Narcolepsy is a chronic neurological disorder caused by the brain’s inability to regulate sleep-wake cycles normally. At various times throughout the day, people with narcolepsy experience fleeting urges to sleep. If the urge becomes overwhelming, patients fall asleep for periods lasting from a few seconds to several minutes. In rare cases, some people may remain asleep for an hour or longer.
Narcoleptic sleep episodes can occur at any time, and thus frequently prove profoundly disabling. People may involuntarily fall asleep while at work or at school, when having a conversation, playing a game, eating a meal, or, most dangerously, when driving an automobile or operating other types of potentially hazardous machinery. In addition to daytime sleepiness, three other major symptoms frequently characterize narcolepsy: cataplexy, or the sudden loss of voluntary muscle tone; vividhallucinations during sleep onset or upon awakening; and brief episodes of total paralysis at the beginning or end of sleep.
Contrary to common beliefs, people with narcolepsy do not spend a substantially greater proportion of their time asleep during a 24-hour period than do normal sleepers. In addition to daytime drowsiness and involuntary sleep episodes, most patients also experience frequent awakenings during nighttime sleep. For these reasons, narcolepsy is considered to be a disorder of the normal boundaries between the sleeping and waking states.
For most adults, a normal night’s sleep lasts about 8 hours and is composed of four to six separate sleep cycles. A sleep cycle is defined by a segment of non-rapid eye movement (NREM) sleep followed by a period of rapid eye movement (REM) sleep. The NREM segment can be further divided into stages according to the size and frequency of brain waves. REM sleep, in contrast, is accompanied by bursts of rapid eye movement (hence the acronym REM sleep) along with sharply heightened brain activity and temporary paralysis of the muscles that control posture and body movement. When subjects are awakened from sleep, they report that they were “having a dream” more often if they had been in REM sleep than if they had been in NREM sleep. Transitions from NREM to REM sleep are governed by interactions among groups of neurons (nerve cells) in certain parts of the brain.
Scientists now believe that narcolepsy results from disease processes affecting brain mechanisms that regulate REM sleep. For normal sleepers a typical sleep cycle is about 100 – 110 minutes long, beginning with NREM sleep and transitioning to REM sleep after 80 – 100 minutes. But, people with narcolepsy frequently enter REM sleep within a few minutes of falling asleep.
Who Gets Narcolepsy?
Narcolepsy is not rare, but it is an under recognized and under diagnosed condition. According to current estimates, the disorder affects about one in every 2,000 Americans-a total of more than 135,000 individuals. After obstructive sleep apnea and restless legs syndrome,* narcolepsy is the third most frequently diagnosed primary sleep disorder found in patients seeking treatment at sleep clinics. But the exact prevalence rate remains uncertain, and the disorder may affect a larger segment of the population than currently estimated.
Narcolepsy appears throughout the world in every racial and ethnic group, affecting males and females equally. But prevalence rates vary among populations. Compared to the U.S. population, for example, the prevalence rate is substantially lower in Israel (about one per 500,000) and considerably higher in Japan (about one per 600).
Most cases of narcolepsy are sporadic-that is, the disorder occurs independently in individuals without strong evidence of being inherited. But familial clusters are known to occur. Up to 10 percent of patients diagnosed with narcolepsy with cataplexy report having a close relative with the same symptoms. Genetic factors alone are not sufficient to cause narcolepsy. Other factors-such as infection, immune-system dysfunction, trauma, hormonal changes, stress-may also be present before the disease develops. Thus, while close relatives of people with narcolepsy have a statistically higher risk of developing the disorder than do members of the general population, that risk remains low in comparison to diseases that are purely genetic in origin.
* Obstructive sleep apnea is a temporary cessation of breathing that occurs repeatedly during sleep and is caused by a narrowing of the airway. Restless legs syndrome is a neurological disorder characterized by unpleasant sensations-burning, creeping, tugging-in the legs and an uncontrollable urge to move when at rest
What are the Symptoms?
People with narcolepsy experience highly individualized patterns of REM sleep disturbances that tend to begin subtly and may change dramatically over time. The most common major symptom, other than excessive daytime sleepiness (EDS), is cataplexy, which occurs in about 70 percent of all patients. Sleep paralysis and hallucinations are somewhat less common. Only 10 to 25 percent of patients, however, display all four of these major symptoms during the course of their illness.
Excessive daytime sleepiness
EDS, the symptom most consistently experienced by almost all patients, is usually the first to become clinically apparent. Generally, EDS interferes with normal activities on a daily basis, whether or not patients have sufficient sleep at night. People with EDS describe it as a persistent sense of mental cloudiness, a lack of energy, a depressed mood, or extreme exhaustion. Many find that they have great difficulty maintaining their concentration while at school or work. Some experience memory lapses. Many find it nearly impossible to stay alert in passive situations, as when listening to lectures or watching television. People tend to awaken from such unavoidable sleeps feeling refreshed and finding that their feelings of drowsiness and fatigue subside for an hour or two. Involuntary sleep episodes are sometimes very brief, lasting no more than seconds at a time. As many as 40 percent of all people with narcolepsy are prone to automatic behavior during such “microsleeps.” They fall asleep for a few seconds while performing a task but continue carrying it through to completion without any apparent interruption. During these episodes, people are usually engaged in habitual, essentially “second nature” activities such as taking notes in class, typing, or driving. They cannot recall their actions, and their performance is almost always impaired during a microsleep. Their handwriting may, for example, degenerate into an illegible scrawl, or they may store items in bizarre locations and then forget where they placed them. If an episode occurs while driving, patients may get lost or have an accident.
The temporary inability to move or speak while falling asleep or waking up also parallels REM-induced inhibitions of voluntary muscle activity. This natural inhibition usually goes unnoticed by people who experience normal sleep because it occurs only when they are fully asleep and entering the REM stage at the appropriate time in the sleep cycle. Experiencing sleep paralysis resembles undergoing a cataplectic attack affecting the entire body. As with cataplexy, people remain fully conscious. Cataplexy and sleep paralysis are frightening events, especially when first experienced. Shocked by suddenly being unable to move, many patients fear that they may be permanently paralyzed or even dying. However, even when severe, cataplexy and sleep paralysis do not result in permanent dysfunction. After episodes end, people rapidly recover their full capacity to move and speak.
Hallucinations can accompany sleep paralysis or can occur in isolation when people are falling asleep or waking up. Referred to as hypnagogic hallucinations when accompanying sleep onset and as hypnopompic hallucinations when occurring during awakening, these delusional experiences are unusually vivid and frequently frightening. Most often, the content is primarily visual, but any of the other senses can be involved. These hallucinations represent another intrusion of an element of REM sleep-dreaming-into the wakeful state.