Objective: To evaluate the reliability and validity of passively monitored arm movements as an approximate measure of bradykinesia.

Background: Clinical assessment of bradykinesia in Parkinson’s disease (PD) using the Movement Disorders Society Unified Parkinson’s Disease Rating Scale part 3 (MDS-UPDRS-III) provides episodic, subjective assessments on an ordinal scale, prone to intra- and inter-rater variability. Wearable sensors enable objective monitoring of passive arm movement in a free-living environment, offering a possible complementary method for evaluating bradykinesia. Research on the reliability and validity of passively measured bradykinesia using wearable sensors is limited, especially in a free-living environment.

Method: Wrist-worn inertial sensors (Axivity AX6) recorded arm mobility (degrees/second), rotation (degrees), and activity (% time moving). Test-retest reliability was assessed using intra-class correlation coefficients (ICC(2,k)) over increasing number of subsequent days (k=1-5). Validity was evaluated by the parameters’ ability to differentiate between people with PD (PwPD) and controls, and by association with MDS-UPDRS-III bradykinesia items of the monitored arm (items 3.4-3.6).

Results: Data from 273 PwPD and 47 control participants from the ProPark cohort were analyzed. ICC was excellent when averaging over one day of measurement for mobility (ICC(2,1)=0.924 (95% CI=0.919-0.945)), two days for rotation (ICC(2,2)=0.950 (95% CI=0.937-0.961)), and five days for activity (ICC(2,5)=0.933 (95% CI=0.919-0.945)). Mobility and rotation best distinguished PwPD from controls (AUC=0.945 (95% CI=0.915-0.975);  AUC=0.942 (95% CI=0.911-0.973), respectively), while activity performed worse (AUC=0.724, 95% CI=0.653–0.724). In PwPD, worse MDS-UPDRS-III bradykinesia subscores were associated with lower mobility (β=-1.318, SE = 0.256, p<0.001) and rotation (β=-0.679, SE = 0.143, p<0.001).

Conclusion: Mobility and rotation are highly reliable, differentiate well between PwPD and controls, and are associated with the MDS-UPDRS bradykinesia items of the arm on which the sensor was worn. This provides support for the reliability and validity of bradykinesia assessment based on passive arm movement. Further studies should evaluate if these measures are sensitive for change over time, and could be used as a prodromal marker.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/evaluating-bradykinesia-in-parkinsons-disease-added-value-of-monitoring-passive-arm-movement-with-a-wrist-worn-wearable-sensor/