Essential tremor (EI) is the most frequent hyperkinetic movement disorder. Its age-adjusted prevalence is 2 to 5 percent y and, as a lifelong illness, EI may affect any age group. It has a bimodal distribution of age at onset, with two peaks, adolescence and the fourth to fifth decade of life. In many families, EI is inherited in an autosomally dominant pattern, and in these patients, the term familial tremor can be used interchangeably with EI. Although central mechanisms are thought to be causative, no specific structural abnormality has been noted in the brains of patients with EI. Because eT can be exacerbated by hyperthyroidism and by caffeine and noradrenergic drugs, alterations in the central norepinephrine chemistry have been suggested.
In contrast to the resting tremor characteristic of PD, EI is a postural tremor, which is characteristically present during maintenance of a position. This tremor is typically an alternating flexion-extension movement, whereas supination-pronation oscillation is more characteristic of Parkinson's tremor. EI is also faster (7 to 10 Hz) than the PD tremor (4 to 7 Hz), and when fully developed, it usually involves the head, neck, and voice. Tremor often interferes
_TABLE 34-6 -- CLASSIFICATION AND DIFFERENTIAL DIAGNOSIS OF TREMORS_
1. Rest tremors a. Parkinson's disease b. Other parkinsonian syndromes (less commonly)
e. Midbrain ("rubral") tremor: Rest < postural < kinetic d. Wilson's disease (aliso acquired hepatocerebral degeneration)
e. Essential tremor--only if severe: Rest < < postural and action
2. Postural and action ("terminal") tremors a. Physiological tremor b. Exaggerated physiological tremor (these factors can also aggravate other forms of tremor) i. Stress, fatigue, anxiety, emotion ii. Endocrine: hypoglycemia, thyrotoxicosis, pheochromocytoma, adrenocorticosteroids iii. Drugs and toxins: Beta agonists, dopamine agonists, amphetamines, lithium, tricyclic antidepressants, neuroleptics, theophylline, caffeine, valproic acid, alcohol withdrawal, mercury ("Hatter's shakes"), lead, arsenic, others c. Essential tremor (familial or sporadic) ?subtypes d. Primary writing tremor e. With other CNS disorders i. Parkinson's disease ii. Other akinetic-rigid syndromes iii. Idiopathic dystonia, including focal dystonias f With penpherai neuropathy i. Charcot-Marie-Tooth syndrome (controversial whether to call this the Roussy-Levy syndrome)
ii. Variety of other peripheral neuropathies (especially dysgammaglobulinemia) g. Cerebellar tremor
3. Kinetic (intention) tremor
Disease of cerebellar "outflow" (dentate nucleus and superior cerebellar peduncle): Multiple sclerosis, trauma, tumor, vascular disease, Wilson's acquired hepatocerebral degeneration, drugs, toxins (e.g., mercury), others
4. Miscellaneous rhythmical movement disorders a. Psychogenic tremor b. Orthostatic tremor e. Rhythmical movements in dystonia (dystonic tremor)
d. Rhythmical myoclonus (segmental myoclonus--e.g., palatal or branchial myoclonus, spinal myoclonus, limb myorhythmia)
e. Oscillatory myoclonus f. Asterixis g. Clonus h. Epilepsia partialis continua i. Hereditary chin quivering j. Spasmus nutans k. Head bobbing with 3rd ventricular cysts l. Nystagmus
Modified from Weiner WJ, Lang AE: Movement Disorders: A comprehensive survey. New York, Futura, 1989.
