Published On: Fri, Jun 29th, 2018

The eye's structure holds information about the health of the mind

BECAUSE it is locked away inside the skull, the brain is hard to study. Looking at it requires finicky machines which use magnetism or electricity or both to bypass the bone. There is just one tendril of brain tissue that can be seen from outside the body without any mucking about of this sort. That is the retina. Look into someone’s eyes and you are, in some small way, looking at their brain.

This being so, a group of researchers at Moorfields Eye Hospital in London, working with others around the world, decided to study the structure of the eye for signs of cognitive decline. Changes in the brain, they reasoned, might lead to changes in the nervous tissue connected to it. They focused on a part of the eye called the retinal nerve-fibre layer (RNFL). This is the lowest layer of the retina and serves to link the light-sensitive tissue above to the synapses which lead to the brain. The team’s results, published in JAMA Neurology this week, show that people with a thin RNFL are more likely to fail cognitive tests than those with a thick one. They are also more likely to suffer cognitive decline as they age.

To discover this, the researchers relied on Britain’s Biobank, a repository of medical data from half a million volunteers. Of these, 32,000 aged between 40 and 69 had provided information pertinent to the study, namely ocular scans and relevant scores from cognitive tests. None of these people had been diagnosed with diabetes or a neurodegenerative disease, both of which could have interfered with the results.

The eye scans had been conducted using a technique called optical coherence tomography. This employs long-wavelength light to penetrate into and scatter from biological tissues, building up a three-dimensional picture of that tissue. Comparing the scans and the cognitive tests showed that those people whose RNFL was in the thinnest fifth of the group were 11% more likely to fail at least one cognitive test than those with an RNFL in the thickest fifth. A small portion of the original cohort sat a second cognitive test three years later. This showed that those in the bottom two-fifths of RNFL thickness were twice as likely to have suffered a decline in cognitive function as those in the top fifth.

Although these results show only a correlation between eye structure and brain health, without an underlying explanation, they do suggest that scanning eyes is a fruitful road to diagnosing and even predicting cognitive decline. That would be warmly welcomed, especially for dementia, which currently lacks any form of predictive test.

In the future, then, a visit to the opticians might also lead to a diagnosis about mental health. Poets say that the eyes are the window to the soul. Perhaps. But this work suggests they are surely a window to the brain.

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The eye's structure holds information about the health of the mind