The Full Wiki

Myopia: Map

Advertisements
  
  
  

Wikipedia article:

Map showing all locations mentioned on Wikipedia article:



Normal vision.
Near-sighted vision.
250 px


Myopia ( , muōpia, "nearsightedness"), also called nearsightedness or shortsightedness, 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 see nearby objects clearly but distant objects appear blurred. With myopia, the eyeball is too long, or the cornea is too steep, so images are focused in the vitreous inside the eye rather than on the retina at the back of the eye. The opposite defect of myopia is hyperopia or "farsightedness" or "long-sightedness"—this is where the cornea is too horizontal or the eye is too small.

Mainstream eye care professionals 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 optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Classification

Myopia has been classified in various manners.

By cause

Borish and Duke-Elder classified myopia by cause:
  • Axial myopia is attributed to an increase in the eye's axial length.
  • Refractive myopia is attributed to the condition of the refractive elements of the eye. Borish further subclassified refractive myopia:
*Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea. In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power.
*Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.


Clinical entity

Various forms of myopia have been described by their clinical appearance:
  • 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. Both genetic and environmental factors, particularly significant amounts of near work, are thought to contribute to the development of simple myopia.
  • 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. This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.
  • 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.
  • Pseudomyopia is the blurring of distance vision brought about by spasm of the ciliary muscle.
  • Induced myopia, also known as acquired myopia, results from exposure to various pharmaceuticals, increases in glucose levels, nuclear sclerosis, or other anomalous conditions. The encircling bands used in the repair of retinal detachments may induce myopia by increasing the axial length of the eye.
*Index myopia is attributed to variation in the index of refraction of one or more of the ocular media. Cataracts may lead to index myopia.
*Form deprivation myopia is a type of myopia that occurs when the eyesight is deprived by limited illumination and vision range, or the eye is modified with artificial lenses or deprived of clear form vision. In lower vertebrates this kind of myopia seems to be reversible within short periods of time. Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.
  • Nearwork Induced Transient Myopia (NITM), is defined as short-term myopic far point shift immediately following a sustained near visual task. Some authors argue for a link between NITM and the development of permanent myopia.


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 classified by degree or severity:
  • Low myopia usually describes myopia of −3.00 diopters or more (closer to 0.00).
  • Medium myopia usually describes myopia between −3.00 and −6.00 diopters. Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma.
  • High myopia usually describes myopia of −6.00 or less (toward -10.00). People with high myopia are more likely to have retinal detachments and primary open angle glaucoma. 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.


Age of onset

Myopia is sometimes classified by the age of onset:
  • Congenital myopia, also known as infantile myopia, is present at birth and persists through infancy.
  • Youth onset myopia occurs prior to age 20.
*School myopia appears during childhood, particularly the school-age years. This form of myopia is attributed to the use of the eyes for close work during the school years.
  • Adult onset myopia
*Early adult onset myopia occurs between ages 20 and 40.
*Late adult onset myopia occurs after age 40.


Epidemiology

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion. The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors. Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.

In some areas, such as Chinamarker, Indiamarker and Malaysiamarker, up to 41% of the adult population is myopic to -1dpt, up to 80% to -0.5dpt.

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

In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%. 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. 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.

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

In Greecemarker, the prevalence of myopia among 15 to 18 year old students was found to be 36.8%.

In Indiamarker, the prevalence of myopia in the general population has been reported to be only 6.9%.

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.

In the United Statesmarker, the prevalence of myopia has been estimated at 20%. Nearly 1 in 10 (9.2%) American children between the ages of 5 and 17 have myopia. Approximately 25% of Americans between the ages of 12 and 54 have the condition. 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.

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

Ethnicity and race

The prevalence of myopia has been reported as high as 70–90% in some Asian countries, 30–40% in Europe and the United States, and 10–20% in Africa.

Myopia is less common in African people and associated diaspora. In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians. Asians had the highest prevalence (18.5%), followed by Hispanics (13.2%). Caucasians had the lowest prevalence of myopia (4.4%), which was not significantly different from African Americans (6.6%). For hyperopia, Caucasians had the highest prevalence (19.3%), followed by Hispanics (12.7%). Asians had the lowest prevalence of hyperopia (6.3%) and were not significantly different from African Americans (6.4%). For astigmatism, Asians and Hispanics had the highest prevalences (33.6% and 36.9%, respectively) and did not differ from each other (P = .17). Blacks had the lowest prevalence of astigmatism (20.0%), followed by whites (26.4%).

