Asterixis is a tremor of the wrist when the wrist is extened (dorsiflexion), sometimes said to resemble a bird flapping its wings. Also called "liver flap", it can be a sign of hepatic encephalopathy, damage to brain cells due to the inability of the liver to metabolize ammonia to urea. The cause is thought to be related to abnormal ammonia metabolism. The term derives from the Greek a, "not" and stērixis, "fixed position".