Purpose: Subjects with Down syndrome (DS) have an anatomical defect within the cerebellum that may impact downstream oculomotor areas. This study characterized gaze holding and gains for smooth pursuit, saccades, and optokinetic nystagmus (OKN) in DS children with infantile nystagmus (IN).
Methods: Clinical data of 18 DS children with IN were reviewed retrospectively. Subjects with constant strabismus were excluded to remove any contribution of latent nystagmus. Gaze-holding, horizontal and vertical saccades to target steps, horizontal smooth pursuit of drifting targets, OKN in response to vertically or horizontally-oriented square wave gratings drifted at 15°/s, 30°/s, and 45°/s were recorded using binocular video-oculography. Seven subjects had additional optical coherence tomography imaging.
Results: Infantile nystagmus was associated with one or more gaze-holding instabilities (GHI) in each subject. The majority of subjects had a combination of conjugate horizontal jerk with constant or exponential slow-phase velocity, asymmetric or symmetric, and either monocular or binocular pendular nystagmus. Six of seven subjects had mild (Grade 0-1) persistence of retinal layers overlying the fovea, similar to that reported in DS children without nystagmus. All subjects had abnormal gains across one or more stimulus conditions (horizontal smooth pursuit, saccades, or OKN). Saccade velocities followed the main sequence.
Conclusions: Down syndrome subjects with IN show a wide range of GHI and abnormalities of conjugate eye movements. We propose that these ocular motor abnormalities result from functional abnormalities of the cerebellum and/or downstream oculomotor circuits, perhaps due to extensive miswiring.