Cataract surgery and cognitive decline

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Tuesday, 1 October 2019

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The visual impairmentsuch as cataracts, which afflicts 1 in 3 people over the age of 50, is considered a risk factor in cognitive decline.

In fact, an American study, which examined 625 adult individuals, found that untreated patients for visual impairment had a risk 9 times more likely to develop Alzheimer's and 5 times more likely to develop cognitive impairment - not dementia (CIND).

Many studies have focused on the visual impairment/cognitive performance association in the older population, but little is yet known about the nature of this relationship and the impact that an intervention on visual impairment would have on cognitive ability.

What is the relationship between sensory impairment and cognitive performance?

The relationship between sensory impairment and cognitive performance can be explained by three main hypotheses:

- common causewhich suggests that the decline in sensory and cognitive functions share common age-related causes, such as degeneration of the central nervous system;

- cascadingaccording to which sensory impairment could have a 'domino effect' on cognitive abilities, possibly through neurobiological mechanisms, social isolation or depression;

– the third alternative hypothesis is that individuals with sensory impairments are penalised in their cognitive performance, providing the cognitive system with inadequate perceptual input that would lead to worse cognitive performance.

The cataract surgery study

A British study published in PlosOne analysed the impact of cataract surgery on cognitive decline to investigate the possibility that treatment of visual impairment may reduce the rate of cognitive decline.

Cataracts are among the most common causes of reversible visual loss and affect around 16 million people worldwide. Cataract surgery consists of replacing the natural lens (crystalline lens) with an artificial intraocular lens (IOL).

This is now the most common form of refractive surgery whose benefits, in terms of vision and quality of life, are well known. The study analysed 2,068 individuals undergoing cataract surgery and 3,636 healthy, demographically similar subjects as a control group.

In these groups, the researchers evaluated the course of cognitive decline before and after cataract surgery between those who underwent surgery and those who did not.

The study, which had a follow-up of 13 years, showed a positive association between cataract surgery and a reduction in the rate of cognitive decline.

The rate of cognitive decline shown by subjects after cataract surgery was reduced compared to before surgery and had become similar to that of healthy subjects.

The positive association between cataract surgery and a lower rate of decline was found among the elderly in England regardless of risk factors for cognitive impairment, including those related to age, gender, education, wealth, chronic diseases, depressive symptoms and physical inactivity.

Study conclusions on cataract surgery

The results of the study explain the association by supporting the 'cascade' hypothesis, according to which lens replacement surgery results in better visual input, which leads to a slower rate of cognitive decline through various potential mechanisms, including the neurobiological one.

Considering the high prevalence of cataracts and cognitive disorders in the older population, the association between cataract surgery and cognitive longevity has important implications for public health, for targeting people at risk of cognitive decline and as a potential means of prevention and treatment to reduce the risk of dementia.