Objective Assessment of Eye Alignment and Disparity-driven Vergence in Parkinson’s Disease

Diplopia is a common non-motor symptom in patients with Parkinson’s disease (PD) affecting 1/3rd of patients. We used high-resolution video-oculography to demonstrate that diplopia in PD arises from disruption of binocular co-ordination of eye movements. Using video-oculography we measured fixation eye movements, the angle and control of eye alignment and its correlation to convergence insufficiency in 13 PD patients and 6 healthy controls. We found that all PD patients had larger amplitude of fixational saccades with increased inter-saccadic drifts particularly of the non-fixing or non-viewing eye. Eye movement recordings suggest that the extent of eye alignment varies across PD subjects, 38% had good eye alignment and control under monocular and binocular viewing similar to controls (Group1). 62% of PD patients had increase in eye position difference, which was outside the threshold window of good alignment under monocular and binocular viewing (Group2 PD). PD subjects with good eye alignment were less likely to exhibit saccadic responses to disparity-driven vergence whereas those with poor control and worse eye alignment were more likely to exhibit saccadic responses with staircase saccades. A cohort of PD patients (Group2) exhibited pure vergence or combined vergence/saccade responses; they had deficits of both fusion initiating (prolonged latencies, reduced gain and peak vergence velocities) and fusion sustaining components (increased eye position variance). The results demonstrate that the vergence abnormalities in PD subjects systematically correlate with both the angle and control of eye alignment. Further, our data suggests an interaction between the vergence and saccade subsystems in PD patients. Abnormalities of vergence in PD may be the result of direct effects of the disease on vergence motor control, coupled with disturbances in the saccadic pathway indirectly leading to effects on vergence.