Adrian
Glasser, Ph.D. - Accommodation and Presbyopia
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Table of Contents Accommodation |
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.
SCLERAL EXPANSION AND PRESBYOPIA
A variety of highly controversial surgical procedures have recently been proposed to "cure" presbyopia. These surgical approaches differ from the treatments (optical compensations) described above in that they claim to actually reintroduce accommodation. The surgical procedures are based on 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 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 has 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 recent studies actually conducted by myself and Paul
L. Kaufman, M.D. in conjunction and in collaboration with Schachar himself have
found no evidence to support his theory. The studies undertaken all support the
classical accommodative mechanism described by Helmholtz.
Further, despite substantial experimental evidence to show that the human lens gets harder with increasing age, Schachar has proposed a novel theory of how presbyopia occurs. Schachar, believes that presbyopia is not due to a hardening of the lens, but is caused by increased growth of the lens. He believes that since the lens continues to grow throughout life, the fibers that hold it in place gradually become slackened with increasing age.
The human lens certainly does continue to grow
throughout life. However, the increasing hardness of the human lens prevents
the lens from being deformed in old eyes as it is during accommodation in young
eyes. If the human lens gets harder, no surgical procedure (short of replacing
the lens) can allow for the reintroduction of accommodation.
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Diagram of the eye showing the PMMA bands inserted into scleral tunnel incisions. |
Photograph of conjunctival opening showing the scleral tunnel incision prior to insertion of the PMMA band. |
Photograph of the PMMA band being inserted into the scleral tunnel incision. |
Schachar's theories of accommodation and presbyopia have not been independently studied or verified. The only support for these theories comes from the work of Schachar himself and his theories are not generally accepted. Despite this, Schachar has proposed that presbyopia can be reversed (not just compensated for, but actually reversed) through a controversial surgical procedure. Schachar believes that if the sclera of the eye is expanded, this will take up the slack in the fibers suspending the lens and will allow the lens to accommodate again. Schachar's surgical procedure entails surgically implanting a PMMA (plastic) band into the sclera of the eye to stretch the sclera over the ciliary muscle. This band is either in the form of a continuous band, or more recently consists of independent bands that are inserted in scleral tunnel incisions in the sclera at four quadrants of the eye as shown in the images above. An alternative surgical procedure that is also intended to expand the sclera of the eye is one proposed by Spencer Thornton, M.D. Thornton believes that the radial scleral incisions (radial sclerotomy) in the sclera of the eye overlying the ciliary muscle will surgically expand the sclera and reintroduce tension on the fibers holding the lens to allow accommodation to be reintroduced. Despite the fact that these surgical procedures do not yet have the US Food and Drug Administration (FDA) approval, they are being offered and performed around the world.
The theoretical bases for these surgical
scleral expansion procedures fly in the face of experimental evidence for the
accommodative mechanism and the causes of presbyopia. There has been no
confirmation that the procedures described do actually serve to expand the
sclera. No independent investigators have verified that accommodation is
actually reintroduced by the scleral expansion procedures. Independent
measurements of three postoperative patients showed that no accommodation
occurred.