Aetiology

The most common explanation for parasomnias is that sleep and wakefulness are not mutually exclusive states but frequently overlap, and intrusion of these states into one another may cause parasomnias.[1][4][6] Intrusion of wakefulness into non-rapid eye movement (NREM) sleep may produce arousal disorders, and intrusion of wakefulness into rapid eye movement (REM sleep) may produce REM sleep parasomnias.[1][6]

The onset of disorders of arousal tends to occur during slow-wave sleep (SWS). Given that SWS predominates during the first third of the sleep period, these disorders are more prevalent at the beginning of the night. However, they may still occur in the second half of the night. They are common in childhood, usually decreasing in frequency with increasing age.[16][17]

Genetic predisposition may give rise to an inherent instability of NREM sleep.

Arousal disorders may be triggered by a variety of conditions, including fever, acute sleep deprivation, emotional stress, and medications. Frequently, medical sleep disorders such as restless legs syndrome (RLS) and obstructive sleep apnoea (OSA) cause NREM parasomnias due to the frequent arousals they cause.[27][28] These precipitators should be viewed as triggering events in susceptible individuals rather than as causal factors.

An underlying psychopathology may play a role in patients with nightmares.[29]

Pathophysiology

The pathophysiology of non-rapid eye movement (NREM) parasomnias is thought to involve disturbances of arousal, and specifically an impaired ability to arouse fully from deep slow-wave sleep (SWS).[30][31][32][33][34][35] Conversely, rapid eye movement sleep behaviour disorder (RBD) often results from serious neuropathology, which affects the area of the brain responsible for inhibiting muscle tone during rapid eye movement (REM) sleep.[11]

Classification

Diagnostic and statistical manual of mental disorders, fifth edition, text revision (DSM-5-TR) classification[12]

The American Psychiatric Association classification of mental disorders DSM-5-TR recognises the following as parasomnias:

  • Non-rapid eye movement (NREM) sleep arousal disorders

    • Recurrent episodes of incomplete awakening from sleep, usually occurring during the first third of the major sleep episode, accompanied by either one of sleepwalking or sleep terrors

    • No or little (e.g., only a single visual scene) dream imagery is recalled

    • Amnesia for the episodes is present

    • The episodes cause clinically significant distress or impairment in social, occupational, or other important areas of functioning

    • The disturbance is not attributable to the physiological effects of a substance (e.g., a drug of abuse or a medication)

    • Coexisting mental disorders and medical conditions do not explain the episodes of sleepwalking or sleep terrors.

  • Rapid eye movement sleep behaviour disorder (RBD)

    • Repeated episodes of arousal during sleep accompanied by vocalisation and/or complex motor behaviours.

    • Behaviours occur during rapid eye movement (REM) sleep. Typically 90 minutes after sleep onset and are more frequent during the later portions of the sleep. Uncommon during daytime naps.

    • Not confused or disorientated during awakening.

    • Either of the following: REM sleep without atonia on polysomnographic recording or a history suggestive of REM sleep behaviour disorder and an established synucleinopathy diagnosis (e.g., Parkinson's disease, multiple system atrophy).

    • May cause clinically significant distress or impairment in social, occupational, or other important areas of functioning (e.g., injury to self).

    • The disturbance is not attributable to the physiological effects of a substance (e.g., a drug of abuse, a medication) or another medical condition.

  • Nightmare disorder

    • Repeated occurrences of long, dysphoric, and well-remembered dreams (these usually involve efforts to avoid threats to survival, security, or physical integrity).

    • On awakening the individual rapidly becomes oriented and alert.

    • The sleep disturbance causes clinically significant distress or impairment in social, occupational, or other important areas of functioning.

    • The nightmare symptoms are not attributable to the physiological effects of a substance (e.g., a drug of abuse, a medication).

International classification of sleep disorders, 3rd edition, text revision (ICSD-3-TR)[11]

The ICSD-3-TR lists parasomnias as follows:

  • NREM-related parasomnias

    • Disorders of arousal (from NREM sleep)

      • Confusional arousals

      • Sleepwalking

      • Sleep terrors

      • Sleep-related eating disorder

  • Rapid eye movement (REM)-related parasomnias

    • REM sleep behaviour disorder

    • Recurrent isolated sleep paralysis

    • Nightmare disorder

  • Other parasomnias

    • Exploding head syndrome

    • Sleep-related hallucinations

    • Sleep-related urological dysfunction

    • Parasomnia due to a medical disorder

    • Parasomnia due to a medication or substance

    • Parasomnia, unspecified

Sleep-related movement disorders (e.g., restless legs syndrome, periodic limb movement disorder, sleep-related leg cramps, bruxism, and benign sleep myoclonus of infancy) are important in the consideration of paediatric parasomnias. The ICSD-3-TR classifies these events as movement disorders, as opposed to true parasomnias. It also classifies sleep-talking and sleep starts as isolated symptoms and normal variants.

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