Sleep paralysis


Sleep paralysis is a state, during waking up or falling asleep, in which a person is conscious but in a complete state of full-body paralysis. During an episode, the person may hallucinate, which often results in fear. Episodes generally last no more than a few minutes. It can recur multiple times or occur as a single episode.
The condition may occur in those who are otherwise healthy or those with narcolepsy, or it may run in families as a result of specific genetic changes. The condition can be triggered by sleep deprivation, psychological stress, or abnormal sleep cycles. The underlying mechanism is believed to involve a dysfunction in REM sleep. Diagnosis is based on a person's description. Other conditions that can present similarly include narcolepsy, atonic seizure, and hypokalemic periodic paralysis.
Treatment options for sleep paralysis have been poorly studied. It is recommended that people be reassured that the condition is common and generally not serious. Other efforts that may be tried include sleep hygiene, cognitive behavioral therapy, and antidepressants.
Between 8% to 50% of people experience sleep paralysis at some point during their lifetime. About 5% of people have regular episodes. Males and females are affected equally. Sleep paralysis has been described throughout history. It is believed to have played a role in the creation of stories about alien abduction and other paranormal events.

Symptoms and signs

The main symptom of sleep paralysis is being unable to move or speak during waking.
Imagined sounds such as humming, hissing, static, zapping and buzzing noises are reported during sleep paralysis. Other sounds such as voices, whispers, screaming, growling, and roars are also experienced. It has also been known that one may feel pressure on their chest and intense pain in their head during an episode. These symptoms are usually accompanied by intense emotions such as fear and panic. People also have feelings of drowning or sinking, being dragged out of bed or of flying, numbness, and feelings of electric tingles or vibrations running through their body.
Sleep paralysis may include hallucinations, such as an intruding presence or dark figure in the room. These are commonly known as sleep paralysis demons or shadow people. It may also include the inability to breathe or the individual feeling a sense of terror, accompanied by a feeling of pressure on one's chest and difficulty breathing.

Pathophysiology

The pathophysiology of sleep paralysis has not been concretely identified, although there are several theories about its cause. The first of these stems from the understanding that sleep paralysis is a parasomnia resulting from dysfunctional overlap of the REM and waking stages of sleep. Polysomnographic studies have found that individuals who experience sleep paralysis have shorter REM sleep latencies than normal along with shortened NREM and REM sleep cycles, and fragmentation of REM sleep. This study supports the observation that disturbance of regular sleeping patterns can precipitate an episode of sleep paralysis, because fragmentation of REM sleep commonly occurs when sleep patterns are disrupted and has now been seen in combination with sleep paralysis.
Another major theory is that the neural functions that regulate sleep are out of balance, causing different sleep states to overlap. In this case, cholinergic sleep "on" neural populations are hyperactivated and the serotonergic sleep "off" neural populations are under-activated. As a result, the cells capable of sending the signals, that would allow for complete arousal from the sleep state, the serotonergic neural populations, have difficulty in overcoming the signals sent by the cells that keep the brain in the sleep state. During normal REM sleep, the threshold for a stimulus to cause arousal is greatly elevated. Under normal conditions, medial and vestibular nuclei, cortical, thalamic, and cerebellar centers coordinate things such as head and eye movement, and orientation in space.
In individuals reporting sleep paralysis, there is almost no blocking of exogenous stimuli, which means it is much easier for a stimulus to arouse the individual. The vestibular nuclei in particular has been identified as being closely related to dreaming during the REM stage of sleep. According to this hypothesis, vestibular-motor disorientation, unlike hallucinations, arise from completely endogenous sources of stimuli.
If the effects of sleep "on" neural populations cannot be counteracted, characteristics of REM sleep are retained upon awakening. Common consequences of sleep paralysis include headaches, muscle pains or weakness or paranoia. As the correlation with REM sleep suggests, the paralysis is not complete: use of EOG traces shows that eye movement is still possible during such episodes; however, the individual experiencing sleep paralysis is unable to speak.
Research has found a genetic component in sleep paralysis. The characteristic fragmentation of REM sleep, hypnopompic, and hypnagogic hallucinations have a heritable component in other parasomnias, which lends credence to the idea that sleep paralysis is also genetic. Twin studies have shown that if one twin of a monozygotic pair experiences sleep paralysis that the other twin is very likely to experience it as well. The identification of a genetic component means that there is some sort of disruption of a function at the physiological level. Further studies must be conducted to determine whether there is a mistake in the signaling pathway for arousal as suggested by the first theory presented, or whether the regulation of melatonin or the neural populations themselves have been disrupted.

Hallucinations

Several types of hallucinations have been linked to sleep paralysis: the belief that there is an intruder in the room, the feeling of a presence, and the sensation of floating. One common hallucination is the presence of an incubus. A neurological hypothesis is that in sleep paralysis the cerebellum, which usually coordinates body movement and provides information on body position, experiences a brief myoclonic spike in brain activity inducing a floating sensation.
The intruder and incubus hallucinations highly correlate with one another, and moderately correlated with the third hallucination, vestibular-motor disorientation, also known as out-of-body experiences, which differ from the other two in not involving the threat-activated vigilance system.

Threat hyper-vigilance

A hyper-vigilant state created in the midbrain may further contribute to hallucinations. More specifically, the emergency response is activated in the brain when individuals wake up paralyzed and feel vulnerable to attack. This helplessness can intensify the effects of the threat response well above the level typical of normal dreams, which could explain why such visions during sleep paralysis are so vivid. The threat-activated vigilance system is a protective mechanism that differentiates between dangerous situations and determines whether the fear response is appropriate.
The hyper-vigilance response can lead to the creation of endogenous stimuli that contribute to the perceived threat. A similar process may explain hallucinations, with slight variations, in which an evil presence is perceived by the subject to be attempting to suffocate them, either by pressing heavily on the chest or by strangulation. A neurological explanation holds that this results from a combination of the threat vigilance activation system and the muscle paralysis associated with sleep paralysis that removes voluntary control of breathing. Several features of REM breathing patterns exacerbate the feeling of suffocation. These include shallow rapid breathing, hypercapnia, and slight blockage of the airway, which is a symptom prevalent in sleep apnea patients.
According to this account, the subjects attempt to breathe deeply and find themselves unable to do so, creating a sensation of resistance, which the threat-activated vigilance system interprets as an unearthly being sitting on their chest, threatening suffocation. The sensation of entrapment causes a feedback loop when the fear of suffocation increases as a result of continued helplessness, causing the subjects to struggle to end the SP episode.

Diagnosis

Sleep paralysis is mainly diagnosed via clinical interview and ruling out other potential sleep disorders that could account for the feelings of paralysis. Several measures are available to reliably diagnose or screen for recurrent isolated sleep paralysis.

Diagnosis

Episodes of sleep paralysis can occur in the context of several medical conditions. When episodes occur independent of these conditions or substance use, it is termed "isolated sleep paralysis". When ISP episodes are more frequent and cause clinically significant distress or interference, it is classified as "recurrent isolated sleep paralysis". Episodes of sleep paralysis, regardless of classification, are generally short, but longer episodes also have been documented.
It can be difficult to differentiate between cataplexy brought on by narcolepsy and true sleep paralysis, because the two phenomena are physically indistinguishable. The best way to differentiate between the two is to note when the attacks occur most often. Narcolepsy attacks are more common when the individual is falling asleep; ISP and RISP attacks are more common upon awakening.

Differential diagnoses

Similar conditions include: