Cataplexy
Cataplexy is an episode of muscular weakness triggered by strong emotions such as laughter, anger and surprise. The loss of muscle tone ranges from a just-perceptible weakening of the facial muscles through weakness at the knees, to total collapse on the floor. Speech is slurred, eyesight impaired (double vision, inability to focus) but hearing and awareness remain undisturbed. Attacks often last less than 2 minutes (see here), and they may only last a few seconds. Some people have repeated attacks of cataplexy which persist for up to 30 minutes. During both mild and severe attacks, the person stays fully conscious.
Cataplexy may be most severe when the subject is tired rather than fully alert and can lead to considerable anxiety although anxiety itself is not a trigger. It is thought that about 75% of patients with narcolepsy have cataplexy. Rarely, it is the first symptom of narcolepsy but typically it does not develop for months or even years after the firs signs of excessive daytime sleepiness.
Essentially, cataplexy is a symptom only found in narcolepsy and thus the presence of it makes diagnosis of narcolepsy much more certain.
There has been considerable debate about whether narcolepsy without cataplexy really is 'proper' narcolepsy. Recently the International Classification of Sleep Disorders has determined that, indeed, narcolepsy without cataplexy does exist. (See diagnosis)