**2. Etiology**

Somnambulism or sleepwalking is one of the 15 parasomnias among the 81 recognized major sleep disorders. It is characterized by sudden arousal occurring at the SWS stages, resulting in walking behavior during a state of altered consciousness [14]. Somnambulism prevalence of 1–6% was indicated by Kleitman [15], "depending on whether the survey applied to the general public, hospital patients or to limited to children with sleep abnormalities." Thirty years later, Shapiro and Dement [16] offered a wider range (1–15%) of somnambulism prevalence, with children at the higher end of the range, relative to adults.

According to Kleitman [15], French physiologist Georges Henri Roger (1860–1946) had provided a good description of a typical sleepwalker, in his 1932 book *Les Troubles du Sommeil—Hypersomnies, Insomnies, parasomnies*. To paraphrase Kleitman [15],

*"The sleeper performs various acts with a certain degree of dexterity and can avoid obstacles. But his behavior is characterized by a rigidity, which gives him the appearance of an automaton. He can answer questions correctly and is quite receptive to suggestions. He obediently carries out rather bizarre orders and tends to preserve attitudes passively imposed on him, exhibiting cataleptic properties. At the end of 15 to 30 minutes of activity, he goes back to bed, sometimes fully clothed, and wakes up next morning, quite surprised to find himself dressed. He can carry out dangerous tasks, like walking along the edge of a roof, which he would be afraid to do when awake" [15].*

Hereditary factors in the prevalence of somnambulism were first recorded in a short note, appearing in *Gazzetta degli Ospitali* 1930 by Clerici in a six-member Italian family (comprising of husband, cousin-wife, and their four children), all suffering from somnambulism. This fact was subsequently confirmed in other countries as well [6–7, 17].

Among a sample of 60 Caucasian subjects with somnambulism disorder, distributed in three countries (France, Switzerland, and Germany), Lecendreux et al. [18] reported that compared to eight (13.3%) age-matched controls, 21 (35%) somnambulists were positive for DQB1 allele of HLA-DQB antigen, and claimed this finding as a first genetic maker for somnambulism. HLA stands for human leukocyte antigen. Subsequently, Licis et al. [19] studied a four-generation Caucasian family in the USA consisting of 22 members, among whom nine were somnambulists and 13 were unaffected. Genomewide linkage analysis from gathered DNA samples revealed the following facts—(1) genetic locus for somnambulism is present at chromosome 20q12-q13.12; (2) sleepwalking may be transmitted as an autosomal dominant trait with reduced penetrance;

(3) seven among the nine somnambulists were males; (4) adenosine deaminase gene (ADA) is the most likely candidate gene in the chromosome 20q12-q13.12 linkage interval due to its association with SWS during with somnambulism occurs.

Since 1956, polysomnographic studies in somnambulist patients have been conducted in the USA, Canada, and a few European countries [7, 8, 20–32]. A chronological compilation of significant polysomnographic studies is presented in **Table 1**. Two


*1 n.r = not reported, presumed to be in late teens to 20s, as the subjects were naval recruits and Electronics school students.*

*2 Patients suffered from sleepwalking only.*

*3 Patients suffered from both sleepwalking and sleep terror.*

*4 Patients suffered from sleepwalking without sexsomnia.*

*5 Patients suffered from sleepwalking and sexsomnia.*

#### **Table 1.**

*Significant polysomnographic studies in somnambulist patients.*

recognizable features revealed were, (1) Earlier studies, with two exceptions, did not have proper control groups; (2) Until 2015, the total number of somnambulist patients was less than 40. In the studies reported by Hurwitz et al. [21] and Oudiette et al. [28], the higher number of patients suffered from sleepwalking *and* sleep terror. The earliest study by Andre-Balisaux and Gonsette [6] in Belgium was omitted in **Table 1**, due to my lack of access in checking the complete text of the original paper.

Among the specific environmental factors precipitating somnambulistic episodes in adults, prescription medication use [33], alcohol [34], emotional stress [35], sleep deprivation [27], and certain psychiatric trauma [36] have been suggested. As such, a multifactorial etiology for somnambulistic disorder deserves recognition.
