H reflex threshold in Parkinson's disease patients for different stimulus duration.

Electrical stimulus, with duration starting at 0.1 ms and gradually increased to 1.0 ms, was used for eliciting the H reflex in 14 normal subjects and 19 patients with Parkinson's disease (PD). In 71.1% of normal subjects and in 13.2% of PD patients the H reflex to M response threshold ratio (H/M TR) was <1 and the H reflex was obtained before the M response for all duration stimuli. For all stimulus durations a significant difference between the H/M TR in normal subjects and PD patients was found (t test 0.002-0.007). The duration effect was found to be highly significant-H/M TR for short stimulus duration was greater than for long stimulus durations (p<0.001). The optimal stimulus duration for evaluating H reflex behavior in PD patient was 0.2 ms. These very significant differences in behavior of the H reflex in PD patients could be used as another parameter in the assessment of extrapyramidal rigidity in PD patients.