The use of drugs in treating this movement disorder has met with variable success. Clonazepam (0.25 to 0.5 mg/d, increasing gradually to 3.0 to 6.0 mg/d), sodium valproate (250 mg/d, increasing to 1000 mg/d), and gabapentin (up to 2100 mg) have suppressed the movement in some cases, particularly the last of these drugs, which reportedly has had a dramatic effect in some patients. Also, tetrabenazine and haloperidol have been helpful on occasion. Selective injection of the palatal muscles with botulinum toxin, while technically demanding, affords modest relief; it is particularly helpful in eliminating the bothersome ear clicking.
The movement disorder known as asterixis was described by Adams and Foley in patients with hepatic encephalopathy but it occurs with a variety of systemic metabolic disorders as mentioned below. It consists of arrhythmic lapses of sustained posture that allow gravity or the inherent elasticity of muscles to produce a sudden movement, which the patient then corrects, sometimes with overshoot. Later, Leavitt and Tyler and then Young and Shahani demonstrated that the initial interruption or lapse in posture is associated with EMG silence for a period of 35 to 200 ms. By interlocking EMG and electroencephalogram (EEG) recordings, Ugawa et al found that a sharp wave, probably generated in the motor cortex, immediately precedes the period of EMG silence. This confirmed that asterixis differs physiologically from both tremor and myoclonus, with which it was formerly confused; it had incorrectly been referred to as a “negative tremor” or “negative myoclonus.”
Asterixis is most readily evoked by asking the patient to hold his arms outstretched with hands dorsiflexed or to dorsiflex the hands and extend the fingers while resting the forearms on the bed or the arms of a chair. Flexion movements of the hands may then occur arrhythmically once or several times a minute. The same lapses in sustained muscle contraction can be provoked in any muscle group—including, for example, the protruded tongue, the closed eyelids, or the flexed trunk muscles. Sometimes, asterixis can be elicited best by asking the patient to place his hand flat on a table and raise the index finger.
Asterixis was first observed in patients with hepatic encephalopathy but was later noted to occur with hypercapnia, uremia, and other metabolic and toxic encephalopathies including those caused by phenytoin and other antiepileptics, usually indicating that these drugs are present in excessive concentrations. Medications in classes other than the antiepileptics, particularly some antibiotics, cause the disorder from time to time, also usually when they are present at toxic levels.
Unilateral asterixis occurs in an arm and leg on the side opposite an anterior thalamic infarction or small hemorrhage, after stereotaxic thalamotomy, and with an upper midbrain lesion, usually as a transient phenomenon after stroke. In two series, Kim and Montalban and colleagues came to a similar conclusion, namely, that unilateral asterixis is usually attributable to an acute thalamic stroke on the contralateral side, but there was an interesting variety of other localizations including the frontal lobe (anterior cerebral artery infarction), midbrain, and cerebellum in a few cases each. Our experience is limited to those arising from thalamic and overlying parietal vascular lesions. Many drugs may unmask unilateral asterixis that has its basis in an underlying lesion of the anterior thalamus. Of course, an individual with a metabolic encephalopathy and a hemiparesis, new or old, will only manifest asterixis on the normal side.