Overview
Presbyopia (from the Greek presbys , elder or old, and, – ops , eye) is a progressive condition where the ability to focus on nearby objects is gradually lost as part of the natural aging process. Although the development of presbyopia appears to begin in the second decade of life, it does not become a significant problem for most people until they reach their 40s, when it becomes increasingly difficult to read, sew, or use a computer without visual assistance. It is sometimes confused with far-sightedness because far vision remains relatively unaffected. However, although the eye’s far point of focus is not significantly altered, the near point (the closest comfortable focus) is gradually receding toward the far point, reducing the range over which clear focus can be attained. Although presbyopia is not life- or vision-threatening, it nevertheless “represents a significant economic cost to society,” as reported by the National Eye Institute ( http://www.nei.nih.gov.easyaccess2.lib.cuhk.edu.hk/resources/strategicplans/neiplan/frm_cross.asp ). It also reduces quality of life and is a universal harbinger of middle age.
Clinical background
Key symptoms and signs
The range over which one can focus – one’s accommodative amplitude – declines throughout life. A small child has an accommodative range of 15–20 D (from infinity to about 5 cm, or the tip of one’s nose), while a young adult’s range is about 10 D (infinity to about 10 cm), reducing to less than 1 D (infinity to about 1 meter) by about 60. Thus diminution of the ability to focus on near objects is the hallmark of presbyopia. This loss becomes particularly apparent to individuals, generally in their 40s, in the process of performing familiar tasks. It may become more difficult to focus on print at normal reading distances, particularly at low light levels, or it may take a perceptible amount of time to shift focus from one distance to another, or prolonged close work might lead to eyestrain or headache ( Box 34.1 ). When people complain that the print is suddenly too small or their arms are too short, they are likely becoming presbyopic.
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Age
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Decrease in ability to focus nearby
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Increase in comfortable reading distance
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Eyestrain
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Headaches after prolonged focus
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Trouble focusing when tired or stressed
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Slow response to change in focus distance
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Need for increased illumination
Historical development
Why does presbyopia occur? It could be argued that the impact of presbyopia was moot for prehistoric hominids, since it is unlikely that their life span in general reached into their 40s, let alone their 50s. However, as will be discussed in more detail later, the changes in the anterior segment associated with visual aging lead specifically to the preservation of far focus at the expense of near. In terms of evolutionary pressure, it is possible that maintenance of distance vision was a survival advantage in spotting and avoiding predators, as well as in locating prey. Indeed, even in modern times, average human refraction around the world is roughly 1 D hyperopic.
Epidemiology
Because presbyopia is an integral part of the human aging process, its impact is theoretically universal once middle age has been reached. There appears to be no significant difference in the progression of the condition between males and females or among different human ethnic groups. What will differ is the apparent age of onset. Some of this difference arises simply from a natural Gaussian distribution of traits across the human population, and some from an individual’s lifestyle (e.g., web page designer versus national park ranger). For an emmetropic (20/20 or 6/6 visual acuity) eye, apparent onset is generally in one’s 40s and progresses until total loss of objective visual range by about 60. A subjective range of about 1 D, due largely to the pinhole effect (a constricted pupil providing an increased depth of focus), remains, but will clearly be dependent on the illumination intensity of ambient light. The impact of presbyopia will be different from emmetropes for those with refractive errors, with far-sighted (hyperopic) individuals more severely affected and near-sighted (myopic) individuals less so. For hyperopes, the location of their far point is further away than for emmetropes, and their most comfortable near point will also be further away for a given age. As a result, their ability to focus on nearby objects will be diminished at an earlier age, and the apparent onset of presbyopia may be in their late 30s or even earlier. In contrast, myopes have a far point that tends to be nearby, so their receding near point does not lead to the functional visual loss experienced by their emmetropic and hyperopic friends. As a consequence, many presbyopic myopes find themselves needing additional optical assistance only for distance viewing (e.g., driving); this “myopic advantage” is lost when invasive procedures like laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) have previously been used for refractive correction.
Diagnostic workup
Diagnosing presbyopia is straightforward, especially since many patients diagnose themselves. A person’s age is of course a major consideration, since it is the primary risk factor. Symptoms will include the gradual onset of several of the following: decreased ability to focus on nearby objects; increase in comfortable reading distance (“arms too short”); eyestrain; headaches after prolonged visual tasks; trouble focusing when tired or stressed; slow visual response to a change in focus distance; and need for increased illumination. Further assessment can be incorporated into a standard eye exam, which may identify additional potential contributory factors. A simple reading test using well-illuminated text of graded sizes at a standard distance (e.g., 35 cm or 14 inches) can be helpful both in characterizing the degree of near-vision loss and in determining the appropriate refractive correction.
Differential diagnosis
Although presbyopia is generally an easily identified condition, there are nevertheless other potential factors affecting changes in visual range that must be considered and eliminated. These factors can include nuclear cataract development, untreated diabetes, central nervous system disorders, macular degeneration, and migraines. In general, a thorough refractive history combined with a standard ophthalmic examination that includes slit-lamp biomicroscopy and inspection of the retina should serve to eliminate most of these possibilities. Presbyopia can also be confused with hyperopia when a subject’s refractive history is unknown; however, hyperopia in an adult over 35 would be indistinguishable from presbyopia, and would be treated as such.
