PRESBYOPIA
Chapter 41
PRESBYOPIA
Guillermo Avalos-Urzua, M.D., Ariadna Silva-Lepe, M.D.
Presbyopia is the age-related normal process where accommodation is lost and the eye is no longer capable to comfortably sustain the accommodation necessary for clear near vision. It is a fact that the range of accommodative amplitude decreases with increasing age, such that the nearest point that can be focused gradually recedes, leading (in humans, at least) to the need for optical prostheses for close work such as reading and, eventually, even for focus in the middle distance. It is the most frequent eye problem in the world, since 40% of the population is presbyopic. In Latin America there are 115 millions of presbyopic, and every year this number increases up to 3 millions; it is said that in 2010 there are going to be almost 145 millions. Even thought it is not a legal blindness cause, the cost from this problem in productivity loss is high in USA.
Figure 41-1. Clear visual zones in an emmetropic eye (a), neutralizing (b) and with add correction (c).
In presbyopia the nearest point that can be
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focused gradually recedes, leading to the need for |
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optical prostheses for close work such as reading and, |
Section 1 |
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eventually, even for focus in the middle distance. For |
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emmetropic persons, presbyopia seems to appear |
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practically overnight when they reach their mid 40s. |
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However, the loss of near focus is actually progressive |
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over a person’s lifetime, whether he or she is |
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emmetropic, myopic, or hyperopic, and the age at |
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which a person requires assistance for near focus will |
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depend in large part on his or her refractive error |
Section 6 |
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(Fig. 41-1). |
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Section 7 |
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Subjects Index |
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LASIK AND BEYOND LASIK |
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Chapter 41
Theories of Accommodation
Helmholtz’s book on Physiological Optics is a careful combination of experimental observations and closely reasoned progressions of logic to demonstrate that accommodation occurs through a change in the shape of the lens, attributing the origin of this hypothesis to Descartes and mentioning an earlier version of this hypothesis, from Leeuwenhoek, which assumed that the lens material was itself contractile like muscle. He observed that the accommodative process is associated with, in addition to pupillary contraction and an anterior movement of the iris, an increased curvature of both the anterior and posterior surfaces of the crystalline lens. The anterior surface of the lens changes curvature with greater amplitude than the posterior surface, and these changes in lens shape result in (1) a thicker lens along the symmetry axis, (2) a thinner lens along the equatorial axis, (3) a shallowing of the anterior chamber as the anterior lens surface is moved forward, and (4) essentially no change in the distance from the cornea to the posterior lens surface along the symmetry axis. Thus, when the human eye is focused on infinity, the lens is under maximum stress and at its thinnest and least sharply curved. As accommodation to a closer focal point proceeds (Fig. 41-2), the contraction of the ciliary muscle is coupled with a controlled elastic recovery or relaxation that allows the lens to “round up” and make a greater refractive contribution to overall globe power through its more sharply curved shape and decreased anterior distance from the cornea.
The zonular geometry described by Farnsworth and Burke is more complex, since some of the zonules from the anterior of the lens pass through the ciliary processes to attach quite posteriorly to the lens, near the ora serrata, while others can attach more anteriorly.
The zonules from the posterior of the lens also attach in a posterior location, and the equatorial zonules attach perpendicular to the lens equator. During the contraction of the ciliary muscle, both the anterior and posterior zonular attachments to the ciliary body would be moved forward and inward, and the resultant change in shape of the lens would be a function of the relative degrees of relaxation of the variously oriented fibers.
Figure 41-2: Schematic representation of retinal image for a near object.
