sections include: essential tremor, other types of tremors, factors, diagnosis and treatment
Although people may think of tremors, spasms and tics as similar, in that all can involve unusual movements, the three conditions have different medical definitions and different origins within the nervous system. Tremors are rhythmic, involuntary, oscillatory movements of muscles around a joint. These movements result from alternating contractions of opposing muscle groups, or from simultaneous contractions of muscles working against each other.
Tremors can appear when
- hands are at rest (for example, parkinsonian tremor)
- hands are outstretched (postural tremors, most commonly essential tremor)
- hands are in motion (action tremors, such as kinetic or writing tremors)
- fingers reach a target (intention tremors, caused by cerebellar disorders)
These variations help physicians diagnose what type of tremor a person has. In addition, different sorts of tremors usually have characteristic frequencies, as measured in hertz (repetitions per second), but there is a large overlap of frequencies among tremor disorders.
Essential tremor (ET) is the most common type; indeed, it’s the most common movement disorder. It is characterized by rhythmic oscillations of the arms or hands when a person holds them in a sustained position (postural tremor) or moves them (kinetic tremor). The tremor usually has a frequency of 4 to 12 Hz. To diagnose ET, doctors must rule out such causes of tremors as toxins or drugs, which can create similar shaking, and such neurological conditions as multiple sclerosis and Parkinson’s disease. ET is not a symptom of another condition but a disorder in its own right, with the tremor its primary characteristic.
About 5 percent of people aged 40 and older have ET, and for elderly individuals in institutions that rate rises to 10 percent. ET usually starts as a postural tremor of the hands or arms, sometimes more on one side than the other. It may progress to affect a person’s head, voice, and, less frequently, trunk and legs. Such tremors usually appear in adulthood, and initially may be more prominent when a person is under emotional stress. ET is most apparent when people hold their arms outstretched. It increases when an individual starts to move (kinetic tremor), and (unlike parkinsonian tremor) rarely shows up when a person is at rest.
ET may be static for many years, but usually progresses slowly in severity. Eventually it can interfere with the activities of daily life, to the point of disabling a person. One of its most obvious manifestations is a rhythmic bobbing of the head, either vertically or horizontally. Despite its progressive nature, however, ET is not associated with increased mortality.
Most patients with ET have postural and kinetic tremors to varying degrees. Other types of tremor disorders appear to be related to the condition. These include:
- combined resting-postural tremor resembling the tremor of Parkinson’s disease
- primary writing tremor—that is, tremor primarily occurs when writing
- isolated chin tremor; “isolated” means the only symptom is chin tremor
- isolated voice tremor
- isolated tongue tremor
- orthostatic truncal tremor—that is, a tremor of the trunk when standing
- kinetic-predominant hand tremor—which appears mostly when people move their hands, with little or no tremor when they are trying to keep still
Often people with these conditions have a family history of ET, and, as with that condition, their tremors frequently improve when they drink alcohol.
Other Types of Tremors
Parkinsonian tremor appears in a limb at rest (resting tremor), is usually asymmetrical, and has a frequency of 4 to 6 Hz. The amplitude of parkinsonian tremor declines during voluntary movements. It typically involves the hands— fingers opening and closing, or fingertips rubbing together continuously (pill-rolling tremor). This tremor is characteristic of Parkinson’s disease.
Cerebellar tremor, so called because it arises from the cerebellum, is a tremor with a frequency of 2 to 8 Hz that occurs when a person performs fine voluntary movements. It appears clearly, for instance, when individuals touch their noses with one finger, or touch one shin with the opposite heel. This tremor is accompanied by other cerebellar features, such as titubation—back-and-forth oscillation of the trunk and head, as if one is losing one’s balance. One cause of cerebellar damage that commonly produces such tremors is multiple sclerosis.
Physiological tremor occurs as a postural tremor and results from peripheral mechanical properties of the musculoskeletal system. The frequency of physiological tremor is between 5 and 15 Hz.
Neuropathic tremor occurs in the context of peripheral neuropathies and arises from problems in how the body’s nerves signal each other. Around 10 percent of people with hereditary motor and sensory neuropathy display a tremor when they move as well. There is no correlation between the tremor and the severity of the neuropathy, however. Some of the clinical features of this tremor—age at onset, response to alcohol, and family history—overlap with those of ET, which suggests an association between the two conditions.
Medications can cause tremors or, more often, intensify a physiological tremor that already exists. Physostigmine or other anticholinesterases can augment or enhance parkinsonian tremor. Other drugs capable of causing tremor include intraventricular baclofen (used to treat cerebral palsy) valproate (for seizures), lithium (for bipolar and other mood disorders), and trimethoprim-sulfamethoxazole (for AIDS).
