суббота, 23 февраля 2013 г.

Myopia
 Classification and external resources
ICD-10 H52.1
ICD-9 367.1
DiseasesDB 8729
MedlinePlus 001023
MeSH D009216
Myopia (Greek: μυωπία, muōpia, [1] from myein "to shut" - ops (gen. opos) "eye"[2]), commonly known as being nearsighted (American English) and shortsighted (British English), is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it. This causes the image that one sees when looking at a distant object to be out of focus, but in focus when looking at a close object.
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, though there are cases of associated side effects. The corrective lenses have a negative optical power (i.e. have a net concave effect) which compensates for the excessive positive diopters of the myopic eye.

Contents

By cause

Borish and Duke-Elder classified myopia by cause:[3][4]
  • Axial myopia is attributed to an increase in the eye's axial length.[5]
  • Refractive myopia is attributed to the condition of the refractive elements of the eye.[5] Borish further subclassified refractive myopia:[3]
  • Curvature myopia is attributed to excessive, or increased, 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]
Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens (hyperphacosorbitomyopicosis) as a result of sorbitol (sugar alcohol) accumulating in the lens. This edema often causes temporary myopia (nearsightedness). A common sign of hyperphacosorbitomyopicosis is blurring of distance vision while near vision remains adequate.[citation needed]

Clinical entity

Various forms of myopia have been described by their clinical appearance:[4][7]
  • Simple myopia, more common than other types of myopia, 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] This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.[9]
  • Nocturnal myopia, also known as night 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.[10][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, oxygen toxicity (e.g., from diving or from oxygen and hyperbaric therapy) 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 occurs when the eyesight is deprived by limited illumination and vision range,[15] or the eye is modified with artificial lenses[16] or deprived of clear form vision.[17][18] In lower vertebrates, this kind of myopia seems to be reversible within short periods of time.[18] Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.[18]
  • Nearwork-induced transient myopia (NITM) is defined as short-term myopic far point shift immediately following a sustained near visual task.[19] Some authors argue for a link between NITM and the development of permanent myopia.[20]

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:[21]

Age at onset

Myopia is sometimes classified by the age at onset:[21]
  • Congenital myopia, also known as infantile myopia, is present at birth and persists through infancy.[8]
  • Youth onset myopia occurs in the early childhood or teenage, and the ocular power can keep varying until the age of 21, before which any form of corrective surgery is usually not recommended by ophthalmic specialists around the world.[8]
  • School myopia appears during childhood, particularly the school-age years.[27] 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]

Signs and symptoms
Near-sighted vision (left), normal vision (right)
Myopia presents with blurry distance vision, but generally gives good near vision. In high myopia, even near vision is affected as objects must be extremely close to the eyes to see clearly, and patients cannot read without their glasses prescribed for distance.

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination by an ophthalmologist, optometrist or orthoptist.[28] 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.

Prevention

The National Institutes of Health says there is no known way of preventing myopia, and the use of glasses or contact lenses does not affect its progression[29] There is no universally accepted method of preventing myopia; proposed procedures have not been studied for effectiveness.[8]
Commonly attempted preventive methods include wearing reading glasses, eye drops and participating in more outdoor activities as 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.[8] The reasoning behind a convex lens's possible effectiveness in preventing myopia is simple to understand: Convex lenses' refractive property of converging light are used in reading glasses to help reduce the accommodation needed when reading and doing close work. Although accommodation is irrelevant in Medina's quantitative model of myopia, it reaches the same conclusion. The model teaches a very simple method to prevent myopia.[30]
For people with presbyopia, whose eyes' lenses 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.[31] 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.[32] Inexpensive nonprescription 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.[33]
A recent Malaysian study reported in New Scientist[34] suggested undercorrection of myopia caused more rapid progression of myopia.[35] However, the reliability of these data has been called into question.[36] 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-controlled, double-blind, prospective-controlled study.[37]
Researchers at the University of Cambridge have found that a lack of outdoor play could be linked to myopia.[38]

Management


Glasses are commonly used to address near-sightedness.

