Sleep Disorders Classification

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The Association of Sleep Disorders Centers initially classified sleep-wake disorders into four categories, including disorders of initiating and maintaining sleep;

disorders of excessive somnolence; disorders of sleep-wake schedule; and dysfunctions associated with sleep, sleep stages, or partial arousals. In 1990, the ICSD described a system, which was slightly revised in 1997, that divided the 84 sleep disorders into four classes: dyssomnias, parasomnias, sleep disorders associated with medical or psychiatric disorders, and proposed sleep disorders.

The latest ICSD was published in 2005 (63). ICSD-2 divides sleep disorders into eight categories: insomnias; sleep-related breathing disorders; hypersomnias of central origin not due to a circadian rhythm sleep disorder, sleep-related breathing disorder, or other cause of disturbed nocturnal sleep; circadian rhythm sleep disorders; parasomnias; sleep-related movement disorders; isolated symptoms, apparently normal variants, and unresolved issues; and other sleep disorders. Some divisions were based according to a common complaint, others on etiology, and still others on the organ system from which the disorder arose.

Insomnias are further subdivided into 11 categories. An adjustment (acute) insomnia is due to an identifiable stressor and has a duration of a few days to a few weeks; once the patient adjusts to or experiences resolution of the inciting stressor, the insomnia resolves. Psychophysiological insomnia is characterized by learned sleep-preventing associations and heightened arousal with decreased functioning during wakefulness. Paradoxical insomnia occurs without either objective sleep disturbance or the level of daytime impairment expected with the degree of reported sleep deficits. Idiopathic insomnia is a chronic problem with insomnia that had an insidious onset appreciated during infancy or childhood. The fifth category of insomnia includes those cases due to an underlying mental disorder. The next category, inadequate sleep hygiene, is produced from an individual's activities, which increase arousal and are inconsistent with sleep organization. Behavioral insomnia of childhood includes both the sleep-association type and the limit-setting type. Insomnia due to a drug or substance includes a suppression or disruption of sleep from a prescribed medicine, recreational drug, alcohol, caffeine, food, or environmental toxin. Insomnia due to a medical condition can entail difficulty with either initiation or maintenance of sleep or be interpreted as poor quality sleep. Insomnia that is not due to substance use or a known physiologic condition (nonorganic insomnia, not otherwise specified) is suspected to be due to an underlying mental disorder, psychological factors, or sleep-disruptive practices. The final category of insomnia, physiologic (organic) insomnia, unspecified, includes those cases thought to be related to an underlying medical disorder, physiological state, or substance use.

Sleep-related breathing disorders include central sleep apnea syndromes (including primary in adults or children as well as those related to Cheyne Stokes, high-altitude, medical conditions, or drugs), obstructive sleep apnea syndromes, sleep-related hypoventilation/hypoxemic syndromes (related to alveolar hypoventilation or a medical condition), and other sleep-related breathing disorders. Hypersomnias of central origin include narcolepsy, recurrent hypersomnia (as in Kleine-Levin syndrome or as related to menses), idiopathic hypersomnia (with or without a long sleep time), behaviorally induced insufficient sleep syndrome, or hypersomnia due to a medical condition, drug, or nonorganic or physiological (organic) factors.

Circadian rhythm disorders are caused by disruption of sleep-wake schedule changes and share an underlying chronophysiological basis. The primary feature of delayed sleep phase syndrome, advanced sleep phases syndrome, and non-24-hour sleep-wake syndrome is that the patient's sleep pattern does not match their desired (or socially acceptable) sleep pattern. The irregular sleep-wake pattern is characterized by intermittent sleep episodes. Time-zone (jet lag) syndrome is familiar to most of us who have crossed multiple time zones. Shift work sleep disorder occurs when the work hours of the employee impose an abnormal shift in their sleep-wake cycle.

Parasomnias usually associated with REM sleep share a common pathophysiological mechanism related to REM sleep. Unlike sleep terrors, sleepwalking, and confusional arousals, which are all associated with SWS, nightmares are REM sleep phenomena. Sleep paralysis is its own entity but can occur in narcoleptics. REM sleep-related sinus arrest is quite rare. REM sleep behavior disorder (RBD) is more common and can manifest in association with other disorders. Acute cases of RBD have toxic, metabolic etiologies, particularly withdrawal from ethanol (64). Chronic cases can be due to neurological disorders [e.g., Parkinson's disease and other neurodegenerative disorders (65), subarachnoid hemorrhage (66), pontine neoplasm (67), or narcolepsy (68)] or may be idiopathic (66). Other types of para-somnias include sleep-related dissociative disorders, sleep enuresis, sleep-related groaning, exploding head syndrome, sleep-related hallucinations, and sleep-related eating disorder.

Sleep-related movement disorders include restless legs syndrome, periodic limb movement disorder, leg cramps, bruxism, and rhythmic movement disorder. These conditions are characterized by relatively simple, usually stereotyped, movements.

Sleep disorders within the isolated symptoms category include long sleeping, short sleeping, snoring, sleeptalking, sleep starts (hypnic jerks), benign sleep myoc-lonus of infancy, hypnangogic foot tremor and alternating leg muscle activation during sleep, propriospinal myoclonus at sleep onset, and excessive fragmentary myoclonus. Sleep disorders that cannot be classified elsewhere are assigned to the final category, other sleep disorders. These include physiological (organic) sleep disorders and environmental sleep disorders.

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