The results of this study demonstrate that many geriatric elective surgical patients do poorly on cognitive screening tests preoperatively. Specifically, 28.61% of patients≥65 years-old scored in a range that suggests probable cognitive impairment.
Preexisting cognitive impairment preoperatively
Cognitive impairment often goes undiagnosed, yet it has significant implications for surgical outcomes. Of the 374 patients included, 107 (28.61%) were identified as having cognitive impairment in this study, lower than previous literature reported. Studies have showed the prevalence of cognitive impairment is up to 35–50%in the community-dwelling older person, including mild cognitive impairment (MCI) as well as dementia.[4,12] The prevalence in elderly patients in surgical wards varies with the disease. A study about 152 subjects 60 yr of age and older who were scheduled for total hip joint replacement surgery underwent preoperative assessment, found 22% were classified as having MCI. And the remarkably high prevalence of preoperative MCI in 70% of vascular surgery patients is a cause for concern, among which with 88％ undiagnosed before admission. These studies confirm that preoperative mild cognitive deficits are common in the older person undergoing major surgery.
Nevertheless, routine preoperative cognitive assessment continues to be overlooked in clinical practice today. Numerous clinical studies confirmed preoperative cognitive impairment in the older patient undergoing major elective surgery is also related to poor postoperative outcomes including infection and bleeding, increased length of stay, and mortality. In a retrospective study of 129 patients undergoing lumbar spine surgery, Lee et al. reported a high prevalence of undiagnosed cognitive impairment that led to higher rates of postoperative delirium, longer LOS, and poorer patient outcomes. In another retrospective study of 82 older patients with spine deformity undergoing elective spinal surgery, Owoicho et al. found that patients with preoperative cognitive impairment were more likely to require an additional stay at a skilled nursing or acute rehabilitation facility. In observational retrospective study of 1258 patients aged older than 69 years undergoing hip surgery, the severity of cognitive impairment was a prognostic factor for mortality and functional recovery. Last but not least, in a study of 5407 patients undergoing cardiac surgery, Tully et al. showed baseline impaired cognitive status was associated with higher risk of long-term mortality.
In addition, from June to November 2018, a similar paper was published in six well-known magazines, suggesting that perioperative neurocognitive disorders (PND) were used to describe the destruction or change of cognitive function during perioperative period to replace postoperative cognitive dysfunction (POCD), which not only extends the timeline of perioperative cognitive follow-up, but also emphasizes the importance of preoperative cognitive assessment.
Clinical Risk Factors for preoperative cognitive impairment
The size and function of the brain decreases with age, causing cognitive decline. In keeping with the clinical practice and literature, our multivariate Logistic regression analysis showed that venerable age was an independent risk factor for cognitive impairment (OR=1.099, P<0.001). In a prospective study of 215 patients undergoing elective surgery of all types, Smith et al. found the effect of ageing on cognitive impairment was apparent. The prevalence of MCI increased with age, with 42% of patients in the 65–69 years age group increasing to 80% of patients aged 80 years and above. Nowadays, more and more elderly patients choose surgery to treat surgical disease. While one or more cardiovascular and cerebrovascular diseases as well as other systemic diseases are always combined in the elderly. Moreover, preoperative multiple medication, frailty, anxiety and depression coexisting further increase the prevalence of cognitive impairment and perioperative complications. Univariate analysis from our data also showed higher ASA grade (P = 0.004) and CCI score (P = 0.048) in the cognitive impairment group when compared with the normal group.
The impact of gender on cognitive dysfunction has been a concern, while the results varied from different studies. Lee et al. have found that gender disparity in cognitive function in India. Compared with male, Indian women have poor cognitive function in their later years. While the cognitive function status of women in developed countries is not significantly different from that of men, and even females are better. Evidence-based analysis indicates that gender has an impact on cognitive impairment in elderly patients, which, on the other hand, might be interfered by differences in BMI, tobacco and alcohol, social and economic activity in different regions, educational attainment, and discrimination against women. The role of gender in cognitive function requires a multi-centered study of larger sample to confirm because of large clinical heterogeneity.
