Visual impairment, age related eye diseases such as glaucoma, and hearing loss have been associated with cognitive decline in older persons. It has been suggested that these conditions may have a common etiology or that sensory loss (vision hearing impairment) leads to disuse atrophy of the brain leading to cognitive decline.
In this investigation Grant and colleagues studied 30,097 community dwelling Canadians aged 45-85 years of age from the Canadian Longitudinal Study of Aging (CSLA). Participants underwent two series of exams and interviews that were conducted 3 years apart (from 2011 to 2015 and from 2015 to 2018). Examinations and interviews included measurement of binocular presenting visual acuity, history of eye diseases (glaucoma, AMD, cataract), history of hearing loss, history of smoking, education, income, and a series of cognitive tests. The cognitive tests were the Rey Auditory Verbal learning tests (RAVLT), the RAVLT delayed test (memory test), the controlled oral word association test (COWAT), the animal naming test (ANT)(verbal fluency test) and the mental alternation test (MAT)(processing speed test). The change in the test scores from the baseline to the followup cognitive tests was measured over the three year follow-up period. Multiple linear regression analyses were used to assess the associated between cognitive decline and the Baseline presence of visual impairment (visual acuity worse than 20/40), history of glaucoma, macular degeneration, cataract, smoking, heart disease, stroke, and diabetes. Effect modification of gender, history of hearing loss, or educational status was also studied.
In general, no relationship was identified between cognitive decline and visual impairment or history of macular degeneration or cataract. Nor was there any effect modifier noted. In only one analysis a relationship was found between one cognitive test (MAT) and glaucoma. This test assesses processing speed and the authors hypothesize that glaucoma and cognitive decline share a common pathological pathway like glial reactivity, neuroinflammation, and oxidative stress. The authors also suggest that with vision loss in glaucoma, patients may perform fewer activities thus leading to cognitive decline.
The authors found one positive result after studying a number of cognitive tests and thus the possibility of a false positive result are relatively high
As the authors acknowledge that their study has some significant limitations including the lack of an objective clinical assessment of eye disease (the study used history of eye disease), a very short follow-up period between exams (3 years) when such cognitive change may take a decade or more, a very homogenous population (over 93% were white) and thus not generalizable. Finally, the authors found one positive result after studying a number of cognitive tests and thus the possibility of a false positive result are relatively high. However, the authors are to be congratulated for studying this important area that deserves further investigation and for acknowledging the limitations of their study.