|Classification and external resources|
Presbyopia is a condition where, with age, the eye exhibits a progressively diminished ability to focus on near objects. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens’ curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause. Like gray hair and wrinkles, presbyopia is a symptom caused by the natural course of aging. The first signs of presbyopia – eyestrain, difficulty seeing in dim light, problems focusing on small objects and/or fine print – are usually first noticed between the ages of 40 and 50. The ability to focus on near objects declines throughout life, from an accommodation of about 20 dioptres (ability to focus at 50 mm away) in a child, to 10 dioptres at age 25 (100 mm), and levels off at 0.5 to 1 dioptre at age 60 (ability to focus down to 1–2 meters only). The expected, maximum, and minimum amplitudes of accommodation in diopters (D) for a corrected patient of a given age can be estimated using Hofstetter's formulas: Expected amplitude (D) = 18.5 - 0.3 x (age in years), Maximum amplitude (D) = 25 - 0.4 x (age in years), Minimum amplitude (D) = 15 - 0.25 x (age in years).
The first symptoms most people notice are difficulty reading fine print, particularly in low light conditions, eyestrain when reading for long periods, blur at near or momentarily blurred vision when transitioning between viewing distances. Many extreme presbyopes complain that their arms have become "too short" to hold reading material at a comfortable distance.
Presbyopia symptoms, like other focus defects, become much less noticeable in bright sunlight due to the action of the iris closing to a smaller diameter. As with any lens, increasing the focal ratio of the lens increases depth of field by reducing the level of blur of out-of-focus objects (compare the effect of aperture on depth of field in photography).
A delayed onset of seeking correction for presbyopia has been found among those with certain professions and those with miotic pupils. In particular, farmers and homemakers seek correction later, whereas service workers and construction workers seek eyesight correction earlier. Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions.
Focusing mechanism of the eye
In optics, the closest point at which an object can be brought into focus by the eye is called the eye's near point. A standard near point distance of 25 cm is typically assumed in the design of optical instruments, and in characterizing optical devices such as magnifying glasses.
Without correction, the near point is at 3 inches (7 cm) at age 10, to 6 inches (16 cm) at age 40, to 39 inches (1 meter) at age 60. As a result, a 60-year-old must use corrective lenses to read books or magazines at a comfortable distance.
There is some confusion in articles and even textbooks over how the focusing mechanism of the eye actually works. In the classic book, Eye and Brain by Gregory, for example, the lens is said to be suspended by a membrane, the 'zonula', which holds it under tension. The tension is released, by contraction of the ciliary muscle, to allow the lens to become more round, for close vision. This implies the ciliary muscle, which is outside the zonula, must be circumferential, contracting like a sphincter, to slacken the tension of the zonula pulling outwards on the lens. This is consistent with the fact that our eyes seem to be in the 'relaxed' state when focusing at infinity, and also explains why no amount of effort seems to enable a myopic person to see farther away. Many texts, though, describe the 'ciliary muscles' (which seem more likely to be just elastic ligaments and not under any form of nervous control) as pulling the lens taut to focus at close range. This has the counter-intuitive effect of steepening the lens valve cytokinesis centrally (increasing its power) and flattening peripherally.
Interaction with myopia
Many people with myopia (near-sightedness) can read comfortably without eyeglasses or contact lenses even after age 40. However, their myopia does not disappear and the long-distance visual challenges remain. Myopes considering refractive surgery are advised that surgically correcting their nearsightedness may be a disadvantage after age 40, when the eyes become presbyopic and lose their ability to accommodate or change focus, because they will then need to use glasses for reading. Myopes with astigmatism find near vision better, though not perfect, without glasses or contact lenses when presbyopia sets in, but the more astigmatism, the poorer their uncorrected near vision.
A surgical technique offered is to create a "reading eye" and a "distance vision eye", a technique commonly used in contact lens practice, known as monovision. Monovision can be created with contact lenses, so candidates for this procedure can determine if they are prepared to have their corneas reshaped by surgery to cause this effect permanently.
Monovision does not work well for spectacles, since the user will not always look through the optical centres of the lenses, causing double images. That problem is not present with contacts or laser surgery.
Also with aging, some people suffer from both myopia and hyperopia. For those people, both type of lenses (concave and convex) are used.
In the visual system, images captured by the eye are translated into electric signals that are transmitted to the brain where they are interpreted. As such, in order to overcome presbyopia, two main components of the visual system can be addressed: 1) the optical system of the eye or 2) the visual processing of the brain.
- Image capturing in the eye - Solutions for presbyopia have advanced significantly in recent years, thanks to widened availability of optometry care as well as over-the-counter vision correction.
