Sleepiness often arises as a result of sleep-related breathing disorders (SRBDs), because of sleep fragmentation and most likely hypoxemia as well. Loud snoring is a cardinal symptom of obstructive sleep apnea (OSA) and upper airway resistance syndrome (UARS) and may be accompanied by mouth breathing, unusual body positions, or visible restlessness during sleep. Respiratory pauses are sometimes witnessed by bed partners or family members, sometimes terminating with a snort or gasp when breathing resumes. Such symptoms are most informative when present, but the absence of observed apnea and even snoring does not preclude the possibility of an underlying SRBD.

Restless legs syndrome (RLS) and periodic limb movement disorder (PLMD) may also be associated with excessive daytime somnolence, especially when the condition impacts the duration, quality, or continuity of nighttime sleep. Restless legs syndrome is characterized by an urge to move the extremities (with or without dysesthesias), motor restlessness, worsening of symptoms at rest, relief with activity, and exacerbation of symptoms during evening or nighttime (3). Children may exhibit “growing pains” as a symptom of RLS (4). The leg movements that accompany PLMD may range from gentle dorsiflexion of the toes or feet to fairly vigorous kicking movements with visible arousal from sleep. Arousals from sleep may accompany periodic limb movements, but the latter may not cause the arousals, and the presence of arousals does not reliably predict associated sleepiness (5,6).

Hypnagogic (at sleep onset) and hypnopompic (at waking) hallucinations are brief, dreamlike episodes that last seconds to minutes. Hallucinations are often vivid and sometimes distressing despite their brevity. Although these hallucinatory episodes are often reported by patients with narcolepsy, they may also occur in association with other sleep disorders, a variety of psychiatric conditions, and as a medication side effect (7,8).

Sleep paralysis is a condition in which muscle atonia, normally restricted to rapid eye movement (REM) sleep, occurs at the interface between sleep and wakefulness. Sleep paralysis may be complete or partial, and sometimes coincides with hypnagogic hallucinations. Although episodes usually last for only about a minute or less, they may be extremely frightening and accompanied by sensations of suff ocation. Episodes can often be terminated by extraneous stimuli such as noise or touch. Sleep paralysis is frequently reported by patients with narcolepsy, but may also be seen with other sleep disorders, as a familial condition, or as an occasional phenomenon in normal individuals (9, 10 and 11).

Automatic behavior refers to episodes of purposeful but sometimes inappropriate behavior occurring during periods of sleepiness, usually with partial or absent subsequent recollection of the activity (12). Some varieties of automatic behavior are common and benign, such as briefly arising from bed while groggy to use
the bathroom and not remembering the event the next day. Other varieties may be more ominous, as when a drowsy driver arrives at a familiar destination with no recollection of how they got there. Automatic behavior that is prolonged, frequent, or potentially unsafe is usually indicative of significant sleepiness.

Cataplexy is characterized by paroxysmal episodes of diffuse muscle weakness or paralysis, triggered by laughing or emotion. The phenomenon occurs when the muscle atonia normally restricted to REM sleep is inappropriately expressed during wakefulness. Unlike sleep paralysis, cataplectic episodes usually occur during sustained wakefulness and are triggered by strong emotions such as laughter, surprise, or anger. Cataplexy is typically brief, lasting seconds to minutes, but successive attacks precipitated by extreme emotional stimuli (status cataplecticus) may rarely last as long as 1 hour (13). Severity of muscle atonia during episodes is highly variable. Mild attacks may consist only of a brief sensation of weakness without loss of posture. Severe episodes may cause complete paralysis sparing only respiration, eye movements, and sphincters. Most often, loss of muscle tone is partial, and may take the form of knee buckling, head drop, slurred speech, or loss of postural tone. Consciousness is maintained during attacks, distinguishing them from syncope and atonic seizure, although some patients report falling asleep during the immobility of severe cataplectic attacks.

Narcolepsy with cataplexy is characterized primarily by excessive daytime somnolence and cataplexy. Sleepiness is usually most evident during sedentary or monotonous activities, but may also take the form of irresistible sleep attacks that may interrupt more active endeavors. Daytime sleep periods in patients having
narcolepsy with cataplexy are usually short and refreshing, but followed within hours by recurrent sleepiness. Severity of somnolence in affected patients ranges from mild to disabling.

Cataplexy is a phenomenon unique to narcolepsy but often develops years after onset of sleepiness. Episodes are most commonly triggered by “positive” emotional experiences such as laughter, elation, or surprise and less consistently by other emotions. Attacks may be localized to specific body areas (e.g., face) or involve all skeletal muscle groups in a generalized fashion. Respiration is never directly affected, but subjective sensations of choking or dyspnea are sometimes reported. The frequency of cataplexy in affected patients ranges from rare and isolated attacks to innumerable daily episodes.

Nonobligate clinical manifestations frequently exhibited by patients having narcolepsy with cataplexy include sleep attacks, sleep paralysis, hypnagogic hallucinations, and automatic behavior (described in the “Clinical Presentation” section). These phenomena affect a small proportion of normal sleepers as well. REM sleep behavior disorder (RBD)—episodes of complex motor behavior arising during REM sleep—has been reported with increased frequency in adults with narcolepsy (22). Nocturnal sleep disruption is frequently seen in individuals having narcolepsy with cataplexy, often taking the form of excessive night waking. Disrupted nighttime sleep in these patients is often sufficient to further exacerbate daytime sleepiness.

