Reflexive eye movement alterations are related to subclinical impulsive-compulsive behaviors in Parkinson's disease

Subclinical impulsive-compulsive behaviors (s-ICBs) in Parkinson's disease (PD) are common, clinically relevant, and frequently underdiagnosed. Objective behavioral measures reflecting vulnerability to impulsive-compulsive behavior are lacking. We investigated whether oculomotor measures of inhibitory control are associated with s-ICBs in dopaminergically treated PD patients without clinically manifest impulse control disorders.

Twenty-nine patients with PD (Hoehn and Yahr stages 1-2) and twenty age-matched healthy controls completed prosaccade and antisaccade eye-tracking tasks. Executive functioning, trait impulsivity, quality of life and s-ICBs were assessed. Group comparisons and association analyses were performed using nonparametric methods with false discovery rate correction. Exploratory mediation analysis examined relationships between levodopa equivalent daily dose (LEDD), s-ICBs severity, and express prosaccades. All assessments were performed in the ON-medication state.

Compared with healthy controls, patients with PD showed a higher frequency of express prosaccades and prolonged antisaccade latencies. Within the PD group, express prosaccades were moderately to strongly associated with s-ICBs severity and LEDD and were also related to reduced quality of life. Mediation analysis revealed a significant statistical indirect effect of dopaminergic medication on express prosaccades through s-ICBs severity, indicating that behavioral symptoms statistically accounted for a substantial proportion of this association.

These findings demonstrate a close relationship between oculomotor control, subclinical impulsive-compulsive behaviors, and dopaminergic treatment in PD. The results highlight the complexity of non-motor manifestations in PD and underscore the need for longitudinal and mechanistic studies to clarify their clinical significance.