Adrian Glasser, Ph.D. - Accommodation and Presbyopia
Table of Contents
This document is intended to serve as a general discussion of accommodation and presbyopia. It is not meant to be a comprehensive discussion of either, but does provide an introduction. A more scientific paper on accommodation and presbyopia can be found here and is also linked through my research web pages.
Accommodation is the process whereby the young eye is able to change the point of focus from a distant object to a near object. When a normal eye (not near-sighted or myopic) is relaxed, it is focused at optical infinity. Optical infinity extends from about 20 ft beyond the eye to infinity (i.e., the distant horizon). So when the normal eye is relaxed, distant objects are in focus.
A short sighted or myopic eye is focused at a point nearer than optical infinity when relaxed and must be corrected with spectacle or contact lenses to again be focused at optical infinity.
Accommodation occurs in young people when they look at a near object, such as when reading. Accommodation is an optical change in the eye. It occurs due to an increase in the optical power of the crystalline lens inside the eye. When the muscles inside the young eye contract, the muscles allow the optical power of the lens to be increased so that near objects can be brought into focus on the retina.
Presbyopia is a gradual age related loss of accommodation. In all normally sighted (i.e., not near-sighted or myopic) individuals, the ability to focus near objects is gradually lost with increasing age. This process occurs continuously, but gradually from soon after birth. The progression of presbyopia generally goes unnoticed until accommodative amplitude is almost completely lost as one nears age 45-50. It is only at this age that the accommodative amplitude is reduced sufficiently to cause a problem for typical near vision tasks such as reading. Although presbyopia really progresses gradually throughout life, it appears to most people that it has a rather sudden onset when one finds that it is becoming more difficult to see near objects in clear focus.
Presbyopia usually does not create a problem for most short-sighted or myopic people. The relaxed, myopic eye is focused at a point closer to the eye than optical infinity. So, rather than needing to accommodate to see near objects clearly, the myopic eye is normally focused at a short viewing distance. In order for elderly or presbyopic myopic people to see clearly at near, they can simply remove their spectacles.
Presbyopia is generally thought to be caused by a gradual hardening of the crystalline lens within the eye. In the young eye, the crystalline lens is quite soft. So, when accommodation occurs, the lens can easily be deformed. As we get older, the lens gradually becomes harder and so can no longer be deformed during accommodation. The hardening of the lens means that we gradually lose the ability to see near object clearly. This is often jokingly referred to as "a shortening of the arms" since it becomes necessary to hold objects further and further from the eye to see them clearly due to the loss of accommodation.
Presbyopia can be alleviated by a variety of methods. Typically these offer optical compensation in the form of spectacle lenses or contact lenses to allow presbyopes to read at near. These include reading glasses, contact lenses, corneal refractive surgery or, in the case of cataracts as described below, intraocular lenses. Spectacles and contact lenses are a non-surgical approach, whereas corneal refractive surgery and intraocular lenses require surgery. The typical approach is to use bifocal spectacles or progression addition spectacle lenses (PAL's), bifocal or multifocal contact lenses or monovison contact lenses. Monovision is when either only one contact lens is worn on one eye to allow that one eye to focus at near or distance, whichever is the opposite of the non-lens wearing eye.
Cataracts also occur as we get older. A cataract causes the normally clear crystalline lens in the eye to lose its clarity and become cloudy or opaque. In an advanced state, cataracts prevent clear vision. The most common approach for treating cataracts is to remove the crystalline lens and to replace it with an artificial lens. These artificial lenses are called intraocular lenses. This is a very common and usually very successful surgical procedure. Although, cataract surgery usually allows clear distant vision, it does not allow both distance and near vision unless a special intraocular lens is used. One such lens that allows both distance and near vision is called a multifocal intraocular lens. This lens does not allow accommodation, but does allow some vision at near and distance simultaneously.
Another approach for alleviating the loss of near vision with the progression of presbyopia is achieved when cataract surgery must be performed on both eyes. In this case, different intraocular lenses can be placed into each eye. One intraocular lens allows one eye to have a fixed focus for distance vision while the other intraocular lens allows the other eye to be focused at near. This situation, where each eye has a fixed focus at different distances, is called monovison. Monovision can also be achieved, as described above, with contact lenses.
A variety of surgical procedures have been proposed to restore accommodation to the presbyopic eye. These surgical approaches differ from the treatments (optical compensations) described above in that they claim to actually restore accommodation to the presbyopic eye. Some surgical procedures are based on alternative theories of accommodation and presbyopia, such as the theories proposed by Ronald Schachar, M.D. Dr. Schachar has proposed that accommodation (near focusing ability) in the human eye occurs through a mechanism that differs fundamentally from what is widely accepted and known to occur.
The classical and widely accepted understanding of how accommodation occurs stems from the early work of Hermann von Helmholtz who in 1856 published Volume 1 of the Handbuch der Physiologischen Optik. Since that time, numerous scientific studies of accommodation have verified the original findings of Helmholtz with a few minor modifications. Despite this substantial body of work, Schachar proposed that accommodation occurs by a mechanism that is fundamentally different. In fact, Schachar believes that the action of the muscles inside the eye (the ciliary muscles) is exactly the opposite of what Helmholtz described. Schachar's theory of accommodation is not supported by independent studies and studies conducted by myself and Paul L. Kaufman, M.D. in conjunction and in collaboration with Schachar himself showed no evidence to support his theory. The studies undertaken all support the classical accommodative mechanism described by Helmholtz.
Despite substantial experimental evidence to show that the human lens gets stiffer with increasing age, Schachar also proposed a novel theory of how presbyopia occurs. Schachar, believes that presbyopia is not due to a stiffening of the lens, but is caused by increased equatorial growth of the lens. He believes that since the lens continues to grow equatorially throughout life, the zonular fibers that hold the lens in place in the eye gradually become slackened with increasing age.
The human lens certainly does continue to grow throughout life. However, the increasing stiffness of the human lens ultimately prevents the lens from being deformed in old, presbyopic eyes as it is during accommodation in young eyes. If the human lens gets stiffer with age to the point where no accommodation can occur, accommodation cannot be restored unless the natural lens is replaced with an artificial lens such as in cataract surgery.
A number of different kinds of so called 'accommodative intraocular lenses' or AIOLs, have been under investigation and in clinical use for some years now. These AIOLs would be used to replace the natural stiffened and cloudy lens in a cataract surgical procedure. These are artificial lenses that are designed to move in the eye or to change shape when the muscles in the eye contract with an effort to focus at near. The expectation is that the muscles in the eye can act on the artificial lenses to allow the eye to focus at near. There is a lot of interest in these AIOLs and research and clinical studies are underway to see if these artificial accommodative intraocular lenses can allow the eye to focus at near.