Narcolepsy Health Article

Definition

Narcolepsy is a neurological disorder characterized by uncontrollable episodes of sleepiness during the day. Episodes can last from a few seconds to more than an hour and can significantly interfere with daily activities.

Description

People with narcolepsy often fall asleep suddenly, anywhere at any time, even in the middle of a conversation. They may sleep for just a few seconds or for up to a half hour, and then reawaken feeling alert until they fall asleep again. The condition affects one of every 2,000 Americans. Sleep apnea (difficulty in breathing while sleeping) is the leading cause of excessive daytime sleepiness. Narcolepsy is the second leading cause.

The attacks of sleepiness that are the hallmark of this condition may be mildly inconvenient or deeply disturbing. Some people continue to function during the sleep episodes, even talking and putting things away, but will reawaken with no memory of what they had been doing while briefly asleep.

Narcolepsy is related to the dreaming part of sleep known as REM (rapid eye movement) sleep. Normally, people fall asleep for about 90 minutes of non-REM sleep followed by REM sleep. However, people with narcolepsy enter REM sleep immediately; upon awakening, REM sleep recurs inappropriately throughout the day.

Causes & symptoms

Recent research suggests that the development of narcolepsy probably involves a combination of environmental factors and several genes, most likely those that are involved with the immune system. The exact gene pattern has not been identified as of 2002.

Cross-ethnic studies indicate significant variations in the prevalence of narcolepsy in different countries, with the Japanese having a very high rate and Israeli Jews one of the lowest in the world. A recent study of five European countries found that the prevalence of narcolepsy is higher in the United Kingdom and Germany than in Italy, Portugal, and Spain.

In the late 1990s, three independent research groups discovered a neuropeptide system in the hypothalamus, the part of the brain that regulates body temperature and appetite. The newly discovered system, which has been called the hypocretinergic system, regulates sleep and wakefulness. The nerve cells, or neurons, in this part of the hypothalamus secrete substances known as hypocretins or orexins, which regulate the sleep/wake cycle in humans. There are two of these compounds, known as orexin-A and orexin-B, or as hypocretin-1 and hypocretin-2. As of 2002, narcolepsy is thought to be an orexin deficiency syndrome; that is, it develops when a person's hypothalamus does not secrete enough orexins to keep the person from falling asleep at inappropriate times. Samples of cerebrospinal fluid taken from patients with narcolepsy contain little or no orexins. MRI scans of these patients indicate that there is some loss of brain tissue in the hypothalamus itself, suggesting that the neurons responsible for secreting orexins have died.

Symptoms of narcolepsy typically appear during adolescence; however, studies have shown that they may also begin in childhood. The disorder itself may not be diagnosed for many years after the first appearance of symptoms. The primary symptom is an overwhelming feeling of fatigue, together with sleep attacks that may occur with or without warning. About 75% of patients also experience cataplexy, a sudden loss of muscle control lasting a few seconds to 30 minutes resulting in physical collapse without any loss of consciousness. Episodes of narcolepsy can be triggered by emotions such as laughter, fear, or anger. Other symptoms include sleep paralysis and hypnogogic (vivid) hallucinations as the person wakes up or falls asleep. Some patients may also have trouble staying asleep at night.