Objectives

1. To understand the clinical classification scheme of parasomnias, under which sleepwalking and periodic limb movement disorders (PLMDs) will be considered.

2. To understand the concept of state dissociations as it applies to disorders of partial arousal, such as as sleepwalking.

3. To understand the nosology and definition of periodic limb movements (PLMs), the possible secondary causes of PLMs, and the controversy surrounding the diagnosis of idiopathic PLMD.

4. To understand how concurrent sleep disorders may influence the expression and management of both sleepwalking and PLMD.

5. To understand the basis of and indications for nonpharmacologic and pharmacologic therapies for sleepwalking and PLMD.

Abbreviations

AASM = American Academy of Sleep Medicine; ADHD = attention deficit hyperactivity disorder; CAP = cyclic alternating pattern; EMG = electromyogram, electromyographic ; ENW = episodic nocturnal wandering; EOG = electro-oculography; ESRD = end-stage renal disease; ICSD = International Classification of Sleep Disorders; NFLE = nocturnal frontal lobe epilepsy; NREM = non-REM; PLM = periodic limb movement (including those occurring during quiet wakefulness); PLMD = periodic limb movement disorders; PLMI = periodic limb movement index; PLMS = periodic limb movements in sleep; PLMW = periodic limb movements of wakefulness; RBD = REM sleep behavior disorder; REM = rapid eye movement; RLS = restless legs syndrome; SRBD = sleep-related breathing disorder; SWS = slow-wave sleep; UARS = upper airway resistance syndrome [a clinical term that is not officially listed as a diagnosis (dyssomnia) in the ICSD]

Sleep is a reward for some, a punishment for others. For all, it is a sanction. -- Isidore Ducasse, Comte de Lautréamont (1846-70)

Introduction

We spend as much as one third of our lifetimes in the sleep state, but despite its obvious importance, a cohesive understanding of the function(s) of sleep remains to be articulated. Believed initially to represent a passive quiescent state intermediate between wakefulness and death,¹ the discovery of rapid eye movements (REMs) in 1953 led to the realization that sleep is actually an active bimodal process, with an inherent architecture and rhythmicity. In the normal adult,^1-3 sleep begins in the non-REM (NREM) state, which has been arbitrarily divided into stages 1 through 4, roughly corresponding to sleep depth; stages 3 and 4 are collectively termed slow-wave sleep (SWS) and are associated with the highest arousal threshold. The REM state, also known as paradoxical sleep , follows approximately 80 min later. REM sleep is characterized by relatively low-voltage, mixed-frequency EEG activity somewhat similar to stage 1 or wakefulness, and by episodic bursts of REMs, but with loss of muscle tone. While this has been traditionally referred to as a dream state , it is now known that dreaming can also occur during NREM sleep. Normal sleep architecture across the night consists of NREM alternating with REM sleep, with an approximately 90- to 120-min cycle length. SWS generally predominates during the first half of the night, while more REM sleep is observed during the second half. Adults typically spend 20% of their total sleep time in REM sleep. Brief arousals punctuate normal sleep, although only the longest of these will be remembered. Indeed, arousals tend to increase in frequency as sleep quality deteriorates, which may be related to external stimuli, inherent sleep instability, or an underlying physiologic sleep disorder.

However, sleep does change with age. In newborns, the 24-h sleep-wake cycle is divided equally between sleep and wakefulness, with the latter increasing in proportion until 4 years of age.⁴ Infants also begin sleep in REM and spend a greater amount of time in this state (50%) until 6 months of age, after which their sleep architecture resembles that of an adult.⁴ As one ages, however, there is a reduction in SWS, while the amount of REM sleep remains relatively constant.² With advanced age, sleep efficiency tends to decline, due mainly to increased arousals and awakenings during the night.²

Superimposed on the wake and sleep states are two intrinsic rhythms that govern physiologic sleepiness and wakefulness: process C and process S. Process C, the circadian rhythm, is dictated by the mammalian biological clock, the suprachiasmatic oscillator. Although relatively rigid, this oscillator can be entrained, mainly by photic-visual influences through primary and secondary retinal afferents, as well as by melatonin via receptors. Process S represents a homeostatic pressure that promotes sleep during prolonged wakefulness or sleep deprivation, and is eliminated by sleep.^1-3

Therefore, like all mammals, we spend most of our lives in three completely different states of being: wakefulness, REM sleep, and NREM sleep.³ As such, research efforts over the last 70 years have expanded our scientific understanding of sleep and revealed various pathologic conditions related to it, leading to the multidisciplinary field of sleep medicine.⁵ Of the many varied and disparate sleep disorders that have been defined and classified thus far, the parasomnias are perhaps the most interesting. Clinical phenomena that tend to occur during the transition from one state to another,⁶ the admixture of behaviors characteristic of both states culminates in the disruption of sleep. As such, parasomnias are defined by the International Classification of Sleep Disorders (ICSD)⁷ as "clinical disorders that are not abnormalities of the processes responsible for sleep and awake states per se but rather, are undesirable physical phenomena that occur predominantly during sleep." Two distinct but associated clinical entities so categorized are sleepwalking and what will be collectively called the periodic limb movement disorders (PLMDs) , which are defined by electromyographically recorded periodic limb movements in sleep (PLMS). Included in this latter category are several secondary causes of PLMS [eg, sleep-related breathing disorder/upper airway resistance syndrome (SRBD/UARS), narcolepsy, and REM sleep behavior disorder (RBD)], as well as idiopathic periodic limb movement disorder (PLMD), which is most often seen in the context of restless legs syndrome (RLS), a related but clinically distinct disorder.

For the purposes of this article, sleepwalking and PLMD will be categorized as primary sleep phenomena within the classification of parasomnias proposed by Mahowald (Table 1) .⁸ Representing a more clinically useful classification scheme compared with the current nosology (Table 2)⁷ espoused by the American Academy of Sleep Medicine (AASM, formerly the American Sleep Disorders Association), the Mahowald classification scheme also emphasizes the potential of each disorder to disrupt normal sleep. Accordingly, these primary phenomena are further subcategorized according to the stages of sleep from which they are thought to originate. These include REM sleep phenomena (eg, RBD) and NREM sleep phenomena (eg, sleepwalking, confusional arousals, and sleep terrors). As PLMD and RLS may occur during both NREM and REM sleep, they are categorized accordingly. The secondary sleep phenomena under this clinical classification scheme are processes that disrupt sleep but represent behavioral manifestations of specific organ systems related to other disorders. Similarly, several parasomnias specifically related to SRBD or its acute treatment have also been identified.⁸

What follows will be a review of pharmacologic and nonpharmacologic treatment options for sleepwalking and the PLMDs, including idiopathic PLMD. For both disorders, pharmacologic therapy is generally symptomatic, with definitive cure possible only in selected secondary cases. Furthermore, comorbid medical conditions as well as sleep disorders may significantly influence the clinical expression of these disorders, and therefore must also be addressed. However, whether specific treatment is even required depends on several clinical factors, including severity and frequency of symptoms, timing, duration, and overall impact on the patient's life; therefore, treatment must be highly individualized. To this end, brief descriptions of the pathophysiologic basis of each disorder, defining clinical features, and associated differential diagnoses will also be included.