Plaques and tangles in the brain are typical manifestations in people with Alzheimer’s disease, as autopsies have shown. So how come some people with these brain features have dementia and some haven’t?
Researchers looked at the brain structure of 38 deceased Catholic nuns, their medical and life history. There were two distinct groups: the asymptomatic and the symptomatic individuals who developed dementia. Autopsy revealed that both groups exhibited brain morphology characteristic of Alzheimer’s, e.g. plaques and tangles. However, the asymptomatic nuns had large neurons with better functionality and more connections compared to their symptomatic counterparts.
Looking into the life history of these nuns, the researchers examined essays written by the deceased nuns in their late teens/early twenties upon entering the convent. Analysis of the written works revealed that the asymptomatic nuns have significant better and richer language skills during the first three decades of their life. The richness was based, among others, on the number of ideas were expressed per 10 words, the number of verbs and adjectives used in one sentence, but not based on grammatical correctness. The asymptomatic nuns scored 20% higher than their symptomatic counterparts.
This is an interesting finding because the language-rich nuns have Alzheimer’s pathology in their brain which somehow did not manifest in clinical symptoms of cognitive decline and dementia. There seems to be a protective mechanism at work and the researchers this has something to do with the larger neurons that compensated for the abnormal plaques and tangles. In addition, the larger neurons may then be linked to better language skills.
According to Dr. Diego Iacono, neuropathology researcher at Johns Hopkins University in Baltimore
“The idea is that we have a sort of cognitive reserve that we start to increase during our second and third decades of life, and you can spend this reserve when you get older. In this way, you can avoid the manifestation of dementia even if you have some pathology. This is something we didn’t expect.”
This is not the first study to find structural differences between symptomatic and asymptomic dementia Alzheimer’s patients. Another study also showed larger neurons in otherwise “plaqued and tangled” brains of men who did not develop Alzheimer’s disease. However, their language skills were not analyzed.
One major limitations of the current study is the sample size. However, it is difficult to conduct such long-term studies on a large scale to include a large number of participants. The current analysis was part of the ongoing Nun Study, which in the coming years, might yield more conclusive data.