Narcolepsy

The Definition of Narcolepsy and What Causes It

凯瑟琳•李
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For individuals with narcolepsy, sleep cycles do not follow the typical pattern — and wake cycles can be disrupted, too. iStock
Narcolepsy is a neurological disorder that interferes with the brain’s ability to control sleep-wake cycles. This lifelong condition can have serious negative effects on physical, psychological, cognitive, emotional, and even social well-being if it isn’t identified and treated, notes the National Institute of Neurological Disorders and Stroke (NINDS) at the National Institutes of Health (NIH).

The condition affects up to 200,000 people in the United States, according to estimates from NINDS.

但人narcoleps的实际数量y is likely much higher, because it often goes undiagnosed or is mistaken for another sleep disorder or medical condition.
Symptoms can appear in children as young as 2, saysEmmanuel Mignot, MD, a professor ofbob ios下载 in the department of psychiatry and behavioral sciences at Stanford University and director of the Stanford Center for Narcolepsy in Palo Alto, California. But the condition is most commonly diagnosed in late adolescence and young adulthood (anywhere from 15 to 35). People who have narcolepsy may fall asleep suddenly and uncontrollably right in the middle of doing something like eating, talking, laughing, or even having sex, saysEric Olson, MD, a a professor of medicine and abob ios下载 specialist at the Mayo Clinic in Rochester, Minnesota, and a member of the board of directors of the American Academy of Sleep Medicine (AASM). These episodes may last for a few seconds up to several minutes — or in rare cases as long as several hours.

Night-time sleep is also disturbed; frequent wake-ups are common.

Here’s a primer on what’s going on when a person has this condition, as well as its potential causes.

By Definition, Narcolepsy Is When Your Brain Can’t Regulate Sleep and Wake Cycles

Understanding what’s happening physiologically during narcolepsy requires a brief rundown ofwhat happens in your brain and body while you sleep.

During sleep we cycle through four stages: three stages of non-rapid-eye-movement (NREM) sleep (ranging from light to deep sleep) and rapid-eye-movement (REM) sleep. Healthy individuals spend the majority of their sleep in the first three stages of NREM, during which their brain waves slow down significantly, as does heart rate and breathing. After that, REM sleep begins: Brain activity speeds back up and dreaming occurs, according to Harvard Medical School.

Our bodies become paralyzed during REM sleep — presumed by sleep experts to be an evolutionary safety feature that prevents us from injuring ourselves or others by unconsciously acting out dreams, according to Mayo Clinic.

Healthy individuals cycle through these stages of NREM and REM sleep multiple times throughout the night (each cycle takes about 90 minutes), with REM periods becoming increasingly longer, according to NINDS.

Forindividuals with narcolepsy, these sleep cycles do not follow the typical pattern — and wake cycles can be disrupted, too.

Forpeople with narcolepsy, sleep cycles can occur out of order and suddenly. Many people with narcolepsy enter REM sleep nearly immediately after falling asleep, rather than cycling through the stages of lighter and deeper NREM sleep first — or they experience the muscle weakness or dream activity associated with REM sleep during the day while they’re awake. And because the boundaries between wakefulness and sleep become blurred, people with narcolepsy can experience extremely vivid dreams and hallucinations while falling asleep and waking up.

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There Are Two Types of Narcolepsy That Make Up the Majority of Cases

According to the American Academy of Sleep Medicine'sInternational Classification of Sleep Disorders, Third Edition, there are two main types of narcolepsy:

  • Type 1 NarcolepsyPeople with type 1 narcolepsy (previously callednarcolepsy with cataplexy) have either or both: abnormally low levels of a brain hormone called hypocretin (also called orexin, it plays a key role in the ability to stay awake and prevents REM sleep from occurring at the wrong time) and signs of cataplexy (sudden loss of muscle tone and control while you’re awake, which is usually triggered by feeling strong emotions).

  • Type 2 NarcolepsyThose who have type 2 narcolepsy (previously called narcolepsy withoutcataplexy) usually do not have cataplexy and have normal levels of hypocretin, but experienceexcessive daytime sleepiness(you may nap frequently but feel tired a short time after waking).

Both types of narcolepsy cause excessive sleepiness in the daytime, although people who have type 2 narcolepsy tend to have symptoms that are less severe, notes StatPearls.

In some cases, a patient may be diagnosed with type 2 narcolepsy and then progress to type 1. “Someone may present without cataplexy and then develop it later,” says Dr. Olson.
Another very rare condition, called secondary narcolepsy, can affect individuals who sustain an injury to the hypothalamus, a region deep in the brain that helps regulate sleep (and where hypocretin is produced). People who have secondary narcolepsy experience typicalnarcolepsy symptoms, as well as other neurological difficulties, and they may also sleep for an excessive amount of time (more than 10 hours at a stretch).

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There’s Strong Evidence Narcolepsy May Be an Autoimmune Disorder, Though There May Be Other Causes, Too

While research has revealed a lot about what’s going on when someone has narcolepsy, experts are less clear about what causes someone to develop narcolepsy in the first place.

In 1999, researchers at Stanford University discovered that type 1 narcolepsy is linked to the lack of hypocretin (the chemical that regulates REM sleep and wakefulness) in the brain.

A separate multicenter team at the Howard Hughes Medical Institute and other institutions identified the same link around the same time.

It’s still less clear what causes the loss of hypocretin in an individual, says Dr. Mignot, who worked on that seminal narcolepsy research in 1999 at Stanford. “There is extremely strong evidence that it’s an autoimmune disease.”

The thinking is that, as is the case with otherautoimmune diseases, in people who are genetically predisposed the body’s immune system mistakenly attacks healthy cells or tissue (in narcolepsy, the neurons deep in the brain that produce hypocretin) in response to an infection. In narcolepsy, the H1N1influenzavirus has been identified as one of the triggering infections that could lead to the disease, for example.

研究人员正在寻找是否年代ARS-CoV-2 virus that causesCOVID-19could also trigger the immune response thought to lead to narcolepsy.

But that work has not yet determined if there's a connection, according toRichard K. Bogan, MD, associate clinical professor at the University of South Carolina School of Medicine and chief medical officer of the sleep diagnostic company SleepMed, based in Columbia, South Carolina, who is not involved in that ongoing work.
Since hypocretin levels are typically normal in people with type 2 narcolepsy, the cause of type 2 narcolepsy remains unclear. Researchers have several theories, including one that suggests that type 2 narcolepsy may actually be an early form of type 1 narcolepsy, as cataplexy (the hallmark of type 1 narcolepsy) can appear years after other symptoms.

And finally, in the rare cases of secondary narcolepsy, brain injury is the cause. In those cases, injury to the brain region that regulates REM sleep and wakefulness (either because of a traumatic brain injury, a tumor, or other disorder) leads to narcolepsy symptoms.

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