Cataplexy

Another common feature of narcolepsy is cataplexy, which is a sudden loss of muscle tone in response to strong emotion (such as laughing, surprise, or anger).

Narcolepsy has been associated with the human leukocyte antigen (HLA) system, a 4-megabase region of chromosome 6, which is divided into three subregions: class I, class II, and class III. Class II HLA antigens have been strongly associated with autoimmune diseases. The class II antigen, DQB1*0602, has been associated with narcolepsy. Most (90%-100%) patients with narcolepsy/cataplexy across all ethnic groups have this allele. In addition, 35% to 56% of patients who have narcolepsy without cataplexy also have this allele. However, 20% of the normal population also has the allele, reducing its usefulness in diagnostic testing.