Tremors

Published on 03/03/2015 by admin

Filed under Neurology

Last modified 03/03/2015

Print this page

rate 1 star rate 2 star rate 3 star rate 4 star rate 5 star
Your rating: none, Average: 0 (0 votes)

This article have been viewed 1463 times

35 Tremors

Tremors are involuntary, rhythmic, and stereotyped oscillatory movements of a body part. These are the most prevalent movement disorders and are usually distinguishable from other abnormal involuntary movements by their rhythmic quality, with concomitant involvement of agonist and antagonist muscle groups. They result from alternating or irregularly synchronous contractions of reciprocally innervated skeletal muscles.

The precise pathogenesis for a tremor is unclear. With neurophysiologic testing, reciprocal bursts of electromyographic (EMG) activity occurring in agonist and antagonist muscles are separated by relative silence. Simultaneous EMG bursts in agonist–antagonist muscle pairs are not characteristic of tremors; the recorded pattern of any tremor can vary over a short period (i.e., co-contraction of the muscles, alternation of contraction, or contraction of the antigravity muscles alone). More complex relations between the agonist and antagonist muscles also occur. Hence, no universal method exists for definitively rating, measuring, or classifying tremors. Clinical examination is still the most important step in tremor evaluation.

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.

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

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

Essential Tremor

This tremor is neither “essential” (an inherent characteristic of the individual) nor “benign.” It is an acquired tremor that usually worsens with age and may eventually significantly interfere with normal activities. This type of action tremor has a lower frequency (4–8 Hz) than do physiologic tremors (7–12 Hz). It often occurs in isolation unrelated to any other neurologic disability. Approximately 50% of these patients have a positive family history, usually involving an autosomal dominant trait with virtually complete penetrance. These are referred to as familial or hereditary tremor. If the ET occurs late in life, it may be called a senile tremor. Although the familial forms tend to begin earlier, they rarely occur during infancy or after the sixth decade.

ET is 20 times more frequent (prevalence, 0.2–33%) than PD. Many adult patients with ET tremors initially fear they have PD, and although classically it has not been thought that ET is an indication that PD will develop, more recent evidence demonstrates a small link between ET and PD. Similarly a postural tremor may appear years before the onset of other extrapyramidal symptoms of PD. Common genes or similar pathologic findings may underlie the development of both ET and PT and later PD. Functional neuroimaging studies demonstrate a dopaminergic deficit in some ET patients. Additionally, autopsy studies support such a relationship as some ET individuals have classic Lewy’s body pathology in their brains similar to that present in PD.

Buy Membership for Neurology Category to continue reading. Learn more here