Postural reactions to soleus muscle vibration in Parkinson's disease: scaling deteriorates as disease progresses.

Previous research has shown that Parkinson's disease (PD) patients, especially those with postural instability, respond hyperactively to visual, vestibular, and neck proprioceptive sensory manipulation. To determine if this impairment of the sensory information scaling holds true for the lower leg proprioceptive system, we studied postural responses to mechanical vibration (which affects the muscle spindle Ia afferents) applied to the soleus muscles of PD subjects and healthy controls. Early-stage and advanced-stage PD patients as well as age-matched control subjects participated. Each group comprised 11 subjects. Nine pulses of 3-s long vibration were applied randomly to both soleus muscles while subjects kept their eyes closed. Postural responses to these stimuli were measured by static posturography. The effect of dopaminergic medication was established by testing patients in both ON and OFF treatment phases. There was no intergroup difference in the pattern or latencies of responses. However, the amplitudes were significantly larger in advanced PD patients; controls did not differ from early-stage PD patients. Dopaminergic medication had no significant effect on any of the measures. The scaling of postural reactions triggered by lower leg proprioception is disturbed in advanced PD. Neither afferent proprioceptive deficits nor inaccurate timing is involved. This study gives further evidence for the generalized impairment of the scaling of postural responses evoked whenever there is a sudden change of sensory conditions, as occurs with the progression of PD. Such impairment could play a significant role in the pathophysiology of postural instability and falls in PD patients.