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Presbyopia
Classification and external resources
ICD-10 H52.4
ICD-9 367.40
MeSH D011305

Presbyopia is a health condition where the eye exhibits a progressively diminished ability to focus on near objects with age. 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's 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.

Similar to grey hair and wrinkles, presbyopia is a symptom caused by the natural course of aging. The first symptoms (described below) are usually first noticed between the ages of 40-50. 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 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 word presbyopia comes from the Greek word presbys (πρέσβυς), meaning "old man" or "elder", and Latin root -opia, meaning "eye"[1])

Contents

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.[2]

Presbyopia, like other focus defects, becomes much less noticeable in bright sunlight. This is a result 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.[3] In particular, farmers and homemakers seek correction later, whereas service workers and construction workers seek eyesight correction earlier.

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.

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 fatten, for close vision. This would seem to imply that 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 further 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 in order 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 or spectacles so that candidates for this procedure can determine if they are prepared to have their corneas reshaped by surgery to cause this effect permanently.

Treatment

Since it is a natural part of the human aging process, presbyopia is not routinely curable. Treatment for presbyopia has advanced significantly in recent years, thanks in no small part to the ready availability of inexpensive over-the-counter reading glasses with corrective lenses that cover a wide gamut of magnification levels. Contact lenses have also been used to correct the focusing loss that comes along with presbyopia. Some people choose to correct the focus problems with bifocals, giving them a wider range of vision without having to use a second set of glasses. As the focusing loss increases, prescription changes become more frequent.

In order to reduce the need for bifocals or reading glasses, 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. There are also newer bifocal or multifocal contact lenses that attempt to correct both near and far vision with the same lens.[4]

Controversially, eye exercises have been touted as a way to delay the onset of presbyopia,[5] but there is no evidence that they work.[6]

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 (IOLs). 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.[7] IntraCOR[8] 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). PresbyLASIK, Conductive Keratoplasty, AcuFocus[9] are also worth consideration, but some use of reading glases will still remain when light is poor or you are reading for extended periods of time etc

See also

References

  1. ^ Direct translation courtesy of Merriam-Webster.com
  2. ^ Robert Abel, The Eye Care Revolution: Prevent and Reverse Common Vision Problems, Kensington Books, 2004.
  3. ^ García Serrano, JL; López Raya; Mylonopoulos Caripidis (Nov 2002). "Variables related to the first presbyopia correction" (Free full text). Archivos de la Sociedad Espanola de Oftalmologia 77 (11): 597–604. ISSN 0365-6691. PMID 12410405. http://www.oftalmo.com/seo/2002/11nov02/03.htm.  
  4. ^ Li, G; Mathine; Valley; Ayräs; Haddock; Giridhar; Williby; Schwiegerling et al. (Apr 2006). "Switchable electro-optic diffractive lens with high efficiency for ophthalmic applications" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 103 (16): 6100–4. doi:10.1073/pnas.0600850103. ISSN 0027-8424. PMID 16597675. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=16597675.  
  5. ^ Eg, Free Eye Exercises for better vision ...Easy, Tried-and-Tested! http://www.eye-exercises-for-good-vision.com/index.html
  6. ^ The lowdown on eye exercises, Harvard Medical School Family Health Guide http://www.health.harvard.edu/fhg/updates/update0903c.shtml
  7. ^ Malecaze, FJ; Gazagne; Tarroux; Gorrand (Dec 2001). "Scleral expansion bands for presbyopia". Ophthalmology 108 (12): 2165–71. ISSN 0161-6420. PMID 11733253.  
  8. ^ www.technolaspv.com
  9. ^ www.acufocus.com

External links








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