Compensating for myopia using a corrective lens.
Eyeglasses, contact lenses, and refractive surgery are the primary options to treat the visual symptoms of those with myopia. Lens implants are now available offering an alternative to glasses or contact lenses for myopics for whom laser surgery is not an option. 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.
Glasses may have the potential to make the eyes worse, as they increase the accommodation needed by the eyes to focus.[citation needed] Evidence of this can be seen when people with higher prescriptions have a harder time with activities like reading because their eyes grow tired faster.[citation needed] Stronger prescriptions require a higher accommodation by the eyes to focus through them, which can, over time, worsen eyesight, requiring yet another prescription, in a continuous but quickening cycle.[citation needed] Contact lenses of equivalent prescription may not result in the same effect as eyeglasses, as they are closer to the eyes and may require less accommodation.[39] This form of myopia is attributed to the use of the eyes for close work during the school years.[5]
Glasses work by using optical lenses bringing the image a viewer closer so that it can be focused by their myopic eyes. Large amounts of near work while wearing glasses can be very detrimental to the eyes and can be a cause of worsening nearsightedness.[citation needed] However, the eyestrain caused by not wearing glasses when they are needed can also be a risk factor. The best way to avoid needing new lenses is by reducing the amount of near work, which forces the eyes into a continuous near-focusing position that eventually causes or increases myopia, by taking frequent breaks from near work, and by only wearing glasses when they are needed.[citation needed] Reading glasses can also be worn during near work to decrease the strain on the eye, especially when already wearing corrective lenses, as they work in the opposite fashion to normal lenses.[40] Using this practice may have the potential to prevent nearsightedness or slow its progression.[citation needed]

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. (left) Close-up of color shifting through corner of eyeglasses. The light and dark borders visible between color swatches do not exist. (right)
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.

Myopia control

Various methods have been employed in an attempt to decrease the progression of myopia.[41] 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.[40][42] 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".[8] In some studies, bifocal and progressive lenses have not shown significant differences in altering the progression of myopia.[41] More recently, robust studies on children have shown orthokeratology[43] and centre distance bifocal contact lenses[44] may arrest myopic development.
Scleral reinforcement surgery is aimed to cover the thinning posterior pole with a supportive material to withstand intraocular pressure and prevent further progression of the posterior staphyloma. The strain is reduced, although damage from the pathological process cannot be reversed. By stopping the progression of the disease, vision may be maintained or improved.[45]

Alternative medicine

A number of alternative therapies exist including eye exercises and relaxation techniques, such as the Bates method. However, the efficacy of these practices is disputed by scientists and eye care practitioners.[46] 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.[47]
In the 1980s and 1990s, biofeedback created a flurry of interest as a possible treatment for myopia. A 1997 review of this biofeedback research concluded "controlled studies to validate such methods ... have been rare and contradictory."[48] One study found 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.[49] In another study, an "improvement" in visual acuity was found, but the authors concluded this could be a result of subjects learning the task.[50] Finally, in an evaluation of a training system designed to improve acuity, "no significant difference was found between the control and experimental subjects".[51]

Epidemiology

The global prevalence of refractive errors has been estimated from 800 million to 2.3 billion.[52] The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.[26][53] Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.[54]
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.[53] Myopia is less common in African people and associated diaspora.[26] In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.[55]

Asia

In some parts of Asia, myopia is very common.
Singapore is believed to have the highest prevalence of myopia in the world; up to 80% of people there have myopia, but the accurate figure is unknown.[56]
China's myopia rates is 31%: 400 million of its 1.3 billion people are myopic. The prevalence of myopia in high school in China is 77.3%, and in college is more than 80%.[57]
In some areas, such as China, India and Malaysia, up to 41% of the adult population is myopic to −1.00 dpt,[58] up to 80% to −0.5dpt.[59]
A study of Jordanian adults aged 17 to 40 found over half (53.7%) were myopic.[60]
However, some research suggests the prevalence of myopia in India in the general population is only 6.9%.[61][62]

Europe

A recent study involving first-year undergraduate students in the United Kingdom found 50% of British whites and 53.4% of British Asians were myopic.[63]
In Greece, the prevalence of myopia among 15- to 18-year-old students was found to be 36.8%.[61]
A recent review found 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.[64]

United States

Myopia is common in the United States, with research suggesting this condition has increased dramatically in recent decades. In 1971-1972, the National Health and Nutrition Examination Survey provided the earliest nationally representative estimates for myopia prevalence in the U.S., and found the prevalence in persons aged 12–54 was 25.0%. Using the same method, in 1999-2004, myopia prevalence was estimated to have climbed to 41.6%.[65]
A study of 2,523 children in grades 1 to 8 (age, 5–17 years) found nearly one in 10 (9.2%) have at least − 0.75 diopters of myopia .[66] In this study, 12.8% had at least +1.25 D hyperopia (farsightedness), and 28.4% had at least 1.00-D difference between the two principal meridians (cycloplegic autorefraction) astigmatism. For myopia, Asians had the highest prevalence (18.5%), followed by Hispanics (13.2%). Caucasian children had the lowest prevalence of myopia (4.4%), which was not significantly different from African Americans (6.6%).[66]
A recent review found 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.[64]

