A total of 10 RCTS were included in this meta-analysis to compare the effects of sevoflurane and desflurane on POCD in patients undergoing general maintenance anesthesia. For the 5-time nodes in which MMSE score was performed, significant differences were observed at 1, 3 and 24h after surgery, but this was not the case for he comparison between preoperative and 6h after surgery. In this meta-analysis, we discovered that more effectively desflurane reduced the incidence of POCD more effectively compared with sevoflurane when applied as the to maintenance general anesthesia. Moreover, it was superior to sevoflurane in terms of time to recovery. The results, revealed no heterogeneity between desflurane and sevoflurane in preoperative MMSE score, but there was some differences between desflurane and sevoflurane at the first, third, sixth and 24h after surgery, and the MMSE score of desflurane was higher than that of sevoflurane. MMSE is a long-term test that is currently used to assess the severity and monitor the progress of cognitive impairment. It tests orientation, memory, attention and numeracy, recall, and language. These six variables are often referred to as cognitive areas, with a maximum score of 30 and a normal score of 27–30. Scores < 27 are classified as cognitive dysfunction [19]. The analysis found, no significant difference in the preoperative MMSE score between the two groups. The incidence of POCD in the desflurane group was lower than that in the sevoflurane group within 24h after the operation, but the difference between the two groups was significantly decreased at 24h after the operation compared with the first hour after the operation. However, the desflurane group had less effect on POCD. For patients, whether there is a difference in the long-term incidence of POCD after the use of two drugs deserves further exploration.
POCD is a common postoperative complication. Although numerous studies have investigated the mechanisms underlying its occurrence in the past decade, its exact mechanism and pathology have not been fully understood. There are many risk factors contributing to the occurrence of POCD, including patient, surgical and anesthetic factors. Anesthesia is an important and integral part of any surgical procedure. The risk factors of POCD include the type of anesthesia, decreased intraoperative brain saturation, and neurotoxic effects of postoperative anesthesia. The use of narcotic drugs may also alter the cognitive function of patients, especially the elderly, because the residual effects of drugs can modify the activity of the central nervous system. Such drugs can influence tau protein modification, inflammatory process calcium regulation and mitochondrial function to alter cognitive function [20, 21]. Anesthetics primarily act on the central nervous system. Currently, the commonly used inhalation anesthetics such as isoflurane, sevoflurane, and desflurane are considered to be safe anesthetics. They have been shown to reduce mortality and morbidity during cardiac surgery, but there is no consensus on which inhalation is more effective [22]. Desflurane is a volatile inhalation anesthetic with a minimum MAC of 6%, low blood gas partition coefficient, and rapid onset and elimination, which leads to faster eye opening and extubation times in the desflurane group than in the sevoflurane group [23]. In animal studies, inhalation of sevoflurane in mice resulted in increased apoptosis of hippocampal neurons and induced hippocampal METTL3 inactivation leading to POCD[24, 25]. However, short exposure time to sevoflurane did not affect cognitive function in mice[26]. In pediatric anesthesia, the incidence of COPD increases significantly after sevoflurane anesthesia for more than 3 hours, as do serum inflammatory cytokines, which peak during the recovery period[27]. In animal studies, Zheng et al. demonstrated that rats anesthetized with desflurane showed decreased levels of neuroinflammation and learning and memory impairments [28]. However, Kilicaslan et al. exposed young adult mice to sevoflurane and desflurane and found that their spatial memory was not impaired [29]. But higher doses of desflurane anesthesia affected learning and memory in adult rats, and more so in older rats, but these effects were temporary and reversible. 10 Chen et al. found that compared with sevoflurane, desflurane caused a shorter extubation, reorientation, and exit from the recovery room, but no significant differences were observed in postoperative cognitive function or time to eye opening after anesthesia [30]. Lertkovit et al. reported that desflurane protected against cognitive decline after anesthesia and was an independent protective factor in POCD, whereas sevoflurane was an independent predictor of POCD [20]. Contrary to our findings, a prospective cohort study in Korea found that desflurane increased the risk of dementia in patients and sevoflurane decreased the risk of dementia [31].
Due to the limited sample size of the study, the relationship between desflurane and sevoflurane and POCD deserves further investigations. There are several shortcomings in the included studies in terms of methodology: (1) Implementation bias and selection bias may exist in the included studies, which decreases the quality of the obtained results. Only 4 studies adopted the random number table method, and the rest only mentioned random allocation. Moreover, the specific randomization method was not explained in detail: only 2 studies used the opaque envelope method to hide the randomization. Therefore, future clinical trials should consider this limitation. (2) The included studies were all published Chinese or English articles, and this may introduce publication bias due to incomplete literature inclusion and missing grey data. (3) Different examiners were involved in the included studies, and the depth of anesthesia was not controlled using a common method, which may weaken the comparison results among indicators; (4) Different types of surgeries were included in the study, which included pediatric surgery, gynecological surgery and orthopedic surgery, among others. Different surgical methods or sites may have exert different effects on the results of the study. These shortcomings and biases can be prevented by inclusion of many studies involving one type of single surgery.