Figure 1. Hypothesized Associations Between Cataracts or Other Eye Conditions and Brain Regions, Leading to Dementia.

With mendelian randomization, variations in genetic risk can be used to distinguish between 2 causal scenarios. A, If incipient neurodegeneration or other physiological markers cause cataracts to form, then genetic risk for Alzheimer disease (AD) should be associated with later cataracts. B, If cataracts have independent effects on processes leading to dementia (ie, cataracts are a causal risk factor for dementia), then genetic risk for AD should be independent of cataracts and that genetic risk for cataracts would be associated with dementia.