Our understanding of narcolepsy was greatly advanced by the demonstration by Dement and his group that this disorder is associated with a reversal in the order of the two states of sleep, with REM rather than NREM sleep occurring at the onset of the sleep attacks. Not all the sleep episodes of the narcoleptic begin with REM sleep, but almost always a number of sleep attacks with such an onset can be identified in narcoleptic-cataplectic patients in the course of a polysomnographic sleep study. The hypnagogic hallucinations, cataplexy, and sleep-onset paralysis (caused by inhibition of anterior horn cells) all coincide with the REM period. These investigators have also shown that the night sleep pattern of patients with narcolepsy-cataplexy may begin with a REM period. This may occur in normal subjects, though infrequently and usually with severe sleep deprivation. Furthermore, the nocturnal sleep pattern is altered in narcoleptics, who have frequent body movements and transient awakenings and a decrease in sleep stage N3, as well as in total sleep duration. Another important finding in narcoleptics is that sleep latency (the interval between the point when an individual tries to sleep and the point of onset of EEG sleep patterns), measured repeatedly in diurnal nap situations, is greatly reduced. Thus, narcolepsy is not simply a matter of excessive diurnal sleepiness (essential daytime drowsiness) or even a disorder of REM sleep but a generalized disorganization of sleep–wake function.
The greatest difficulty in diagnosis relates to the problem of separating narcolepsy from the daytime sleepiness of sedentary, obese adults who, if unoccupied, doze readily after meals, while watching television or in the theater. Many of these patients prove to have obstructive sleep apnea. Excessive daytime somnolence, easily mistaken for narcolepsy, may also attend heart failure, hypothyroidism, excessive use of soporific, other medications including antihistamines, use of alcohol, cerebral trauma, and certain brain tumors (e.g., craniopharyngioma; see Table 18-2). A more serious form of recurrent daytime sleepiness, referred to as independent narcolepsy or essential narcolepsy, is described further on. However, both of these forms of daytime drowsiness are isolated disturbances, lacking the other disturbances of sleep and motor activity that characterize the narcolepsy syndrome. The brief attacks of automatic behavior and amnesia of the narcoleptic must be distinguished from hysterical fugues and complex partial seizures.
Cataplexy must also be distinguished from syncope, drop attacks (see Chap. 17), and atonic seizures; in atonic seizures, consciousness is temporarily abolished. The careful documentation of narcolepsy by laboratory techniques is imperative when the diagnosis is in doubt, in part because of the potential for abuse of stimulant drugs used for treatment. Overnight polysomnography followed by a standardized multiple sleep latency test, in which the patient is afforded opportunities for napping at 2-h intervals, permit the quantification of drowsiness and increase the probability of detecting short-latency REM activity (within 15 min from the onset of each sleep period). According to some investigators, a reduced level (below 110 pg/mL) of hypocretin in the spinal fluid is virtually diagnostic of narcolepsy in the proper clinical circumstances (see Mignot et al). We would comment, however, that it is not necessary to resort of these studies in clinically typical cases.