The small print in an ingredients list is hard to read in patients with presbyopia.
|Classification and external resources|
Presbyopia is a condition associated with aging such that the eye exhibits a progressively diminished ability to focus clearly on an object close to the eye. The first signs of presbyopia – eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print – are usually first noticed between the ages of forty and fifty.
Its cause is not exactly understood. Research most strongly suggests a loss of elasticity of the crystalline lens, although changes in the lens's curvature from continual growth and diminished of strength of the ciliary muscles (the muscles that flex and flatten the lens) have also been postulated as its cause. It is a type of refractive error.
The word derives from the Greek πρέσβυς (presbys, "old man") and ὤψ (ops, "see like," "sight"), to mean "trying to see as old men do."
Signs and symptoms
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, like other focal imperfections, becomes less noticeable in bright sunlight when the pupil becomes smaller. 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).
The onset of correction for presbyopia varies 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.
Interaction with myopia
Many people with near-sightedness can read comfortably without eyeglasses or contact lenses even after age forty. 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 forty, 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 the 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 wearer will not always look through the optical centres of the lenses, causing double images. That problem is not present with contacts or laser surgery.
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.
There is some confusion in articles over how the focusing mechanism of the eye 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.
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).
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. Multifocal contact lenses can be used to correct vision for both the near and the far. Some people choose contact lenses to correct one eye for near and one eye for far with a method called monovision.
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. 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, using scleral band expansion (SBE) were proposed by Scharcker and Lin, using a laser scleral ablation in US Pat # 6,258,082 (2001). These procedures are different from PresbyLasik, and claim to create the true increase of accommodation of the eye by increasing the space between the ciliary body and lens. SBEs have not been found to provide predictable or consistent results in the treatment of presbyopia.
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.
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