Is visual impairment from glaucoma a risk factor for cognitive decline?
While a simple question to ask, providing a robust and reliable answer is quite complicated. Glaucoma and cognitive decline are both associated with ageing. Many cognitive tests require good vision to complete. Glaucoma is associated with depression and reduced social interaction both of which may speed cognitive decline.
Grant et al acknowledge this and propose to avoid some of these problems by analyzing changes in cognition over 3 years in the Canadian Longitudinal Study on Ageing (CSLA) Comprehensive Cohort.
Participants in this very large (27 412) cohort had five different cognitive tests which did not require vision to complete. Each test measured a slightly different aspect of cognition; immediate memory, medium term memory, verbal fluency, category fluency and processing speed when having to be flexible.
In contrast to this comprehensive set of cognitive measures, the vision measures were basic, a binocular ETDRS vision with habitual distance correction and asking participants whether they recalled a personal history of glaucoma, age-related macular degeneration, or cataract. Visual impairment was defined as a binocular acuity of less than 6/12 (20/40).
When the researchers analyzed the changes in the five cognitive tests across the four ocular variables (vision impairment and the three eye diseases), using a 5% statistical cut-off, they found exactly 5% (1 out of 20) of the comparisons showed a statistically significant relationship, the one between glaucoma history and the Mental Alternation Test (MAT), which measures processing speed. On average, the MAT score was 0.6 lower than in those without a glaucoma history. This seems small compared with a baseline MAT of 27.6 and a decline of about 0.5 (standard deviation 6.4) over 3 years in the whole cohort.
How do we interpret this finding? Could this be a statistical chance outcome? Certainly. Is it reasonable to use a p=0.05 cut off in a study of 27 000 people? Most studies of this size set a much higher bar for statistical certainty. Could glaucoma specifically affect mental processing speed but not any other measure of cognitive function? This is certainly possible, and Grant speculates on possible reasons, but in the absence of corroborative data and in a field with inconsistent findings, it must for now remain as speculation.