The incidence of POCD has been widely reported, with studies showing an incidence of 18–32% within 1 week after undergoing general anaesthesia and noncardiac surgery in elderly patients (Moller et al. 1998; Yuan et al. 2019; Jiao et al. 2021; Li et al. 2021; Kim et al. 2016). In the results of this study (Fig. 2), it was found that the duration of surgical anaesthesia was approximately twice that of other common major surgeries. The incidence of POCD at 1 week in elderly patients with oral malignancies was 33.1%, which is higher than that observed in other types of noncardiac surgery. This was possibly because of surgery for oral malignancy requiring tissue repair and reconstruction of the defect after tumour resection and extensive surgical traumas. And excessive stretching and rotation in cervical lymphatic surgery can cause intimal injury to the arterial lining of head and neck, thereby promoting the formation of microembolism, creating carotid artery stenosis, cerebral tissue hypoperfusion, and other risks to cause cerebral ischemia (Evered et al. 2011; Leiendecker et al. 2010). These factors often lead to brain damage in patients, causing a decline in cognitive function.

Comparison of the incidence of POCD with the duration of surgical anaesthesia in several common major surgeries. Note: Data were obtained from references (Yuan et al. 2019; Jiao et al. 2021; Li et al. 2021; Kim et al. 2016)

Venerable age is currently a more explicit risk factor for POCD, and similar results were obtained in this study. Several studies suggest that the pathogenesis of POCD is neuroinflammation, and it is age related (Luo et al. 2019). The mechanism may be a decrease in brain volume and white matter integrity with increasing age. Moreover, cerebral blood flow decreases with age, resulting in reduced oxygen delivery, slow metabolism, and age-induced central nervous system apoptosis, which affects neurons, synapses, and neurotransmitters, leading to an increased incidence of POCD in elderly patients. At the same time, elderly patients often combined with a variety of diseases, and the ability to cope with the injuries caused by surgery and anaesthesia is reduced. Therefore, the risk of perioperative complications and POCD also increases accordingly (Otomo et al. 2013). In conclusion, elderly patients are more likely to develop POCD after surgery.

The level of education is closely related to the occurrence of POCD (Huang et al. 2020). Feinkoh et al. (2017) found that in middle-aged and elderly patients undergoing surgery, years of education are inversely proportional to the incidence of cognitive impairment. Patients with high cognitive reserve display more brain activity and can better adjust or activate synaptic connections between neurons using neuronal reserve, bypassing damaged areas and increasing synaptic efficacy to deal with injury. In addition, with low education, they have more harmful factors in their living environment and a more unhealthier lifestyle. They may present more severe brain pathological manifestations than their peers, such as amyloid β-protein deposition, which exacerbated the cognitive deficiency by anaesthesia (Evered et al. 2016).

In recent years, studies have focused on the association between dyssomnia and POCD. The results obtained in this study showed that sleep disturbance is an independent risk factor for POCD. Sleep accounts for approximately one-third of an individual’s lifespan and is closely related to human health. Various aspects of sleep are affected in varying degrees in most elderly adults (Kang et al. 2017). Dyssomnia is not conducive to protein synthesis and establishment of new synaptic connections in the brain, affecting the cognitive change in cerebral cortical, leading to neuroendocrine disorders, decreased immune function, deterioration of behaviour, anxiety, depression, irritability, and other complications, thereby inducing or aggravating POCD (Gogenur et al. 2008). A meta-analysis revealed that various types of sleep disorders, such as difficulty remaining asleep, reduced sleep duration, reduced sleep efficiency, and daytime dysfunction, can significantly increase the risk of cognitive impairment (Bubu et al. 2017). Studies have demonstrated that the intraoperative use of dexmedetomidine can significantly stabilise patients’ haemodynamics, reduce the occurrence of inflammatory reactions, inhibit free radical generation, and reduce sleep disorders caused by the use of other anaesthetic drugs and has a certain protective effect on the sleep of patients under general anaesthesia (Guldenmund et al. 2017).

