The medical imaging and pathological changes of MCI are very similar to the early stage of Alzheimer’s [25,26]. Some studies believed that MCI was the transitional stage of dementia and the basis for the occurrence of dementia. At present, disease-modifying drugs are not available and symptomatic medications have been found to have only modest benefit. Primary prevention of dementia is therefore of great importance. Mild cognitive impairment (MCI) is an intermediate state between normal cognitive aging and dementia. Identification of MCI is thought to be crucial to early intervention. Indeed, MCI is associated with an increased risk of dementia, as well as with future disability and mortality in some studies.
It is a crucial intervention stage to delay senile dementia, and MCI has become an important research topic worldwide. Studies by Brodaty H et al. found that about 30% of MCI patients diagnosed by Petersen criteria progressed to dementia within 3 three years . Early screening of cognitive impairment in the elderly is helpful for early prevention and intervention and can delay the occurrence of Alzheimer’s to a large extent .
The overall prevalence of MCI
A total of 18 studies were included this time, and the total number of studies was 33042 and the combined prevalence rate of MCI was 18%. The prevalence rate of MCI in the Yangtze River Delta was lower than the national average (20.8%) calculated by the Jianping Jia team. First, the lack of homogeneity in the prevalence of MCI is partially due to the differences in the diagnostic criteria and the heterogeneity of the population in China. The Yangtze River Delta locates on the southeast coast of China, with a relatively developed economy, a warm and humid climate, good public health policies, and generally high health literacy among the elderly. These factors would also lead to a low prevalence of cognitive impairment in the elderly. Secondly, cultural and traditional diversity could also explain the differences between these results and those found in previous studies in China. As we all known, a positive relationship between Mediterranean diet (MeDi) adherence and reduced risk for cognitive decline or dementia has been demonstrated by most cross-sectional and prospective epidemiological studies. The Jiangnan diet is very similar to the Mediterranean diet and is a healthy diet suitable for Chinese people, which the native inhabitants in the Yangtze River Delta region have eaten for several hundreds of years. Besides, people in the Yangtze River Delta generally have the habit of drinking tea . Experiments and animal studies have shown that catechins can promote neural progenitor cells’ proliferation, improve spatial cognitive learning ability, and reduce amyloid-mediated cognitive impairment [34,35].
Differences in the spatial distribution
There were also considerable differences in the prevalence of MCI between the regions in the Yangtze River Delta. Generally speaking, the prevalence of MCI in Shanghai [23% (95% CI:17- 30)] was higher than that in other regions. Several factors likely underlie the high prevalence of MCI in Shanghai. According to the seventh national census data, Shanghai’s elderly accounted for 23.38% of the population, making it the most aging area in the Yangtze River Delta. Secondly, the area of Shanghai is smaller than other provinces. The difference is small between factors (diet, education, culture, etc.) that affect the prevalence of MCI. The data deviation between the studies was small, which eliminates the influence of extreme data on the combined prevalence. Third, compared with Shanghai, Zhejiang and Anhui have a lower proportion of the elderly population (18.70%, 18.79%), and the prevalence of MCI in the elderly population is lower than that of Shanghai correspondingly.
Time distribution of the prevalence
By analyzing the development trend of the prevalence of MCI in the elderly in the Yangtze River Delta over the past ten years, we found that the prevalence rate has shown an increasing trend over time, reaching the highest in 2013-2014, and the data dropped slightly since then. Does this fall have any significance? Furthermore, research was needed. The trend of the prevalence of MCI in the elderly with time in this study was consistent with the reports of Emma F and Xue J et al. [36,37]. However, other studies have shown that in many high-income countries, including the United States, the risk of cognitive impairment in later life was declining. For example, Jo Mhairi Hale et al. found that the prevalence of MCI in the elderly showed a downward trend under models that only considered gender, age, and race . The possible reason was related to the improvement of the test score for the subjects who received repeated measurements. Similarly, Kenneth M et al. found that from 1993 to 2002, the prevalence of cognitive impairment in the elderly in the United States dropped from 12.2% to 8.7%, an absolute drop of 3.5% . Given the development trend of the prevalence of MCI in the elderly in the Yangtze River Delta, long-term investigation and more researches are needed. In addition to the prevalence, we should also pay more attention to the incidence for it will reflect the fact of MCI of the elderly in current.
Educational level and prevalence of MCI
In terms of education level and prevalence of MCI in the elderly, the present study revealed a steep increase in the MCI prevalence as the educational level declined. We found that the prevalence difference between the illiterate group and other groups was statistically significant. Most studies also have shown that the lower the level of education, the higher the prevalence of MCI. Leggett A et al. proved that those who have received university education maintain a high level of cognitive function throughout their lives, and the incidence of cognitive dysfunction only increases rapidly after the age of 80 . The higher the level of education, the more developed the knowledge reserve and the brain’s cognitive network, and the higher the health information literacy. A 10-year longitudinal, multicenter, prospective cohort study in Korea showed that low Cognitive Reserve (CR) increased the risk of post-stroke cognitive impairment. The level of education, occupation and CR scores all alleviated the slope of the cognitive impairment curve: the higher the level of education, the higher the vocational or composite CR score, the faster the recovery. It is suggested that higher education can minimize the decline of long-term cognitive ability, especially in elderly patients. Therefore, increasing people’s educational opportunities or mental activities is an important strategy to reduce the risk of cognitive dysfunction in later life.
