Delirium is defined as a disturbance in mental abilities that can be characterized by restlessness, confused thinking, and reduced awareness of the environment.1 Postoperative delirium is a common complication in the geriatric population and occurs with an overall prevalence of 10-37% in elderly patients.2 Despite typically resolving within 48 hours of surgery, postoperative delirium is associated with increased length of stay, poorer recovery, and a greater likelihood of being placed in an assisted living facility after surgery.3 Spinal anesthesia, in contrast to general anesthesia, is commonly used for procedures involving the lower abdomen, pelvis, and lower extremities.4 Level of sedation has been to found to correlate with the severity of delirium for intensive care patients, but this relationship has yet to be thoroughly explored for spinal anesthesia administered in context of surgery.5
A leading hypothesis in the field is that limiting or optimizing the depth of anesthesia may be a strategy for limiting postoperative delirium. Depth of anesthesia is defined as the degree to which the central nervous system is depressed by an anesthetic agent, which is affected by the potency of the agent, the concentration at which it is administered, and the probability of non-response to stimulation.6 The bispectral index (BIS) is used to measure the depth of anesthesia. The BIS monitor processes EEG signals and outputs a value between 0 (indicating the absence of brain activity) and 100 (the awake state).7 Anesthesia may induce delirium through its proclivity to increase neural apoptosis8 and production of Alzheimer’s-associated amyloid-beta protein,9 which motivate the hypothesis that decreased depth of anesthesia may concomitantly decrease postoperative delirium.
In a study published in December 2021, Brown et al.10 sought to determine whether depth of anesthesia has an effect on postoperative delirium. Patients 65 years old or greater undergoing lumbar spine fusion were separated into two groups, one of which received spinal anesthesia with targeted sedation to a BIS value greater than 60 to 70, while a control group received general anesthesia with masked, or unmeasured, BIS (which is assumed to be lower than 60-70). Delirium was assessed postoperatively using the Confusion Assessment Method. The researchers found that there was not a significant difference in postoperative delirium between the two groups. The authors concluded that further studies are needed to expand on this finding and optimize anesthesia strategies, especially in cognitively impaired patients, who are at higher risk for delirium.
A prior study investigated the effects of lighter sedation in patients undergoing hip fracture surgery. Sieber et al.11 administered deep (BIS = 50) or light (BIS > 80) spinal anesthesia to two different groups of patients and monitored postoperative delirium using the Confusion Assessment Method. In contrast to Brown et al., Siebert et al. found that the prevalence of postoperative delirium was significantly lower in the light group compared to the deep group. Brown et al. qualify this finding by noting that most of the adults in the study were cognitively impaired, and the results therefore may not be generalizable to most older adults that undergo surgery with spinal anesthesia. What is clear, though, is that further research into the relationship between spinal anesthesia and delirium is needed.
References
1. Delirium – Symptoms and causes. Mayo Clinic https://www.mayoclinic.org/diseases-conditions/delirium/symptoms-causes/syc-20371386.
2. Dyer, C. B., Ashton, C. M. & Teasdale, T. A. Postoperative delirium. A review of 80 primary data-collection studies. Arch. Intern. Med. 155, 461–465 (1995).
3. Zakriya, K., Sieber, F. E., Christmas, C., Wenz, J. F. & Franckowiak, S. Brief postoperative delirium in hip fracture patients affects functional outcome at three months. Anesth. Analg. 98, 1798–1802 (2004).
4. Olawin, A. M. & M Das, J. Spinal Anesthesia. in StatPearls (StatPearls Publishing, 2021).
5. Shehabi, Y. et al. Sedation Intensity in the First 48 Hours of Mechanical Ventilation and 180-Day Mortality: A Multinational Prospective Longitudinal Cohort Study. Crit. Care Med. 46, 850–859 (2018).
6. Shafer, S. L. & Stanski, D. R. Defining depth of anesthesia. Handb. Exp. Pharmacol. 409–423 (2008) doi:10.1007/978-3-540-74806-9_19.
7. Mathur, S., Patel, J., Goldstein, S. & Jain, A. Bispectral Index. in StatPearls (StatPearls Publishing, 2021).
8. Xie, Z. et al. The common inhalation anesthetic isoflurane induces apoptosis and increases amyloid beta protein levels. Anesthesiology 104, 988–994 (2006).
9. Zhang, B. et al. The inhalation anesthetic desflurane induces caspase activation and increases amyloid beta-protein levels under hypoxic conditions. J. Biol. Chem. 283, 11866–11875 (2008).
10. Brown, C. H., IV et al. Spinal Anesthesia with Targeted Sedation based on Bispectral Index Values Compared with General Anesthesia with Masked Bispectral Index Values to Reduce Delirium: The SHARP Randomized Controlled Trial. Anesthesiology 135, 992–1003 (2021).
11. Sieber, F. E. et al. Sedation Depth During Spinal Anesthesia and the Development of Postoperative Delirium in Elderly Patients Undergoing Hip Fracture Repair. Mayo Clin. Proc. 85, 18–26 (2010).