cataplexy

cataplexy, a sudden brief impairment of muscle tone that can vary from subtle slackening—such as a limpness of the arms or legs—to a complete loss of strength that causes collapse. Cataplexy is often precipitated by an emotional response such as laughter or startle. It occurs in an estimated 60 to 75 percent of people affected by narcolepsy type 1, in which irresistible episodes of sleep can strike anywhere, anytime, even when standing or walking. Cataplexy is the primary distinguishing feature between narcolepsy type 1 and type 2.

Episodes of cataplexy usually last less than two minutes and can occur anywhere from multiple times in a single day to less than once a month. During an episode, the individual remains fully conscious and afterward regains the ability to speak and move. The condition is caused by the autoimmune destruction of neurons in the hypothalamus of the brain that secrete hypocretin (orexin), which regulates wakefulness. The loss of hypocretin destabilizes the division of sleep and wake states, allowing muscle paralysis typical of REM sleep (rapid eye movement sleep) to occur while awake.