Gender and Prevalence of MCI
In terms of gender and the prevalence of MCI in the elderly, most studies currently show that women’s prevalence was higher than that of men, suggesting that women may be a risk factor for cognitive impairment. As mentioned above, considering the relationship between education level and the prevalence of cognitive impairment, the possible reason for the higher prevalence of women than men may be related to the fact that women’s education level was generally lower than that of men. Based on the current research on the elderly, their birth date was before 1960, which was in the early days of China’s liberation. Under the socio-economic conditions at that time, women’s education level was generally lower than that of men, and their knowledge reserves and health knowledge literacy were lower than men’s. So the prevalence of cognitive impairment in older women was higher than that in men.
In 1986, China began to implement nine-year compulsory education. Since then, the level of female education has gradually improved. According to data from the National Bureau of Statistics, as of 2019, there were 18.82 million female students in high school education, accounting for 47.1% of all students. The gender gap in education has been eliminated. With the improvement of women's education level, the prevalence of cognitive impairment in Chinese older women will decline, but further empirical research is still needed.
Age and Prevalence of MCI
All included studies found that the prevalence of MCI increased with age. After comprehensive prevalence, we found that the prevalence of MCI of people over 80 years old was statistically different from that of the 60-69 years old group and the 70-79 years old group. Highly consistent with the results of various studies at home and abroad, advanced age was a risk factor for cognitive impairment in the elderly [42-44]. First, The gray matter atrophy of the prefrontal cortex of the medial brain increases with age, and these areas are related to cognitive function associated with impaired long-term memory retention in older adults . Secondly, brain tissue atrophy related to cognition has also changed the sleep patterns of the elderly, and the reduction in the proportion of slow-wave sleep further promotes the impairment of memory function . Also, as the age increases, various organs and tissues of the elderly have undergone degeneration, which indirectly leads to cognitive impairment. Nowadays, due to a lack of an effective treatment regimen, lots of evidence shows that early intervention measures are considered to be the most cost-effective way for managing dementia . It is necessary to study and formulate effective comprehensive intervention measures for the elderly to delay this change’s occurrence and development as much as possible.
Other factors and prevalence of MCI
This study also compared the relationship between the prevalence of MCI in rural and urban elderly. We found a higher prevalence of MCI in the rural than in the urban population. First, The
less-advanced conditions (the income, living conditions, education level, social and health rights enjoyed by most rural residents, and low-skill occupations) in rural areas may explain the high prevalence of MCI. Besides, with the increase of young and middle-aged rural adults leaving their hometown for work, the number of “empty nester” elderly in rural areas has gradually increased. These older people have long been in loneliness and social isolation which also exacerbated the decline in cognitive function of rural elderly to a certain extent. The higher MCI prevalence in rural than in urban areas indicates that special attention must be given to implementing new strategies for these areas. At the same time, we found that the prevalence of MCI in the elderly without a spouse (including unmarried, widowed, divorced, and separated elderly) was higher than that of the elderly with a spouse. This may be related to the long-term lack of communication in the elderly, leading to emotional anxiety and depression [48-50].
More and more evidence indicated that lifestyle changes and early treatment of cardiovascular risk factors might lead to delayed cognitive decline . With our society’s progress, improving people’s living conditions, education, and medical conditions has positively impacted people’s physical, mental, and cognitive health. These effects will reduce the difference in prevalence caused by regions and education, which may reduce the overall prevalence of MCI in the elderly population in China. The sample size included in this study was large enough to cover most of China’s provinces. To a certain extent, it can reflect the development trend and distribution of the prevalence of MCI in China.
Our research still has certain limitations. First of all, the included studies were mainly regional, lack of the study which samples covered the entire Yangtze River Delta region. Therefore, the pooled prevalence will still have a certain deviation from the actual situation. Secondly, there was a lack of data in some cities, especially in underdeveloped regions, because regional economic levels would impact the prevalence, reducing the reliability of the results. Finally, MCI was further categorized by imaging into MCI caused by prodromal Alzheimer’s disease (MCI-A), MCI resulting from cerebrovascular disease (MCI-CVD), MCI with vascular risk factors (MCI-VRF), and MCI caused by other diseases (MCI-O). This study did not reflect the differences between various types of cognitive impairment, and we will continue future studies. Accordingly, we will carry out a large-scale epidemiological survey in the Yangtze River Delta region to accurately assess the prevalence of MCI and subtypes in the Yangtze River Delta region.
In summary, the prevalence of MCI among the elderly population in the Yangtze River Delta was relatively high, and there were differences in the prevalence of MCI among the elderly in different regions, genders, ages, and education levels. The following research on cognitive impairment in the elderly should focus on: The following research directions: (1) For areas not covered by the study, we must strengthen the screening of elderly cognitive impairment and policy attention; (2) Research and formulate unified and reliable standards for the diagnosis of various types of cognitive impairment; (3) Screen high-risk populations from mild cognitive impairment to dementia, and improve the pertinence of prevention and intervention; (4) Formulate relevant public health policies, and accelerate the establishment of a socialized service system for the elderly with cognitive impairment, such as a comprehensive intervention system with a three-level linkage between “family -community-hospital”; (5) Increase efforts to train high-level home care service personnel to make up for the lack of care resources caused by changes in family structure.