Asterixis is a neurological symptom characterized by a sudden, temporary loss of muscle tone that results in an involuntary lapse of a sustained posture. It is often referred to as the “flapping tremor.” This symptom is a form of negative myoclonus, involving a brief cessation of muscle contraction rather than an active spasm. Its presence alerts medical professionals to a potential underlying systemic or metabolic issue affecting the central nervous system.
What the Flapping Tremor Looks Like
The physical manifestation of asterixis is most commonly observed in the hands and wrists. To check for the symptom, a clinician asks the patient to extend their arms straight out, wrists cocked backward, and fingers spread wide. This position requires the continuous, voluntary contraction of the wrist extensor muscles.
After holding this sustained posture for a few moments, a patient with asterixis exhibits a brief, jerky movement. The hands suddenly drop forward due to an involuntary loss of muscle tone, only to immediately return to the original, extended position. This irregular, arrhythmic motion creates the appearance of a flap or a tremor.
While the hands are the most common site, asterixis can affect any muscle group attempting to hold a fixed position, including the feet, tongue, or trunk. The duration of the muscle lapse is very brief, typically lasting between 35 and 200 milliseconds.
Underlying Causes of Asterixis
Asterixis is a hallmark sign of metabolic encephalopathy, a brain dysfunction caused by toxins or imbalances circulating in the bloodstream. The most frequent cause is hepatic encephalopathy, which arises from advanced liver failure. When the liver fails to clear toxins, substances like ammonia build up in the blood, cross the blood-brain barrier, and disrupt neuronal function.
The failure of other major organs can also precipitate this neurological symptom. Uremic encephalopathy, a complication of severe kidney failure, occurs when waste products accumulate and affect brain activity. Respiratory failure leading to hypercapnia (abnormally high levels of carbon dioxide) can also induce asterixis.
An excess of carbon dioxide alters the acid-base balance in the central nervous system, impairing normal brain function. Certain medications, including specific sedatives and anticonvulsant drugs like phenytoin or valproate, are also known to trigger the flapping tremor as a form of drug-induced toxicity by interfering with neurological signaling.
How Doctors Identify and Address Asterixis
Diagnosis relies on clinical observation, specifically demonstrating the flapping motion during the sustained posture test. Once identified, the diagnostic process shifts to determining the underlying systemic cause. Clinicians order laboratory tests to evaluate major organ function.
These tests include liver function tests for hepatic failure, kidney function tests for uremia, and blood gas analysis for carbon dioxide levels. Toxicology screens rule out drug-induced causes. Asterixis is a reversible symptom that cannot be treated directly.
Managing the tremor involves treating the underlying metabolic imbalance. For example, hepatic encephalopathy treatment focuses on lowering elevated ammonia levels. Once systemic toxicity is controlled, the brain’s function improves, and the symptom of asterixis resolves.