Treatment
Because of the universality of presbyopia in older individuals, there are many different options available for mitigating its effects. Historically, the very first contribution to the treatment of presbyopia was developed in 1784, when Benjamin Franklin combined two pairs of glasses – one for distance vision and one for close work – into the first set of bifocals (bifocles). Reading glasses, bifocals, progressives, and even trifocals, remain the most common methods for providing one or more different comfortable refractive distances for eyes that can no longer focus on near objects, and are the most versatile option for a visual system that continues to change from the onset of presbyopia into the 60s. It is possible for presbyopes to test out and buy their own reading glasses from a pharmacy or supermarket; if both eyes have similar or identical refractions, this is a reasonable if limited strategy for the long term. However, if there are significant differences between the two eyes due to refractive or other differences, or if there are special visual requirements (e.g., sustained focus on a computer monitor) it becomes important to have glasses or other optical prostheses professionally prescribed.
A second set of options involves contact lenses. There are a growing number of different designs of contact lenses available for presbyopes, either bifocal (one refractive range within the circumference of a second) or multifocal (simultaneous images at two or more different refractive powers), which have received a mixed response from users. An additional alternative is monovision contact lenses, where the refractive correction for one eye is set for focus at a distance and the other for focus nearby; although the image received by each eye is clear, the concomitant loss of depth perception has a vertiginous effect which some people cannot overcome. A general drawback to the use of contact lenses for presbyopia is the decrease in tear production with increasing age in the target population, but, like glasses, an advantage is that a change in prescription is easy to do.
A third set of alternatives involves direct alteration of the visual system. These currently include monovision through LASIK, conductive keratoplasty, or other procedures altering corneal shape (and thus refractive power), and scleral relaxation and scleral expansion surgeries. The same problem as experienced in contact lens-mediated monovision – loss of depth perception – is a consequence of surgical monovision, so it is important to ensure that this can be tolerated before the procedure is performed. Modifying scleral shape has not been very successful up to this point in treating presbyopia, although it continues to have its fervent advocates and detractors. In development are methods for altering lens properties in situ using laser techniques or replacement of the natural lens with an intraocular lens implant that restores accommodation. These and other novel approaches for the treatment of presbyopia and restoration of accommodative range continue to be designed, developed, and tested, and it is likely that effective new options for treatment will be available within the next decade ( Chapter 35 ).
Physiology and pathophysiology
Introduction
Image formation by the human eye involves refractive contributions from both the cornea and the crystalline lens, with the cornea a passive component and the lens and associated structures an active contributor. In order to understand presbyopia, which is a natural consequence of the aging process, it is first necessary to understand accommodation, the mechanism by which this focusing is effected, and the age-related changes in this mechanism that are associated with loss of accommodative amplitude.
Accommodation
The human focusing mechanism is the subject of qualitative, if not quantitative, agreement. Focus on points closer than infinity (for the human eye, about 6 meters or 20 feet) involves an increase in the sharpness of curvature of the crystalline lens surfaces, an increased thickening of the lens along the optical axis, a shallowing of the anterior chamber, and essentially no change in the distance from the cornea to the posterior lens surface along the axis ( Box 34.2 ). This process was, in essence, first described in the 19th century by Helmholtz ( Figure 34.1 ) in his Treatise on Physiological Optics , although Helmholtz was certainly not the first to develop hypotheses about the mechanism. It is the causative factors through which these alterations in lens shape, thickness, and position relative to the cornea occur that are the subject of intense debate, and that lead to presbyopia.
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When unaccommodated, the lens is at its thinnest and flattest, while the ciliary muscle is relaxed
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Closer focus couples ciliary muscle contraction with lens elastic recovery, or “rounding up”
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The lens becomes thicker and more sharply curved
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Anterior-segment length remains almost the same, so lens center of mass is moved forward and anterior-chamber depth decreases
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Internal lens curvatures are directly related to surface curvatures, and these relationships are maintained during accommodation
The crystalline lens is located in the anterior segment of the eye behind the iris, suspended in place by the zonules of Zinn ( Figure 34.2 ), which connect the lens to the ciliary muscle through insertions into the collagenous lens capsule surrounding the lens fiber cells. Light enters the eye through the cornea, which provides the major refractive component of the system, due in part to its small radius of curvature and to the comparatively large increase in refractive index in going from air to the cornea. The light emerging from the posterior surface of the cornea, after passing through the circular slit of the iris, arrives at the lens, which provides a variable refractive contribution to the system ( Figure 34.3 ). When focused at infinity, the lens is at its flattest and thinnest along the optical axis, while the ciliary muscle is relaxed; closer focus involves a carefully controlled relaxation of the forces acting upon the lens, coupled to ciliary muscle contraction, and allows the lens to “round up” and increase its refractive contribution ( Figure 34.4 ).
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