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|Year : 2003 | Volume
| Issue : 3 | Page : 272-275
Gokula RM, Khasnis A
Division of Geriatrics, Department of Internal Medicine, University of Michigan, Ann Arbor, Michigan
5421, Wild Oak Drive, East Lansing, MI 48823
|How to cite this article:|
Gokula R M, Khasnis A. Asterixis. J Postgrad Med 2003;49:272-5
Adams and Foley first described asterixis in 1949 in patients with advanced hepatic encephalopathy. It is a disorder of motor control characterised by brief, arrhythmic interruptions of sustained voluntary muscle contraction usually observed due to brief lapse of posture. It manifests as a bilateral flapping tremor affecting various parts of the body independently and occurs at a rate of 3-5 Hz during active maintenance of posture. Except for the facial muscles, the tremors occur in an asymmetric fashion on either side of the body. Leavitt & Tyler took the next step when they defined the electromyographic (EMG) abnormality associated with what they called metabolic tremor in hepatic encephalopathy. As with asterixis, they found brief irregular pauses or reductions in ongoing tonic EMG activity in patients with metabolic tremor. Later, Young and Shahani were the first to describe unilateral asterixis and they classified the involuntary movements as a form of “mini-asterixis” and the pauses as negative myoclonus. Negative myoclonus is characterised by a brief loss of muscle tone in agonist muscles followed by a compensatory jerk of the antagonistic muscles. Asterixis, posthypoxic myoclonus and stiff man syndrome are examples of negative myoclonus. This negative myoclonus correlated with intermittent pauses of 50-200 milliseconds on EMG tracings. Asterixis can be either unilateral or bilateral. This is described in detail in the sections below.
The postural stability or tonic control of the extremities is related to multiple brainstem-spinal pathways such as the vestibulospinal, reticulospinal, or rubrospinal tracts. These systems are, in turn, regulated by supratentorial structures. The ventro-lateral nucleus of the thalamus is the area in which cerebello-rubral or vestibulocerebellar fibres converge, and is also heavily connected with the prefrontal area. There is evidence that the projections from the medial frontal cortex to the brainstem reticular formation may have a role in regulating muscle tone or posture. The occasional occurrence of bilateral asterixis and the transient nature of the symptoms suggest that the system regulating posture maintenance is not strictly unilateral. The occurrence of ipsilateral asterixis in patients with cerebellar lesions can be explained by crossing of the cerebello-rubral fibres at the superior cerebellar peduncle. Despite several postulations, the mechanism of asterixis has not yet been systematically explained.
The exact mechanism underlying asterixis remains elusive and many explanations have been forthcoming. The following pathogenic mechanisms have been suggested:
1. Result of diffuse, widespread derangement of CNS function.
2. A “receptive inattentiveness to incoming information”, which could thus result from a dysfunction of the sensorimotor integration occurring in the contralateral parietal lobe and midbrain.
3. Episodic dysfunction within neural circuits concerned with maintenance of sustained or tonic muscle contraction, due to focal, specific brain lesions or by a generalised neurochemical imbalance. The existence of a possible neural subsystem whose dysfunction could result in asterixis rather than “non-specific” CNS lesions was hypothesised. Drowsiness in normal people and diffuse central nervous system lesions can also produce asterixis, perhaps by their effects on alerting or arousal mechanisms rather than by non-specific CNS actions.
4. Electrophysiological evaluation of asterixis using silent period locked averaging method revealed negative sharp waves in the contralateral central area. It was suggested that asterixis is due to abnormal activity in the motor field in the cerebral cortex.,
5. Recently mini-asterixis which is a part of the spectrum of the gross flapping tremor seen in hepatic encephalopathy was proposed as being due to the involvement of motor cortex causing a pathologically slowed and synchronised motor cortical wave.
6. A failure in arm posture maintenance that is comparable to failure in leg posture control in patients with astasia. (Astasia means inability to stand due to muscular incoordination).
Metabolic encephalopathies, especially hepatic and renal, are the most common causes of bilateral asterixis. Other causes of asterixis include cardiac and respiratory disease, electrolyte abnormalities and drug intoxication. Asterixis is a relatively frequent sign in structural CNS disorders and is a valuable sign in neurological examination. However, it is not helpful in localising intracranial pathology or in determining the aetiology. The most common CNS cause of asterixis is ischaemia or haemorrhage in the CNS; most frequently in the thalamus.
