We examine the prevalence and subtype of SWJs in drug-naïve PD and explore their association with motor and non-motor features across PD subtypes.

Square wave jerks (SWJs) are pairs of small saccades that takes the gaze away from  the target and then return it, commonly observed in neurodegenerative disorders. Although prominent in progressive supranuclear palsy, their clinical significance in Parkinson’s disease (PD) is not well defined.

We retrospectively reviewed 145 de novo PD patients who underwent video-oculography, UPDRS part Ⅲ, Hoehn and Yahr staging, cognitive testing, and assessments for orthostatic hypotension (OH) and probable REM sleep behavior disorder (RBD). SWJs were categorized as macro (≥5˚), staircase, or dynamic overshoot. Associations with motor severity, cognition, non-motor features, and PD motor subtypes (tremor-dominant [TD], postural instability/gait difficulty [PIGD], indeterminate) were analyzed.

Among 67 TD, 41 PIGD, and 37 indeterminate patients, RBD (47.6%) and OH (29.7%) were most frequent in PIGD, though without statical significance. Overall SWJ frequency did not differ across motor subtypes. Macro SWJs were rare (3.4%). Staircase SWJs (49.7%) correlated with higher SWJ counts (p<0.001) but no with clinical parameters. Dynamic overshoot SWJs (42.1%) were associated with higher SWJ frequency (p<0.001), worse motor severity (UPDRS part Ⅲ, p<0.001), and higher PIGD scores (p=0.032). No associations were observed with cognition, OH, or RBD.

Dynamic overshoot SWJs, unlike other subtypes, were linked to more severe motor impairment in de novo PD. These findings suggest that dynamic SWJs may reflect cortical inhibitory dysfunction, in which diminished inhibitory control from the frontal eye field results in disinhibition and subsequent hyperactivation of the superior colliculus.