The preservation of muscle tone during REM phases is the single most important symptom of RBD. It entails involuntary myoclonic jerks throughout the body as well as movements of limbs and head, likely resembling motor dream enactment [6]. Affected individuals execute possibly aggressive or violent motions that may lead to self-injury, harm to third parties and damage to objects [7]. More complex presentations have also been described. Patients may gesture, punch, kick, sit up or leap from bed, grab somebody or something [8]. Their behavior is generally unintentional and not directed against a determined person or item. They may or may not wake up and recall their vivid dreams. Fatigue and excessive daytime sleepiness are a common consequence of RBD [9].

Ventilation is usually not affected.

Diagnostic criteria for RBD have been defined as follows by the American Academy of Sleep Medicine [10]:

• Involuntary movements are associated with dreams

As well as either one of the following:

• Harmful or potentially harmful behavior during sleep
• Motor dream enactment
• Disruption of sleep continuity

The diagnosis of RBD may be based on these minimal criteria, but is ideally supported by the following polysomnographic findings as obtained during REM sleep:

• Increased chin electromyography tone
• Enhanced chin or limb phasic electromyography twitching associated with excessive myoclonic jerks or complex, vigorous, or violent behavior or the absence of epileptiform activity

These criteria apply if mental disorders have been ruled out. In contrast, concomitant neurodegenerative disease or other neurological disorder are not to be considered at this point. In fact, a considerable subset of RBD patients suffers from additional pathologies like Parkinson's disease.

Physiologically, longer periods of non-REM sleep are interrupted by short REM phases. A complete sleep cycle comprising all four stages of non-REM sleep as well as a REM phase usually lasts about 100 minutes [1]. Both disturbances of non-REM or REM sleep and irregular cycling are related to sleep disorders.

Dreaming typically occurs during REM sleep. Accordingly, REM phases are characterized by bursts of rapid eye movement, increase of the brain and sympathetic nerve activity, heart rate, and blood pressure, as well as irregular respiration and the absence of muscle tone [2]. Enhanced brain activity is reflected in wake-like electroencephalograms, loss of muscle tone may be depicted by means of electromyography [1]. The preservation of muscle tone during REM phases is pathologic, and affected individuals are diagnosed with REM sleep behavior disorder (RBD), a type of parasomnia [3].

Although the majority of cases is deemed idiopathic, incidence rates are markedly increased among patients suffering from neurodegenerative disorders like Parkinson's disease, dementia with Lewy bodies and multiple system atrophy. In this context, RBD has been proposed as a risk factor for neurodegenerative diseases, and have been shown to precede the latter by decades [4]. This may have major implications for the diagnosis and early treatment of the respective entities. RBD may also be seen in individuals taking antidepressants and in those abusing or withdrawing from alcohol or drugs [5].

1. Penzel T, Kantelhardt JW, Lo CC, Voigt K, Vogelmeier C. Dynamics of heart rate and sleep stages in normals and patients with sleep apnea. Neuropsychopharmacology. 2003; 28 Suppl 1:S48-53.
2. Institute of Medicine Committee on Sleep M, Research. The National Academies Collection: Reports funded by National Institutes of Health. In: Colten HR, Altevogt BM, eds. Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem. Washington (DC): National Academies Press (US); 2006.
3. Coeytaux A, Wong K, Grunstein R, Lewis SJ. REM sleep behaviour disorder - More than just a parasomnia. Aust Fam Physician. 2013; 42(11):785-788.
4. Claassen DO, Josephs KA, Ahlskog JE, Silber MH, Tippmann-Peikert M, Boeve BF. REM sleep behavior disorder preceding other aspects of synucleinopathies by up to half a century. Neurology. 2010; 75(6):494-499.
5. McCarter SJ, St Louis EK, Boeve BF. REM sleep behavior disorder and REM sleep without atonia as an early manifestation of degenerative neurological disease. Curr Neurol Neurosci Rep. 2012; 12(2):182-192.
6. Blumberg MS, Plumeau AM. A new view of "dream enactment" in REM sleep behavior disorder. Sleep Med Rev. 2015; 30:34-42.
7. Lloyd R, Tippmann-Peikert M, Slocumb N, Kotagal S. Characteristics of REM sleep behavior disorder in childhood. J Clin Sleep Med. 2012; 8(2):127-131.
8. Arnulf I. REM sleep behavior disorder: motor manifestations and pathophysiology. Mov Disord. 2012; 27(6):677-689.
9. Arnulf I, Neutel D, Herlin B, et al. Sleepiness in Idiopathic REM Sleep Behavior Disorder and Parkinson Disease. Sleep. 2015; 38(10):1529-1535.
10. American Academy of Sleep Medicine. International Classification of Sleep Disorders, revised: Diagnostic and Coding Manual. Chicago, Illinois: American Academy of Sleep Medicine; 2001.