Education, intelligence, and IQ

A number of studies have shown that the incidence of myopia increases with level of education and 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 as they are caused or influenced by the same genes. No specific mechanism that could cause a relationship between myopia and IQ has yet been identified.

Also other personal characteristics, as value systems, school achievements, time spent in reading for pleasure, language abilities and time spent in sport activities correlated to the occurrence of myopia in studies.

Etiology and pathogenesis

Because in the most common, "simple" myopia, the eye length is too long, any etiologic explanation must account for such axial elongation. To date, no single theory has been able to satisfactorily explain this elongation.

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".

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

There are currently two basic mechanisms believed to cause myopia: form deprivation (also known as pattern deprivation) and optical defocus. Form deprivation occurs when the image quality on the retina is reduced; optical defocus occurs when light focuses in front of or behind the retina. Numerous experiments with animals have shown that myopia can be artificially generated 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. The exact mechanism of this image-controlled elongation of the eye is still unknown. It has been suggested that accommodative lag leads to blur (i.e. optical defocus) which in turn stimulates axial elongation and myopia.

Theories

  • Combination of genetic and environmental factors—In China, myopia is more common in those with higher education background and some studies suggest that near work may exacerbate a genetic predisposition to develop myopia. Other studies have shown that near work (reading, computer games) may not be associated with myopic progression, however. A "genetic susceptibility" to environmental factors has been postulated as one explanation for the varying degrees of myopia among individuals or populations, but there exists some difference of opinion as to whether it exists. 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 slightly 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. 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. 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. 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. Some recommend a variety of eye exercises to strengthen the ciliary muscle. 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.


Near work and nightlight exposure in childhood have been hypothesized as environmental risk factors for myopia. Although one initial study indicated a strong association between myopia and nightlight exposure, recent research has found none.
*Near work. Near work has been implicated as a contributing factor to myopia in some studies, but refuted in others. One recent study suggested that students exposed to extensive "near work" may be at a higher risk of developing myopia, whereas extended breaks from near work during summer or winter vacations may retard myopic progression. Near work in certain cultures (e.g. Vanuatumarker) does not result in greater myopia It has been hypothesized that this outcome may be a result of genetics or environmental factors such as diet or over-illumination, changes which seem to occur in Asian, Vanuatu and Inuit cultures acclimating to intensive early studies.
*Time spent indoors - A number of studies have shown that children who spend more time outdoors have lower rates of myopia, possibly explaining the observed increase in myopia. It is theorized that the higher brightness or the larger distances outdoors play a role.
*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.
*Stress has been postulated as a factor in the development of myopia.


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.
  • A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creation of myopia.


Presbyopia and the 'payoff' for the nearsighted

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

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination by an ophthalmologist, optometrist or orthoptist. 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

Glasses are commonly used to address near-sightedness.
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. Occasionally, pinhole glasses are used by patients with low-level myopia. These work by reducing the blur circle formed on the retina, but their adverse effects on peripheral vision, contrast and brightness make them unsuitable in most situations.

Chromatic aberration of strong eyeglasses

Prismatic color distortion shown with a camera set for nearsighted focus, and using -9.5 diopter eyeglasses to correct the camera's myopia.
For people with a high degree of myopia, very strong eyeglass prescriptions are needed to correct the focus error. However, strong eyeglass prescriptions have a negative side effect in that off-axis viewing of objects away from the center of the lens results in prismatic movement and separation of colors, known as chromatic aberration. This prismatic distortion is visible to the wearer as color fringes around strongly contrasting colors. The fringes move around as the wearer's gaze through the lenses changes, and the prismatic shifting reverses on either side, above, and below the exact center of the lenses. Color fringing can make accurate drawing and painting difficult for users of strong eyeglass prescriptions.

Strongly nearsighted wearers of contact lenses do not experience chromatic aberration because the lens moves with the cornea and always stays centered in the middle of the wearer's gaze.

