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Myopia in Wikipedia

Note:Wikipedia is a user-contributed encyclopedia and may not have been reviewed by professional editors (See full Wikipedia disclaimer)

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Myopia". (Source - Retrieved 2006-09-07 14:15:32 from http://en.wikipedia.org/wiki/Myopia)

Introduction

Myopia, sometimes called short-sightedness[1] or near-sightedness, is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed.

Those with myopia typically can see nearby objects clearly but distant objects appear blurred. The opposite defect of myopia is hyperopia or "far-sightedness" or "long-sightedness" — this is where the cornea is too flat or the eye is too short.

Mainstream ophthalmologists and optometrists most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Classification

Myopia has been classified in various manners.[2][3][4]

Etiology

Borish and Duke-Elder classified myopia by cause:[3][4]

  • Axial myopia is attributed to excessive, or increase in, the eye's axial length.[5]
  • Refractive myopia is attributed to the condition of the refractive eelements of the eye.[5] Borish further subclassified refractive myopia:[3]
  • Curvature myopia is attributed to excessive, or increase in, curvature of one or more of the refractive surfaces of the eye, especially the cornea.[5] In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power. [6]
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[5]

Clinical entity

Various forms of myopia have been described by their clinical appearance:[4][7]

  • Simple myopia is more common than other types of myopia and is characterized by an eye that is too long for its optical power (which is determined by the cornea and crystalline lens) or optically too powerful for its axial length.[8] Both genetic and environmental factors, particularly significant amounts of near work, are thought to contribute to the development of simple myopia.[8]
  • Degenerative myopia, also known as malignant, pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma, and associated with a high refractive error and subnormal visual acuity after correction.[5] Myopia which gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.[9] Myopia with degenerative changes has been described as being very common in certain races, such as Chinese, Japanese, Arab, and Jewish people. [10]
  • Nocturnal myopia, also known as night myopia or twilight myopia, is a condition in which the eye has a greater difficulty seeing in low illumination areas, even though its daytime vision is normal. Essentially, the eye's far point of an individual's focus varies with the level of light. Night myopia is believed to be caused by pupils dilating to let more light in, which adds aberrations resulting in becoming more nearsighted. A stronger prescription for myopic night drivers is often needed. Younger people are more likely to be affected by night myopia than the elderly[11].
  • Pseudomyopia is the blurring of distance vision brought about by spasm of the ciliary muscle.[12]
  • Induced myopia, also known as acquired myopia, results from exposure to various pharmaceuticals, increases in glucose levels, nuclear sclerosis, or other anomalous conditions.[8] The encircling bands used in the repair of retinal detachments may induce myopia by increasing the axial length of the eye.[13]
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[5] Cataracts may lead to index myopia.[14]
  • Form deprivation myopia is a type of myopia that occurs when the eye is deprived of clear form vision.[15] Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.[15]

Degree

Myopia, which is measured in diopters by the strength or optical power of a corrective lens that focuses distant images on the retina, has also been classfied by degree or severity:[2]

  • Low myopia usually describes myopia of –3.00 diopters or less.[5]
  • Medium myopia usually describes myopia between –3.00 and –6.00 diopters.[5] Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma.[16]
  • High myopia usually describes myopia of –6.00 or more.[5] People with high myopia are more likely to have retinal detachments[17] and primary open angle glaucoma.[18] They are also more likely to experience floaters, shadow-like shapes which appear singly or in clusters in the field of vision. Roughly 30% of myopes have high myopia.[10]

Age of onset

Myopia is sometimes classified by the age of onset:[2]

  • Congential myopia, also known as infantile myopia, is present at birth and persists through infancy.[8]
  • Youth onset myopia occurs prior to age 20.[8]
  • School myopia appears during childhood, particularly the school-age years.[19] This form of myopia is attributed to the use of the eyes for close work during the school years.[5]
  • Adult onset myopia
  • Early adult onset myopia occurs between ages 20 and 40.[8]
  • Late adult onset myopia occurs after age 40.[8]

Epidemiology

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion.[20] The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.[10][21]

In some countries, such as Japan, Singapore and Taiwan, up to 44% of the adult population is myopic.

