H reflex behavior in Parkinson's disease patients and patients with extrapyramidal and pyramidal signs combined.

The H reflex was elicited in 21 normal subjects, 48 patients with Parkinson's disease (PD) and 22 patients with pyramidal and extrapyramidal signs combined (PESC). In most normal subjects (90.5%), in 29.2% of PD patients and 54.5% of patient with PESC 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 both legs. In 9.5% of normal subjects, 39.6% of PD patients with mild and moderate rigidity (according to the motor part of UPDRS) and 31.8% of patients with PESC, 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 31.2% of PD patients (most of them with severe rigidity) and 13.7% of patients with PESC, the threshold for M response was lower for all stimulus duration in at least one of the legs, and it was obtained before H reflex. These very significant differences in behavior of the H reflex in PD patients (Fisher exact test, p<0.0001) that almost disappear in patients with PESC, could be possibly explained by changes in agonist-antagonist inhibition.