What Causes Presbyopia?

What Causes Presbyopia? Presbyopia is an age-related impairment of near vision. To see clear images, the eye must focus light and it has two structures to do this – the cornea and the lens. The cornea does about 60 percent of focusing and the lens fine-tunes with about 40 percent. Together, in a 20/20 eye, they focus images on the retina regardless of how near or distant an object is.

Whereas the corneal curvature is fixed, the lens is flexible and changes its curvature according to how close your focus is.

For near objects, the lens becomes steeper (bending light more sharply)
For distant objects it becomes flatter (bending light less sharply)

This continual change in curvature is known as accommodation. When a person reaches the age of about 40, the lens starts to become less able to change its curvature. It remains flat enough to give clear distance vision but is less able to steepen for near vision. Ophthalmologists have several theories as to why this happens.

Hardening of the Lens

Around the lens and attached to it is a circular muscle called the ciliary muscle. It controls the lens curvature by contracting to make it steeper and relaxing to flatten it.

The lens consists of proteins and water. One theory of presbyopia causation is that with age, changes happen in the proteins that make the lens harder and less flexible. It is less able to respond to the contraction of the ciliary muscle which would normally steepen its curvature for near vision. This theory has been long held by most eye doctors.

Weakening Ciliary Muscle

Another long-held theory is that the ciliary muscle becomes weaker with age. By being less able to contract strongly, it is less able to steepen the lens curvature for near vision. This muscle atrophy could be happening at the same time as the lens is becoming stiffer.

Whether one theory is true, or both or neither, the treatments used to date for presbyopia have been effective.

By steepening the cornea, Conductive Keratoplasty compensates for the reduced steepness in the lens.
Laser Thermal Keratoplasty takes a similar approach using a laser instead of radio waves.
LASIK Monovision corrects one eye for near vision and the other for distance vision if that is necessary; otherwise it leaves the other eye as it is.
An Intraocular Lens (IOL) replaces the aging natural lens and provides clear vision at all distances

Continuous Lens Growth

A newer theory that does not have wide acceptance at this point suggests that the lens continues to grow throughout a person’s life. By age 40 or a little older, it has grown wide enough to invade the space needed by the ciliary muscle around it. With insufficient space, the ciliary muscle cannot contract enough to steepen the lens shape.

This theory has generated a new surgical procedure called Surgical Reversal of Presbyopia with Scleral Expansion Bands which is not yet FDA-approved.

There are several other new presbyopia treatments currently being tested for FDA approval. You can read more at Possible Future Presbyopia treatments.

If you would like to learn more about presbyopia and how it might be treated, please contact an eye surgeon in your area.