In this prospective cohort study, the incidence of POD was 15.38%, a figure that similar to the previously reported 19% incidence in a retrospective study of elderly cancer patients [22]. Our study identified ACCI, preoperative sleep disorder, postoperative pain, and postoperative infection as factors that can predict the occurrence of POD. We constructed and validated a predictive model for POD and depicted the impact of different factors on POD through a nomogram.
At present, most research on POD focuses on patients undergoing cardiac and orthopedic surgeries [23]. The prevalence of POD in cardiac surgery patients varies from 25–50%[24], while orthopedic surgery patients have a POD incidence of 17.3% [25]. There is a noticeable disparity in the occurrence rates of POD among different surgical procedures, indicating that the nature of the surgery itself may pose a considerable risk factor for POD. Therefore, developing a predictive model for POD in elderly patients after a specific type of surgery may be more feasible and valuable in clinical practice.
A recent study extracted 576 variables and concluded that older age was the most influential factor in the onset of POD [26]. Similarly, studies found that older age and the presence of comorbidities were associated with an increased risk of POD [27, 28]. Elderly patients typically have multiple comorbidities and tend to become more frail as they age [29]. ACCI is a reliable predictor of overall survival and mortality across various surgeries, and it has the potential to accurately predict the onset of POD [30]. Researchers observed that 25% of spinal surgery patients developed POD, and 24% were frail, with frail patients having a 6.6 times higher probability of developing POD compared to robust patients [31]. In the older population undergoing major planned noncardiac surgical procedures, the probability of developing POD was 2.7 times higher in frail patients compared to robust patients. However, frailty was not found to be related to postoperative cognitive decline [32].
According to previous studies, preoperative anxiety has been recognized as a contributing factor for POD [33, 34]. There is an evident association between preoperative anxiety and sleep disorder [35]. Postoperative sleep disorder was among one of the risk factors for POD in elderly patients who underwent elective spinal surgery [36]. Another study also determined that sleep disorder increased the probability of developing POD [37]. Perioperative individuals with sleep disorders demonstrated heightened neuroinflammation, increased oxidative stress, compromised blood-brain barrier integrity, and impaired glymphatic pathway function, and accumulation of amyloid-beta proteins, all of which were linked to postoperative neurocognitive dysfunction [38]. Moreover, patients who experienced sleep disturbances prior to surgery exhibited notably elevated occurrences of delayed neurocognitive improvement and postoperative infection compared to those without sleep disorders [39]. Melatonin, a hormone produced by the pineal gland, has been shown to regulate sleep disturbances. Studies demonstrated that melatonin can notably reduce the occurrence of delirium [40], suggesting that improving perioperative sleep may decrease the likelihood of POD and postoperative cognitive dysfunction (POCD). Our study solely concentrated on the influence of preoperative sleep disorder on POD and did not assess postoperative sleep quality. Subsequent research should further investigate the connection between postoperative sleep disorder and POD.
Researchers discovered that after surgery, the pain experienced by the patient may lead to an increased likelihood of POD. However, the effect of pain on other postoperative neurocognitive disorders remained uncertain [41]. Advanced age, smoking, cognitive impairment, and postoperative pain were identified as risk factors for subsyndromal delirium in a descriptive correlational study [42]. Postoperative acute pain could worsen neuroinflammation and associated cognitive functional impairments in an animal study [43]. However, studies found that preoperative chronic pain was not an independent predictor of POD [44]. Our research found that elevated postoperative pain levels in elderly individuals were linked to a higher likelihood of developing POD. The association between resting pain and POD was even more pronounced, with higher scores and longer duration of resting pain potentially elevating the risk of POD.
In our study, we found that postoperative infection was the most prominent independent indicator for POD. Factors such as trauma, surgical manipulation, intraoperative bleeding, hypoxia, electrolyte disturbances, acid-base imbalance, and the use of specific anesthetic drugs can stimulate the body's immune system and peripheral inflammatory response, resulting in the liberation of a substantial quantity of inflammatory mediators. These factors can disrupt the blood-brain barrier, allowing the inflammatory factors to reach the brain parenchymal cells through various pathways and ultimately contributing to the occurrence of POD [45]. Previous research has shown an association between POD and POCD with the levels of inflammatory markers in the peripheral tissues. Certain markers, including IL-6 and CRP, have been associated with both POD and POCD [46]. Additionally, studies have linked the elevation of inflammatory markers CPAR and S100B to the occurrence of POD [47]. Furthermore, it has been discovered that the axon guidance molecule Netrin-1 possesses properties that are anti-inflammatory and neuroprotective. Netrin-1 was found to decrease the levels of IL-6 and HMGB-1 in peripheral blood, prefrontal cortex, and hippocampus, while concurrently increasing the expression of IL-10. Additionally, Netrin-1 was observed to diminish the activation of microglial cells in the prefrontal cortex and hippocampus, and ameliorate behavior characteristic of POD in mice [48]. Another study observed a significant association between the increase in systemic immune-inflammation index and the presence of POD [49], although this finding was not validated in our study. The study has a limited sample size and did not encompass perioperative drug use that lacked statistical significance. Subsequent research should enlarge the sample size and include it. Moreover, the onset of POD varies, and may occur beyond the duration of our follow-up. Many assessment scales rely on subjective accounts from patients, potentially leading to biased outcomes. It is expected that more comprehensive research will be conducted to provide early intervention for high-risk patients with POD in the future.