Responses to dynamic head-and-body tilts are enhanced in Parkinson's disease.
BACKGROUND
Previous studies demonstrated that destabilizing responses to slow perturbations were enhanced in patients with Parkinson's disease (PD). Our objectives were to investigate the influence of PD on responses 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 soleus (SO) H-reflex.


METHODS
Ten PD patients and 10 age-matched normal subjects assumed a standing position on an L-shaped tilting apparatus. Their head and shoulders were firmly attached to the back support of the apparatus, while their feet were fixated to the standing platform. With their vision occluded, the subjects's whole head-and-body was suddenly tilted forward to 20 degrees, at a peak head acceleration of 0.7 g +/- 0.1 g. Tilt-evoked responses were recorded from the lower limb muscles bilaterally. In addition, 40 H-reflexes were elicited in the SO muscle at 30-190 ms intervals after the onset of head acceleration. The M response amplitude was kept within +/- 15% of its control value.


RESULTS
PD patients demonstrated an abnormally high responsiveness to whole 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 muscles (p < 0.05), as well as their significantly larger SO response area (413%; p < 0.01). In contrast, the amplitude of the SO H-reflex was significantly increased by only 14% (p < 0.05) in these patients, and only at 30-70 ms after head acceleration onset.


CONCLUSIONS
The overexcitable tilt-evoked responses of PD patients could originate from a reduced ability to suppress responses when the body is supported. This enhanced excitability of tilt-evoked responses was probably not due to motoneuronal hyperexcitability or decreased presynaptic inhibition of the group Ia terminals involved in the mainly monosynaptic H-reflex pathway. Thus, we hypothesize that the control of spinal interneurons involved in the tilt-evoked responses may be defective in PD.