RESPONSES TO DYNAMIC HEAD-AND-BODY TILTS ARE ENHANCED IN PARKINSONS-DISEASE

Background: Previous studies demonstrated that destabilizing responses to slow perturbations were enhanced in patients with Parkinson's dise ase (PD). Our objectives were to investigate the influence of PD on re sponses to faster whole head-and-body tilts in the standing position, and to establish whether any modification of tilt-evoked responses in PD patients was related to possible changes in the modulation of soleu s (SO) H-reflex. Methods: Ten PD patients and 10 age-matched normal su bjects assumed a standing position on an L-shaped tilting apparatus. T heir head and shoulders were firmly attached to the back support of th e apparatus, while their feet were fixated to the standing platform. W ith their vision occluded, the subject's whole head-and-body was sudde nly tilted forward to 20 degrees, at a peak head acceleration of 0.7g +/- 0.1g. Tilt-evoked responses were recorded from the lower limb musc les bilaterally. In addition, 40 H-reflexes were elicited in the SO mu scle at 30-190 ms intervals after the onset of head acceleration. The M response amplitude was kept within +/-15% of its control value. Resu lts: PD patients demonstrated an abnormally high responsiveness to who le head-and-body tilts in comparison with age-matched normal subjects. This was shown by the significantly larger proportion of PD patients manifesting responses in the SO, biceps femoris and vastus lateralis m uscles (p<0.05), as well as their significantly larger SO response are a (413%; p<0.01). In contrast, the amplitude of the SO H-reflex was si gnificantly increased by only 14% (p<0.05) in these patients, and only at 30-70 ms after head acceleration onset. Conclusions: The overexcit able tilt-evoked responses of PD patients could originate from a reduc ed ability to suppress responses when the body is supported. This enha nced excitability of tilt-evoked responses was probably not due to mot oneuronal hyperexcitability or decreased presynaptic inhibition of the group Ia terminals involved in the mainly monosynaptic H-reflex pathw ay. Thus, we hypothesize that the control of spinal interneurons invol ved in the tilt-evoked responses may be defective in PD.