General anesthesia is intended to induce unconsciousness to allow patients to safely undergo major surgeries. Studying the brain’s behavior while under anesthesia produces a better understanding of both anesthesia and the brain. One area of interest for research is dreaming under anesthesia. Because patients are typically unable to discern exactly when or how many times dreaming occurred, it is a difficult subject for investigation. Regardless, patients often report experiencing dreams or altered states of consciousness during surgical procedures with anesthesia. The occurrence and content of these dreams may be influenced by preoperative suggestive communication, the patient’s anxiety levels, the type of anesthetic agent used, the duration of anesthesia, and more.¹ Such experiences challenge our understanding of consciousness and anesthesia and suggest the brain retains a degree of awareness even in the absence of conscious sensory input.

A prospective, randomized controlled trial of 200 female patients undergoing elective abortion surgery compared the incidence and nature of dreaming under sevoflurane and propofol anesthesia. Basic demographic and personal data, including information on typical dreaming frequency and anxiety level, were collected prior to the surgical procedure and administration of either propofol or sevoflurane.² The incidence of dreaming was significantly higher in patients receiving sevoflurane (60%) compared to their counterparts receiving propofol (33%). Dreaming content was classified using a five-point Likert scale describing emotional content/nature, memorability, visual vividness, emotional intensity, meaningfulness, strangeness, and amount of movement. Of all dreamers (n=93), 46 reported positive dreams, while only 9 reported experiencing dreams with negative emotional content. Compared to those in the propofol group, dreamers in the sevoflurane group reported a higher frequency of dreams without sound, dreams without movement, and dreams with little to no memorability. However, there was no significant difference in dream emotional content (positive or negative content) or visual vividness between anesthesia groups.² Higher memorability of dreams with propofol anesthesia may be associated with propofol’s effect of enhancing endocannabinoid signaling, which may increase the consolidation of emotional memory.³ Consistent with previous studies, the authors conclude the differences in dreaming content may be associated with the different pharmacological effects of the various anesthetic agents on the CNS.²

A retrospective re-evaluation of an original prospective randomized trial demonstrated that dreams during anesthesia can be influenced by suggestions communicated immediately preceding anesthesia induction. Suggested topics that were communicated to the patient prior to induction included loved ones, work, sports, holidays, erotica, fairy tales, religion, and surgery. The topics most successfully applied through suggestion were loved ones, holidays, and sports, with 83.8%, 77.8%, and 63.6% of dreamers, respectively, reporting a dream matching that of the suggestion. The researchers suggest this influence could be attributed to the difference between REM and NREM dreams. While REM dreams are well-known and typically consist of memory fragments and bizarre events that are difficult, if not impossible, to tie to real events, NREM dreams often contain elements of episodic memory, especially those occurring most recently. The dreams experienced by the patients in this study were NREM dreams, as shown by their inclusion of episodic memory (i.e., work, recreation, joint activities with loved ones).⁴ These results are consistent with prior research that states functional connections between the thalamus and cortex are reduced during deep sedation and anesthesia, while connections mediating sensory transfer from the periphery are preserved.⁵ As a result, the perception of environmental stimuli remains intact.

The phenomenon of dreaming and experiencing altered states of consciousness under anesthesia presents an intriguing challenge to our current understanding of anesthesia and awareness. Studies have shown that various factors, including the type of anesthetic agent, depth of anesthesia, and preoperative suggestions, may influence the occurrence and content of dreams under anesthesia. Results from prospective and retrospective studies not only shed light on the intricate processes of consciousness under anesthesia but also suggest that the brain may retain a level of awareness under short-term sedation. As research on this topic continues, it may lead to a deeper understanding of anesthesia, dreaming, awareness, and consciousness.

References

1. Brice, D. D., et al. “A Simple Study of Awareness and Dreaming Under Anesthesia.” British Journal of Anaesthesia, 42(6), 1970, 535–42. https://doi.org/10.1093/bja/42.6.535
2. Xu, G. H., et al. “Dreaming during Sevoflurane or Propofol Short-Term Sedation: A Randomised Controlled Trial.” Anaesthesia and Intensive Care, 40(3), 2012, 505–10, https://doi.org/10.1177/0310057X1204000317
3. Hauer, D., Ratano, P., Morena, M., Scaccianoce, S., Briegel, I., Palmery, M., et al., “Propofol Enhances Memory Formation via an Interaction with the Endocannabinoid System.” Anesthesiology, 2011, 114, 1380-1388.
4. Gyulaházi, J., et al. “Dreaming under Anesthesia: Is It a Real Possibility? Investigation of the Effect of Preoperative Imagination on the Quality of Postoperative Dream Recalls.” BMC Anesthesiology, 16(1), 2016, 53. https://doi.org/10.1186/s12871-016-0214-1
5. Liu, Xiaolin, et al. “Differential Effects of Deep Sedation with Propofol on the Specific and Nonspecific Thalamocortical Systems.” Anesthesiology, 118(1), 2013, 59–69.  https://doi.org/10.1097/ALN.0b013e318277a801