Asterixis

Asterixis is a motor disorder characterized by the intermittent loss of muscle tone when attempting to maintain a set position. It most commonly refers to a clinical sign consisting of a tremor of the hand that is best seen when the individual attempts to extend their wrists. Asterixis is almost always bilateral, although in some cases, it may affect only one side of the body. 

Asterixis is pronounced as-teh-rik-sis. 

The exact cause of asterixis is not completely understood; however, it may be caused by impaired function of the diencephalic motor centers, which are brain centers that provide a feedback mechanism to cortical motor areas for initiation and control of voluntary movements. 

Asterixis is a classic sign of metabolic encephalopathy, which is when brain function is affected by the presence of an underlying medical condition or the build-up of toxic metabolites. In particular, asterixis can occur with hepatic encephalopathy in liver disease when the build-up of ammonia and other toxic metabolites affects the functioning of the central nervous system. Other causes of metabolic encephalopathy include uremic encephalopathy in kidney disease and carbon dioxide retention in lung disease. In addition, asterixis may be a sign of electrolyte abnormalities (e.g., hypokalemia, hypomagnesemia) as well as a side effect of medications (e.g., antiepileptics, like phenytoin, valproate, or carbamazepine; antipsychotics, like lithium and clozapine; and some antibiotics, like ceftazidime). Finally, although less commonly, unilateral asterixis may occur in individuals with focal brain lesions, such as an ischemic or hemorrhagic stroke, neurological tumors, or encephalitis. 

Clinically, asterixis produces flapping tremors, which refers to sudden lapses of sustained posture caused by involuntary interruption in muscle contraction. In a flapping tremor, an individual will flap their wrists like a bird flapping its wings. These tremors most commonly affect the upper limbs, but they can also affect the lower limbs; trunk; face; or in some cases, the tongue. Although asterixis is typically asymptomatic, it may occasionally cause jerking movements of the dominant hand, irregular handwriting, and falls (in the case of lower limb or truncal asterixis). 

Asterixis is a physical exam finding that can be elicited by asking the individual to extend the arms, flex the wrists, and spread the fingers wide. There may be a latent period between adopting the posture and the beginning of asterixis, so it is important to wait at least 30 seconds before concluding the test. The appearance of a flapping tremor at the wrist is sufficient to diagnose asterixis; however, further diagnostic tests may be done to identify any underlying medical conditions. 

Treatment of asterixis involves managing the underlying disorder. In cases of hepatic encephalopathy, treatment generally includes medications like lactulose, which decrease the reabsorption of excess ammonia in the gastrointestinal tract, as well as antibiotics like rifaximin or neomycin, which reduce the amount of gut bacteria that produce ammonia. In cases of uremic encephalopathy, treatment may involve dialysis or a kidney transplant, whereas individuals may be put on mechanical ventilation if asterixis is due to carbon dioxide retention. Finally, in cases where asterixis is caused by medication use, treatment may involve changing or stopping the offending medication when possible. 

Asterixis is a tremor of the hand that resembles a bird flapping its wings and is best seen when the individual attempts to extend their wrists. Asterixis can be caused by a wide variety of metabolic and structural brain disorders, or it can represent an adverse effect of medications. Since asterixis is a sign of an underlying medical condition, treatment focuses on addressing the underlying cause when possible.