Parasomnia Overlap Disorder
In 1934, French researcher Henri Roger coined the term parasomnie (in English, parasomnia; from the Greek para meaning “alongside” and Latin somnum meaning “sleep”) for phenomena that occur in the transition from sleep to wake or vice versa. A parasomnia can occur during the transition between nonrapid eye movement (NREM) sleep and wake (i.e., NREM parasomnias such as sleepwalking, sleep terrors, confusional arousal, sleep-related eating disorder) or during the transition between rapid eye movement (REM) sleep and wake (i.e., REM parasomnias such as REM sleep behavior disorder [RBD], recurrent isolated sleep paralysis, nightmare disorder). A parasomnia has the following features: recurrent episodes of incomplete awakening from sleep, an inappropriate or lack of response to intervention or redirection during an episode, limited or no cognition of dream imagery and partial or complete amnesia for the event. In addition, the nocturnal disturbance is not explained by another sleep, psychiatric or medical disorder or medication/substance use. Some people experience REM parasomnias and NREM parasomnias, a condition called parasomnia overlap disorder (POD). A person with POD has a disorder of arousal (e.g., sleepwalking confusional arousal, sleep terror) and rapid eye movement sleep behavior disorder (RBD; which involves vivid, often unpleasant dreams; vocalization during sleep and sudden, often violent, arm and leg movements during REM sleep [i.e., dream-enacting behavior]).
In 1997, POD was first described by Carlos Schenck et al., who initially proposed that it was a subtype of RBD. In 2013, Dumitrascu et al. were the first scientists to propose that POD was a distinct parasomnia rather than a subtype of RBD. Based on their experience with five patients with POD, they noted that the onset of POD symptoms occurred earlier in life than did the symptoms of someone with RBD only, the NREM parasomnia symptoms were more pronounced than the RBD features and the POD symptoms were not associated with any neuropathy.
Why an overlap between NREM and REM parasomnia occurs in some people is unclear, although genetic factors and impaired neuroactivation in the brain may contribute. In addition, recent case reports indicate that it can be secondary to (i.e., caused by) another problem such as obstructive sleep apnea (OSA) or the use of certain drugs and that treating the causative factor may eliminate or reduce symptoms of POD.
Investigations of possible genetic factors associated with NREM or REM parasomnias have indicated a higher prevalence of human leukocyte antigen (HLA) genes among people with non-REM parasomnias such as sleepwalking, sleep terrors, and confusional arousals, and among people with REM parasomnias such as RBD. Anna Heidbreder et al. found a higher prevalence of the HLA DQB1*05:01 allele (an alternate form of a gene) among patients with NREM parasomnias (e.g., sleepwalking, sleep terrors, confusional arousals), regardless of the type of parasomnia, than in patients without NREM parasomnias. Licis et al. reported a higher prevalence of a gene on chromosome 20 (i.e., 20q12-q13.12) among several generations of sleepwalkers in a large family. Some research indicates that the prevalence of DQB1*05 and DQB1*06 alleles is higher among people with RBD than among people without RBD.