Two back-to-back studies show that the eyes are not only the window to the soul, as the saying goes, but also an early marker of pathological changes that affect the brain. Alzheimer’s and Parkinson’s diseases would be detectable in the eyes long before the first symptoms of these neurodegenerative diseases.
The decline of cognitive functions and visual acuity are two major problems associated with aging: it is predicted that by 2050 the number of people affected by dementia will increase from 50 to 132 million, while that of people with blindness will increase from 38 to 115 million.
Some observations suggest that these two conditions do not develop independently, but may instead be related. For example, autopsy studies carried out on people affected by Alzheimer’s disease have shown the presence of significant damage to the optic nerve and retina of these patients. A link between cognition and vision is also suggested by an analysis of data acquired by two large American studies (more than 30,000 participants in total), which shows that the risk of being affected by a loss of visual acuity is 2 to 3 times higher in people with cognitive dysfunction. However, these studies do not make it possible to determine whether the loss of vision is the cause or the consequence of the decline in cognitive functions.
Vision problems herald Alzheimer’s disease
To better establish the sequence of events linking these two phenomena, an American research group undertook a prospective study (the Salisbury Eye Evaluation Study), among 2520 people aged 65 to 84 years. Participants’ visual acuity and the presence of cognitive dysfunction were measured by clinically validated tests (Early Treatment Diabetic Retinopathy Study charts for vision and the Mini-Mental State Examination [MMSE] for cognition). By measuring these parameters over an eight-year period, the researchers were able to establish a correlation between the deterioration of vision during this period and the decline in cognitive functions. On the other hand, when they analyzed the results in the opposite way, that is to say by associating cognitive decline with a decrease in visual acuity, the correlation was much weaker, suggesting that it is vision problems that precede cognitive loss, not the other way around.
According to the authors, these results suggest that the management of visual acuity problems that occur with age (myopia, cataracts, macular degeneration) could therefore represent a simple way to prevent, or at the very least slow down, the onset of cognitive dysfunction.
Thinning retina: the first stage of Parkinson’s disease
Another ocular anomaly which has been associated with certain neurological disorders such as Alzheimer’s disease, multiple sclerosis or even Parkinson’s disease is the thinning of the retina. To better characterize this phenomenon, researchers used the technique of optical coherence tomography to precisely measure the thickness of the retina of 49 patients suffering from Parkinson’s disease, and compared the values obtained with 54 healthy people of the same age. (mean age 69). They discovered that thinning of the retina could be observed in the early stages of the disease and was correlated with the severity of neurological damage. According to the authors, this measurement could represent a diagnostic tool for Parkinson’s disease from its earliest stages and thus facilitate the management of patients.