Presbyopia is the main condition that affects the human eye with age. This condition usually begins anywhere between the late thirties and the early fifties, depending on a person’s refractive error, specific visual needs and the health of the eyes.
The crystalline lens inside the eye, which contributes about a quarter of the focussing power of the eye, is responsible for accommodation (the ability to focus clearly at different distances from the eyes). In very young children, the crystalline lens is extremely flexible so that a child may be able to focus on an object just a few centimetres from the eye. Over the first six to eight decades of life, the lens gradually hardens, eventually behaving like a lump of optical rock, and this hardening process causes the eye to become less efficient in changing focus from one distance to another.
The amount of correction required for presbyopes depends on the extent to which the crystalline lens has hardened. For example, most people require about 60 units of refractive power (called dioptres) in the total eye for them to be able to see in the distance and the ability to change this to about 63 units for close visual tasks. If a person has the required 60 units, but only has two extra units of accommodation, their near vision is deficient by one unit and their near refractive error (usually called their “add”) is said to be +1.00D (“add one dioptre”).
There has been less research into what actually causes presbyopia than there has for myopia. There is some evidence that the hardening of the crystalline lens is slower in healthy eyes and exposure to ultraviolet light may also play a part in the rate of presbyopic change.
There are various ways to correct presbyopia. The traditional forms of correction, spectacles and monovision or multifocal contact lenses (or a combination of contact lenses and spectacles) are still the main ways of correcting this condition. Advanced technology has widened our options to now include orthokeratology ( the overnight wearing of rigid contact lenses) as an effective alternative. All forms of correction have their advantages and disadvantages, which our optometrist at Vision Excellence at Castle Hill will discuss with you after carefully measuring your distance and near refractive errors.
Presbyopia is a totally normal part of the aging process of the human eye. Cataract formation is the other change in the crystalline lens, but unlike presbyopia is an abnormal change and is classified as an eye disease.