A recent study involving first-year undergraduate students in the United Kingdom found that 50% of British whites and 53.4% of British Asians were myopic.[22]

In Australia, the overall prevalence of myopia (worse than –0.50 diopters) has been estimated to be 17%.[23] In one recent study, less than 1 in 10 (8.4%) Australian children between the ages of 4 and 12 were found to have myopia greater than –0.50 diopters.[24] A recent review found that 16.4% of Australians aged 40 or over have at least –1.00 diopters of myopia and 2.5% have at least –5.00 diopters.[25]

In Brazil, a 2005 study estimated that 6.4% of Brazilians between the ages of 12 and 59 had –1.00 diopter or myopia or more, compared with 2.7% of the indigenous people in northwestern Brazil.[26] Another found nearly 1 in 8 (13.3%) of the students in one city were myopic.[27]

In Greece, the prevalence of myopia among 15 to 18 year old students was found to be 36.8%[28]

In India, the prevalence of myopia in the general population has been reported to be only 6.9%. [29][28]

A recent review found that 26.6% of Western Europeans aged 40 or over have at least –1.00 diopters of myopia and 4.6% have at least –5.00 diopters.[30]

In the United States, the prevalence of myopia has been estimated at 20%.[10] Nearly 1 in 10 (9.2%) American children between the ages of 5 and 17 have myopia.[31] Approximately 25% of Americans between the ages of 12 and 54 have the condition.[32] A recent review found that 25.4% of Americans aged 40 or over have at least –1.00 diopters of myopia and 4.5% have at least –5.00 diopters.[33]

A study of Jordanian adults aged 17 to 40 found that over half (53.7%) were myopic.[34]

Ethnicity and race

The prevalence of myopia in has been reported as high as 70-90% in Asia, 30-40% in Europe and the United States, and 10-20% in Africa.[21]

Myopia is less common in black, Nubians, and Sudanese people.[10] In Americans between the ages of 12 and 54, myopia has been found to affect whites more than blacks.[32]

Gender

Lower levels of myopia appear to affect males only slightly more so than females, however, women are more likely to have higher amounts or degenerative changes.[10] In Americans between the ages of 12 and 54, myopia has been found to affect women more than men.[32]

Education, intelligence, and IQ

A number of studies have shown that the prevalence of myopia increases with level of education.[32][28]

Many studies have shown a relationship between myopia and IQ. According to Arthur Jensen, myopes average 7-8 IQ points higher than non-myopes. The relationship also holds within families, and siblings with a higher degree of refraction error average higher IQs than siblings with less refraction error. Jensen believes that this indicates myopia and IQ are pleiotropically related (both myopia and IQ are caused by the same genes). The mechanism that has caused a relationship between myopia and IQ is not yet known with certainty (Jensen, 1998).

Etiology and pathogenesis

In the early $1900s$, William Bates asserted that myopia, as with all refractive errors, resulted from a particular type of "eyestrain" that was itself a result of "mental strain"[35] He stated that the shape of the eyeball responded instantaneously to the action of the extraocular muscles upon it[36] and that myopia was produced due to contraction of the inferior oblique and superior oblique muscles which lengthened the eye.[37] According to Bates, myopia was associated with a "strain" to see distance objects rather than near work.[38]

In the mid-1900s, mainstream ophthalmologists and optometrists believed myopia to be primarily hereditary; the influence of near work in its development seemed "incidental" and the increased prevalence of the condition with increasing age was viewed as a "statistical curiosity".[3][4]

Among mainstream researchers and eye care professionals, myopia is thought to be a combination of genetic and environmental factors.[8][19]

There are two basic mechanisms now thought to cause myopia: form deprivation and optical defocus.[39] Form deprivation occurs when the image quality on the retina is reduced; optical defocus occurs when light focus in front of or behind the retina. Numerous experiments with animals have shown that myopia can be artificially generated either by inducing either of these conditions. In animal models wearing negative spectacle lenses, axial myopia has been shown to occur as the eye elongates to compensate for optical defocus.[39] The exact mechanism of this image-controlled elongation of the eye is still unknown.

