Myopia

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Myopia
File:Myopia.gif
Classification and external resources
Specialty Ophthalmology
ICD-10 H52.1
ICD-9-CM 367.1
DiseasesDB 8729
MedlinePlus 001023
Patient UK Myopia
MeSH D009216
[[[d:Lua error in Module:Wikidata at line 863: attempt to index field 'wikibase' (a nil value).|edit on Wikidata]]]

Myopia, also known as near-sightedness and short-sightedness, is a condition of the eye where the light that comes in does not directly focus on the retina but in front of it, causing 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.

When used colloquially, 'myopia' can also refer to a view on or way of thinking about something which is—by extension of the medical definition—hyper-focused and fails to include a larger context beyond the focus.

Myopia is most commonly corrected 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. Negative diopters are generally used to describe the severity of the myopia, as this is the value of the lens to correct the eye. High-degree myopia, or severe myopia, is defined as -6 diopters or worse.[1]

The term myopia is Ancient Greek: μυωπία, muōpia, from myein "to shut (like a mole - mys/mus in Greek)" – ops (gen. opos) "eye, look, sight"[2][not in citation given]), literally meaning "trying to see like a mole" (mys/mus). The opposite of myopia is hyperopia (long-sightedness).

Signs and symptoms

Near-sighted vision (top/left), normal vision (bottom/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 people with myopia cannot read without their glasses prescribed for distance. On fundoscopic examination of the eye, the optic nerve appears to be tilted and an area of white sclera could be seen on next to the disc with a line of hyperpigmentation separating this area from normal retina. The macula will have some retinal pigmentary changes and sometimes will have subretinal hemorrhages. The retina in myopic patients is thin and thorough evaluation of the periphery might show retinal holes and lattice degeneration. In addition, people with myopia might develop choroidal neovascularization in the macula.

Causes

A 2012 review could not find strong evidence for any cause, although many theories have been discredited.[3] Because twins and relatives are more likely to get myopia under similar circumstances, there must be a hereditary factor, but because myopia has been increasing so rapidly throughout the developed world, environmental factors must be more important.[4]

“Near work” hypothesis

The "near work" hypothesis, also referred to as the “use-abuse theory” states that spending time involved in near work strains the eyes and increases the risk of myopia. Some studies support the hypothesis while other studies do not.[5] While an association is present it is unclear if it is causal.[5]

Quite similarly, it is still unclear what is the mechanism of emmetropization, that is, the mechanism by which the degree of hyperopia and myopia change during childhood, such that a farsighted eye tends to become less farsighted and a nearsighted eye tends to become less nearsighted in the course of development.

"Visual stimuli" hypothesis

Although not mutually exclusive with the other hypotheses presented, the visual stimuli hypothesis adds another layer of mismatch to explain the modern prevalence of myopia. There is evidence that lack of normal visual stimuli causes improper development of the eyeball. In this case, “normal” refers to the environmental stimuli that the eyeball evolved for over hundreds of millions of years.[6] These stimuli would include diverse natural environments—the ocean, the jungle, the forest, and the savannah plains, among other dynamic visually exciting environments. Modern humans who spend most of their time indoors, in dimly or fluorescently lit buildings are not giving their eyes the appropriate stimuli to which they had evolved and may contribute to the development of myopia.[6] Experiments where animals such as kittens and monkeys had their eyes sewn shut for long periods of time also show eyeball elongation, demonstrating that complete lack of stimuli also causes improper growth trajectories of the eyeball.[7][8] Further research shows that people, and children especially, who spend more time doing physical activity and outdoor activity have lower rates of myopia,[6][9][10][11] relating the increased magnitude and complexity of the visual stimuli encountered during these types of activities.

Other risk factors

In one study, heredity was an important factor associated with juvenile myopia, with smaller contributions from more near work, higher school achievement and less time in sports activity.[12]

Long hours of exposure to daylight appears to be a protective factor.[4][13] Researchers at the University of Cambridge have found that a lack of outdoor play could be linked to myopia.[14]

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.[12][15][16]

Another explanation is that pleiotropic gene(s) affect the size of the brain and the shape of the eye simultaneously.[citation needed]

High consumption of whole grains might also contribute to myopia.[citation needed]

Diagnosis

A diagnosis of myopia is typically confirmed during an eye examination performed by a specialized doctor who is an expert in refractive conditions of the eye, the optometrist, or by an ophthalmologist or orthoptist.[17] 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.