Australia

In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%.[67] In one recent study, less than one in 10 (8.4%) Australian children between the ages of four and 12 were found to have myopia greater than −0.50 diopters.[68] A recent review found 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.[64]

Brazil

In Brazil, a 2005 study estimated 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.[69] Another found nearly 1 in 8 (13.3%) of the students in the city of Natal were myopic.[70]

Education and IQ

A number of studies have shown the incidence of myopia increases with level of education,[55][61] and many studies[71] have shown a correlation between myopia and a higher intelligence quotient (IQ).
A 2008 literature review reported studies in several nations have found a relationship between myopia and higher IQ and between myopia and school achievement. A common explanation for myopia is near-work. Regarding the relationship to IQ, several explanations have been proposed. One is that the myopic child is better adapted at reading, and reads and studies more, which increases intelligence. The reverse explanation is that the intelligent and studious child reads more, which causes myopia. Another is that myopic children have an advantage at IQ testing which is near-work because of less eye strain. Still another explanation is that pleiotropic gene(s) affect the size of both brain and eyes simultaneously. According to the two most recent studies, higher IQ may be associated with myopia in schoolchildren, independent of books read per week.[72]
Other personal characteristics, such 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.[73][74][75]

Other risk factors

Heredity appears as an important factor associated with juvenile myopia, with smaller contributions from more near work, higher school achievement and less time in sports activity.[76]

Myopia as metaphor

The terms "myopia" and "myopic" (or the common terms "shortsightedness" or "shortsighted", respectively) have been used metaphorically to refer to cognitive thinking and decision making that is narrow in scope or lacking in foresight or in concern for wider interests or for longer-term consequences.[77] 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. Hyperopia, the biological opposite of myopia, may also be used metaphorically for a value system or motivation that exhibits "farsighted" or possibly visionary thinking and behavior; that is, emphasizing long-term interests at the apparent expense of near-term benefit.[78]

Research

Normally eye development is largely genetically controlled, but it has been shown that the visual environment is an important factor in determining ocular development.

Genetic basis for myopia

Genetically, linkage studies have identified 18 possible loci on 15 different chromosomes that are associated with myopia, but none of these loci are part of the candidate genes that cause myopia. Instead of a simple one-gene locus controlling the onset of myopia, a complex interaction of many mutated proteins acting in concert may be the cause. Instead of myopia being caused by a defect in a structural protein, defects in the control of these structural proteins might be the actual cause of myopia.[79] A collaboration of all myopia studies worldwide, identified 16 new loci for refractive error in individuals of European ancestry, of which 8 were shared with Asians. The new loci include candidate genes with functions in neurotransmission, ion transport, retinoic acid metabolism, extracellular matrix remodeling and eye development. The carriers of the high-risk genes have a tenfold increased risk of myopia.[80]

Visual environment

To induce myopia in lower as well as higher vertebrates, translucent goggles can be sutured over the eye, either before or after natural eye opening.[81] Form-deprived myopia (FDM) induced with a diffuser, like the goggles mentioned, shows significant myopic shifts.[82] Anatomically, the changes in axial length of the eye seem to be the major factor contributing to this type of myopia.[83] Diurnal growth rhythms of the eye have also been shown to play a large part in FDM. Chemically, daytime retinal dopamine levels drop about 30%.[84]
Normal eyes grow during the day and shrink during the night, but occluded eyes are shown to grow both during the day and the night. Because of this, FDM is a result of the lack of growth inhibition at night rather than the expected excessive growth during the day, when the actual light deprivation occurred.[85] Elevated levels of retinal dopamine transporter (which is directly involved in controlling retinal dopamine levels) in the RPE have been shown to be associated with FDM.[86]