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The classical Helmholtz theory of |
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accommodation has, over the years, not gone |
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unchallenged and most recently has been opposed by |
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Schachar et al. who suggest that increased zonular |
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tension increases rather than decreases the power of |
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the lens. This view is supported by a numerical |
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analysis of the lens based on a linear form of the |
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governing equations. |
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Burd et al proposed an alternative numerical |
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model in which the geometric non-linear behavior of |
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the lens is explicitly included. Their results differ |
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from those of Schachar and are consistent with the |
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classical Helmholtz mechanism. |
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Age-Related Changes of
Accommodation
Accommodative amplitude (the difference between the nearest and farthest point of comfortable focus) decreases with increasing age. The far point is essentially unaffected, while the near point gradually recedes (Fig. 41-3), resulting in an accommodative amplitude of only about 2 diopters by age 50 years, much of which comes from the pinhole effect of a constricted iris and the intrinsic depth of field of the crystalline lens itself.
As the lens grows and thickens in the polar and equatorial directions, the location of internal regions relative to the cornea changes. Cortical growth, which along the optical axis is the same for the anterior and posterior regions, shifts the location of the lens nucleus in the anterior direction. The central
438 SECTION VI
For some patients, standard optical correction for presbyopia is not satisfactory. That is why surgeons
SURGICAL METHODS
Multifocals (Simultaneous Vision)
The monovision approach to contact lens use utilizes one eye for distance and the other for near
vision. Although the dominant eye is usually selected Simultaneous vision lens designs are based for distance correction, this is not mandatory. This on the concurrent presentation of distance and near method of correction provides functional distance images on the retina, and their success is dependent and near visual acuity but may compromise stereopsis, on the patient’s ability to ignore the undesired image. contrast sensitivity, and other aspects of binocular Several designs are available: annular-concentric, visual function. Patients with uncompromising vision aspheric, and diffractive.
requirements who need higher add powers (more than 2.00 D) are usually not good candidates for monovision correction unless the reading add is prescribed in steps over a period of time.
CONTACT LENSES: MONOVISION, BIFOCALS, MULTIFOCALS
Monovision
Segment bifocal contact lenses resemble their spectacle counterparts in having superiorly located distance and inferiorly located reading powers. The translation of the lens that occurs with a change in gaze is accompanied by the shift of the visual axis into the appropriate zone as the eye rotates under the lens. Proper orientation of segment bifocal designs is achieved by prism ballasting, lens truncation, and periballasting. Lens wearing comfort may be compromised by the tendency of these lenses to position low. Patients having hyperopic refractive errors tend to be more successful with these lens designs.
Concentric translating bifocal contact lenses incorporate the reading power circumferentially and therefore do not require a specific rotational orientation. The transition from the center distance power to the peripheral reading add is abrupt. Since translating bifocal designs are gaze-dependent, near acuity is achieved only in the traditional reading posture. Patients requiring near acuity in primary gaze are poor candidates for this design. Since predictable translation is required, translating soft lenses are less effective than translating rigid lenses.
Beginning with the refraction for distance, an estimated add is chosen by subtracting half the amplitude of accommodation from the number of diopters needed for the desired working distance. With the estimated add in place, the resulting range of accommodation is measured and add adjusted so that the patient’s near tasks are brought within the zone of clarity and comfort. Usually the patient is best served by placing most of the range of clarity farther away than the chosen working distance, choosing a lesser add (it is better to undercorrect than overcorrect). This avoids blurring of the middle distance and gives a larger range of accommodation. For adds less than +1.25 D, separate reading glasses may be preferable to bifocals, as the patient is likely to want the add only for prolonged reading.
Eyes with unequal visual acuities, but equal AA, should be given equal adds. On the other hand, when the amplitudes of accommodation are unequal, adds are prescribed so that each eye is using half of its respective amplitude of accommodation for clarity at the desired reading distance. The presumption here is that this amount of accommodation will be produced by equal accommodative innervation of the two eyes.
Optical Devices
Bifocals
Translating designs use temporally spaced images through two distinct contact lens zones: one for distance and one for near. The retina is expected to receive only one focused image at a time. Translating lenses have either a segment or an annular-concentric design.
Chapter 41
Contents
Section 1
Section 2
Section 3
Section 4
Section 5
Section 6
Section 7
Subjects Index
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