High blood pressure is generally considered to be a risk factor for developing dementia. However, new research challenges this belief, instead suggesting that hypertension may lower the risk - especially in older patients.
The illness, which manifests as a loss of cognitive function and behavioral abilities, affects 1 in 3 U.S. seniors.
However, new research published in the journal Alzheimer's & Dementia suggests that the opposite may be true.
A team of researchers at the University of California-Irvine - led by Prof. Maria Corrada, a professor of neurology and epidemiology - set out to investigate the link between hypertension and dementia in patients aged 90 and above.
Analyzing the link between hypertension and dementia in older patients
Prof. Corrada and team examined 559 patients from a long-term, population-based longitudinal study of people aged 90 and above, known as The 90+ Study.
The 90+ Study participants were mostly highly educated (75 percent), Caucasian (99 percent), and female (71 percent). They were selected from the survivors of the Leisure World Cohort Study, which comprised 13,978 members of a California retirement community.
At the start of The 90+ Study, the respondents - who, on average, were 93 years old - did not have dementia. The researchers followed the participants for 2.8 years, assessing them every 6 months to check if they developed the disease.
The assessment included neurological and neuropsychological examinations, as well as a review of their medical record to check for a hypertension diagnosis.
The researchers also measured their BP at the beginning of the study.
Prof. Corrada and team first examined the association between dementia risk and a history of hypertension, as reported by the participants.
Then, researchers estimated the link between the age of hypertension onset and dementia risk, using the "no hypertension" group as reference, and the association between baseline hypertension stage and risk of dementia, using normal BP as a reference.
Dementia risk lowest among those with hypertension at age 90 or above
Prof. Corrada and team adjusted all these measurements to account for antihypertensive medications.
In the follow-up period, 40 percent of the participants received a dementia diagnosis, and 61 percent reported a hypertension diagnosis.
Most participants reported having been diagnosed with hypertension after the age of 70, but 19 percent of participants said that the hypertension onset was at the age of 80 and above.
Overall, individuals in all stages of hypertension at baseline seemed to have a lower risk of dementia compared with those with normal BP, but these results were not statistically significant. However, the scientists noticed an inverse correlation between dementia risk and hypertension severity.
Patients who had been diagnosed with hypertension between the ages of 80-89 were at a significantly lower risk of developing dementia, compared with those with normal BP. Additionally, those who received their hypertension diagnosis at the age of 90 and over had the lowest risk of dementia.
The results stayed the same after the scientists adjusted for other parameters (including antihypertensive medication).
What could explain the findings?
As the authors note, this is the first time a study reports on the associations between dementia in "the oldest old" and age of hypertension onset.
"These new findings suggest some risk factors for dementia may change over the course of our lives," says Maria Carrillo, Ph.D., Alzheimer's Association chief science officer. "We have seen similar results in past studies comparing body mass in older adults with dementia risk."
Although the study is observational, Prof. Corrada and team ventured possible explanations for the results.
One reason for the association could be that in order to maintain normal cognition, the brain needs a certain blood flow level, which may change with age. According to this explanation, those who develop hypertension later in life may have developed a compensatory mechanism to keep the necessary blood flow level. The authors reference other studies showing that individuals with lower blood flow may have higher rates of cognitive decline.
Another possible but unlikely explanation includes antihypertensive medications, and the possibility of a certain class of medication lowering dementia risk. However, the authors explain that if this was the case, the lowest risk would have been noticed in those taking the medication for a long period of time - such as with hypertension onset earlier in life, not later.
Finally, a third possible explanation is a form of reversed causality; rather than lower BP causing poor cognition, a neurodegenerative process may cause a decline in BP. This way, those who have dementia as a consequence of the deterioration of brain cells would also have higher BP.
The authors concede that more studies are needed to explain their findings.
"Before we can make the leap to suggesting changes to blood pressure recommendations for reducing dementia risk in clinical care, we need more research to confirm and explain our findings. This includes investigations into the underlying biology of hypertension and brain function."
Prof. Maria Corrada
"We need to understand the bigger picture of what protects brain health throughout our entire lives, including our later years," adds Dr. Carrillo. "Looking at dementia in this group is critical since it is the fastest growing segment of the U.S. population with the highest rate of dementia."