Etiology
The following parasomnias do not have a known etiology but are considered disorders of arousal:
Confusional arousals
Sleep terrors
Sexsomnia
Exploding head syndrome
Sleep-related hallucinations
Sleep-related eating disorder
Nightmares are postulated to be simulations of threatening events and to serve a rehearsal function important for survival.[2][18] Recurrent nightmares are thought to occur due to a combination of impaired fear extinction and hyperarousal, which result in a nightmare script that is replayed and causes distress.[2][18][19]
Enuresis is associated with central nervous system immaturity, disorders of arousal, increased fluid intake, urinary problems both functional and structural, male sex, maternal smoking, mother’s age less than 20 at the time of the child’s birth, psychosocial stressors, and attention deficit hyperactivity disorder.[20] Sleepwalking has been associated with the human leukocyte antigen gene DQB1.[21]
Rapid eye movement sleep behavior disorder (RBD) can occur in an isolated form or secondary to other etiologies such as neurodegenerative diseases, narcolepsy, and other disorders that impact the brainstem nuclei involved in rapid eye movement (REM) sleep. The isolated RBD often precedes the development of neurodegenerative disorders, especially the synucleinopathies, in which insoluble protein (alpha-synuclein) is deposited in brain tissue.[22] The prototype condition in this category is Parkinson disease.[23]
RBD symptoms may also be precipitated by medications including tricyclic antidepressants, selective serotonin-reuptake inhibitors, cholinergic agents, and monoamine oxidase inhibitors.[24] Recurrent isolated sleep paralysis does not have a known etiology and may be associated with sleep deprivation (e.g., irregular sleep-wake schedules, mental stress, anxiolytic medication).[2][25]
Pathophysiology
The precise pathophysiology is unknown for the following:
Exploding head syndrome
Nightmare disorder
Sleep-related eating disorder
Enuresis
Nonrapid eye movement (NREM) sleep parasomnias
Confusional arousals and sleepwalking are characterized by NREM sleep instability, noted even on the nonsleepwalking nights. In electroencephalographic-neuroimaging studies, sleepwalking motor events are also associated with arousal-related activation of cingulate motor area.[26] This is similar to the instability noted in patients with the upper airways resistance syndrome.[27]
High levels of fragmentation of slow-wave sleep underlie the pathophysiology of sleep terrors and sleepwalking.[28]
Patients with sleep-related eating disorder tend to share psychological characteristics of people with daytime eating disorders, and they tend to have mildly fragmented sleep and reduced sleep efficiency.
Patients with sleepwalking have simultaneous wakefulness originating from the motor and cingulate cortices and persistent sleep in associative cortical regions. There is a decline in gray matter volume in the dorsal posterior and posterior midcingulate cortices in sleepwalkers representing a neuroanatomic correlate to this pathophysiologic finding.[30]
Rapid eye movement (REM) sleep parasomnias
RBD is a type of REM sleep parasomnia and its pathophysiology is complex, but early research supports the hypothesis that lesions of the tegmentum of the pons may result in loss of atonia and motor activity during REM sleep.[31] It is postulated that the pontine tegmentum accommodates an atonia mechanism and a mechanism that suppresses brainstem motor pattern generation. Different behaviors (atonia alone versus atonia with more complex, unsuppressed motor activity) could therefore be expected depending on the precise location of injury.
Recurrent isolated sleep paralysis arises from REM sleep, and may be viewed as a dissociated state in which REM sleep elements persist into wakefulness.[2]
Other parasomnias
Although REM dream intrusion into the wake-sleep-wake transition is presumed to occur in sleep-related hallucinations, the exact pathophysiology is also unknown.[2]
Classification
The international classification of sleep disorders (3rd edition), text revision (ICSD-3-TR)[2]
1) Nonrapid eye movement (NREM) parasomnias
Confusional arousals: characterized by confusion noted when the patient arouses or is aroused from sleep, usually slow-wave sleep.
Sleepwalking: results in patient ambulation during sleep. Sleep or an altered level of consciousness will persist during the episode.
Sleep terrors: presents with manifestations of fear and autonomic hyperactivity (e.g., tachycardia, pupillary dilation, increased blood pressure) during an arousal from slow-wave sleep. Often heralded by a loud vocalization sometimes described as a blood-curdling scream.
Sleep-related eating disorder: patients eat and/or drink after sleep arousal. The behavior is involuntary and may or may not be subsequently recalled.
2) Rapid eye movement (REM) parasomnias
RBD is a type of REM parasomnia manifested by vigorous and even violent behavior occurring during REM sleep. Associated with abnormal tone during REM sleep and seems to represent vivid dream enactment. Patients often develop neurodegenerative disorders that are usually synucleinopathies, such as Parkinson disease. A variant of RBD also exists that presents with both dream-enacting behavior out of REM sleep and either confusional arousal or sleepwalking, and tends to happen in a younger group of subjects without a clear association with synucleinopathies. It is called parasomnia overlap syndrome.
Recurrent-isolated sleep paralysis: results in the inability to speak or move at the time of sleep onset or on waking, typically from REM sleep. Diagnosis implies that narcolepsy is not also present and that no other disorders that could result in transient paralysis are present. Typically occurs during REM sleep.
Nightmare disorder: patients report recurrent unpleasant dreams resulting in sleep disruption. The nightmares usually occur during REM sleep.
3) Other parasomnias
Exploding head syndrome: patients describe a loud noise or a sensation of explosion in the head occurring during the wake-sleep or sleep-wake transitions.
Sleep-related hallucinations: may occur at sleep onset (hypnagogic) or on waking from sleep (hypnopompic) and are usually visual experiences.
Enuresis or bedwetting tends to occur more in children, but adults can suffer from it as well.
Diagnostic and statistical manual of mental disorders, 5th ed., text revision (DSM-5-TR) classification[1]
DSM-5-TR classifies parasomnias as follows:
Nonrapid eye movement sleep arousal disorders
Nightmare disorder
Rapid eye movement sleep behavior disorder
For further details, see Diagnostic criteria.
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