Discussions
Long-term hypertension can cause cerebral arteriosclerosis and cerebral blood circulation disorders, cause brain tissue function damage, and is related to cognitive dysfunction31. This meta-analysis has included thirteen case-control studies on the correlation between blood pressure variability and cognitive function, and the quality of the included literature is high. The synthesized outcomes have showed that the increase in blood pressure coefficient of variation is correlated with cognitive dysfunction, and cognitive function is also correlated with 24-hour blood pressure standard deviation and average blood pressure, but there is no correlation with the morning peak of blood pressure. The increase of blood pressure variability will increase the risk of cognitive dysfunction and adversely affect the prognosis of hypertensive patients32. It is worth noting that the large heterogeneity of the analysis indicators in this study may be related to the sample size and age or the inconsistent follow-up time of each study. Therefore, the results of this meta-analysis should be treated with cautions.
Short-term and long-term blood pressure variability are independently related to the development, progression, and severity of heart, blood vessel, and kidney damage, and are associated with an increased risk of cardiovascular events and death33,34. Blood pressure variability has been considered that it has a better predictive effect on cardiovascular events than the average blood pressure [20]. Cognitive dysfunction covers the entire progression from mild cognitive impairment to dementia, and it is also a major problem in the prevention and treatment of hypertension complications. There has been a large amount of evidence in the past that hypertension is the most common risk factor for cognitive impairment, and recent studies35,36 has shown that blood pressure variability has a close clinical relationship with cognitive function. At present, the mechanism between blood pressure variability and cognitive function is not clear, but some scholars believe that it may be related to the white matter. Blood pressure variability is related to target organ damage, including white matter that has nothing to do with the 24-hour average blood pressure value, leading to the occurrence of white matter hyperintensity, and white matter lesions are the inducing conditions for cognitive impairment37. The presence of high-strength lesions and cavities around the ventricles of the elderly with asymptomatic hypertension suggests the relationship between diurnal blood pressure changes and cerebrovascular injury, and cerebrovascular injury will further accelerate the progress of cognitive dysfunction, which is also a common predictor of stroke8.
Previous study38 has showed that increased systolic blood pressure and increased sleep systolic pressure variability were related to the aggravation of brain atrophy. The aggravation of cerebral arteriosclerosis and cerebral arterial remodeling caused by long-term systolic hypertension may be related to the disorder of brain autoregulation39. It leads to a decrease in cerebral blood flow and aggravation of brain atrophy, which leads to a decline in cognitive function40. Recent studies41,42 have shown that increased blood pressure variability is associated with stroke and advanced carotid artery remodeling, and carotid artery remodeling can lead to cognitive impairment. The absolute dynamic systolic blood pressure level especially during sleep and the decrease in night systolic blood pressure are important indicators of brain volume and cognitive function. Based on the above results, it is believed that chronic ischemia caused by hypertension may cause the interruption of the day and night changes in blood pressure through the damage of the brain’s self-regulatory function, and then the blood pressure does not drop during sleep43. Previous studies44,45 have found that systolic blood pressure variability is more predictive of cognitive dysfunction than systolic blood pressure. It’s been reported that through community experiments that higher blood pressure variability is related to the decline in cognitive function of patients assessed by simple mental status examination46. Although the systolic blood pressure variability cannot predict whether the risk of dementia in the overall population increases, it is not found in the subgroup of individuals without a history of cardiovascular disease47. The systolic blood pressure variability is positively correlated with the risk of dementia48. Previous studies49,50 have reported that high or low blood pressure will lead to cognitive dysfunction. The above conclusions are consistent with the analysis results of this research. Therefore, it is speculated that blood pressure variability reflects the degree of cognitive decline to a certain extent, and can be used as an important clinical reference index in the prognosis of middle-aged and elderly hypertensive patients51,52.
Several limitations in this present meta-analysis should be concerned. Firstly, most included studies were reported from China, therefore population and region biases may be existed, future studies from different region and populations should be conducted. Secondly, studies have reported that cognitive impairment is related to factors such as pulse pressure index and ankle brachial index. Due to the limitations of the included research data, we cannot further analyze and discuss the correlation of these indicators. Future research should be further conducted to elucidate the correlation between blood pressure related indicators and CI.