It is almost always caused by toxic-metabolic encephalopathies that induce generalised cerebral dysfunction, especially hepatic and renal, but the possibility of a focal lesion cannot be ruled out. It may also be seen during the recovery phase following general anesthesia, following ingestion of sedative medications, electrolyte abnormalities, advanced cardiac or respiratory disease and drug intoxication [Table - 1]. The flapping tremor (Liver flap) is characteristically seen in hepatic encephalopathy.
It is most commonly due to focal structural brain lesions in the genu and the anterior portion of the internal capsule or ventrolateral thalamus., There is a good correlation between the presence of unilateral asterixis and structural intracranial disease; nevertheless it is not helpful in localising intracranial pathology or in determining its aetiology. Lesions in the medial frontal lobe, parietal lobe, midbrain, pons, medulla oblongata, basal ganglia, insular lesions, may also cause unilateral asterixis.
Pseudoasterixis is defined as brief, rapid, voluntary action tremors of the hands and fingers, elicited by slow flexion and extension movements of the hands at the wrists, while keeping the fingers in full hyperextension. Because subtle movements can trigger pseudoasterixis, it mimics asterixis, a primary disorder of muscle tone. However in pseudoasterixis, the patient is aware of the hand twitching in contrast to asterixis that is involuntary.
Postural abnormalities like limb paresis, proprioceptive loss or involuntary movements that may disturb the maintenance of position must be distinguished from asterixis.
It is typically elicited by asking the patient to hold the arm outstretched, spread the fingers, dorsiflex the wrist, keep the eyes closed and mouth open for 30 seconds if necessary and then observe for the abnormal brief downward flaps of the hands that returns rapidly to the original position. If not immediately apparent, the patient can be asked to keep the arms straight while the examiner gently hyperextends the patient's wrist with a sweeping motion.
Another method to test for asterixis is to ask the patient to lie in a supine position and have the knees in a bent position on the bed. The patient is asked to relax his legs. The feet should be flat on the table and as the legs fall to the sides, watch for flapping of the legs at the hip joint. This repetitively brings the knees back together. It is also observed that gravity is required to elicit asterixis effectively.
a) Request the patient to squeeze the doctor's hand or the doctor's extended fingers. Patients who are unable to maintain a posture usually are unable to maintain a steady squeeze.
b) Have the patient squeeze a semi-inflated blood pressure cuff with instructions to maintain the reading. The readings bounce dramatically in patients with asterixis.
The laboratory investigations suggested are complete blood count, electrolytes, glucose, urea, renal function tests, liver function tests and arterial blood gases to rule out metabolic disorders associated with causing asterixis as elucidated in the [Table - 1]. A search for drug-induced asterixis should be carried out if history is suggestive of drug intoxication. Radiological investigations like Computerised Tomographic (CT) scan, Magnetic Resonance imaging (MRI) of brain in CNS lesions are very helpful in localising the lesion especially in asterixis of vascular pathology. Non-vascular causes like CNS infections and tumors can also be evaluated for, in appropriate situations.
The Electromyographic (EMG) features consist of cessation of electrical activity of 35 to 200 msec in multiple muscles during which posture may be overcome by gravity or tendinous elastic forces followed by equally abrupt reactivation of motor units and restorative jerk of the affected body part. Ugawa et al. described Silent Period Locked Averaging (SPLA) method, where backward averaging technique is used for the analysis of asterixis. This method will be useful to study the origin of asterixis and will also help to study various kinds of EMG silences that are seen in asterixis. Recently Timmerman et al evaluated the cortical origin of mini-asterixis in hepatic encephalopathy by studying the hand muscle EMG recordings and brain activity recorded by magneto-encephalography (MEG) and noted that this technique could be used to distinguish different tremor syndromes.
It usually consists of correction of the underlying disorder. Clonazepam has been reported to improve asterixis and prevented astatic seizures in a patient who presented with viral encephalitis and bilateral insular lesions.
The importance of asterixis is not in the length of the differential diagnosis it suggests, but rather its implication that, whatever the diagnosis, the condition is at a serious juncture and merits aggressive investigation and management. In one study of alcoholic liver disease, asterixis was the only physical finding on admission that had a statistically significant predictive value for mortality, the rate being 56% in those with asterixis, as opposed to 26% in those without it.
Asterixis is an important clinical sign and an invaluable clue to a number of underlying serious diseases. It is not pathognomonic of any condition and therefore a list of possible differential diagnoses tailored to the patient's clinical presentation must be thought of. It follows that the treatment of asterixis is the treatment of the underlying condition. However, it should be borne in mind that asterixis is a prognostic marker of severity of underlying disease in some conditions and should therefore be meticulously sought in those cases. The early detection of asterixis can help to initiate effective treatment that may help avoid complications in these patients.
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