Repetitive stepping in place identifies and measures freezing episodes in subjects with Parkinson's disease.

Freezing of gait (FOG) in Parkinson's disease (PD) is challenging to measure. We asked whether a repetitive stepping in place (SIP) task on force plates could identify freezing episodes (FEs) in PD subjects, self-classified as "freezers", using the validated FOG questionnaire (FOG-Q) and whether a computerized algorithm could provide automatic detection of FEs during SIP. Thirty PD subjects and nine age-matched controls completed the SIP task. PD subjects were assessed using the Unified Parkinson's Disease Rating motor Scale (UPDRS-III) and the FOG-Q. The identification of "freezers" using the SIP task correlated with the FOG-Q (r=0.80, P<0.001). The specificity and sensitivity of identifying freezers using the SIP task reached 93% and 87%. The number and duration of FEs detected by the algorithm correlated with visual inspection (r=0.97, r=0.998, P<0.001). Freezers had larger SIP asymmetry compared to controls (P=0.02) and non-freezers (P=0.03) as well as larger arhythmicity (P=0.003 and P<0.001, respectively). UPDRS subscores were higher in freezers compared to non-freezers (P<0.05). These results suggest that the SIP task is a useful tool to detect freezing in PD and is correlated with FOG-Q. SIP cycle asymmetry and stride time variability were worse in freezers, similar to that shown in FOG studies. Detection of the number and duration of FEs using a computerized algorithm correlated with independent visual inspection of records.