Spinal and supraspinal stretch responses of postural muscles in early Parkinsonian patients

Early Parkinson's disease (PD), in which the motor disorder is mostly unilateral, allows to investigate the presence of side-related pathophysiological changes in the responses to balance perturbations prior to any confounding effect of therapy. These patients offer the possibility of answering the question whether the initial abnormalities affect spinal circuits or supra-spinal sensori-motor loops. Toe-down rotation of a supporting platform evokes in standing subjects a double-burst medium-latency response (MLR) in the stretched tibialis anterior (TA) muscle. The former burst (MLR1) is fed by group II spindle fibres through a spinal circuit, the latter (MLR2) travels through supraspinal pathways. This perturbation was used to separately test both reflex pathways in both the unaffected and affected side. Ten patients with unilateral PD (7 of which de-novo) and 10 normal subjects (NS) were recruited. 1. Body sway and centre of feet pressure (CFP) were recorded during quiet standing; 2. Rotations of the supporting platform were administered with both legs on the platform during free stance (Control) and 3. While holding onto a stable rail (Holding); 4. Separate perturbations to either leg (Leg-on or Leg-off) were also delivered to avoid the effect of concurrent ipsi- and contralateral inputs on MLR. The EMG of TA and Sol muscles were recorded bilaterally. 1. Position and sway of CFP were similar in NS and PD. 2. Under Control condition, there were no differences in area of TA MLR1 and MLR2 of both limbs between NS and PD. 3. During Holding, both bursts were reduced in amplitude in both NS and PD, but less so for the MLR1 on the PD affected side. 4. During single-leg perturbation, both bursts were reduced in amplitude in both NS and PD, both ipsi- and contralateral to the perturbation; however, in PD, MLR2 was much less reduced in the unperturbed affected side Leg-off. The differences in the responses to stretch between NS and early PD are not accounted for by changes in postural attitude, or by different amplitudes of the Control responses. The asymmetrical reduction of the MLR1 by Holding in PD indicates mostly unilateral impairment of the descending pathways modulatory to spinal group II circuits. Single-leg perturbation unveils a larger excitability of the supraspinal loop mediating the MLR2 on the affected side. These changes are early markers of basal ganglia malfunctions, and are related to both their descending effect on the spinal cord and their ascending influence onto the cortex.