Theories

  • Combination of Genetic and Environmental Factors — In China, myopia is more common in those with higher education background[40] and some studies suggest that near work may exacerbate a genetic predisposition to develop myopia[41]. This "genetic susceptibility" to environmental factors has been postulated as one explanation for the varying degrees of myopia among individuals or populations[42], but there exists some difference of opinion as to whether it exists.[19][43]. High heritability simply means that most of the variation in a particular population at a particular time is due to genetic differences. If the environment changes — as, for example, it has by the introduction of televisions and computers — the incidence of myopia can change as a result, even though heritability remains high. From a little bit different point of view it could be concluded that — determined by heritage — some people are at a higher risk to develop myopia when exposed to modern environmental conditions with a lot of extensive near work like reading. In other words, it is often not the myopia itself, which is inherited, but the reaction to specific environmental conditions — and this reaction can be the onset and the progression of myopia.
  • Genetic Factors — The wide variability of the prevalence of myopia in different ethnic groups has been reported as additional evidence supporting the role of genetics in the development of myopia. [44]. Measures of the heritability of myopia have yielded figures as high as 89%, and recent research has identified genes that may be responsible: defective versions of the PAX6 gene seem to be associated with myopia in twin studies [1]. Under this theory, the eye is slightly elongated front to back as a result of faults during development, causing images to be focused in front of the retina rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescence and then levels off as a person reaches adulthood. Genetic factors can work in various biochemical ways to cause myopia, a weak or degraded connective tissue is a very essential one. Genetic factors include an inherited, increased susceptibility for environmental influences like excessive near work, and the fact that some people do not develop myopia in spite of very adverse conditions is a clear indication that heredity is involved somehow in any case.
  • Environmental Factors — It has been suggested that a genetic susceptibility to myopia does not exist.[19] A high heritability of myopia (as for any other condition) does not mean that environmental factors and lifestyle have no effect on the development of the condition. Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak. Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. Other theories suggest that the eyes become strained by the constant extra work involved in "nearwork" and get stuck in the near position, and eye exercises can help loosen the muscles up thereby freeing it for far vision. These primarily mechanical models appear to be in contrast to research results, which show that the myopic elongation of the eye can be caused by the image quality, with biochemical processes as the actuator. Common to both views is, however, that extensive near work and corresponding accommodation can be essential for the onset and the progression of myopia.
One Austrian study confirmed that the axial length of the eye does mildly increase while reading, but attributed this elongation due to contraction of the ciliary muscle during accommodation (the process by which the eye increases optical power to maintain a clear image focus), not “squeezing” of the extraocular muscles.[45] Near work and nightlight exposure in childhood have been hypothesized as environmental risk factors for myopia.[46] Although one initial study indicated a strong association between myopia and nightlight exposure,[47] recent research has found none.[46][48][49][50]
  • Near work. Near work has been implicated as a contributing factor to myopia in many studies. New research from NSU College of Optometry shows that students exposed to extensive "near work" are at a higher risk of developing myopia, whereas taking summer or winter vacations (which amount to extended breaks from near work) will either reduce or stop myopic progression [2]. Near work in certain cultures (e.g. Vanuatu) does not result in greater myopia[3][4][5][6]. It has been hypothesized that this outcome may be a results of genetics or environmental factors such as diet or over-illumination, changes in which seem to occur in Asian, Vanuatu and Inuit cultures acclamating to intensive early studies[7].
  • Diet and nutrition - One 2002 article suggested that myopia may be caused by over-consumption of bread in childhood, or in general by diets too rich in carbohydrates, which can lead to chronic hyperinsulinemia. Various other components of the diet, however, were made responsible for contributing to myopia as well, as summarized in a documentation.

Relevant research

  • A Turkish study found that accommodative convergence, rather than accommodation, may be a factor in the onset and progression of myopia in adults[8] [9].
  • A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creation of myopia.[51]

Presbyopia and the 'payoff' for the short-sighted

Many people with myopia are able to read comfortably without eyeglasses. Myopes considering refractive surgery are advised that this may be an advantage after the age of 40 when the eyes become presbyopic and lose their ability to accommodate or change focus.

Perceptual effects of extreme myopia

At the extremes of nearsightedness (-9 diopter shown here) a new perceptual ability arises out of what would otherwise appear to be a disability. When a person with -9 diopter myopia views light sources at night without corrective optics, the light source is spread out into a large unfocused disk of light.

However, the disk of light is not flat and featureless, but instead reveals details about the viewer that are not otherwise within the range of perception. These drawings attempt to approximate what is seen separately in the left and right eye. (In normal stereoscopic vision, these two views of a single light source are overlapped and merge together, but can be seen separately by covering one of the eyes.)

Due to the extreme vision error, the person is instead able to see microscopic details within their own eyeball, such as the protein strings that naturally float freely in the clear aqueous humour of the eye. Rapid movements of the eye will swirl up the protein strings, which continue to swirl around for a few seconds when the viewer suddenly stops to look at the light source. The motion and shape of the strings is clearly visible.

The microscopic surface of the cornea is also visible in the light disk. Objects such as tiny air bubbles, lint, and other debris can be seen, and partially closing and reopening the upper eyelid will leave a visible ridge of liquid where the lid has pushed the liquid along in front of it. The eyelashes hanging down from the upper eyelid in front of the pupil can also be seen.

Other visible details include the size of the pupil, which can be seen as the light disk growing and shrinking in size, and the shape of the cornea, which may not necessarily be perfectly circular and shows what is being corrected by astigmatic cylinder adjustments in the lens prescription.

One final detail that is visible is a distortion of the upper part of the cornea, caused by the eyelid normally lying across the top edge of the cornea when the eyes are open. This slight distortion usually disappears after sleeping all night with the eyes closed, then slowly reappears again in the daytime when the eyes are open.

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination by an ophthalmologist or an optometrist. Frequently an autorefractor or retinoscope is used to give an initial objective assessment of the refractive status of each eye, then a phoropter is used to subjectively refine the patient's eyeglass prescription.

