Square-wave jerks are small saccades which interrupt fixation, moving the eye away from the primary position and then returning. This instability of ocular fixation is a disorder of saccadic eye movements in which there is a saccadic interval (of about 200 ms; cf. ocular flutter, opsoclonus). Very frequent square-wave jerks may be termed Square-wave oscillations. Very obvious square-wave jerks (amplitude > 7°) are termed macro-square-wave jerks.
Square-wave jerks are often best appreciated on ophthalmoscopy. Their name derives from the appearance they produce on electro-oculographic recordings.
Although square-wave jerks may be normal in elderly individuals, they may be indicative of disease of the cerebellum or brainstem, e.g., Huntington’s disease, Parkinson’s disease, progressive supranuclear palsy, cerebellar degeneration.