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Introduction

Hypertension (HT, also known as high blood pressure, BP) is defined as a BP of 140/90. But more recently, the guidelines were changed making HT being defend as a BP over 130/90 (Carey et al, 2022; Iqbal and Jamal, 2022). One 2019 study showed that in a sample with an age range of 20-79, 24 percent of men and 23 percent of women could be classified as hypertensive based on the old guidelines (140/90) (Deguire et al, 2019). Having consistent high BP could lead to devestating consequences like (from the patient’s perspective) hot flushes, dizziness, and mood disorders (Goodhart, 2016). However, one serious problem with HT is the issue that consistently high BP is associated with a decrease in brain volume (BV). This has been seen in two systematic reviews and meta-analyses (Alosco et al, 2013; Beauchet et al, 2013; Lane et al, 2019; Alateeq, Walsh and Cherbuin, 2021; Newby et al, 2022) while we know that long-standing hypertension has deleterious effects on brain health (Salerno et al, 1992). However, it’s not only high BP that’s related to this, it’s also lower BP in conjuction with lower pulse pressure (Muller et al, 2010; Foster-Dingley, 2015). So what this says to me is that too much or too little blood flow to the brain is deleterious for brain health.I will state the hypothesis and then I will state the predictions that follow from it. I will then provide three reasons why I think this relationship occurs.

The hypothesis

The hypothesis is simple: high BP (hypertension, HT) is associated with a reduced brain volume. This relationship is dose-dependent, meaning that the extent and duration of HT correlates with the degree of BV changes. So the hypothesis suggests that there is a relationship—an association—between HT and brain volume, where people with HT will be more likely to have decreased BVs than those who lack HT—that is, those with BP in the normal range.

The dose-dependent relationship that has been observed (Alateeq, Walsh and Cherbuin, 2021), and this shows that as HT increases and persists over time, the effects of decreased BV become more pronounced. This relationship suggests that it’s not a binary, either-or situation, present or absent situation, but that it varies across a continuum. So people with shorter-lasting HT will have fewer effects than those with constant and consistent elevated BP and they will then show subsequent higher decreases in BV. This dose-dependent relationship also suggests that as BP continues to elevate, the decrease in BV will worsen.

This dose-dependent relationship implies a few things. The consequences of HT on BV aren’t binary (either or), but are related to the severity of HT, how long one has HT, and at what age they have HT and that it varies on a continuum. For instance, people with mild or short-lasting HT would experience smaller reductions in BV than those that have severe or long-standing HT. The dose-dependent relationship also suggests that the longer one has HT without treatment, the more severe and worse the reduction in BV will be if it is uncontrolled. So as BP continues to elevate, it may lead to a gradual reduction in BV. So the relationship between HT and BV isn’t uniform, but it varies based on the intensity and duration of high BP.

So the hypothesis suggests that HT isn’t just a risk factor for cardiovascular disease, but it’s also a risk factor for decreased BV. This seems intuitive, since the higher one’s BP, the more likely it is that there is the beginnings of a blockage somewhere in the intricate system of blood vessels in the body. And since the brain is a vascular organ, then by decreasing the amount of blood flowing to it, this then would lead to cell death, white matter lesions which would lead to a smaller BV. One newer study showed, with a sample of Asians, whites, blacks, and “Latinos” that, compared to those with normal BP, those who were transitioning to higher BP or already had higher BP had lower brain connectivity, decreased cerebral gray matter and frontal cortex volume, while this change was worse for men (George et al, 2023). Shang et al (2021) showed that HT diagnosed in early and middle life but not late life was associated with decreased BV and increased risk of dimentia. This, of course, is due to the slow cumulative effects of HT and it’s effects on the brain. While Power et al (2016) “The pattern of hypertension ~15 years prior and hypotension concurrent with neuroimaging was associated with smaller volumes in regions preferentially affected by Alzheimer’s disease.” But not only is BP relevant here, so is the variability of BP at night (Gutteridge et al, 2022; Yu et al, 2022). Alateeq, Walsh and Cherbuin (2021) conclude that:

Although reviews have been previously published in this area, they only investigated the effects of hypertension on brain volume [86]. To the best of our knowledge, this study’s the first systematic review with meta-analysis providing quantitative evidence on the negative association between continuous BP and global and regional brain volumes. Our results suggest that heightened BP across its whole range is associated with poorer cerebral health which may place individuals at increased risk of premature cognitive decline and dementia. It is therefore important that more prevention efforts be directed at younger populations with a greater focus on achieving optimal BP rather than remaining below clinical or pre-clinical thresholds[5].

