Strokes + Dementia + The Nun Study

My father is still is hospital after the stroke he sustained on 13 Feb, 2016. He remains very confused and unable to move his left arm or leg. After a short stint in the rehabilitation unit - doctors have deemed him a non-viable rehabilitation patient due to his dementia, which is significantly worse since the stroke.
I have been reading up on the topic of dementia and stroke. It appears that for people with Alzheimer's disease (one type of dementia) the chance of confusion dissipating is much higher than for people who have a vascular dementia. Makes sense since strokes (including TIAs) and vascular dementia are common bed-buddies.
Below is an extract taken from a blogpost about strokes on Welcome Trust - about a study conducted by Dr David Snowdon looking at the brains of hundreds of nuns comparing brains damaged by strokes and dementia. I love this study as it isn't contaminated by additional, uncontrollable risk factors. Actually, the entire post is fascinating and worth reading. References for the Nun Study are available on the original post.

‘Ageing with Grace’

In the 1990s, a landmark US study began to show the power of the interplay between vascular disease and dementia. The Nun Study has followed several hundred School Sisters of Notre Dame, who were originally recruited by epidemiologist Dr David Snowdon. Remarkably, these nuns all agreed to donate their brains to the study after they died. When allied with information from mental and physical tests in life, the post-mortem findings allow the researchers to assess the connections between potential risk factors, signs of dementia and brain pathology.
In a book about the Nun Study, Snowdon wrote: “Strokes can bring on the sudden appearance of dementia symptoms that are caused by the rupture or blockage of a blood vessel. In contrast, the symptoms of Alzheimer’s disease usually progress slowly. However, with advancing age, these two diseases may become intertwined and confused. How stroke and Alzheimer’s knot together is far from an academic question. Effective prevention and treatment strategies might result from untying that knot.”
Because the nuns have lived very similar lives in the same environment, the degree of confounding by other factors is minimal. For example, none of them smoked and they all had essentially the same diet and lifestyle. Differences between those who developed dementia and those who didn’t can therefore be picked up more readily than in broader population studies.
The results were startling. In a 1997 paper published in the Journal of the American Medical Association, Snowdon and colleagues presented findings on the links between dementia in life, signs of vascular damage and the characteristic signs of Alzheimer’s disease – amyloid plaques and tau tangles – in the nuns’ brains.
Of those with the plaques and tangles of Alzheimer’s disease but no signs of stroke, 57 per cent had had dementia. But of those with both Alzheimer’s and stroke damage, the proportion with dementia was 93 per cent. “What we found,” wrote Snowdon, “went beyond loosening the knot of confusion regarding how Alzheimer’s disease and stroke interact – it challenged some of the most common diagnostic truisms.”
Stewart says there are two possible explanations for how vascular disease exacerbates the dementia caused by Alzheimer’s disease: “Vascular changes in the brain might be directly linked to amyloid plaques and tau tangles – there is some evidence for this in studies where animals with ischaemia in the brain seemed to produce more amyloid.
“Or, and there is more evidence for this, there are two parallel processes combining to cause dementia. Each causes damage to the brain and, as the damage accumulates, the resulting dementia becomes more severe.”
It is also possible that strokes impair the brain in such a way as to reveal undiagnosed dementia. Stewart explains that, in the early stages of Alzheimer’s disease, people can often compensate for signs of dementia such as memory loss for quite a long time. Compensating becomes much harder when the effects of vascular disease are causing problems in other brain functions as well.
Either way, vascular damage caused by strokes, whether detected in life or not, increased the nuns’ risk of dementia, and it was clear that the classic hallmarks of Alzheimer’s disease were not enough on their own to guarantee dementia.