What is Asterixis?
Asterixis is a negative myoclonus characterized by brief, involuntary lapses of sustained posture caused by intermittent loss of postural muscle tone, most commonly manifesting as a "flapping tremor" when patients extend their wrists with fingers separated. 1
Clinical Characteristics
Asterixis is not actually a tremor but rather a negative myoclonus consisting of involuntary 50-200 millisecond silent periods in tonically contracting muscles. 1, 2 The key distinguishing feature is that these are lapses in muscle contraction rather than active involuntary movements.
How to Elicit Asterixis
The sign is easily demonstrated by:
- Hyperextension of the wrists with fingers separated (classic method)
- Rhythmic squeezing of the examiner's fingers
- Dorsiflexion of the feet
- Other postures requiring sustained muscle tone 1
Asterixis can appear in multiple body regions including the feet, legs, arms, tongue, and eyelids, though upper extremity testing is most commonly performed. 1
Patterns of Presentation
- Bilateral asterixis (81.4% of cases): Usually indicates metabolic encephalopathy 3
- Unilateral asterixis (18.6% of cases): Suggests a focal structural lesion in the contralateral cerebral hemisphere 3, 2
Clinical Significance and Causes
Asterixis is NOT pathognomonic of hepatic encephalopathy, despite its historical association as a "liver flap." 1 It can occur in multiple conditions:
Common Causes:
- Hepatic encephalopathy (most classic association)
- Renal failure/uremia
- Respiratory failure
- Medications causing hyperammonemia (valproic acid, carbamazepine, phenytoin)
- Cerebrovascular disease
- Structural brain lesions (tumors, subdural hematoma, epidural abscess)
- Electrolyte imbalances 1, 4, 3
Timing in Disease Progression
Asterixis appears in the early to middle stages of hepatic encephalopathy, preceding stupor or coma. 1 The ISHEN consensus specifically uses the onset of disorientation or asterixis as marking the beginning of overt hepatic encephalopathy. 1
Critical caveat: Asterixis disappears as encephalopathy progresses to deeper stages of coma, so its absence does not rule out severe encephalopathy. 1
Clinical Implications
Asterixis is usually asymptomatic and not spontaneously reported by patients, making active examination essential in any encephalopathic patient. 4 Its presence should prompt investigation for underlying toxic or metabolic causes.
The finding is typically reversible upon treatment of the underlying condition, making it a useful marker for both diagnosis and monitoring treatment response. 4
Diagnostic Approach When Asterixis is Present:
- If bilateral: Investigate for metabolic/toxic encephalopathy (liver function, renal function, ammonia level, medication review, arterial blood gas)
- If unilateral: Obtain neuroimaging to evaluate for focal structural lesion in contralateral hemisphere
- Consider the clinical context and associated symptoms to guide specific testing