Treatment, management, and prevention

Eyeglasses, contact lenses, and refractive surgery are the primary options to treat the visual symptoms of those with myopia. Orthokeratology is the practice of using special rigid contact lenses to flatten the cornea to reduce myopia.

Practitioners and advocates of alternative therapies often recommend eye exercises and relaxation techniques such as the Bates method[52][53], however, the efficacy of these practices are disputed by mainstream eye care practitioners.[54]

There is no universally accepted method of preventing myopia.[8] Some clinicians and researchers recommend plus power lenses in the form of single vision reading lenses or bifocals[8][55] Although recent Malaysian study suggested that undercorrection of myopia caused more rapid progression of myopia[56], the reliability of the data have been called into question.[57]

Myopia control

Various methods have been employed in an attempt to decrease the progression of myopia.[39] Altering the use of eyeglasses between full-time, part-time, and not at all does not appear to alter myopia progression.[58][59]

References

  1. ^ The term "short-sightedness" is often used in the context of "lack of foresight"
  2. Grosvenor T. "A review and a suggested classification system for myopia on the basis of age-related prevalence and age of onset." Am J Optom Physiol Opt. 1987 Jul;64(7):545-54. PMID 3307441
  3. Borish, Irvin M. (1949). Clinical Refraction. Chicago: The Professional Press.
  4. Duke-Elder, Sir Stewart (1969). The Practice of Refraction (8th ed.). St. Louis: The C.V. Mosby Company. ISBN 0-7000-1410-1.
  5. Cline D; Hofstetter HW; Griffin JR. Dictionary of Visual Science. 4th ed. Butterworth-Heinemann, Boston 1997. ISBN 0-7506-9895-0
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  28. Mavracanas TA, Mandalos A, Peios D, Golias V, Megalou K, Gregoriadou A, Delidou K, Katsougiannopoulos B. "Prevalence of myopia in a sample of Greek students." Acta Ophthalmol Scand. 2000 Dec;78(6):656-9. PMID 11167226.
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  31. ^ Kleinstein RN, Jones LA, Hullett S, Kwon S, Lee RJ, Friedman NE, Manny RE, Mutti DO, Yu JA, Zadnik K; Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error Study Group. "Refractive error and ethnicity in children." Arch Ophthalmol. 2003 Aug;121(8):1141-7. PMID 12912692.
  32. Sperduto RD, Seigel D, Roberts J, Rowland M. "Prevalence of myopia in the United States." Arch Ophthalmol. 1983 Mar;101(3):405-7. PMID 6830491.
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  35. ^ "Chapter X: Strain."
  36. ^ "Chapter VII: The Variability of the Refraction of the Eye."
  37. ^ Chapter IV : The Truth about Accommodation as Demonstrated by Experiments on the Eye Muscles of Fish, Cats, Dogs, Rabbits and Other Animals."
  38. ^ "Chapter I: Introductory."
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  43. ^ Morgan I, Megaw P. Using natural STOP growth signals to prevent excessive axial elongation and the development of myopia. Ann Acad Med Singapore. 2004 Jan;33(1):16-20. PMID 15008556
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  46. Saw SM, Wu HM, Hong CY, Chua WH, Chia KS, Tan D. "Myopia and night lighting in children in Singapore." Br J Ophthalmol. 2001 May;85(5):527-8. PMID 11316706.
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  51. ^ Czepita D, Filipiak D. [The effect of the type of astigmatism on the incidence of myopia]. Klin Oczna. 2005;107(1-3):73-4. PMID 16052807.
  52. ^ Eye Exercises for Myopia
  53. ^ Chinese Eye Exercises
  54. ^ Rawstron JA, Burley CD, Elder MJ. "A systematic review of the applicability and efficacy of eye exercises." J Pediatr Ophthalmol Strabismus. 2005 Mar-Apr;42(2):82-8. PMID 15825744.
  55. ^ The International Myopia Prevention Association
  56. ^ Chung K, Mohidin N, O'Leary DJ. "Undercorrection of myopia enhances rather than inhibits myopia progression." Vision Res. 2002 Oct;42(22):2555-9. PMID 12445849.
  57. ^ The Wildoset Lab. "Controlling Myopia Progression - A Confusing Story." Retrieved September 1, 2006.
  58. ^ Ong E, Grice K, Held R, Thorn F, Gwiazda J. "Effects of spectacle intervention on the progression of myopia in children." Optom Vis Sci. 1999 Jun;76(6):363-9. PMID 10416930.
  59. ^ Parssinen O, Hemminki E, Klemetti A. Effect of spectacle use and accommodation on myopic progression: final results of a three-year randomised clinical trial among schoolchildren. Br J Ophthalmol. 1989 Jul;73(7):547-51. PMID 2667638.

See also

  • Astigmatism (eye)
  • Lens (optics)
  • Optician
 

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