Protective stepping response in Parkinsonian patients and the effect of vibrotactile feedback

The objectives of this study were (1) to characterize protective stepping responses to unpredictable forward/backward postural perturbation in Parkinson's disease (PD) and (2) to assess whether vibrotactile cues of the impending fall improve the stepping response. Twenty mild PD patients, 7 advanced PD patients, and 17 age-matched controls stood on a platform moving unpredictably forward and backward, requiring a protective step to maintain balance. Direction-coded vibrotactile cues, triggered by leg tilt, were provided to prompt step generation. All subjects showed quicker reaction time, shorter steps, and smaller total trunk displacement when stepping backward than when stepping forward. Advanced PD patients took shorter, slower, and an increased number of protective steps. The only abnormality observed in mild PD patients was slightly slower backward steps. Vibrotactile feedback reduced the amount of trunk displacement observed before taking a protective step but did not improve any abnormality in PD patients. Early PD patients had near-normal protective stepping responses to unpredictable perturbations, but advanced patients made slow and short steps both forward and backward. Given that latencies were preserved even in unpredictable conditions, step slowness and hypometria are the primary abnormalities of the stepping response in PD. As voluntary locomotor stepping in PD is reported to improve with sensory feedback, the lack of such improvement in our study implies that additional sensory cues cannot help automatic reflex-like stepping reactions in PD patients.