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Tremor is generally defined as “rhythmic involuntary oscillatory movement of a body part.”1 The amplitude of tremor is not critical to the definition. Tremor is most commonly seen in the upper extremities, but tremor of the lower extremities, head, trunk, lips, chin, tongue, and vocal cords can also occur. Functionally, rest tremor is present when the affected part of the body is in repose and fully supported against gravity, requiring no active muscle contraction.2 Tremor seen in parkinsonism is typically rest tremor and often assumes a “pill-rolling quality.” This tremor typically disappears with onset of movement but, once partial stability is attained in the new position, the tremor returns (reemergent tremor). When tremor occurs with maintained posture such as holding arms perpendicular to the body, it is called postural tremor. Postural tremor is possibly the most common form of tremor and can be seen in physiological tremor, essential tremor, cerebellar postural tremor, and others. When tremor occurs with movement from one point to another, it is referred to as kinetic tremor. Kinetic tremor that appears near termination of movement is known as terminal kinetic tremor (intention tremor). Kinetic tremor present during specific tasks but absent with other activities involving the same limb is referred to as task-specific tremor (TST). Examples include primary writing tremor, vocal tremor, and orthostatic tremor. Tremor may arise from several anatomic locations within the central nervous system (CNS) or peripheral nervous system, including the cerebral cortex, white matter, basal ganglia, thalamus, midbrain, cerebellum, and peripheral nerves. In physiological tremor, no known lesion is present. Intention tremor is usually caused by damage to the deep cerebellar nuclei (dentate and globose-emboliform) or their efferent pathway to the contralateral ventrolateral thalamus. Alterations in neurotransmitters such as dopamine deficiency (as seen in Parkinson's disease [PD]), excess epinephrine as seen in anxiety, decreased level of substrate such as glucose (hypoglycemia), reduced level of electrolyte (hyponatremia), and excess level of a hormone such as thyroxin (thyrotoxicosis) can induce tremor attesting to the presence of biochemical nonspecificity.

We will begin the discussion with physiological tremor that represents a normal variant as a lead in to uncommon forms of tremor.

Physiological tremor is present in normal people and is usually invisible to the naked eye. It is thought to reflect a mechanical vibration of body parts. It is symptomatic only during activities that require extreme precision. Physiological tremor has two distinct oscillations. The 8–12-Hz level3 is very resistant to frequency change. Internal loads,4,5 elastic loads,5 limb cooling,6 and torque loads7 produce less than a 1–2-Hz frequency change. This frequency variability and the intense synchronous motor unit modulation suggest that the neuronal oscillator is responsible for the 8–12-Hz tremor.6 It occurs during the maintenance of study limb postures and has a low amplitude. The functional significance of this tremor is not known. The mechanical reflex component is the larger of the two distinct oscillations of ...

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