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Narcolepsy (cont.)

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What causes narcolepsy?

Advances have been made in the last few years in determining the cause of narcolepsy. The newest discovery has been the finding of abnormalities in the structure and function of a particular group of nerve cells, called hypocretin neurons, in the brains of patients with narcolepsy. These cells are located in a part of the brain called the hypothalamus and they normally secrete neurotransmitter substances (chemicals released by nerve cells to transmit messages to other cells) called hypocretins.

Abnormalities in the hypocretin system may be responsible for the daytime sleepiness and abnormal REM sleep found in narcolepsy. (See the section below on sleep laboratory tests for a discussion of REM sleep.)

Experiments in dogs and mice with narcolepsy suggest an abnormal hypocretin system as a cause for the development of their narcolepsy. People with narcolepsy have been found to have a markedly decreased number of hypocretin nerve cells in the brain. They also have a decreased level of hypocretins in the cerebrospinal fluid (the fluid that surrounds the brain and the spinal cord).

Narcolepsy is associated with a specific type of human leukocyte antigen (HLA). HLAs are genetically determined proteins on the surface of white blood cells. They are a part of the body's immune (defense) system. The finding of a very high HLA- association in narcolepsy led to the proposal that narcolepsy is an autoimmune disease, similar to other HLA-associated diseases such as multiple sclerosis and ankylosing spondylitis.

It is theorized that an autoimmune reaction causes the loss of nerve cells in the brain in patients with narcolepsy. The environment (for example, infection or trauma) might trigger an autoimmune reaction where normal brain cells are attacked by the body's own immune system. As a result, the neurons are damaged and ultimately destroyed, and they and their neurotransmitter chemicals disappear. Whether narcolepsy is an autoimmune disease remains to be proven.

The role of heredity in humans with narcolepsy is not completely understood. No consistent pattern of heredity has been recognized in families so far. It is estimated that relatives of patients with narcolepsy may have a higher predisposition to develop narcolepsy or sleep-related abnormalities, such as increased daytime sleepiness, increased REM sleep, or others. In dogs with narcolepsy, the disease is inherited in a predictable pattern. In these animals, the narcolepsy is caused by a mutation in a particular gene that is normally responsible for producing a receptor (binder) in the brain for the hypocretin neurotransmitter.

How is narcolepsy diagnosed?

The diagnosis of narcolepsy is based on a clinical evaluation, specific questionnaires, sleep logs or diaries, and the results of sleep laboratory tests.

Clinical evaluation

Clinical evaluation includes a detailed medical history and physical examination by a physician.

Questionnaires

Questionnaires may be used in the assessment of patients with symptoms that suggest narcolepsy. The Stanford Narcolepsy Questionnaire is an extensive questionnaire that can provide the physician with valuable information on all symptoms of narcolepsy, but especially on cataplexy. The Epworth Sleepiness Scale is a brief self-administered questionnaire that provides an estimate of the degree of daytime sleepiness. A person rates the likelihood of falling asleep during specific activities. Using the scale from 0 to 3 below, the person ranks their risk of dozing in the chart below.

0 = Unlikely to fall asleep
1 = Slight risk of falling asleep
2 = Moderate risk of falling asleep
3 = High likelihood of falling asleep

SituationRisk of Dozing
Sitting and reading
Watching television
Sitting inactive in a public place
As a passenger in a car riding for an hour, no breaks
Lying down to rest in the afternoon
Sitting and talking with someone
Sitting quietly after lunch, without alcohol
In a car, while stopped for a few minutes in traffic

After ranking each category, the total score is calculated. The range is 0 to 24, with the higher the score the more sleepiness.

Scoring:

  • 0 to 9 = Average daytime sleepiness
  • 10 to 15 = Excessive daytime sleepiness
  • 16 to 24 = Moderate to severe daytime sleepiness

Sleep logs or sleep diaries

Sleep logs or sleep diaries for 2 to 3 weeks are recommended in the evaluation of any patient with excessive daytime sleepiness. Sleep diaries record the patient's usual sleep patterns (sleep deprivation, irregular sleep/wake pattern, interrupted sleep), alcohol and/or drug use, and common behaviors that cause the patient to lose sleep (for example, Internet syndrome - surfing the Internet until late at night, causing sleep deprivation and daytime sleepiness.) This information may be helpful in the evaluation of a patient with excessive daytime sleepiness.

Sleep laboratory tests

Sleep laboratory tests ("sleep studies") for narcolepsy include polysomnography (PSG) and the multiple sleep latency test (MSLT). Polysomnography (PSG) is a full night recording of several different physiological factors of a patient's sleep. A PSG is necessary to insure good quality sleep to identify any untreated causes for daytime sleepiness such as obstructive sleep apnea (OSA). The PSG is followed the next day by the multiple sleep latency test (MSLT), which is a recording of the patient's tendency to fall asleep during the day. These procedures provide objective measures of daytime sleepiness and REM sleep abnormalities.

Daytime sleepiness is measured in the MSLT by the sleep latency (SL) time. This is the time from the beginning of the recording to the onset of sleep. In healthy individuals, the SL time is more than 10 minutes, whereas in narcolepsy, it could be as short as 0.5 minutes (an almost immediate onset of sleep).

REM sleep

REM sleep is named for the rapid eye movements (REM) that characterize this phase of sleep. In REM sleep dreams are vivid, muscle activity is suppressed, and brain activity is high. The REM sleep abnormality that is characteristic of narcolepsy is referred to as sleep onset REM periods. In healthy individuals, the first REM sleep period occurs about 80 to 120 minutes after the onset of sleep. By contrast, in narcolepsy, the initial REM sleep period usually occurs within 15 minutes of the onset of sleep. In addition, narcolepsy patients will have two or more sleep onset REM periods during the multiple sleep latency test (MSLT) in the daytime.

PSG is also helpful in excluding other causes of daytime sleepiness, such as sleep apnea syndrome (SAS), periodic limb movements in sleep (PLMS), and sleep disruptions. In some cases, repeat tests may be recommended if there is worsening of the symptoms of narcolepsy despite treatment or if an additional sleep disorder is suspected (for example, sleep apnea syndrome).

Maintenance of wakefulness test (MWT)

Maintenance of wakefulness test (MWT) may be used to evaluate the effects of the treatment for narcolepsy. This test is a recording that measures the ability of a subject to stay awake during the day.

The diagnostic criteria for narcolepsy are described in the International Classification of Sleep Disorders. These criteria indicate that the diagnosis may be based on clinical symptoms alone if both excessive daytime sleepiness and cataplexy are present. If cataplexy is not present, however, the diagnosis should be based on the clinical symptoms and polysomnographic findings.

The blood test for a type of HLA

The blood test for a type of HLA (Human Leukocyte Antigen) has been observed to have a very high association with narcolepsy. Certain types of HLA are part of an individual's genetic or hereditary makeup and can be characteristic of certain conditions, especially autoimmune diseases. The particular HLA type associated with narcolepsy is not unique for this condition as it is also found in 20% of the general population. Therefore, HLA typing is used for the diagnosis of narcolepsy. However, there is a strong association with certain genes in patients diagnosed with narcolepsy and cataplexy. Many patients are known to have a specific HLA gene variant called DQB1*0602. Scientists continue to work in this area to better understand the pathogenesis of narcolepsy.

Medically Reviewed by a Doctor on 10/2/2017

Source: MedicineNet.com
https://www.medicinenet.com/narcolepsy/article.htm

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