Reciprocal inhibition in essential tremor and dystonic tremor

Abstract

Background: Reciprocal inhibition (RI) is that the antagonist muscles do not behave passively but are actively inhibited by central mechanism while on contraction of agonist muscles (Sherringtos`s principle). RI is mediated by Ia afferents of the antagonist motor neurons, which disynaptically inhibit alpha motor neurons. So, the antagonist muscles are actively inhibited via Ia inhibitory interneurons by agonist activation. Excessive and inappropriate co-contraction of antagonist muscles is characteristic of both voluntary and involuntary movements in patients with dystonia. But, Patients with essential tremor (ET) do not have excessive and inappropriate co-contraction of antagonist muscles. So, we build hypothesis that RI may be generated normally in patients with ET , but patients with dystonic tremor (DT) may not be. In this background, we examined the Ia inhibition from extensor to flexor muscles of the forearm in patients with classic ET, ET variant( or isolated head tremor), and DT, and compared those results of RI. Methods: The subjects of the study were 12 patients with classic ET, 12 patients with DT and 5 patients with ET variant. The RI between the wrist flexors and extensors was obtained at one time. The FCR H-reflex was tested by electrical stimulation of the median nerve in antecubital fossa. A stimulation of the radial nerve in spiral groove, at the time of 1ms before stimulation of median nerve, was used to evoke disynpatic inhibition of FCR motor neurons. We compared the ratio of conditioned RI and unconditioned RI in patients with ET, ET variant, DT. Results: Significant difference of RI was observed between classic ET and DT group (P<0.001). In DT group, a less pronounced inhibition was found at first period of RI, which means that disynaptic inhibition via Ia afferents of the antagonist motor neurons is inappropriate. In ET group, the inhibition was significant compared with DT group. There is much intersubject variation in patients with ET variant( or isolated head tremor). Conclusions: Our data support the previous findings that a less inhibition was observed in patients with dystonia. Also, we found that there is a relatively pronounced inhibition in patients with classic ET than petients with DT. Interestingly, ET variant ( or isolated head tremor) have intersubject variation. We therefore subdivided this group into two physiologically defined groups- one group with relatively pronounced inhibition shows tendency to ET, the other group with less pronounced inhibition shows tendency to DT. We suggested RI may be helpful in distinguishing isolated head tremor and DT.