16 Jan 2020

Three new studies add to growing evidence that damaged blood vessels wreak havoc on the brain, but not by exacerbating Aβ deposition. One, published in JAMA Neurology on December 20, found no correlation between intracerebral atherosclerosis and overall amyloid plaque burden in cognitively normal older adults. Another, in the January issue of Annals of Neurology, reported that midlife atherosclerosis in the carotid artery upped future risk of vascular dementia, but not AD. A third, published in Neurology on December 30, found that white-matter hyperintensities—a proxy for damage to small vessels in the brain—had no bearing on future changes in AD biomarkers.

“These studies can all be interpreted to support the hypothesis that vascular risk influences the risk for development of cognitive impairment and dementia principally via non-amyloidogenic pathways,” wrote Jonathan Schott of University College London. “They provide further evidence for, and are compatible with, the growing body of evidence that the timing of vascular risk also matters, with midlife being the most sensitive period.”

To Anja Soldan, Johns Hopkins University, Baltimore, who led one of the studies, all three converge on a similar conclusion. “They all suggest that cerebrovascular disease and AD affect cognitive decline through distinct pathways,” she said.

On their own, faulty blood vessels in the brain can cause cognitive impairment and dementia. Blood-vessel disease is also thought to contribute to the clinical symptoms of AD, since people with Alzheimer’s often have vascular pathology along with amyloid plaques and neurofibrillary tangles (Honig et al., 2005; Yarchoan et al., 2012; Toledo et al., 2013). However, if, and at what disease stage, vascular dysfunction contributes to the amyloid deposition is unclear.

While the Atherosclerosis Risk in Communities (ARIC) study tied midlife vascular risk to subsequent Aβ accumulation, recent data from the British birth cohort indicated no such relationship (Apr 2017 news; Aug 2019 news; Nov 2019 news). 

Now, as reported in JAMA Neurology, researchers led by Rebecca Gottesman at Johns Hopkins looked for cross-sectional correlations in ARIC between Aβ load and intracerebral atherosclerotic disease (ICAD)—a manifestation of vascular disease in the brain that has been tied to stroke and cognitive decline. They used black-blood MRI, which detects atherosclerotic plaques, to measure ICAD throughout the brains of 300 cognitively normal ARIC participants in their 70s. Each of them also had an amyloid PET scan in the same year. MRI indicated that 105 people had at least one vessel containing an atherosclerotic plaque. Though the global Aβ burden generally trended higher in people with ICAD, this association disappeared once the researchers adjusted for age, sex, and other demographic factors.

Gottesman did pick up some associations in specific regions of the brain. Among 22 people with at least one ICAD plaque who also had narrowing, aka stenosis, of the middle or anterior cerebral arteries, chances of Aβ deposition in regions directly supplied by those vessels more than doubled. However, Gottesman cautioned that this intriguing hint came from a small subset of people in the cohort, and needs to be tested in a larger sample.

If midlife vascular risk correlated with future Aβ deposition in ARIC, why didn’t late-life ICAD? Gottesman thinks that the midlife vascular risk factors that influence ICAD and Aβ may be different. Costantino Iadecola of Weill Cornell Medical College in New York noted that because the study excluded people with cognitive impairment, it likely excluded those with more advanced ICAD—especially people with stenosis. Perhaps an association with Aβ would be detected in advanced ICAD, he said.

In contrast, researchers led by Katarina Nägga and Oskar Hansson of Lund University in Mälmo, Sweden, found no relationship between midlife atherosclerosis in the carotid artery and future AD diagnoses. As they reported in Annals of Neurology 6,103 participants in the Mälmo Diet and Cancer Study had baseline ultrasound to measure the thickness of their intima media—the innermost layers of the carotid artery—in midlife. Over the following 20 years, 462 developed dementia. Of those, 138 were classified as pure AD, 147 as AD with cerebrovascular pathology, and 109 as vascular dementia. The researchers found that neither midlife intima media thickness nor the number of carotid atherosclerotic plaques associated with AD or mixed dementia. However, a thicker intima media in midlife did associate with all-cause and with vascular dementia. People with the most carotid plaques in midlife also had higher chances of vascular dementia later on.