By cause

Borish and Duke-Elder classified myopia by cause:[18][19]

  • Axial myopia is attributed to an increase in the eye's axial length.[20]
  • Refractive myopia is attributed to the condition of the refractive elements of the eye.[20] Borish further subclassified refractive myopia:[18]
  • Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea.[20] In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power.[21]
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[20]

Elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens as a result of sorbitol (sugar alcohol) accumulating in the lens. This edema often causes temporary myopia (near-sightedness).

Clinical entity

Various forms of myopia have been described by their clinical appearance:[19][22][23]

  • 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.[24] Both genetic and environmental factors, particularly significant amounts of near work, are thought to contribute to the development of simple myopia.[24]

  • 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.[20] This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.[25]
  • 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 near-sighted. A stronger prescription for myopic night drivers is often needed. Younger people are more likely to be affected by night myopia than the elderly.[26][27]
  • Pseudomyopia is the blurring of distance vision brought about by spasm of the ciliary muscle.[28]
  • 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.[24] The encircling bands used in the repair of retinal detachments may induce myopia by increasing the axial length of the eye.[29]
  • Index myopia is attributed to variation in the index of refraction of one or more of the ocular media.[20] Cataracts may lead to index myopia.[30]
  • Form deprivation myopia occurs when the eyesight is deprived by limited illumination and vision range,[31] or the eye is modified with artificial lenses[32] or deprived of clear form vision.[33][34] In lower vertebrates, this kind of myopia seems to be reversible within short periods of time.[34] Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.[34]
  • Nearwork-induced transient myopia (NITM) is defined as short-term myopic far point shift immediately following a sustained near visual task.[35] Some authors argue for a link between NITM and the development of permanent myopia.[36]
  • Instrument myopia is defined as over-accommodation when looking into an instrument such as a microscope.[23]

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:[37]

Age at onset

Myopia is sometimes classified by the age at onset:[37]

  • Congenital myopia, also known as infantile myopia, is present at birth and persists through infancy.[24]
  • 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.[24]
  • School myopia appears during childhood, particularly the school-age years.[42] This form of myopia is attributed to the use of the eyes for close work during the school years.[20]
  • Adult onset myopia
  • Early adult onset myopia occurs between ages 20 and 40.[24]
  • Late adult onset myopia occurs after age 40.[24]

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.[43] There is no universally accepted method of preventing myopia; proposed procedures have not been studied for effectiveness.[24]

Various methods have been employed in an attempt to decrease the progression of myopia, although studies show mixed results.[44] 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.

Glasses and contact lenses

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.[45][46] 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".[24] In some studies, bifocal and progressive lenses have not shown significant differences in altering the progression of myopia.[44]

Medication

Anti-muscarinic topical medications in children under 18 years of age may slow the worsening of myopia.[47][48] These treatments include pirenzepine gel, cyclopentolate eye drops, and atropine eye drops. While these treatments were shown to be effective in slowing the progression of myopia, side effects included light sensitivity and near blur.[47]

Other methods

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.[49]

Management

File:Briller2.JPG
Glasses are commonly used to address near-sightedness.
File:Myopia color.png
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.[50] 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.

Eyeglasses

Prismatic color distortion shown with a camera set for near-sighted 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 near-sighted 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.

Refractive surgery

Refractive surgery includes procedures which alter the corneal curvature or which add additional refractive means inside the eye.

PRK / LASEK

Ablation of corneal tissue from the corneal surface using an Excimer Laser. The amount of tissue ablation corresponds to the amount of myopia. Advantage: Relatively safe procedure up to 6 dioptres of myopia. Disadvantage: postoperatively painful.[51][52]

LASIK

In a preprocedure a corneal flap is cut into the cornea and lifted to allow the Excimer laser beam access to the exposed corneal tissue. After that the Excimer laser ablates the tissue according to the required correction. When the flap again covers the cornea the change in curvature generated by the laser ablation proceeds to the corneal surface. Advantage: Not painful and short rehabilitation time. Disadvantage: Potential flap complications and potential loss of corneal stability (post-LASIK Keratectasia).[53][54]

Phakic intra-ocular lens (IOL)

Instead of modifying the corneal surface, as in laser vision correction (LVC), an additional lens is implanted inside the eye (i.e., in addition to the already existing natural lens). Advantage: Relatively good control of the refractive change. Disadvantage: Potential serious long-term complications such as glaucoma, cataract and endothelial decompensation.[55][56][57]

