Objective: To disentangle bradykinesia from apathy through analysis of ocular movements.

Background: While Parkinson’s disease (PD) is mainly characterized by motor symptoms such as bradykinesia, non-motor symptoms such as apathy are frequent and debilitating. Bradykinesia is a movement disorder encompassing a motor component and a motivational aspect. The purpose of this study was to disentangle bradykinesia from apathy through analysis of eye movement patterns.

Method: Seventeen PD patients and thirteen controls were recruited. Both groups were matched regarding age and social-cultural level. Apathy was assessed with the Dimensional Apathy Scale (DAS) with its three subscores (executive, emotion, and initiative), and bradykinesia was measured with the MDS-UPDRS part III.
Subjects were asked to fixate successively two green points (cues, 40° apart) that appeared alternating at a 1Hz rhythm. After 20 seconds, all stimuli disappeared, and participants were required to continue fixating the previous locations of the cues at the same frequency for another 20 seconds. The maximal amplitude was defined as the amplitude of a gaze shift from side to side of the screen.
We have run the linear Mixed-Effects Models and subsequent contrasts analysis to assess the effect of participant group (patient vs. control), the cue presence, the DAS, and the MDS-UPDRS score.

Results: The DAS executive subscore (Das_Ex) was significantly different between both groups and used for the analysis.
The main finding is a triple interaction between group, DAS_Ex, and cue presence (F (1, 2155) = 14.41, p < 0.001). The contrast analysis between group and cue presence showed that the saccade amplitude was larger when the cue was absent but was decreased by high score in DAS_Ex in the patient group only. In the patient group, the executive score (-0.48, p = 0.0002) and the bradykinesia (-0.185, p < 0.001) significantly affected the maximal amplitude in the absence of cue.

Conclusion: The maximal amplitude of saccades depends on the degree of apathy and the motor score in PD patients. The higher the apathy or the motor scores, the smaller the maximal amplitude. However, the apathy score had a greater effect on maximal amplitude, suggesting that when saccades have to be generated without visual stimulation, their amplitude is mainly driven by apathy. The analysis of ocular movement is a promising tool to quantify apathy objectively.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/ocular-ping-pong-movement-to-disentangle-apathy-from-bradykinesia-in-parkinsons-disease/