with handwriting, drawing, holding a spoon, using a drinking cup, and manipulating tools. 74I The vocal cords may be affected, resulting in a wavering voice. Although ET usually occurs in isolation, it can be associated with various other neurological conditions including dystonia, parkinsonism, and certain inherited peripheral neuropathies (e.g., Charcot-Marie-Tooth disease). The differential diagnosis includes normal physiological tremor and the tremors associated with anxiety, thyrotoxicosis, and alcohol withdrawal as well as drug-induced tremors resulting from bronchodilator use, corticosteroids, various CNS stimulants, lithium, and sodium valproate. ET diminishes with rest, ethanol, beta-noradrenergic blockers (usual dose, propranolol 80 to 240 mg/day), primidone (25 to 750 mg/day), and benzodiazepines. In addition, some patients may benefit from gabapentin
and injections of botulinum toxin directly into the contracting muscles. y Thalamotomy or high-frequency thalamic stimulation may be an option in patients with severe tremors that are refractory to other treatment.y
Orthostatic tremor and primary writer's tremor are two particular forms of tremor that are highly dependent on posture. In the former, patients can usually walk with only mild discomfort, but when asked to stand in place for several seconds, they develop hard and cramping calves and thighs that shake uncontrollably. Although considered by some to be a variant of ET, this tremor does not respond to the usual drugs described above, and clonazepam (1 to 10 mg/day) is usually far more effective. Baclofen can also be used. The differential diagnosis includes dystonia and stiff-person syndrome. Similarly, primary writer's tremor does not occur outside the very specific activity of writing. It is thought that the complicated posture and muscle activation of writing induces the contractions in the agonist and antagonist hand muscles that induce the tremor. Sometimes the hand cramps at the same time, suggesting the presence of an underlying dystonia. The handwriting becomes sloppy, shaky, and large. Patients may respond to the medications used for ET, but often they improve more with anticholinergic medicines, again suggesting that dystonia may underlie these tremorous movements. Importantly, changing to a large, fat pen that requires different muscles for holding and writing with it, or switching to typing is often the most helpful of all therapies.
Cerebellar kinetic tremor, sometimes referred to as an intention tremor, is most apparent during a goal-directed limb movement such as that used in the finger-to-nose or heel-to-shin test. In contrast to postural tremor, the patient's greatest disability occurs at the "end point," when a sudden destabilization occurs with severe shaking. This tremor is usually caused by a lesion in the cerebellar outflow (dentatorubral) tracts, y and therefore it is seen most typically in patients with multiple sclerosis, cerebrovascular accidents, or primary cerebellar disorders. Likewise, rubral tremor, a tremor that mixes rest, postural, and kinetic tremors, is generally not an idiopathic tremor and is induced by structural disease in the midbrain or fiber pathways connecting this region with the cerebellar and thalamic nuclei. Treatment of these tremors is frustrating, and no reliable pharmacological success has been reported. Because these patients are generally not weak and overshoot their targets as they move, stabilization of the involved limbs with wrist weights can reduce the amplitude and disability of the tremor.
Dystonic tremor is a prominent form of shaking that occurs as a compensation for dystonic spasms and must be recognized so that the underlying dystonia can be treated (see earlier discussion). Finally, palatal tremor, also termed palatal myoclonus, is characterized by continuous and synchronous contractions of the soft palate that occur at frequencies of 100 to 150 per minute (range, 20 to 600 per minute). Patients may not notice the jerking movements unless they are accompanied by persistent ear clicks caused by the repetitive opening and closing of the eustachian tubes. Concomitant contractions of other muscles, including the larynx, extraocular muscles, neck, diaphragm, tongue, and face, may be observed. These patients may have additional symptoms including postural tremor, oscillopsia, dysphagia, and dysarthria. The movements, which generally persist throughout life with infrequent remissions, may be idiopathic or due to brain stem or cerebellar disease. The anatomical and physiological bases underlying palatal tremor involve the hypertrophic degeneration of the olivary nucleus located within the medulla oblongata. The alteration of the inferior olive develops secondary to a lesion within the Guillain-Mollaret triangle, a pathway extending from the contralateral dentate nucleus via the superior cerebellar peduncle and ipsilateral central tegmental tract. Palatal tremor is usually resistant to therapy and is unnecessary in most patients. 5-Hydroxytryptophan, the serotonin precursor, and carbamazepine are variably effective. Others have reported success with clonazepam, tetrabenazine, and trihexyphenidyl. Surgical intervention, including perforation of the tympanic membrane and tamponade of the eustachian tubes, is unsuccessful.
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