H reflex behavior in Parkinson's disease patients. Effect of stimulus duration

Electrical stimulus, with duration starting at 0.1 ms and gradually increasing to 1.0 ms, was used for eliciting the H reflex in 21 normal subjects and 48 patients with Parkinson's disease (PD). In 19 normal subjects (90.5%), the threshold for sensory fibers was lower than for motor fibers, and the H reflex was obtained before the M response for all duration stimuli. In 19 PD patients (39.6%), with mild or moderate rigidity (according to the motor part of UPDRS), the threshold for the H reflex and M response was the same or the M response threshold was lower in at least one of the legs for short stimulus duration (0.1–0.2 ms). In 15 PD (31.2%) patients (most of them with severe rigidity), the threshold for M response was lower for all stimulus duration, and it was obtained before the H reflex. In 14 PD (29.2%) patients, the H reflex behavior was the same as in most normal subjects in one or both legs.

These very significant differences in the behavior of the H reflex in PD patients (Fisher exact test, P<0.0001) could possibly be explained by changes in agonist–antagonist inhibition, and could be used as another parameter in the clinical assessment of extrapyramidal rigidity in PD patients.