One would think that a high BP would actually increase blood flow to the brain, but HT actually causes alterations in the flow of blood to the brain which leads to ischaemia and it causes the blood-brain barrier to break down (Pires et al, 2013). Essentially, HT has devestating effects on the brain which could lead to dimentia and Alzheimer’s (Iadecola and Davisson, 2009).

So the association between HT and decreased BV means that individuals with HT can experience alterations in BV in comparison to those with normal BP. The hypothesis also suggests that there are several mechanisms (detailed below), which may lead to various physiological and anatomic changes in the brain, such as vascular damage, inflammation and tissue atrophy.

The mechanisms

(1) High BP can damage blood vessels in the brain, which leads to reduced blood flow. This is called “cerebral hypoperfusion.” The reduced blood flow can deprive the cells in the brain of oxygen and nutrients, which cause them to shrink or die which leads to decreased brain volume (BV). Over time, high BP can damage the arteries, making them less elastic

(2) Over a long period of time having high BP, this can cause hypertensive encephalopathy, which is basically brain swelling. A rapid increase in BP could over the short term increase BV, but left untreated it could lead to brain damage and atrophy over time.

And (3) Chronically high BP can lead to the creation of white matter lesions on the brain, and the lesions are areas of damaged brain tissue which could result in microvascular changes caused by high BP (hypertension, HT). Thus, over time, the accumulation of white matter lesions could lead to a decrease in brain volume. HT can contribute to white matter lesions in the brain, which are then associated with cognitive changes and decreased BV, and these lesions increase with BP severity.

So we have (1) cerebral hypoperfusion, (2) hypertensive encephalopathy, and (3) white matter lesions. I need to think/read more on which of these could lead to decreased BV, or if they all actually work together to decrease BV. We know that HT damages blood vessels, and of course there are blood vessels in the brain, so it then follows that HT would decrease BV.

I can also detail a step-by-step mechanism. The process beings with consistently elevated BP, which could be due to various factors like genetics, diet/lifestyle, and underlying medical conditions. High BP then places increased strain on the blood vessels in the body, including those in the brain. This higher pressure could then lead to structural change of the blood vessels over time. Then, chronic HT over time can lead to endothelial dysfunction, which could impair the ability of blood vessels to regulate blood flow and maintain vessel integrity. The dysfunction can result in oxidative stress and inflammation.

Then as a response to prolonged elevated BP, blood vessels in the brain could undergo vascular remodeling, which involves changes im blood vessel structure and thickness, which can then affect blood flow dynamics. Furthermore, in some cases, this could lead to something called cerebral small vessel disease which involves damage to the small blood vessels in the brain including capillaries and arterioles. This could impair delivery of oxygen and nutrients to brain tissue which could lead to cell death and consequently a decrease in BV. Then reduced blood flow along compromised blood vessel integrity could lead to cerebral ischaemia—reduced blood supply—and hypoxia—reduced oxygen supply—in certain parts of the brain. This can then result in neural damage and eventually cell death.

Then HT-related vascular changes and cerebral small vessel disease can trigger brain inflammation. Prolonged exposure to neural inflammation, hypoxia and ischemia can lead to neuronal atrophy, where neurons shrink and lose their functional integrity. HT can also increase the incidence of white matter lesions in the brain which can be seen in neuroimages, which involve areas of white matter tissue which become damaged. Finally, over time, the cumulative effects of the aforementioned processes—vascular changes, inflammation, neural atrophy, and white matter changes could lead to a decrease in BV. This reduction can manifest as brain atrophy which is then observed in parts of the brain which are susceptible and vulnerable to the effects of HT.