Onset of narcolepsy with cataplexy is most commonly observed during adolescence or young adulthood, but occasionally is encountered in younger children (23). Cataplexy usually develops several years after onset of sleepiness, but may sometimes arise before or long after the onset of somnolence. Laboratory evidence suggests that loss of hypocretin-1 secreting cells in the hypothalamus, possibly on an autoimmune basis, plays a pathogenetic role in the majority of cases (24, 25 and 26).

Narcolepsy without cataplexy is similar in most clinical respects to narcolepsy with cataplexy except for the lack of definite cataplectic episodes. As some patients in
this category eventually develop cataplexy later in the course of their disease, the diagnostic classification may also change at that time (13).

Despite the clinical similarities between narcolepsy without cataplexy and narcolepsy with cataplexy, some evidence suggests that the underlying pathophysiology of the two conditions is not identical. Cerebrospinal fluid (CSF) hypocretin-1 levels are most often normal for patients who have narcolepsy without cataplexy, whereas they are substantially decreased or undetectable in those who have narcolepsy with cataplexy (25,27). Recent postmortem analyses identified partial loss of hypocretin-secreting neurons in the hypothalamus in some cases of narcolepsy without cataplexy (28).

Narcolepsy with and without cataplexy has been reported in association with a variety of medical and neurological conditions (29). Genetic disorders associated with narcolepsy and cataplexy include Niemann-Pick disease type C (21,30), Prader-Willi syndrome (31), and possibly Coffin-Lowry syndrome (32). Structural lesions in the hypothalamic region, including tumors, sarcoidosis, multiple sclerosis, and neurocysticercosis may also cause secondary narcolepsy (33, 34 and 35). Symptomatic narcolepsy has also been reported in several neurological disorders not having demonstrable hypothalamic involvement, including acute disseminated encephalomyelitis, multiple system atrophy, and traumatic brain injury (36, 37, 38 and 39).

Narcolepsy, unspecified is defined by the ICSD-2 as a temporary classification for patients who meet clinical and multiple sleep latency testing (MSLT) criteria for narcolepsy but require additional evaluation for more precise classification.

The recurrent hypersomnias are rare conditions in which prolonged episodes of excessive sleepiness are separated by periods of normal alertness and function. In Kleine-Levin syndrome, which most often affects adolescent males, patients may sleep for all but a few hours daily during episodes that last days to weeks (40). Somnolence is often accompanied by variable disturbances of mood, cognition, and temperament, often including increased appetite and significantly aggressive or hypersexual behavior (41). Episodes may occur up to 10 times yearly, but in most cases subside with advancing age (13,42). No specific etiology for Kleine-Levin syndrome has been established, but intermittent hypothalamic dysfunction or autoimmune etiologies have been postulated (43).

Menstrual-associated hypersomnia represents a poorly characterized condition in which episodic sleepiness coincides with the menstrual cycle, and is postulated to be secondary to hormonal influences (44).

Idiopathic hypersomnia with long sleep time is characterized by pervasive daytime sleepiness despite longer-than-average nighttime sleep duration (45,46). Affected patients often report feeling unrefreshed or confused upon morning waking (sleep drunkenness) despite prolonged nighttime sleep periods of 10 or more hours. Daytime sleepiness is pervasive and often severe. Daytime naps are long and unrefreshing compared to those of narcoleptic patients. The condition most often develops during the early adult years with a chronic but usually stable course. The pathophysiology of the condition is unknown, but in contrast to narcolepsy with cataplexy, defects of hypocretin-1 metabolism have seldom been identified in affected patients (47).

Idiopathic hypersomnia without long sleep time is characterized by constant, often severe daytime sleepiness in the context of nighttime sleep of normal or only slightly prolonged duration (45,48). Although the pervasive somnolence, sleep drunkenness, and unrefreshing naps seen in this condition are otherwise comparable to those seen in idiopathic hypersomnia with long sleep time, the nighttime sleep period is less than 10 hours.

Excessive daytime sleepiness often results primarily from habitually insufficient nighttime sleep (49). Review of the sleep history in affected patients reveals a chronically shortened nighttime sleep period that is either less than the patient’s premorbid baseline, and/or substantially less than normal for age. Daytime symptoms are those of chronic sleep deprivation, including sleepiness, irritability, disturbed mood, and impaired school or work performance. Symptoms remit with sustained lengthening of the nighttime sleep period, but transient periods of longer sleep on weekends or holidays are seldom sufficient to provide complete relief.

Hypersomnia due to medical condition may be diagnosed when chronic sleepiness represents the direct result of a coexisting medical or neurological condition, but clinical and MSLT criteria for narcolepsy are not fulfilled. Severity of daytime somnolence and length of nighttime sleep vary considerably among patients. Associated symptoms may include automatic behavior or sleep paralysis in the absence of cataplexy.

A variety of causes may underlie this disorder. Associated neurological conditions may include stroke, brain tumor, encephalitis, head trauma, and Parkinson’s disease (50, 51, 52 and 53). Genetic conditions associated with excessive sleepiness most notably include Prader-Willi syndrome and myotonic dystrophy (54, 55 and 56). Endocrine and toxic-metabolic causes include hypothyroidism, hypoadrenalism, hepatic encephalopathy, and renal failure. Drug-induced and psychiatric causes are classified elsewhere, as outlined in the following sections.

Hypersomnia due to drug or substance

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