The degree of education has a great impact on cognitive function. Studies have shown that good education and cultural background have a positive effect on the ability of concept formation, vocabulary expression, spatial structure perception and memory; while cultural restriction may contribute to a negative effect. Highly educated people often have high reserve of neurons. The more people receive education, the better subjective initiative and ability to adapt to the external environment, which may stimulate brain cells. The amount of nerve connections (neurons) and information hubs (synapses) are likely to be more numerous in people who are highly educated. Alternatively, even if the quantity of neurons and synapses is no different, the synapses are likely to be more efficient and/or alternative circuitry is likely to be operating in those who are highly educated. Cognitive reserve is an emerging dynamic concept and is thought to be modifiable in keeping with the concept of brain plasticity; A recent clinical study demonstrated that preoperative cognitive reserve might have protective effects on long-term cognitive function after surgery.
A variety of vascular risk factors such as diabetes, hypertension and hyperlipidemia are closely related to cognitive impairment. Nevertheless, the results of the present study showed that there was no difference in diabetes, hypertension and hyperlipidemia between patients with and without cognitive impairment(P>0.05). There is a possibility that severity of the disease and the intervention subjects received are not the same. Clinical research design in future is supposed to filter the enrolled subjects strictly, expand the sample size, and use subgroup analysis to explore the effects of these comorbidities and their intervention on cognitive function.
There is growing evidence that higher Hcy levels are involved in age-related cognitive deficits and various types of central nervous system disorders, including Alzheimer´s disease, Parkinson disease, multiple sclerosis, cerebrovascular diseases and strokes. A review by Esther et al. revealed a positive trend between cognitive decline and increased plasma Hcy concentrations in general population and in patients with cognitive impairments. Homocysteine is produced in all cells, and mechanisms of Homocysteine-induced cognitive impairment include neurotoxicity and vascular injury. Some studies suggest the post-translational modification of proteins by homocysteine, termed homocysteinylation, contributes to its toxicity, while others shown that homocysteinylation induces cellular damage via oxidative stress, as well as disrupts astrocytic end-feet.[26,28] Animal models have shown that high plasma levels of homocysteine contribute to ultrastructural changes to cerebral capillaries, endothelial damage, swelling of pericytes, basement membrane thickening, and fibrosis. In keeping with the literature, patients in cognitive impairment group had a higher level of homocysteine, even though multivariate regression model did not find the difference.
Sleep disorders are quite common in the elderly and are mostly associated with neurodegenerative processes. Moreover, sleep disorders and cognitive impairment often coexist and interact with one another in the early stages of Alzheimer's disease.[31,32] Sleep disorders in patients with MCI are associated with changes in memory and execution, suggesting that sleep dysfunction may be a precursor to cognitive changes. Structure of sleep and EEG may be also abnormal, even in the early stage of MCI. On our study, the elderly often complained sleep disruption due to frequent nocturia, easy to wake up or early awakening. Electroencephalo-graph (EEG) studies also show that such patients have reduced nighttime slow wave sleep, weakened sleep promotion process and enhanced wakefulness process. Altered sleep affects normal sleep patterns seriously: patients frequently recounted that they were sleepy at daytime, and several rapid-eye-movement sleep were exhibited in EEG during their naps. In this study, compared with normal group, subjective sleep quality of impairment group was poorer.
This study has several important limitations. One is that MMSE, the most widely used cognitive screening test, is affected by significant ceiling effects and has insufficient sensitivity for detecting MCI and mild dementia, especially in individuals with higher education levels.[13,35] The Montreal Cognitive Assessment (MoCA) can be used instead of MMSE in order to improve the sensitivity, while with higher requirements for health status and longer test time. Another issue is that other potential confounding biases still remained. For example, anxiety during the preoperative period is the most common problem (the prevalence up to 80%) with a number of perioperative complications such as an increase in cognitive dysfunction and delayed postoperative recovery. While we did not quantify to further analysis the effect on cognition. As risk factors for cognitive impairment, impairments in hearing and vision have impact on perioperative complications in older.[38,39] We excluded these patients for the feasibility of assessment, which may underestimate the prevalence of preoperative cognitive impairment.