Does cataract surgery really decrease the risk of dementia?

Our vision appears to have a complicated relationship to dementia. Posterior cortical atrophy is a rare variant of Alzheimer’s disease in which the amyloid plaques and neurofibrillary tangles first form in the back of the brain, in the visual cortex, and migrate towards the front of the brain, just the opposite of the more typical front to back progression in most Alzheimer’s cases.  The result is a progressive distortion and loss of vision sometimes progressing to blindness even before the onset of significant cognitive impairment. I have only seen one patient with posterior cortical atrophy, and I was completely surprised by her normal MRI, at least initially.  I was sure she had a stroke or brain tumor.  Subtle visual distortion is also fairly common in the more standard presentation of Alzheimer’s disease. Visual-spatial perception can be altered affecting one’s ability to locate an object in space.  The inability to recognize faces, prosopagnosia or face blindness, is common even early in Alzheimer’s. In Lewy body dementia, very realistic visual hallucinations are common, often as the first sign of the disease. 

But does vision loss increase the risk of dementia? Several studies have suggested that it can, but the effect is relatively small. A recent paper in JAMA Internal Medicine looked at over 3,000 subjects averaging 75 years of age who had a diagnosis of cataracts. Based on nearly 24,000 person-years of follow-up, cataract extraction was associated with a 29% reduced risk of getting dementia compared with participants with cataracts who did not have surgery. As a control, patients who had eye surgery for treatment of glaucoma had no change in their risk of dementia.  The decreased risk of getting dementia after cataract surgery was attributed to the improvement in vision.  Although no specific mechanism has been proposed, it seems possible to me that keeping the neuronal pathways for vision active may help preserve these visual processing centers in the brain, just as staying mentally active appears to have a protective effect in slowing the impairment of other cognitive functions. One caveat about interpreting this paper and most others I have seen about possible effects of visual loss on dementia risk is that the studies do not look at specific types of dementia. They are all lumped together. It is possible that the effects might be stronger for one dementia than another, say Alzheimer’s vs frontotemporal dementia or Lewy body dementia. Still, it seems to be important to keep our senses as keen as possible, along with the rest of our brain.

Thanks to Jim Read’s dog training class for posing this question.