H-reflex recovery curves differentiate essential tremor, Parkinson's disease, and the combination of essential tremor and Parkinson's disease.

The purpose of this study was to examine H-reflex parameters among the pathophysiologic conditions of essential tremor (ET), Parkinson's disease (PD), combined essential tremor with Parkinson's disease (ETPD), and a control group. H-reflex latencies, amplitude of maximum H-reflex to maximum M-response ratio (H:M), vibration H-reflex to control H-reflex (Hv:Hc), and H-reflex recovery curves (HRRCs) were recorded and compared between a control group and patient groups with ET, early-stage PD, and with ETPD. No statistically or clinically significant differences were found between the patient groups and the control group for latency, H:M ratio, or Hv:Hc ratio. Significantly greater ratio values were observed for the PD group over the other groups for the HRRC tests at each interstimulus interval between 200 and 300 msec (p < 0.05), but values were not different between PD and ETPD patients for intervals between 350 and 1,000 msec. Patients with ET, PD, and ETPD apparently have different underlying pathologies. HRRC tests do not distinguish ET patients from normal, but differentiates specifically between PD and ETPD, and normal individuals. HRRC testing may be a useful method for evaluating pathologies between ET, PD, and ETPD patients.