Objective: To determine whether motor impairment and variability extend from isolated REM sleep behaviour disorder (iRBD) to probable RBD (pRBD), and whether instability can be detected using remote digital testing.
Background: PREDICT-PD is an online cohort identifying individuals at risk of Parkinson’s disease (PD). Isolated REM sleep behaviour disorder (iRBD) is a prodromal marker of PD associated with motor impairment detectable using remote finger tapping. The Bradykinesia Akinesia Incoordination (BRAIN) test assesses PD motor dysfunction.¹ However, mean decline may not capture early progression.² Whether motor variability is increased in PSG-confirmed iRBD (PSG-iRBD) and probable RBD (pRBD; defined using the single-item RBD questionnaire³), by olfactory status, remains unclear.
Method: This longitudinal study compared BRAIN test performance in PSG-iRBD, pRBD with hyposmia, normosmic pRBD, and controls. Participants completed the BRAIN and Distal Finger Tapping (DFT) test remotely via the PREDICT-PD website, with four BRAIN and two DFT annual assessments. Outcomes included kinesia score (KS), akinesia time (AT), and incoordination score (IS). Cases were propensity score-matched to controls (1:5) by age and sex. Variability across visits was quantified using mean square successive differences (MSSD) and heteroscedastic mixed-effects models assessing whether variance differed across visits. Models were compared using likelihood ratio tests (LRT).
Results: Compared with controls, PSG-iRBD (n=38 vs 249) showed greater variability across visits in BRAIN measures using heteroscedastic modelling (IS LRT=30.72, p<0.001; AT LRT=12.36, p=0.006; KS LRT=9.90, p=0.019), despite no differences detected by MSSD. Hyposmic pRBD (n=97 vs 228) also showed significant variability differences across visits for all BRAIN measures (IS LRT=101.90; AT LRT=29.39; KS LRT=22.22; all p<0.001). Normosmic pRBD (n=637 vs 2030) showed greater within-participant variability in IS and AT (MSSD p=0.013, 0.017), with modelling indicating variability differences across visits for all measures (all LRT p<0.001). No variability differences were observed using the DFT (all MSSD p≥0.085; all LRT p≥0.091).
Conclusion: Motor instability, rather than consistent decline, is detectable in prodromal PD using remote finger tapping. Variability is present in PSG-iRBD and pRBD, particularly in hyposmic individuals, supporting a continuum of motor dysfunction.
References: References:
1 Noyce AJ, Nagy A, Acharya S, Hadavi S, Bestwick JP, Fearnley J, Lees AJ, Giovannoni G. Bradykinesia-akinesia incoordination test: validating an online keyboard test of upper limb function. PLoS One. 2014 Apr 29;9(4):e96260.
2 H. Chohan, T. Tocino, L. Pérez-Carbonell, G. Leschziner, J. Bestwick, A. Schrag, A. Noyce, C. Simonet. Longitudinal Analysis of BRAIN test in isolated REM sleep behaviour disorder with and without hyposmia. [abstract]. Mov Disord. 2025; 40 (suppl 1).
3 Postuma RB, Arnulf I, Hogl B, Iranzo A, Miyamoto T, Dauvilliers Y, Oertel W, Ju YE, Puligheddu M, Jennum P, Pelletier A. A single‐question screen for rapid eye movement sleep behavior disorder: a multicenter validation study. Movement Disorders. 2012 Jun;27(7):913-6.
To cite this abstract in AMA style:
H. Chohan, S. Water, L. Pérez-Carbonell, G. Leschziner, T. Periñán, B. Huxford, S. Eriksson, J. Bestwick, A. Schrag, A. Noyce, C. Simonet. Motor variability in isolated and probable REM sleep behaviour disorder using remote finger tapping [abstract]. Mov Disord. 2026; 41 (suppl 1). https://www.mdsabstracts.org/abstract/motor-variability-in-isolated-and-probable-rem-sleep-behaviour-disorder-using-remote-finger-tapping/. Accessed October 1, 2026.« Back to 2026 International Congress
MDS Abstracts - https://www.mdsabstracts.org/abstract/motor-variability-in-isolated-and-probable-rem-sleep-behaviour-disorder-using-remote-finger-tapping/