THE VESTIBULO-OCULAR REFLEX IN CHIARI TYPE II MALFORMATION

Background and Objectives: Chiari type II malformation (CII) is a congenital anomaly of the cerebellum and brainstem, both important structures for processing the vestibulo-ocular reflex (VOR). Little information is available on the VOR in CII. The study objectives were to determine VOR characteristics in darkness and during visual fixation (visually-enhanced VOR or VVOR) in CII.

Methods: We recorded the angular VOR using an infrared eye tracker and magnetic head tracker in 20 participants aged 8–19 years with CII. Thirty-eight age-matched typically developing children constituted the control group. Participants made active sinusoidal head on body rotations in yaw (side-to-side) and pitch (up-down) at about 0.5Hz and 2Hz frequencies, in darkness while viewing an imaginary target 25cm away, and in the light while viewing an earth-fixed target 57cm away. Parametric and non-parametric tests were used to compare VOR parameters between the two groups.

Results: VOR and/or VVOR gains, the ratios of eye velocity to head velocity, were abnormally high or low (i.e. more than two SDs outside the control group's mean) in eight participants with CII. VOR and VVOR gains above unity in both groups are attributed to near viewing in darkness and during visual fixation.

Conclusion: The deformity of CII can impair the angular VOR in darkness and during visual fixation.

Keywords: Vestibulo-ocular reflex, Chiari type II malformation, eye movements