Clinical Vignette

A 31-year-old woman had the spontaneous onset of a relatively mild horizontal (“no-no”) head tremor. She had no family history of neurologic disorders. This tremor was initially inconsequential but it gradually increased in severity. Turning her head to the right seemed to increase the tremor, whereas turning her head to the left decreased the tremor. Mild finger pressure on her left chin dampened the tremor and the involuntary movements of her head. Beta blockers, primidone, and alcohol had little effect. Eighteen months later, driving had become difficult because her head tended to turn involuntarily. Attempts to hold her head in the neutral position as well as stress markedly increased the tremor, leading to neck discomfort. Over time, the involuntary movements were continuously present. Neurologic examination demonstrated right sternocleidomastoid muscle hypertrophy. A trial of anticholinergic medication increased the tremor and precipitated a psychotic reaction. Eventually, the patient was treated with botulinum toxin in the right sternocleidomastoid muscle and some of the left paracervical muscles. This treatment controlled the involuntary movements and dampened the involuntary tremor.

Physiologic Tremor

This is normally present in healthy individuals and is related to a number of factors. Physiologic tremor typically has an 8- to 12-Hz frequency in young adults and decreases to 6–7 Hz in the senior population (Table 35-1).

Table 35-1 Approximate Frequencies of Tremors

Frequency Range, Hz	Tremor Type
1–5	Holmes cerebellar tremor
2–10	Multiple sclerosis
2–12	Drug-induced tremor
2–12	Neuropathic tremor
3–10	Parkinsonian tremor
3–10	Task- and position-specific tremor
3–12	Dystonic tremor
4–10	Psychogenic tremor
4–8	Essential tremor
7–12	Physiologic and enhanced physiologic tremor
16–25	Orthostatic tremor

Adapted from Bain PG. The management of tremor. J Neurol Neurosurg Psychiatry. 2002;72(Suppl. 1):i3-i16.

Peripheral components include muscle mass and stiffness, long and short loop reflexes, grouped motor neuron firing rates, and the inertia of muscles and other structures. The primary component of a physiologic tremor, the central generator, contributes weak 8- to 12-Hz low-amplitude movement not affected by inertial loading or physical manipulation. Other components, including the heartbeat (cardioballistics), may also contribute. This normal tremor is best seen by holding an arm straight in front of the body and placing a sheet of paper across the outstretched fingers.

The most common tremor is referred to as an enhanced physiologic tremor (EPT). It may result when motor units become discharged in groups. Typically this type of tremor becomes enhanced during muscular fatigue, fear, excitement, or emotional distress. It may occur in various medical conditions, for example, thyrotoxicosis, pheochromocytoma, catecholamine intake, methylxanthine use, drug withdrawal, and alcohol intoxication. Beta receptor agonists enhance physiologic tremor, and β-receptor blockade or β₂ receptor antagonists effectively decrease it. Early on, essential tremors (ETs) are difficult to separate from exaggerated physiologic tremors.

Pathophysiology

The etiology of many tremors is unknown. Although Parkinson disease (PD) lesions predominate in the substantia nigra, experimental animal lesions within the substantia nigra do not cause tremor, and not all patients with lesions at this level have a tremor. Moreover, a tremor develops in only half of patients poisoned with the analog of meperidine (MPTP) that preferentially destroys part of the substantia nigra and classically leads to a mimic of PD. However, even with MPTP, the tremors are not the typical resting pill rolling ones of PD tremor; these have more of an action or postural component. In contrast, ventromedial tegmentum lesions made in a monkey midbrain do produce a resting-type tremor. Pathologic tremors such as ET, dystonic tremor, and the PD tremors are thought to have multiple central generators, resulting in variable frequencies of approximately 1–26 Hz (see Table 35-1).

Pathologic Tremor

The most useful classification of pathologic tremors is based on their clinical features, especially anatomic distribution (proximal or distal, body part involved), symmetry, and the conditions that best activate them (Table 35-2). Rest tremors occur when a body part is completely supported against gravity (Fig. 35-1). Action tremors occurring during voluntary muscular contraction are further classified as (1) postural (occur in a body part maintained in position against gravity), (2) kinetic (occur during voluntary movement), or (3) isometric (occur within a muscle contracting against a stationary object).

Table 35-2 Classification of Tremors

Type of Tremor	Clinical Features	Common Examples
Postural	A posture is maintained against gravity	Physiologic tremor Essential tremor Drug-induced tremor

Kinetic With voluntary movements Isometric With voluntary muscle contraction against a rigid, stationary object Orthostatic tremor Orthostatic tremor Tremor of lower limbs on standing and remits on walking or sitting Dystonic tremor Tremor in body part affected by dystonia Spasmodic torticollis Resting tremor Limb fully supported against gravity; improves with voluntary movement Parkinson disease Psychogenic tremor Acute onset, inconsistent, fatigues, decreases amplitude with distraction Asterixis Arrhythmic lapses of sustained postures Toxic and metabolic encephalopathies