Eye movement abnormalities in Parkinson’s disease motor subtypes: a video-oculographic study

Introduction: Eye movement dysfunction has been described in Parkinson’s disease (PD), but differences between tremor-dominant (TD) and postural instability/gait difficulty (PIGD) PD motor subtypes remain poorly understood. The aim of this study was thus to compare video-oculographic (VOG) features between PD motor subtypes. Methods: Two hundred and four PD patients and 55 age-matched healthy control subjects (HC) were enrolled in this study. PD patients were stratified into PIGD and TD motor subtype groups. VOG amplitude, peak velocity of upward, downward, and vertical saccades, and square wave jerks (SWJ) number and amplitude were compared across groups. Multivariate linear models also investigated associations between VOG parameters and motor severity, gait/balance disturbances, dopaminergic treatment, and cognitive function. Results: The final cohort included 180 PD patients classified as PIGD (n = 121) or TD subtype (n = 59) and 55 HC. Both PD subtypes showed reduced upward and downward amplitude compared to HC, with normal peak velocity. PIGD patients exhibited significantly decreased upward saccadic amplitude compared to TD patients, with no differences in other VOG parameters. Moreover, the upward saccadic amplitude was associated with motor severity, particularly slowness of gait and bradykinesia/rigidity scores, as well as with levodopa equivalent daily dose (LEDD) in the PIGD group. Discussion: This study provides evidence of greater saccadic hypometria in PIGD than in TD patients in upward gaze, contributing to a better understanding of oculomotor impairment in PD. The association of saccadic amplitude with bradykinesia/rigidity severity and LEDD may suggest a role of underlying dopaminergic deficits in ocular dysfunction in PD patients.