Hypertension is a common cardiovascular complication in elderly patients, and studies suggest that hypertension is often accompanied by cerebrovascular and carotid atherosclerotic plaque, which leads to cerebrovascular autoregulation function. Under the stimulation of various related factors during the perioperative period, there can be local or whole cerebral blood flow, cerebral oxygen content further declines, central nervous system transmitter release reduced, and particularly, the cholinergic nervous system function declined, leading to impaired brain function, making hypertensive patients more prone to postoperative cognitive function damage (Shaw et al. 2003). Spence et al. (2004) demonstrated that for every 10-mmHg increase in systolic BP, the risk of cognitive dysfunction increased by 7% compared with that of the control group. The systolic BP was > 160 mmHg, and the cognitive decline was significantly increased.

The duration of operation was also an independent risk factor for POCD onset in this group, and the mean duration of operation of patients in the POCD group was higher than that in the non-POCD group, and perhaps the more complex surgical steps led to the occurrence of POCD. More complex surgical steps imply a prolonged surgical anaesthesia duration. A previous study revealed a significant increased incidence of POCD in patients with surgery longer than 450 min, and a long-time surgery is a further important predictor of POCD (Otomo et al. 2020). A systematic review by Freddi Segal-Gidan et al. (2017) concluded that a shorter duration of operation was associated with a less risk of POCD. Animal studies confirmed that anaesthesia exposure can change amyloid and tau protein in mice, which leads to cognitive dysfunction (Segal-Gidan 2017).

Currently, the relationship between intraoperative blood pressure and postoperative cognitive function is controversial. Intraoperative hypotension in this study was an independent risk factor for POCD. Hypotension leads to a low perfusion state of the brain blood supply and induces free radical damage and other pathological changes. In addition, oxidative stress response can cause various changes in neuronal degeneration and protein apoptotic genes, reducing the molecular expression that creates and maintains synaptic connections, thereby impacting memory and cognitive function. A prospective clinical study demonstrated that a single longest cerebrovascular modulation of BP change over 5.03 min was associated with decreased postoperative cognitive function (Kumpaitiene et al. 2019). Therefore, sustained hypotension during general anaesthesia may cause damage to the nervous system of patients with chronic insufficient cerebral perfusion (such as elderly patients), and preventing intraoperative hypotension helps to prevent POCD (Yamamoto et al. 2018).

Due to the oral surgery, language testing could not be performed. To mitigate the impact on neuropsychology, the cognitive function of patients could not be studied within 7 postoperative days. In the multivariate analysis, some insensitive indicators were not analysed. In this study, diabetes, coronary disease, cerebral infarction, and hypokalaemia were associated with POCD but were not independent risk factors for POCD. Few studies have found postoperative cognitive impairment risk factors may also include intraoperative bleeding, prolonged ICU stay, second-time surgery, and postoperative infection. The above two conditions may be related to the present study. This study was a single-centre experiment with small sample size; therefore, the results showed no significant statistical differences. Consequently, multicentre observational studies involving large cohorts are required to determine whether current risk factors have high predictive value.

“Perioperative Brain Health Initiative”, which aims to focus on the anaesthesia-related brain health of elderly patients, explores effective perioperative brain protection measures and reduces the incidence of central nervous system complications, such as POCD in elderly patients. Therefore, the active prevention of POCD in elderly patients with oral malignancies is a problem that clinicians should pay significant attention to and resolve. In this study, the incidence of POCD 1 week after oral malignancy surgeries of the elderly patients was higher than that in other noncardiac surgeries. Venerable age, low education level, hypertension, sleep disturbance, long-time surgery, and intraoperative hypotension were independent risk factors for POCD in elderly patients with oral malignancies. Clinicians should understand the solutions to reduce the incidence of POCD, identify and manage the perioperative risk factors early, and provide effective preventive interventions and treatments for high-risk groups.