Presbyopia
Presbyopia | |
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Classification and external resources | |
MedlinePlus | 001026 |
The word presbyopia comes from the Greek word presbys (πρέσβυς), meaning "old man" or "elder", and the ancient Greek word ops (ὤψ), meaning "eye".[2]
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.[3]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.[4] 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.[5] 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.[6]
Focusing mechanism of the eye
Main article: Accommodation (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.[7]
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,[8] 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.[citation needed] 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.[9]
Treatment
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.[10]
Corrective lenses
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.[11]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.
Exercises
Available treatment programs based upon exercising the ciliary muscles have not proven effective.[12]Surgery
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[13] 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.[14]
Surgical procedures for the correction of presbyopia without reshaping the cornea, using scleral band expansion (SBE) were proposed by Scharcker, M.D.and J.T. Lin, Ph.D., 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.[15]
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.[16][17][18]
IsoVision: developed since 2008 by Dr. Frederic HEHN, laser surgery used to correct all refractive errors (myopia, hyperopia, astigmatism) associated with presbyopia. The treatment consists of two phases: one for distance vision and one for near vision. This allows an efficient, sustainable and safe treatment in time.[19]
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.[20] 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.[21]
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