Tremor, the most common movement disorder, is defined as a rhythmic, oscillatory movement of a body part produced by alternating or synchronous contractions of agonist and antagonist muscles. It ranges from a normal, barely noticeable, physiologic phenomenon to a severe, disabling movement disorder. Tremors can be classified according to their phenomenology, distribution, frequency, amplitude or etiology (Deuschl et al., 1998). Phenomenologically, tremors are subdivided into two major categories: rest tremors and action tremors. Rest tremors occur when the body part is fully supported against gravity and not actively contracting. In contrast, action tremors manifest during voluntary muscle contraction on an antigravity posture (postural tremor) or a goaldirected movement (kinetic tremor) (Table 35.1). This phenomenologic classification is far from ideal since there are many overlapping features among different tremors, but it remains the most widely accepted classification.

Rest tremor is present predominantly in Parkinson's disease (PD). It may also occur in other conditions such as different forms of parkinsonism, severe essential tremor (ET) and midbrain lesions. Postural tremors are typical of physiologic tremor, enhanced physiologic tremor, and ET. Task- or position-specific tremors are action tremors that occur only during specific motor activities, such as writing (‘primary writing tremor’) or maintaining at a certain posture. Kinetic tremors exist in cerebellar or midbrain disorders. Isometric tremor is seen during a voluntary isometric contraction, such as making a tight fist or contracting abdominal muscles. Tremors associated with dystonia, myoclonus, tardive dyskinesia, and other movement disorders may exhibit mixed phenomenology. Other disorders that produce rhythmic, but not necessarily oscillatory movements include segmental myoclonus, myorhythmia, asterixis, fasciculations, clonus, epilepsia partialis continua, shivering, head bobbing, and titubation. In this chapter we will first review the current notions of the tremor pathophysiology and then discuss the clinical features and treatment of the different types of tremors.

Pathophysiologic mechanisms of tremors

The broad clinical spectra of tremors suggest that different pathophysiologic mechanisms underlie various forms of tremors. Based on a large body of evidence from experimental and clinical physiologic studies, tremors originate from two types of mechanisms: (i) central and (ii) peripheral. The central oscillators consist of neuronal networks with auto-rhythmic properties and spontaneous bursting propagated through central nervous system (CNS) motor pathways. The peripheral components of tremors are influenced by mechanical characteristics of the affected body parts (muscles, tendons, and joints) and sensorimotor reflex mechanisms (Hallett, 1998).