Dopamine

Dopamine is a major neurotransmitter in the retina involved in signal transmission in the visual system. In the retinal inner nuclear layer, a dopaminergic neuronal network has been visualized in amacrine cells. Also, retinal dopamine is involved in the regulation of electrical coupling between horizontal cells and the retinomotor movement of photoreceptor cells.[87] Although FDM-related elongations in axial length and drops in dopamine levels are significant, after the diffuser is removed, a complete refraction recovery is seen within four days in some laboratory mice. Although significant, what is even more intriguing is that within just two days of diffuser removal, an early rise and eventual normalization of retinal dopamine levels in the eye are seen. This suggests dopamine participates in visually guided eye growth regulation, and these fluctuations are not just a response to the FDM.[88]
L-Dopa has been shown to re-establish circadian rhythms in animals whose circadian rhythms have been abolished. Dopamine, a major metabolite of levodopa, releases in response to light, and helps establish circadian clocks that drive daily rhythms of protein phosphorylation in photoreceptor cells. Because retinal dopamine levels are controlled on a circadian pattern, intravitreal injection of L-dopa in animals that have lost dopamine and circadian rhythms has been shown to correct these patterns, especially in heart rate, temperature, and locomotor activity.[84] The occluders block light completely for the animals, which does not allow them to establish correct circadian rhythms, which leads to dopamine depletion. This depletion can be rectified with injections of L-dopa and hopefully contribute to the recovery from FDM.
  • L-Dopa metabolism is important to consider due to its extensive presystemic metabolism, rapid absorption in the proximal small intestine and short plasma half-life. The major metabolites of L-dopa are dopamine, dihydroxyphenylacetic acid, homovanillic acid, and 3-O-methyldopa and 3-methoxytyramine.[89] Levodopa can be converted into dopamine in the presence of aromatic L-amino acid decarboxylase (L-AAAD).[90] L-AAAD activity in rat retinas is modulated by environmental light,[91] and this modulation is associated with dopamine D1 receptors[92] and alpha 2 adrenoceptors.[93] Also, the synthesis and release of dopamine are light dependent, and light accelerates the formation of dopamine from exogenous L-Dopa.
  • Past treatments with dopamine has been used as the gold-standard drug in the treatment of Parkinson's disease and low-dose administration of the drug has been the most effective treatment of Parkinson’s. Possible treatments involving dopamine in preventing a decrease in visual acuity have been shown to be successful in the past. L-Dopa treatment in children with amblyopia showed an improvement in visual acuity.[94] In rabbits, injections of dopamine prevented the myopic shift and vitreous chamber and axial elongation typically associated with FDM.[95] In guinea pigs, systemic L-dopa has been shown to inhibit the myopic shift associated with FDM, and has compensated for the drop in retinal dopamine levels.[96] These experiments show promise in treating myopia in humans.
  • Side effects of L-dopa have been experimentally determined. L-Dopa and some of its metabolites have been shown to have pro-oxidant properties, and oxidative stress has been shown to increase the pathogenesis of Parkinson's disease.[97] This promotion of free-radical formation by L-dopa does seem to directly affect its possible future treatment of myopia because free-radicals could cause further damage to those proteins responsible for controlling structural proteins in the eye. Levodopa and some of its metabolites such as dopa/dopamine quinone have also been shown to be toxic for nigral neurons.[98] This toxic effect must be analyzed before treatment with levodopa for myopia to prevent damaging effects to these neurons.

L-Dopa inhibits myopic shifts

In guinea pigs, intraperitoneal injections of L-dopa have shown to inhibit the myopic shift associated with FDM and have compensated to the drop in retinal dopamine levels. In this study specifically, 60 animals were used and the L-dopa treatments inhibited the myopic shift (from −3.62 ± 0.98 D to −1.50 ± 0.38 D; p < 0.001) due to goggles occluding and compensated retinal dopamine (from 0.65 ± 0.10 ng to 1.33 ± 0.23 ng; p < 0.001). Daily L-dopa (10 mg/kg) was shown to increase the dopamine content in striatum. The axial length and retinal dopamine changes were positively correlated in the normal control eyes, deprived eyes, and L-dopa-treated deprived eyes. The increase in retinal dopamine and subsequent retardation of myopia may be associated with the fact that exogenous L-dopa was converted into dopamine. This suggests retinal dopaminergic function in the development of form-deprivation myopia in guinea pigs. The inhibitory effect of L-dopa on FDM may be associated with the fact that retinal L-AAAD can convert it into dopamine to balance the deficiency in the retina of the deprived eyes.[96]
  • Areas of future research include intraperitoneal injection of L-dopa; its use at 10 mg/kg could not completely suppress the development of form-deprivation myopia. Perhaps the dose may be too low to completely suppress myopia. Another possibility of the incomplete suppression of myopia may be because it is a complex process, and retinal dopamine content is only one factor. It is also unclear whether systemic application of L-dopa is able to suppress the development of form-deprivation myopia.

See also

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