Tremor is an involuntary, rhythmic, oscillatory movement of a body part. Tremor results from contractions of agonist and antagonist muscles entrained by a signal pattern originating from an oscillator in the central nervous system. Resting tremor occurs in a body part that is not activated and completely supported against gravity. Action tremor refers to tremor occurring with voluntary muscle contraction and is subdivided into postural tremor, isometric tremor and kinetic tremor. The latter is further subdivided into simple kinetic, intention tremor, and task specific tremor, such as writing tremor.
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The etiology of tremor is diverse, and includes essential tremor (with 50% having a positive FH but no single gene identified); as part of genetic disorders eg Wilson’s disease; degenerative such as Parkinson’s disease, Metabolic diseases (such as thyroid-, parathyroid-, liver disease and hypoglycemia); peripheral neuropathies (eg associated with Charcot-Marie-Tooth; drug-induced (including narcoleptics, tricyclics, lithium, cocaine, alcohol, adrenaline, bronchodilators, theophylline, caffeine, steroids, valproate, amiodarone, thyroid hormones, vincristine); toxins (heavy metals) and psychogenic/functional disorders.
The original consensus criteria proposed by MDS from 1998 were recently revised to better overcome inconsistencies, particularly with regard to essential tremor (ET), tremor associated with dystonia or other additional neurological abnormalities, and focal tremors. The new consensus proposes classification has two main axes: clinical features (Axis 1) and etiology (Axis 2).
Clinically, a syndrome consisting only of tremor is called an isolated tremor syndrome. On the other hand, a syndrome of consisting of tremor and other systemic or neurological signs is called a combined tremor syndrome.
Etiologically, tremor can be classified into physiological tremor, enhanced physiological tremor, essential tremor syndromes (including classical ET), dystonic tremor, parkinsonian tremor, cerebellar tremor (often associated with multiple sclerosis), Holmes’ tremor (a.k.a. rubral tremor), palatal tremor, neuropathic tremor, toxic or drug-induced tremor, and psychogenic tremor.
Contributed by Leo Verhagen, MD, PhD
Associate Professor of Neurological Sciences
Rush University Medical Center
Chicago, IL, USA
2019 Updates Contributed by:
Susanne A. Schneider, MD
LMU University Hospital Muenchen
Susan Fox, MRCP (UK), PhD
Professor of Neurology
University of Toronto, Toronto Western Hospital
Toronto, ON, Canada