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One of the universal and better known changes associated with ageing is a decline in visual function. Starting at around the age of 40, all adults experience a noticeable difficulty focussing on objects that are close to them (though this change may begin unnoticed in one’s late teens). This is called presbyopia, and occurs primarily because of a stiffening of the lens of the eye.
When focussing on near objects, the ciliary muscles of the eye contract, allowing the lens to bulge and thicken, which focusses the image on the retina at the back of the eye. Stiff lenses are not able to bulge as much, making it harder to bring near objects into focus. Lens stiffening also narrows the accommodation range, which is the range of optical power (measured in dioptres) that the lens is able to achieve.
Ciliary muscles also weaken with age, though how much this contributes to presbyopia is still an unresolved question. For most of the developed world, presbyopia is an inconvenience that can be corrected with glasses or contact lenses. However, in parts of the world without easy access to vision care, presbyopia has a major impact on quality of life. Lens stiffening happens due to biochemical changes in the eye, including decreasing levels of the structural protein alpha-crystallin. The lens also continues to grow with age, making it thicker and therefore less flexible.
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