What about CSF biomarkers? Among 330 participants in the Swedish BIOFINDER cohort—a cognitively normal subgroup of the Mälmo study whose members had CSF biomarker measurements—the researchers found no connection between midlife carotid atherosclerosis and CSF Aβ42 or Aβ42/p-tau measured 20 years later. Iadecola noted that, similar to the ARIC study, the BIOFINDER cohort was subject to selection bias, as people with dementia were excluded.

The third study, published in Neurology and led by Soldan, investigated in the BIOCARD study whether the size of a person’s white-matter hyperintensities—a gauge of small-vessel disease in the brain—correlated with cognitive impairment or CSF biomarkers of AD. WMH volume and CSF biomarkers were measured at baseline in 274 participants, when they were in their late 50s and cognitively normal. Over the next decade, 37 had developed MCI and 23 dementia. Clinical records indicated that 24 had vascular disease. Those 60 people had larger WMHs, less CSF Aβ42, and more CSF t-tau and p-tau at baseline than the 214 who did not go on to develop MCI.

However, WMH volume did not sway a person’s time to MCI independently of baseline Aβ or p-tau. Hence, the data suggest that small-vessel pathology did not influence subsequent cognitive decline in people who had AD, Soldan said. In people with low baseline total tau, and hence a lesser degree of neurodegeneration, baseline WMH volume did correlate with earlier symptom onset. However, overall, people with low t-tau at baseline had lower odds of future cognitive impairment, regardless of their WMH burden, than did people with high t-tau or other AD biomarkers.

Soldan followed a subset of 132 participants who donated at least two CSF samples over the next two to four years. Their baseline WMH volume did not correlate with any fluid biomarker changes. Essentially, this study suggests that damage to small vessels in the brain exacerbates neither AD pathology nor cognitive decline in people who tested positive for AD biomarkers.

Anna-Märta Gustavsson, first author of the Mälmo study, also thinks cerebrovascular disease and AD work through different pathways. “My interpretation of these joint findings is that vasculopathy may contribute to cognitive decline in AD dementia by cerebrovascular alterations, rather than influencing Aβ pathology directly,” she wrote. Gustavsson added that further studies should examine relationships between midlife vascular risk and subsequent AD biomarkers in more diverse cohorts, and include people who have MCI and dementia.

Prashanthi Vemuri of the Mayo Clinic in Rochester, Minnesota, said that findings from the three studies dovetail with those from her group, which cast good vascular health not as a protector against amyloid, but as shield against synaptic damage and neurodegeneration, regardless of its cause (Vemuri et al., 2017; Vemuri et al., 2017; Vemuri et al., 2019). “Good vascular health may not stop Ab deposition, but it likely delays the onset of clinical symptoms in people with AD,” she said.

Iadecola thinks that regardless of whether vascular dysfunction has an additive or synergistic relationship with AD pathology in influencing cognitive decline, the crucial point is that good blood vessel health benefits the brain. A person’s vascular risk is highly modifiable by way of lifestyle choices or, if need be, medication.—Jessica Shugart

Comments

1. • Timothy Hughes
     Wake ADRC
   • Posted: 21 Jan 2020

   This a wonderful discussion of an important topic in the vascular contributions to dementia. It is important to contextualize that these results pertain specifically to atherosclerosis and not other forms of vascular dysfunction. Gottesman et al. is consistent with the literature showing atherosclerosis is weakly or not associated with dementia and its pathology.

   Atherosclerosis is one focal form of vascular disease that can impact brain health, most often through acute cerebrovascular insult. In the case of severe stenosis, a diffuse pattern of ischemia is also observed. It is more difficult to measure and disentangle more diffuse forms of vascular pathology and dysfunction (e.g. arterial stiffness, arteriolosclerosis, endothelial dysfunction) from atherosclerosis.