- Image processing in the brain - Scientific solutions for overcoming the symptoms of presbyopia were developed in recent years and tested successfully in multiple studies. These solutions are available thanks to significant progress in the understanding of the human brain plasticity and the field of perceptual learning.
Corrective lenses provide a range of vision correction, some as high as +4.0 diopter. Some with presbyopia choose varifocal or bifocal lenses to eliminate the need for a separate pair of reading glasses; specialized preparations of varifocals or bifocals usually require the services of an optometrist. Some newer bifocal or varifocal spectacle lenses attempt to correct both near and far vision with the same lens.
Contact lenses can also be used to correct the focusing loss that comes along with presbyopia. Some people choose contact lenses to correct one eye for near and one eye for far with a method called monovision, which can interfere with depth perception due to loss of focusing ability in the other eye.
New surgical procedures may also provide solutions for those who do not want to wear glasses or contacts, including the implantation of accommodative intraocular lenses. Scleral expansion bands, which increase the space between the ciliary body and lens, have not been found to provide predictable or consistent results in the treatment of presbyopia. INTRACOR has now been approved in Europe for treatment of both eyes (turning both corneas into multifocal lenses and so dispensing with the need for reading glasses).
Surgical treatments that reshape the cornea, such as PresbyLASIK and conductive keratoplasty, are also worth consideration, but some use of reading glasses will still remain when light is poor or when reading for extended periods of time, since such procedures do nothing for crystalline lens deterioration.
Surgical procedures for the correction of presbyopia, without reshaping the cornea, were proposed by Scharcker, M.D, using scleral band expansion (SBE), and J.T. Lin, Ph.D, using a laser scleral ablation in US Pat # 6,258,082 (2001). These procedures are different from the so-called Presby-Lasik (monovision or bifocal)and are able to cause the true increase of accommodation of the eye.
Another treatment option for the correction of presbyopia in patients with emmetropia, as well as in patients with myopia, hyperopia and astigmatism is laser blended vision. This procedure uses laser refractive surgery to correct the dominant eye mainly for distance vision and the nondominant eye mainly for near vision, while the depth of field (i.e. the range of distances at which the image is in focus) of each eye is increased. As a result of the increased depth of field, the brain merges the two images, creating a blend zone, i.e. a zone which is in focus for both eyes. This allows the patient to see near, intermediate and far without glasses. Some literature also suggests the benefits achieved include the brain learning to adapt, assimilating two images, one of which is out of focus. Over time, many patients report they are unaware one eye is out of focus.
Surgically implanted corneal inlays are another treatment option for presbyopia. Corneal inlays typically are implanted in the nondominant eye to minimize impact to binocular uncorrected distance vision. They seek to improve near vision in one of three ways: The Flexivue Microlens (Presbia) changes the central refractive index, the Kamra (AcuFocus) increases the depth of focus through the use of a pinhole, and the Vue+ (Revision Optics) reshapes the central cornea.
Brain image processing enhancement
Scientists' understanding of the ability of the adult brain to change, termed neuroplasticity, has been vastly advanced during the last 20 years, giving rise to solutions based on perceptual learning. Perceptual learning, standing for repeated practice on a demanding task, improves visual skills by enhancing the image processing in the brain. Several great achievements in this field include FDA-approved solutions for severe visual problems such as amblyopia, loss of vision resulting from a stroke, and others. Academic research on perceptual learning has been published in leading academic journals, including Nature, Science, PNAS, Vision Research and others, including studies showing improved visual performance in presbyopes. Improvements in visual functions following perceptual learning are achieved without changing the optical characteristics of the presbyopic eye, implying enough retained plasticity in the aging brain to offset the natural biological deterioration of the eyes. Thus, the unavoidable deterioration of the eyes with age can be compensated by boosting the speed and quality of image processing in the visual cortex of the brain.
- Adjustable-focus eyeglasses
- Astigmatism (eye)
- Conductive keratoplasty
- Corrective lens
- Eye examination
- Eyeglass prescription, especially the section on distant vision and near vision
- Lens (optics)
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|Look up presbyopia in Wiktionary, the free dictionary.|
|Look up presbyopia in Wiktionary, the free dictionary.|
- Vision Over 40
- Presbyopia at MedLinePlus Medical Encyclopedia
- Binocular Presbyopia Diagnosis with Dynamic Stimulation Aberrometry
- *Ray Gottlieb, O.D., Ph.D. "Presbyopia Reduction" (PDF).—Chart with exercise proposed to reverse presbyopia.