The Aging Retina, a Mirror of the Aging Brain

Retinal degeneration is a feature of old age, and here researchers show that it correlates quite well with a loss of volume in portions of the visual cortex of the aging brain. These two portions of the nervous system are are connected and related, but it is unclear as to whether there is a direction of causation, or whether this is a case of similar structures being similarly affected by the same underlying mechanism of aging. Chronic inflammation, for example, operates throughout the body, and many aspects of aging are correlated because inflammation accelerates tissue dysfunction in a systemic, whole-body manner.

Age-related retinal diseases, such as late-stage macular degeneration have an impact on cortical morphology. For example, researchers found that the regions of the striate cortex that usually sample the visual input from the injured area of the retina were thinner in participants with macular disease when compared to controls. On the contrary, regions corresponding to non-damaged areas demonstrated a significant increase in cortical thickness. Overall, these findings suggest a strong retinocortical coupling of structural and functional changes in retinal disorders.

Healthy aging is characterized by diverse structural changes in both brain and retina, which association remains to be studied. Concerning the former, there is substantial evidence suggesting that widespread cortical shrinkage takes place with increasing aging. The nature of such effects in early visual areas remains controversial. Whereas some studies have shown volume loss or cortical thinning of visual cortices, others have reported a certain sparing of these areas during aging. Specifically to the primary visual cortex such discrepancy in the literature could be explained by methodological issues, since they demonstrated that different sub-regions of primary visual cortex were unequally affected by aging, depending on their retinotopic eccentricity. Nonetheless, studies directly examining the structure and function of primary visual areas are scarce in healthy aging, in spite of the fact that the organization of early visual areas is well documented in young adults.

The retina also undergoes substantial modifications throughout the lifespan. Histological studies have reported a reduction in the density of photoreceptors, ganglion cells, and pigment epithelial cells with age. Overall, retinal thickness studies using optical coherence tomography (OCT) imaging have revealed regional age-dependent differences in global macular integrity. The improvement of OCT image processing techniques has now allowed for the automatic segmentation of retinal individual layers, which provides a more detailed and specific perspective on such alterations.

we aimed to investigate the association of retinal layer and cortical integrity, in a healthy cohort aged between 20-80 years old. To that end, we performed magnetic resonance structural data imaging measurements of cortical thickness in the primary visual cortex - BA17, the cortical area that receives direct retinal input - and OCT to measure the thickness of the macula and their individual layers, in the same set of participants. We found an age-related decay of primary visual cortical thickness that was significantly correlated with a decrease in global and multiple layer retinal thicknesses. The atrophy of both structures might jointly account for the decline of various visual capacities that accompany the aging process. Furthermore, associations with other cortical regions suggest that retinal status may index cortical integrity in general.

Link: https://doi.org/10.3389/fnagi.2019.00360

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