   Yet, there are data emerging that more diffuse function disorders (e.g., arterial stiffness) are associated with both cerebral small-vessel disease and amyloid pathology in older adults, while atherosclerosis is not. Additional basic models are needed to determine if arterial dysfunction and stiffening plays a potential role inhibiting amyloid clearance or as a consequence of amyloid deposition in the vascular wall. Currently, our amyloid PET ligands cannot differentiate vascular from parenchymal fibrillar amyloid deposits. Given the common nature of extensive amyloid deposition in older adults, it is important to specify what forms of amyloid deposition are exhibited in older adults.

   For more detailed reading: Hughes, Craft and Lopez. "Review of 'The Potential Role of Arterial Stiffness in the Pathogenesis of Alzheimer's Disease.'" 

   References:

   Hughes TM, Craft S, Lopez OL. Review of 'the potential role of arterial stiffness in the pathogenesis of Alzheimer's disease'. Neurodegener Dis Manag. 2015;5(2):121-35. PubMed.

   View all comments by Timothy Hughes

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References

News Citations

1. Vascular Disease in 50s Begets Brain Amyloid in 70s 14 Apr 2017
2. Blood Pressure: How Low to Prevent Dementia—and When? 23 Aug 2019
3. Already in Mid-30s, Poor Vascular Health Means Small Brain at 70 13 Nov 2019

Paper Citations

1. Honig LS, Kukull W, Mayeux R. Atherosclerosis and AD: analysis of data from the US National Alzheimer's Coordinating Center. Neurology. 2005 Feb 8;64(3):494-500. PubMed.
2. Yarchoan M, Xie SX, Kling MA, Toledo JB, Wolk DA, Lee EB, Van Deerlin V, Lee VM, Trojanowski JQ, Arnold SE. Cerebrovascular atherosclerosis correlates with Alzheimer pathology in neurodegenerative dementias. Brain. 2012 Dec;135(Pt 12):3749-56. PubMed.
3. Toledo JB, Arnold SE, Raible K, Brettschneider J, Xie SX, Grossman M, Monsell SE, Kukull WA, Trojanowski JQ. Contribution of cerebrovascular disease in autopsy confirmed neurodegenerative disease cases in the National Alzheimer's Coordinating Centre. Brain. 2013 Sep;136(Pt 9):2697-706. Epub 2013 Jul 10 PubMed.
4. Vemuri P, Lesnick TG, Przybelski SA, Knopman DS, Lowe VJ, Graff-Radford J, Roberts RO, Mielke MM, Machulda MM, Petersen RC, Jack CR Jr. Age, vascular health, and Alzheimer disease biomarkers in an elderly sample. Ann Neurol. 2017 Nov;82(5):706-718. Epub 2017 Oct 26 PubMed.
5. Vemuri P, Knopman DS, Lesnick TG, Przybelski SA, Mielke MM, Graff-Radford J, Murray ME, Roberts RO, Vassilaki M, Lowe VJ, Machulda MM, Jones DT, Petersen RC, Jack CR Jr. Evaluation of Amyloid Protective Factors and Alzheimer Disease Neurodegeneration Protective Factors in Elderly Individuals. JAMA Neurol. 2017 Jun 1;74(6):718-726. PubMed.
6. Vemuri P, Lesnick TG, Knopman DS, Przybelski SA, Reid RI, Mielke MM, Graff-Radford J, Lowe VJ, Machulda MM, Petersen RC, Jack CR Jr, MD. Amyloid, Vascular, and Resilience Pathways Associated with Cognitive Aging. Ann Neurol. 2019 Dec;86(6):866-877. Epub 2019 Oct 17 PubMed.

Further Reading

News

1. Vascular Problems in 40s, 50s Beget Dementia Down the Road 9 Aug 2017
2. SPRINT MIND Data Published, Follow-Up Extended 30 Jan 2019