First described in 1877 (15) and given its name by Gelineau in 1880,(!6,> narcolepsy is not a rare condition. According to most recent evaluations its prevalence is 0.21 to 0.26 per 1000,(1 18) i.e. slightly less than the prevalence of multiple sclerosis.

The two cardinal symptoms of narcolepsy are excessive daytime sleepiness culminating in recurrent daily irresistible and refreshing episodes of sleep, and cataplexy, a sudden loss of muscle tone triggered by an emotional factor, most often a positive one, such as laughter, compliments, or dry humour, and less often a negative one, such as stress or anger. Cataplexy occurs with a frequency anywhere between once a year or less to several times a day. The loss of tone may be total, causing the subject to fall, but much more frequently it is partial when the subject is unable to articulate words, has a strange feeling in the face, or experiences unlocking of the knees. It may last from a split second to several minutes. Other symptoms are not necessary for the diagnosis. Hypnagogic (at sleep onset) or hypnopompic (at sleep offset) hallucinations are somesthetic, auditory, or sometimes visual. The impression of a human presence in the room is frequent. Hypnagogic hallucinations are sometimes so frightening that the subject keeps a weapon on the bedside table or a dog in the room for reassurance. Sleep paralysis is an inability to move the arms, legs, and head or to breathe normally. It is often associated with an hypnagogic hallucination. It is terrifying, and may last for 10 min or more. Finally, narcoleptic subjects experience disturbed nocturnal sleep with recurrent awakening, sleeptalking, and frequently rapid eye movement ( REM) sleep behaviour disorder.

Diagnosis of narcolepsy is based on clinical features. However, there are cases where there is uncertainty due to rare or atypical cataplexies. In these cases laboratory tests (polysomnography and HLA typing) are particularly helpful. Polysomnography includes monitoring of night sleep followed by the multiple sleep latency test procedure. Night sleep is characterized by increased Stage I sleep, increased number of awakenings, and often a sleep-onset REM period. The multiple sleep latency test typically shows a mean sleep latency less than 7 min over the five sessions of the test and two sleep-onset REM periods at least. However, subjects with typical narcolepsy may show only one or even no sleep-onset REM period in some cases. HLA typing shows an association with HLA DR15-DQ6 (DRB1*1501-DQB1*0602) in 96 to 98 per cent of Caucasian patients and in 60 per cent of African-American patients.

The natural history of narcolepsy is chronic. Excessive daytime sleepiness is lifelong, although subjects cope with it more easily after retirement. Cataplexy may vanish with age in some subjects. Hypnagogic hallucinations and sleep paralysis are most often temporary. Poor sleep does not show a tendency to improve.

The mainstay of treatment is pharmacological, although taking naps may alleviate excessive daytime sleepiness, and refraining from emotion may prevent cataplexy. Excessive daytime sleepiness is treated by stimulants (methylphenidate, methamphetamine, mazindol, pemoline) or by modafinil, a new drug with awakening properties. Cataplexy and other REM-related symptoms, hypnagogic hallucinations, and sleep paralysis respond to tricyclic or selective serotonin reuptake inhibitor antidepressants. Poor sleep may be improved by the use of hypnotics.

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