So the step-by-step mechanism goes like this: elevated BP —> increased vascular strain —> endothelial dysfunction —> vascular remodeling —> cerebral small vessel disease —> ischemia and hypoxia —> inflammation and neuroinflammation —> neuronal atrophy —> white matter changes —> reduction in BV.

Hypotheses and predictions

H1: The severity of HT directly correlates with the extent of BV reduction. One prediction would be that people with more severe HT would exhibit greater BV decreases than those with moderate (less severe) HT, which is where the dose-dependent relationship comes in.

H2: The duration of HT is a critical factor in BV reduction. One prediction would be that people with long-standing HT will show more significant BV changes than those with recent onset HT.

H3: Effective BP management can mitigate BV reduction in people with HT. One prediction would be that people with more controlled HT would show less significant BV reduction than those with uncontrolled HT.

H4: Certain subpopulations may be more susceptible to BV decreases due to HT. One prediction is that certain factors like age of onset (HT at younger age), genetic factors (some may have certain gene variants that make them more susceptible and vulnerable to damage caused by elevated BP), comorbities (people with diabetes, obesity and heart problems could be at higher risk of decreased BV due to the interaction of these factors), ethnic/racial factors (some populations—like blacks—could be at higher risk of having HT and they could be more at risk due to experiencing disparities in healthcare and treatment.

The hypotheses and predictions generated from the main proposition that HT is associated with a reduction in BV and that the relationship is dose-dependent can be considered risky, novel predictions. They are risky in the sense that they are testable and falsifiable. Thus, if the predictions don’t hold, then it could falsify the initial hypothesis.

Blacks and blood pressure

Due to this, for populations like black Americans, this is significant. About 33 percent of blacks have hypertension (Peters, Arojan, and Flack, 2006), while urban blacks are more likely to have elevated BP than whites (Lindhorst et al, 2007). Though Non, Gravlee, and Mulligan (2012) showed that racial differences in education—not genetic ancestry—explained differences in BP in blacks compared to whites. Further, Victor et al (2018) showed that in black male barbershop attendees who had uncontrolled BP, that along with medication and outreach, this lead to a decrease in BP. Williams (1992) cited stress, socioecologic stress, social support, coping patterns, health behavior, sodium, calcium, and potassium consumption, alcohol consumption, and obesity as social factors which lead to increased BP.

Moreover, consistent with the hypothesis discussed here (that chronic elevated BP leads to reductions in BV which lead to a higher chance of dementia and Alzheimer’s), it’s been shown that vulnerability to HT is a major determinate in the risk of acquiring Alzheimer’s (Clark et al, 2020; Akushevic et al, 2022). It has also been shown that “a lifetime of racism makes Alzheimer’s more common in black Americans” and consistent with the discussion here since racism is associated with stress which is associated with elevated BP, then consistent events of racial discrimination would lead to consistent and elevated BP which would then lead to decreased BV and then a higher chance of acquitting Alzheimer’s. But, there is evidence that blood pressure drugs (in this case telmisartan) reduce the incidence of Alzheimer’s in black Americans (Zhang et al, 2022) while the same result was also seen using antihyperintensive medications in blacks which led to a reduction in incidence of dementia (Murray et al, 2018), which lends credence to the discussed hypothesis. Stress and poverty—experiences—and not ancestry could explain higher rates of dementia in black Americans as well. Thus, since blood pressure could explain higher rates of dementia in black populations, this then lends credence to the discussed hypothesis.

Conclusion

The evidence that chronic elevated BP leads to reductions in BV are well-studied and the mechanisms are well-known. I discussed the hypothesis that chronically elevated BP leads to reduced blood flow to the brain which decreases BV. I then discussed the mechanisms behind the relationship, and then hypotheses and predictions that follow from them. Lastly, I discussed the well-known fact that blacks have higher rates of BP, and also higher rates of dementia and Alzheimer’s, and linked the fact that they have higher rates of BP to those maladies.

So by catching chronically elevated BP in the early ages, since the earlier one has high BP the more likely they are to have reduced brain volume and the associated maladies, we can then begin to fight the associated issues before they coalesce, since we know the mechanisms behind them, along with the fact that blood pressure drugs and antihypertensive medications decrease incidences of dementia and Alzheimer’s in black Americans.