The pathophysiology of tremor

Tremor is defined as rhythmic oscillatory activity of body parts. Four phys iological basic mechanisms for such oscillatory activity have been describe d: mechanical oscillations; oscillations based on reflexes; oscillations du e to central neuronal pacemakers; and oscillations because of disturbed fee dforward or feedback loops. New methodological approaches with animal model s, positron emission tomography, and mathematical analysis of electromyogra phic and electroencephalographic signals have provided new insights into th e mechanisms underlying specific forms of tremor. Physiological tremor is d ue to mechanical and central components. Psychogenic tremor is considered t o depend on a clonus mechanism and is thus believed to be mediated by refle x mechanisms. Symptomatic palatal tremor is most likely due to rhythmic act ivity of the inferior olive, and there is much evidence that essential trem or is also generated within the olivocerebellar circuits. Orthostatic tremo r is likely to originate in hitherto unidentified brainstem nuclei. Rest tr emor of Parkinson's disease is probably generated in the basal ganglia loop , and dystonic tremor may also originate within the basal ganglia. Cerebell ar tremor is at least in part caused by a disturbance of the cerebellar fee dforward control of voluntary movements, and Holmes' tremor is due to the c ombination of the mechanisms producing parkinsonian and cerebellar tremor. Neuropathic tremor is believed to be caused by abnormally functioning refle x pathways and a wide variety of causes underlies toxic and drug-induced tr emors. The understanding of the pathophysiology of tremor has made signific ant progress but many hypotheses are not yet based on sufficient data. Mode rn neurology needs to develop and test such hypotheses, because this is the only way to develop rational medical and surgical therapies. (C) 2001 John Wiley & Sons, inc.