Exploring Recurrent Isolated Sleep Paralysis

Figure 1. Examples of anomalous perceptual experiences associated with SP

Figure 2. The classical symptom tetrad of narcolepsy

Take home points

Dr Buzzi is a practicing neurologist with a special interest in sleep disorders, including sleep paralysis, with a private practice in Ravenna, Italy. He has co-authored numerous publications in the fields of sleep apnea and restless legs syndrome.

Occasionally, during sleep, I start to wake up . . . or try to force myself to wake up. During these episodes, I can hear what is going on around me, but I’m unable to move or open my eyes. I’m not completely awake, but I’m not really asleep either. It’s an extremely frightening and stressful incident when it occurs. I’m wondering if this is normal-not being able to move is almost painful, as I can feel my muscles fighting to move and they can’t.

Clinical presentation and physiopathology

Sleep paralysis (SP) is quite easy to recognize once there is some familiarity with the disorder. Clinically, it is characterized by a transient inability to move or speak at sleep onset or upon awakening. The episodes last seconds to several minutes, and termination is spontaneous or induced by external stimulation (eg, if the subject is touched by another person). The patient may be unable to open their eyes and have a feeling of suffocation, although ocular and respiratory movements are usually intact. SP is commonly associated with intense fear and anxiety, which may be enhanced by the concomitant occurrence of multisensorial hallucinations. These hallucinations typically include an intruder (eg, a threatening presence; see Figure 1, Case Example 1) and an incubus (eg, a malevolent agent exerting pressure by sitting on the chest).

During SP, the patients are awake or half-awake, with varying degrees of awareness; they usually engage in a mental struggle to move or to cry out, yet they appear to be sleeping. They may experience a false awakening phenomenon (see Figure 1, Case Example 2).

SP may occur in an isolated form (ie, isolated sleep paralysis or ISP) or in the context of narcolepsy. Recurrent isolated sleep paralysis (RISP) is the recurrent form of ISP; it is listed by the International Classification of Sleep Disorders (ICSD) as a REM sleep-related parasomnia.

Sleep paralysis and associated hallucinations occur when REM sleep elements (such as muscular atonia and dream imagery) intrude into waking, with an overlap between the two states. SP can be facilitated by sleep schedule alteration, mental or physical fatigue, stress, shift work, jet lag, alcohol, sleep-disrupting medication. These factors cause sleep disruption with frequent awakenings, facilitating REM-wake overlap, and/or REM sleep deprivation with subsequent REM rebound with enhanced dreams at sleep onset. SP is also more likely to occur when the subject sleeps in a supine position.

SP and associated hallucinations have been interpreted in a variety of cultural contexts, with supernatural explanations ranging from malevolent spirits to alien encounters. As such, some patients may be reluctant to report these events.

Recent areas of interest

Recent research has focused on epidemiology, risk factors and neurocognitive aspects of this condition; the latter will not be considered in this brief overview (for more information, please see this report).

A systematic review¹ of SP prevalence found that 7.6% of the general population experienced at least one episode of sleep paralysis, with higher prevalence rates in students (28.3%) and in psychiatric patients (31.9%). In particular, 34.6 % of the patients with panic disorder reported lifetime sleep paralysis. The authors speculated that irregular sleep patterns, which is a risk factor for SP, are often associated with students and psychiatric patients and thus may be related to the higher prevalence rates in these populations. It should be noted that this study was unable to ascertain rates of recurrent SP or isolated SP (ie, SP not related with narcolepsy or other disorders).

Researchers also have observed a possible genetic influence. Although a familial form of ISP had previously been reported, Denis and colleagues² recently performed the first study to examine genetic factors associated with SP. This twin and molecular genetics study revealed a moderate genetic influence on SP.

On the other hand, a systematic review of the literature³ found a series of factors associated with both the frequency and intensity of SP episodes, including sleep problems and disorders as well as psychiatric illness. SP was found to be particularly prevalent in posttraumatic stress disorder and panic disorder. The causality of these relationships, however, has yet to be determined. It is not clear, for example, whether high levels of stress and anxiety are the cause or the consequence of SP.

Taken together, these findings suggest that SP may result from a concomitance of genetic, experiential, and environmental factors.

Clinical management

Differential diagnosis of SP is usually not difficult. The ICSD lists a few conditions that should be easy to rule out:

• Epileptic atonic seizures are unlikely to occur only during sleep;
• Cataplectic attacks of narcolepsy are triggered by intense emotions and occur during wakefulness;
• Periodic paralyses are characterized by attacks that last several hours and are not terminated by external stimulation.

In patients with recurrent episodes of SP, narcolepsy and other sleep disorders must be ruled out. Narcolepsy is characterized by excessive daytime sleepiness and cataplectic attacks (Figure 2) and by specific criteria in polysomnographic studies (which must include multiple sleep latency test). Other sleep disorders that may cause sleep disruption and thus facilitate the occurrence of SP also must be ruled out (Figure 2, Case Example 3).

Most individuals do not require treatment. Most importantly, the patients must be reassured of the benign nature of the disorder. Patients may be concerned about a possible psychiatric or even paranormal origin of the disorder, in particular when hallucinatory experiences are present. For those who experience frequent SP episodes, instructions on sleep hygiene (eg, avoiding irregular sleep patterns, avoiding alcohol and caffeine close to bedtime) may serve as preventive measures. Patients also should be reminded to avoid sleeping in a supine position.

To date, there are no randomized controlled trials for the pharmacologic treatment of RISP. Tricyclic antidepressants and selective serotonin reuptake inhibitors have been used to treat RISP due to their REM sleep-suppressing effect as seen in narcolepsy studies and small case studies of RISP. A manual for cognitive behavioral therapy, which seems to be a promising way to address SP, has been proposed,⁴ though it has yet to be validated.

Take Home Points

• SP may occur in an isolated form or in the context of narcolepsy
• SP is commonly associated with intense fear and anxiety, which may be enhanced by the concomitant occurrence of frightening hallucinations
• In patients with recurrent episodes, narcolepsy and other disorders must be ruled out
• Patients should be reassured that ISP is not harmful nor is it a mental disorder
• Most patients do not require treatment; the disorder can be managed with sleep hygiene
• SSRIs may be used in some cases
• Cognitive behavioral therapy seems to be a promising treatment

REFERENCES
1. Sharpless BA and Barber JP. Lifetime prevalence rates of sleep paralysis: a systematic review. Sleep Med Rev. 2011;15: 311-315.
2. Denis D, French CC, Rowe R et al. A twin and molecular genetics study of sleep paralysis and associated factors. J Sleep Res. 2015;24: 438-446.
3. Denis D, French CC, Gregory AM. A systematic review of variables associated with sleep paralysis. Sleep Med Rev. 2018;141-157.
4. Sharpless BA, Doghramji K. Sleep paralysis: Historical, Psychological and Medical Perspectives. New York: Oxford University Press; 2015.