376 Sleep-Wake Disorders
ducing narcolepsy a few months later. Head trauma and abrupt changes in sleep-wake
patterns (e.g., job changes, stress) may be additional triggers. 
Genetic and physiological. Monozygotic twins are 25%–32% concordant for narcolepsy.
The prevalence of narcolepsy is 1%–2% in firs t-degree relatives (a 10- to 40-fold increase
overall). Narcolepsy is strongly  associated with DQB1*06:02  (99% vs. 12%–38% in control
subjects of various ethnic gr oups; 25% in the general U.S. population). DQB1*03:01 in-
creases, while DQB1*05:01, DQB1*06:01, and DQB1*06:03 reduce risk in the presence of
DQB1*06:02, but the effect is small. Polymorphisms within the T-cell receptor alpha gene
and other immune modulating gene s also modulate risk slightly.
Culture-Related Diagnostic Issues
Narcolepsy has been described in all ethnic groups and in many cultures. Among African
Americans, more cases present without cataplexy or with atypical cataplexy, complicating
diagnosis, especially in the presence of obesity and obstructive sleep apnea.
Diagnostic Markers
Functional imaging suggests impaired hypo thalamic responses to humorous stimuli.
Nocturnal polysomnography followed by an MS LT is used to confirm the diagnosis of
narcolepsy, especially when the disorder is first being diagnosed and before treatment has
begun, and if hypocretin deficiency has not been documented biochemically. The poly-
somnography/MSLT should be performed after the individual is no longer taking any