slip. Signal increases in the frontal and supplementary eye fields may reflect saccadic resetting and the increased effort to stabilize gaze in space. The relative decreases in striate visual cortex activity may be related to a central mechanism to suppress oscillopsia (illusory motion of the visual scene). This corresponds to increased visual motion detection thresholds, which were observed earlier in individuals with congenital nystagmus.