Tremor is defined as an involuntary rhythmic oscillation of ≥1 body parts, mediated by alternating contractions of reciprocally acting muscles.[1] It commonly affects the upper extremities but can also affect the head, chin, voice, or legs. Tremor, if severe, may impair activities of daily living or occupation, and may lead to embarrassment and social withdrawal.
Its rhythmicity differentiates it from other involuntary movements such as myoclonus, clonus, chorea, dystonia, and tics, although patients with these other conditions may complain of "shaking" or "tremulousness". For appropriate diagnosis and treatment, it is critical to determine the positional properties that make the tremor most noticeable. Most tremors can be classified as "rest" or "action".
- Rest tremors occur when the body region is relaxed (i.e., not actively moving, contracting, sustaining a position, or opposing gravity). Examples include tremor in a hand relaxed by the side when walking or resting in a lap when sitting.
- Action tremors occur during voluntary muscle contraction. They are further divided into postural, kinetic, isometric, or task-specific subtypes.
- Postural tremors occur during maintenance of a posture, usually against gravity (e.g., holding up a newspaper).
- Kinetic tremors occur during active movement. A subtype of kinetic tremor, intention tremor, is seen with a goal-directed movement (e.g., finger-to-nose testing). Action tremors are often both postural and kinetic.
- Isometric and task-specific subtypes are uncommon. Isometric tremors occur during muscle contractions without movement (e.g., fist-clenching or standing). Task-specific tremors occur with a specific activity (e.g., writing or playing an instrument).
