Presbyopia is the term which denotes the medical condition whereby a person loses the ability to see objects at near distances clearly. It is a fairly predictable part of aging, because everyone will develop it at some point. It develops most commonly after the fifth decade of life and people typically find themselves having to hold reading material further away from their eyes in order to read. Presbyopia does not occur overnight. It is a process that occurs gradually and progressively with increasing age, hence the meaning of presbyopia which is literally “old eye” in the Greek language.
In youth, the lens is flexible and soft. It readily changes shape during the process of accommodation, which allows one to focus objects at both near and far distances. As we age, however, the lens becomes increasingly rigid, with a corresponding inability to readily accommodate. As a result, activities that require near distance focusing, such as reading and sewing, become more difficult to carry out. The hardening of the lens that occurs with aging, together with the physiological decline in the muscular function of the muscles involved in accommodation, are central to the etiology of presbyopia.
Our lenses grow continually throughout our lives, because they are of ectodermal (i.e. the most external of the primary germ layers) origin. Mitosis gives rise to new nerve ibers at the lens equator, which causes its growth to expand at a rate of approximately 20 micrometers per year. This expansion is accompanied by a decrease in the length of the ciliary muscle and a directly proportional decrease in the strength of the muscle. This muscle is pivotal in the accommodative process according to the Helmholtz theory.
This theory postulates that during accommodation there is ciliary muscle contraction and a release in zonular tension. This causes the elastic lens capsule to contract with a resultant decrease in diameter and an increase in its anterior and posterior surface curvatures. All in all, the lens becomes more rounded, which increases its dioptric power. This is crucial for near distance vision. Once accommodation effort is no longer necessary, the ciliary muscle relaxes and the tension in the zonular fibers returns, thereby flattening the lens equator and decreasing dioptric power.
Age is the single most important risk factor for presbyopia. This unfortunately is something none of us can ever escape, should we be privileged enough to advance in years. Other factors that may contribute to speeding up the process of presbyopia are certain medical conditions and drugs, such as antihistamines and antidepressants.
People who are hyperopic (farsighted) typically tend to experience the onset of presbyopia earlier than their emmetropic (normal vision) counterparts. Morbidities such as cardiovascular disease, diabetes and multiple sclerosis have also been implicated in the earlier onset of presbyopia.
Reviewed by Liji Thomas, MD. References