Eye-exercises and biofeedback

Practitioners and advocates of alternative therapies often recommend eye exercises and relaxation techniques such as the Bates method. However, the efficacy of these practices is disputed by scientists and eye care practitioners. A 2005 review of scientific papers on the subject concluded that there was "no clear scientific evidence" that eye exercises were effective in treating myopia.

In the 1980s and 1990s, there was a flurry of interest in biofeedback as a possible treatment for myopia. A 1997 review of this biofeedback research concluded that "controlled studies to validate such methods ... have been rare and contradictory." It was found in one study that myopes could improve their visual acuity with biofeedback training, but that this improvement was "instrument-specific" and did not generalise to other measures or situations. In another study an "improvement" in visual acuity was found but the authors concluded that this could be a result of subjects learning the task. Finally, in an evaluation of a training system designed to improve acuity, "no significant difference was found between the control and experimental subjects".

Prevention

There is no universally accepted method of preventing myopia. Commonly attempted preventative methods include wearing reading glasses, eye drops and participating in more outdoor activities are described below. Some clinicians and researchers recommend plus power (convex) lenses in the form of reading glasses when engaged in close work or reading instead of using single focal concave lens glasses commonly prescribed. The reasoning behind convex lense's possible effectiveness in preventing myopia is simple to understand: Convex lenses's refractive property of converging light are used in reading glasses to help reduce the accommodation needed when reading and doing close work. For people with Presbyopia whose eye's lens can not accommodate enough for very near focus; reading glasses help converge the light before it enters the eye to complement the refractive power of the eye lens so near objects focus clearly on the retina. [13157] By reducing the focusing effort needed (accommodation), reading glasses or convex lenses essentially relax the focusing ciliary muscles and may consequently reduce chances of developing myopia. source: [13158] Inexpensive non prescription reading glasses are commonly sold in drug stores and dollar stores. Alternatively, reading glasses fitted by optometrists have a wider range of styles and lens choices. source: [13159]A recent Malaysian study reported in New Scientist suggested that undercorrection of myopia caused more rapid progression of myopia. However, the reliability of these data has been called into question. Many myopia treatment studies suffer from any of a number of design drawbacks: small numbers, lack of adequate control group, failure to mask examiners from knowledge of treatments used, etc.

Pirenzepine eyedrops had a limited effect on retarding myopic progression in a recent, placebo-control, double-blinded prospective controlled study.

Daylight

Daylight may prevent myopia. Australian researchers had concluded that exposure to daylight appeared to play a critical role in restricting the growth of the eyeball, which is responsible for myopia or short-sightedness. They compared children from other developed countries such as Singapore and Australian children spent about 2–3 hours a day outdoors which could increased dopamine in the eyes that restrict distorted shaping of the eyes.

Myopia control

Various methods have been employed in an attempt to decrease the progression of myopia. Dr Chua Weihan and his team at National Eye Centre Singapore have conducted large scale studies on the effect of Atropine of varying strength in stabilizing, and in some case, reducing myopia. The use of reading glasses when doing close work may provide success by reducing or eliminating the need to accommodate. Altering the use of eyeglasses between full-time, part-time, and not at all does not appear to alter myopia progression. The American Optometric Association's Clinical Practice Guidelines for Myopia refers to numerous studies which indicated the effectiveness of bifocal lenses and recommends it as the method for "Myopia Control". In some study, bifocal and progressive lenses have not shown significant differences in altering the progression of myopia.

Myopia as metaphor

The terms myopia and myopic (or the common terms short sightedness or short sighted) have also been used metaphorically to refer to cognitive thinking and decision making that is narrow sighted or lacking in concern for wider interests or longer-term consequences. It is often used to describe a decision that may be beneficial in the present but detrimental in the future, or a viewpoint that fails to consider anything outside a very narrow and limited range (see pragmatism, which tends to be myopic). Some antonyms of short sightedness are foreseeing, "forward thinking" and prophecy. Hyperopia, the biological opposite of myopia, is also used as a metaphor for those who exhibit "far-sighted" behavior; that is, over-prioritizing long-term interests at the expense of present enjoyment.

See also



References

External links




Embed code:
Advertisements






Got something to say? Make a comment.
Your name
Your email address
Message