CISIS / MyoRing

After creation of an almost completely closed corneal pocket, a compressible yet rigid complete ring is inserted 0.3 mm under the cornea surface into the cornea. This procedure changes the central corneal curvature required for the myopic correction. Advantage: Safe and reversible. Disadvantage: Good predicatability of the refractive result only in moderate and high myopia above 5 dioptres.[58][59]

Alternative medicine

A number of alternative therapies exist including eye exercises and relaxation techniques, such as the Bates method by William H. Bates, an American ophthalmologist who discovered adrenaline's usage for eye surgeries. He states in his book that "It is as natural for the eye to see as it is for the mind to acquire knowledge, and any effort in either case is not only useless, but defeats the end in view".[60] However, the efficacy of these practices is disputed by scientists and eye care practitioners.[61] 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.[62]

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."[63] One study found that myopes could improve their visual acuity with biofeedback training, but that this improvement was "instrument-specific" and did not generalize to other measures or situations.[64] In another study, an "improvement" in visual acuity was found, but the authors concluded this could be a result of subjects learning the task.[65] Finally, in an evaluation of a training system designed to improve acuity, "no significant difference was found between the control and experimental subjects".[66]

Epidemiology

Global refractive errors have been estimated to affect 800 million to 2.3 billion.[67] The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.[68][69] Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.[70]

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.[69] Myopia is about twice as common in Jews than in people of non-Jewish ethnicity.[71] Myopia is less common in African people and associated diaspora.[68] In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.[72]

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.[73]
  • China's myopia rate 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%.[74]
  • In some areas, such as China and Malaysia, up to 41% of the adult population is myopic to 1.00 dpt,[75] and up to 80% to 0.5 dpt.[76]
  • A study of Jordanian adults aged 17 to 40 found over half (53.7%) were myopic.[77]
  • Some research suggests the prevalence of myopia in India in the general population is only 6.9%.[78][79]

Europe

  • In first-year undergraduate students in the United Kingdom found 50% of British whites and 53.4% of British Asians were myopic.[80]
  • In Greece, the prevalence of myopia among 15- to 18-year-old students was found to be 36.8%.[78]
  • 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.[81]

North America (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%.[82]

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 .[83] 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) of 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%).[83]

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.[81]

Oceania (Australia)

In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%.[84] 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.[85] A recent review found 16.4% of Australians aged 40 or over have at least −1.00 diopters of myopia and 2.8% have at least −5.00 diopters.[81]

South America (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.[86] Another found nearly 1 in 8 (13.3%) of the students in the city of Natal were myopic.[87]

Society and culture

The terms "myopia" and "myopic" (or the common terms "short-sightedness" or "short-sighted", 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.[88] 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.[89]

Correlations

Numerous studies have found correlations between myopia, on the one hand, and intelligence and academic achievement, on the other; it is not clear whether there is a causal relationship.[90]

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 .[91] Some research suggests that myopia may be inherited from one's parents.[92]

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.[93] 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.[94]

Human population studies suggest that contribution of genetic factors accounts for 60%-90% of variance in refraction.[95][96][97][98] However, the currently-identified variants account for only a small fraction of myopia cases suggesting the existence of a large number of yet unidentified low-frequency or small-effect variants, which underlie the majority of myopia cases.[99]

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.[100] Form-deprived myopia (FDM) induced with a diffuser, like the goggles mentioned, shows significant myopic shifts.[101] Imposing retinal blur (or defocus) with positive (myopic defocus, that causes the image to be focussed in front of the retina) and negative lenses (hyperopic defocus, that causes the image to be focussed behind the retina) has also been shown to result in predictable changes in eye growth of various animal models, whereby the eye alters its growth to effectively eliminate the lens induced blur.[102][103][104][105] Anatomically, the changes in axial length of the eye seem to be the major factor contributing to this type of myopia.[106] Diurnal growth rhythms of the eye have also been shown to play a large part in FDM, and have been implicated in refractive error development of human eyes.[107] Chemically, daytime retinal dopamine levels drop about 30%.[108]

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.[109] 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.[110]

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.[111] 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.[112]

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.[108] 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.[113] Levodopa can be converted into dopamine in the presence of aromatic L-amino acid decarboxylase (L-AAAD).[114] L-AAAD activity in rat retinas is modulated by environmental light,[115] and this modulation is associated with dopamine D1 receptors[116] and alpha 2 adrenoceptors.[117] 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.[118] In rabbits, injections of dopamine prevented the myopic shift and vitreous chamber and axial elongation typically associated with FDM.[119] 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.[120] 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.[121] 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.[122] This toxic effect must be analyzed before treatment with levodopa for myopia to prevent damaging effects to these neurons.

See also

References

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