A feasibility study of objective outcome measures used in clinical trials of freezing of gait

BACKGROUND: Freezing of gait (FOG) is notoriously difficult to quantify, which has led to the use of multiple markers as outcomes for clinical trials. The instrumented timed up and go (TUG) and the many parameters that can be derived from it are commonly used as objective markers of FOG severity in clinical trials; however, it is unknown if they represent actual FOG severity. OBJECTIVE: To determine the specificity and responsiveness of objective surrogate markers of FOG severity commonly utilized in FOG studies. METHODS: Study design: We compared the specificity and responsiveness of commonly used markers in FOG clinical trials. Markers compared included velocity, step/stride length, step/stride length variability, TUG, and turn duration. Data was collected in four conditions (ON and OFF dopaminergic drugs, with and without a dual task). Unified Parkinson’s Disease Rating Scale (UPDRS) was administered in the ON and OFF states. RESULTS: Thirty-three subjects were recruited (17 PD subjects without FOG (PD-control) and 16 subjects with PD and dopa-responsive FOG PD-FOG). The UPDRS motor scores were 24.9 for the PD-control group in the ON state, 24.8 for the FOG group in the ON state, and 42.4 for the FOG group in the OFF state. Significant mean differences between the ON and OFF conditions were observed with all surrogate markers (p < 0.01). However, only dual task turn duration and step variability showed trends toward significance when comparing PD-control and ON-FOG (p = 0.08). Test–retest reliability was high (ICC > 0.90) for all markers except standard deviations. Step length variability was the only marker to show an area under the ROC curve analysis > 0.70 comparing ON-FOG vs. PD-control. CONCLUSIONS: Multiple candidate surrogate markers for FOG severity showed responsiveness to levodopa challenge; however, most were not specific for FOG severity.