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Editors Selection IGR 13-2

Imaging Techniques: GDx and cognitive functioning

Comment by George Lambrou on:

24830 Association of cognitive functioning with retinal nerve fiber layer thickness, van Koolwijk LM; Despriet DD; Van Duijn CM et al., Investigative Ophthalmology and Visual Science, 2009; 50: 4576-4580

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Is the eye a 'window to the mind' ? Since the middle of the 19^th century, we can easily see a part of the brain ‐ the retina ‐ by using some very simple optical equipment ‐ the ophthalmoscope ‐ and a very conveniently placed magnifying and focusing system ‐ the subject's cornea and lens. This has proved an invaluable diagnostic procedure for most conditions affecting the retina and some affecting the central nervous system. But can this be pushed one step further? Can we conceive that because of their common origin ‐ the embryonic prosencephalon ‐ the healthy retina and brain might maintain such a strong link during childhood development and adult life, that the thickness of the first would mirror the most complex functions of the second? This is what Van Koolwijk et al. (1745) have set out to explore, by investigating the association between GDx-measured RNFL and cognitive function as assessed by six standard neuropsychological tests, addressing memory, executive functioning, visuospatial skills and general cognitive ability. Their study sample consisted of 1485 healthy subjects from a genetically isolated population in the Netherlands, all descendants of a 400-strong founder group in the 18^th century. In this subject sample, better cognitive performance was significantly and consistently associated with thicker RNFL, independent of possible confounding factors such as age, sex, refraction, level of inbreeding and common cardiovascular or metabolic parameters. The association was stronger in younger adults (18-39 years), less strong in middle age (39-53 years) and weaker in older (53+) persons. This fact, and the inclusion of healthy subjects only, is consistent with the hypothesis that a higher level of cognitive skills and a thicker RNFL may reflect a more robust development of prosencephalic tissue during childhood, a relation that may gradually break down with age, as diseases or insults predominant on one or the other of these tissues (like increased IOP or cerebrovascular insufficiency) take their toll.

The eye as a 'window to the mind'may turn out to be more than poetic license, after all

Still, and in spite of the statistical significance, the sum total of all investigated variables could only explain 16-37% of the total variance in cognitive function, whereas RNFL thickness itself explained no more than 2.8% of this variance. This, and the study limitations that the authors acknowledge (very particular population; crosssectional design) do not permit to position retinal thickness as a reliable predictor of cognitive ability. Yet their results, demonstrated for the first time in healthy subjects, are interesting enough to warrant further research into potential clinical applications of this intriguing association. The eye as a 'window to the mind' may turn out to be more than poetic license, after all.

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