P118. Speed- and phase-dependent suppression of downbeat nystagmus during locomotion

Publication date: August 2018

Source: Clinical Neurophysiology, Volume 129, Issue 8

Author(s): H. Dietrich, M. Wuehr, C. Pradhan, R. Schniepp

Introduction

Downbeat nystagmus (DBN) is the most common form of acquired fixation nystagmus. Patients typically suffer from oscillopsia and postural instability. Postural symptoms are known to improve during walking, in particular at fast speeds (Schniepp et al., 2014); however, it is unknown whether locomotion also influences ocular motor symptoms in these patients. We thus investigated the frequency and timing of DBN occurrence during standing and walking at different speeds in patients with DBN.

Methods

Eye movements of DBN patients with clear central-gaze nystagmus were recorded using video-oculography during standing as well as treadmill walking at individual slow, preferred and fast speeds. Subjects were instructed to fixate on a target straight in front of them at 2 m distance. We analyzed the frequency and timing of occurrence of compensatory quick-phases (downbeats) throughout the gait cycle as well as the angular and linear components of the vestibulo-ocular-reflex (VOR) to assess general gaze stabilization performance during walking in these patients. Results were compared to healthy controls.

Results

Compared to rest, slow walking significantly lowered the frequency of DBN occurrence in all patients. With increasing walking speed, DBN frequency was further reduced and in some subjects even fully suppressed during fast walking. In addition, DBN events predominantly occurred at the start of the swing phase, indicating that DBN occurrence is phase-coupled to distinct periods of the gait cycle. Whereas angular VOR gains of patients during walking were comparable to those of healthy subjects, linear VOR components were found to be significantly decreased.

Conclusion

DBN ocular-motor symptoms improve during walking, in particular at fast speeds, indicating a speed-dependent re-weighting of the involved vestibulo-cerebellar pathways during locomotion. In addition, compensatory quick-phase eye movements (downbeats) most frequently occur during distinct phases of the gait cycle, suggesting that a phase-coupled locomotor feedback such as spinal motor efference copies influences the brain networks associated with downbeat nystagmus. Notable deficits in compensating linear head motion during walking indicate impairments in pathways processing otolithic inputs, despite an intact integration of vestibular cues from the semicircular canals (Glasauer et al., 2004).

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