ET may be sporadic or familial, but it tends to run in families. Studies have found that between 17 percent and 70 percent of people with such a tremor have a relative with the same condition. Swedish geneticists conducted a comprehensive study of people in the geographically and ethnically restricted north of their country, tracing 210 cases of ET to nine ancestral families. The inheritance pattern of ET was found to be autosomal dominant, meaning that only one parent needs the unusual gene for children to inherit the condition. Other studies have identified specific chromosomes linked to forms of ET and related conditions.
We do not know where in the brain or nervous system the possible neural generators of ET reside. The condition may result from the activity of an abnormal oscillator, a sort of “tremor pacemaker,” within the central nervous system.
Diagnosis and Treatment
Usually a person with a tremor is aware of the condition when he or she goes to a doctor for advice. Sometimes the shaking is simply mysterious and embarrassing, and sometimes it is interfering with the person’s daily life. The physician’s first task is to determine the nature of the tremor, which will indicate the most appropriate treatment and the long-term prognosis.
If doctors diagnose a serious disease underlying a person’s tremor, the treatment they recommend will be focused primarily on that disease. In many cases, however, ET or a related tremor appears on its own. In this case, a thorough neurological examination will eliminate other possible diagnoses. Doctors should evaluate all people with tremors for an overactive thyroid gland (thyrotoxicosis) and, if necessary, screen their urine for toxins. In cases of neuropathic tremor, they should also perform nerve-conduction studies.
ET may be misdiagnosed as parkinsonian tremor, especially in elderly people. But the lack of such other parkinsonian features as resting tremor, rigidity, bradykinesia, and loss of postural reflexes should help neurologists differentiate these two diseases. Other neurological diseases that may be associated with tremor include multiple sclerosis, Wilson’s disease, Huntington’s disease and cerebellar degenerative diseases. In addition, physicians must exclude the possibility that the tremors are induced by drugs, toxins and systemic illness. Occasionally, ET may be misdiagnosed as anxiety disorder.
Some people with ET do not require treatment, just reassurance that the problem is not more serious, and advice about how to deal with it. One useful step is to minimize caffeine consumption. Drinking small quantities of alcohol may lessen the tremor, though of course such a drinking habit can affect us in other ways. ET disappears during sleep. A person’s tremor may also vary, not only over the course of years but even over the course of a single day.
The main pharmacological options include primidone and beta-blockers. Some people respond only to high doses of primidone, but physicians must start with a small initial dose (25 mg) to avoid its possible side effects: nausea, vertigo, weakness, and ataxia. If an individual shows no sign of those problems, the physician can then gradually increase the dose to the usual range, 100 to 250 mg at bedtime. If the tremor remains, the doctor may add or switch to another medication, such as propranolol.
Propranolol, which blocks peripheral receptors of adrenaline, can suppress tremors. The medication may be administered as a short-acting formulation, as a person needs it (for example, 10 to 20 mg half an hour before a public event) or a few times daily. However, propranolol has possible adverse effects, including depression, weight gain, orthostatic hypotension (low blood pressure), sedation, and impotence. People with asthma, chronic obstructive pulmonary disease, and congestive heart failure should not take propranolol. Benzodiazepines may reduce anxiety but do not have a direct effect on ET.
Botulinum toxin type A is effective for a variety of movement disorders, including dystonia and hemifacial spasms. It works by relaxing the muscles involved in these disorders. The drug appears to have some good effect on ET of the head and voice, but less for hand tremors. Research continues into new therapeutic agents for people whose tremors do not respond to the currently used medications.
For people with severe ET that interferes with their daily activities and is resistant to medications, doctors may recommend surgical options. These operations all involve manipulating areas of the brain involved in the tremors and therefore carry risks of severe complications. The target for some of the surgeries is the ventralis intermedius nucleus of the thalamus. In one procedure, a thalamotomy, surgeons use magnetic resonance imaging (MRI) to guide them in placing a probe at that site that destroys a small area of brain tissue through radiation or freezing. This can stop the errant signals that produce tremors. However, side effects may include temporary intellectual deficits and transitory paralysis on the corresponding side of the body, though a lasting weakness is unusual. Other rare adverse outcomes include seizures, involuntary movements, and cerebellar dysfunction.
Bilateral thalamotomy—performing the same operation on both sides of the brain—is associated with much more serious complications. In particular, people can experience a severe, persistent difficulty articulating words (dysarthria) and permanent mental changes. Therefore, people should undergo bilateral operations only with the utmost caution.
A more modern surgical intervention consists of implanting a permanent electrode in the appropriate part of the brain that is connected to an electronic stimulator placed under the skin (deep brain stimulation, or DBS). The configuration of the electrode and the intensity of its stimulation can be adjusted to best suppress an individual’s tremors. DBS of the thalamus is highly effective in reducing ET and appears to produce fewer adverse reactions than thalamotomy. This procedure can be done on one or both sides of the brain.
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