From Joseph in the book of Genesis to the 2010 blockbuster Inception, dreams have been a source of fascination. The attraction seems to derive, in part, from their bizarreness. Indeed, it is rare to hear someone muse that “I had the craziest wakefulness last night.” But we are also compelled by the allure that dreams are imbued with meaning or that they reveal something about our psyche. Although this notion would seem to be an anachronism that went the way of the psychoanalytic dinosaurs, the belief is very much alive as evidenced by the popularity of films such as Inception. A science fiction thriller with premises relating to a flat earth, a geocentric paradigm of the cosmos, or lipids that transmitted genetic information would lack both credibility and appeal, even to the lay public. Unlike physics or molecular biology, the science of dreaming has yet to achieve full maturity.
Dreaming might be considered the purest form of consciousness, because it is generally disconnected from both sensory input and motor output. In other words, it is pure subjectivity. It is thus strange that dreams make their often dramatic entrance on the dimly lit stage of unconsciousness. Despite the apparent contradiction, dreams enjoy unqualified acceptance as a canonical trait of the unconscious state of sleep. Should it be any more surprising if the same were true of anesthesia or sedation?
DREAM THEORY AND NEUROBIOLOGY
The modern era of dream theory commenced with the publication of Freud's Interpretation of Dreams in 1899.1 Freud regarded dreams as a “royal road” to the unconscious because they reflected the wish fulfillment of repressed desires that the dream both revealed and concealed. A little more than 50 years later, the field of sleep neurobiology was born with the identification of rapid eye movement (REM) sleep by Aserinsky and Kleitman in 1953.2 In this same publication, the association of REM sleep and dreaming was identified. It is important to note that, despite the strong association between the two, dreams and REM sleep are doubly dissociable.3 In other words, one can have REM sleep without dreams and dreams without REM sleep. Nonetheless, the identification of a neurophysiologic process that strongly correlated with dreaming established the groundwork for a neurobiological theory. Hobson et al.4,5 developed the AIM model of REM sleep and dreaming, consisting of a 3-dimensional state-space plot of Activation via pontine cholinergic transmission, Input gating of sensory information and motor output, and Modulation (Fig. 1). Other current theories relating to sleep “switches” suggest GABAergic and glutamatergic control of REM sleep generation.6 Neuroimaging has further contributed to our understanding of REM sleep by (among other important findings) identifying the deactivation of the dorsolateral prefrontal cortex, which accounts for the decreased insight, judgment, and self-awareness during dreams.7
DREAMING DURING GENERAL ANESTHESIA
There is literature from the earliest history of anesthesiology suggesting that dreaming occurs during general anesthesia.8,9 In a more recent study by Leslie et al.,10 22% of subjects undergoing general anesthesia reported dreams, with dreamers tending to be young, healthy males with high dream recall at home, who received propofol-based anesthetics. It is of interest that propofol has a stronger association with dreaming than inhaled anesthetics, because animal studies suggest that propofol satisfies REM sleep homeostasis,11 whereas isoflurane and sevoflurane do not.12,13
A recent electroencephalographic study demonstrated REM-like activation in association with dreams during both propofol- and desflurane-based anesthesia.14 Consistent with the AIM model, a role for the acetylcholine system in generating anesthetic dreams is suggested by the finding that scopolamine eliminates dreaming during propofol- and N2O-based anesthesia.15 However, the neural correlates of dreaming during anesthesia are still unclear, especially because the dream reports do not typically have the vivid or bizarre qualities that are characteristic of REM sleep–associated dreams.16
DREAMING DURING SEDATION
Eer et al.17 and Stait et al.18 found that approximately 20% to 25% of patients undergoing sedation for colonoscopy had dreams, which is an incidence comparable to that found during general anesthesia.10 The article by Kim et al.19 in this issue of Anesthesia & Analgesia supports this literature by demonstrating a similar incidence of dreaming during sedation. Kim et al. studied 215 patients receiving supplemental sedation during spinal anesthesia, randomizing them to either propofol or midazolam infusions. Of the total cohort, 26% dreamed, with a higher incidence found in the propofol group (39.8%) compared with the midazolam group (12.1%). Within the propofol group, dreamers had a higher satisfaction than nondreamers, which was not found in the midazolam group. Our choice of anesthetic drug can therefore influence the incidence of dreaming during sedation and, possibly, patient satisfaction. But can anesthesia providers influence the content of dreams? The Brief Report by Cheong et al.20 in this issue of Anesthesia & Analgesia suggests that we can.
Cheong et al. also studied adjunct sedation during spinal anesthesia, in this instance with the drug ketamine. Ketamine is well known to be associated with a high incidence of dreaming,21 but in this investigation, the authors explored whether the affective valence of dreams during sedation could be influenced by suggestion. The test group was told, “After being treated with this drug, you will have very good dreams,” whereas the control group was told, “You will be given a sedative drug before the operation, and then you will fall asleep. You may or may not dream.” The investigators found that 3 times as many patients reported “very pleasant” dreams after this simple suggestion. Although not specifically analyzed in the article, considerably more patients in the suggestion group remembered their dreams. The role of suggestion in anesthetic dreaming or dream recall returns us to the psychological origins of modern dream theory.
FUTURE RESEARCH IN ANESTHETIC DREAMING
A number of questions remain regarding dreaming during anesthesia and sedation. First, what is the neurobiology of anesthetic dreams? Leslie et al14 and Aceto et al.22 have taken important first steps by identifying associations with cortical activation and responsiveness, but the lack of frank REM sleep correlates during general anesthesia will make the task challenging. After characterizing the neurophysiologic markers of anesthetic dreaming, a second important question can be resolved: What is the incidence of dream experience as opposed to dream recall? Currently, we can only study the latter. The phenomenon of dreaming during sedation may facilitate the investigation of dream experiences; the sedative state is more readily reversible and the dreaming subject can thus be interviewed when such markers appear. Third, do dreams matter? Although it is clear that REM sleep is important based on the consequences of deprivation, sleep neurobiology has yet to elucidate whether the accompanying subjective experience of dreaming is important or merely epiphenomenal. Current studies related to anesthesia have focused on patient satisfaction, but perhaps future studies could assess whether the underlying neurobiology of anesthetic dreams bears any cognitive relevance. Finally, we must address the question: Are anesthetic drugs the stuff that dreams are made of? If dreaming is established as an intrinsic trait of the anesthetic or sedative state, then the hypothesis of a fundamental connection between sleep and anesthesia will be strengthened.
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