Signs and symptoms Edit

Near-sighted vision (top/left), normal vision (bottom/right) A myopic individual can see clearly out to a certain distance, but everything further becomes blurry. If the extent of the myopia is great enough, even standard reading distances can be affected. Upon routine examination of the eyes, the vast majority of myopic eyes appear structurally identical to nonmyopic eyes. In cases of high myopia, a staphyloma can sometimes be seen on fundoscopic examination. Because the most significant cause of myopia is the increase in axial length of the eye, the retina must stretch out to cover the increased surface area. As a result, the retina in myopic patients can become thin and might develop retinal holes and lattice degeneration in the periphery. High myopia increases the risk of retinal tears and detachment.

Causes Edit

A 2012 review could not find strong evidence for any single cause, although many theories have been discredited.[10] A 2015 review found that spending greater time looking at objects that are close appears to increase risk.[11] Because twins and relatives are more likely to get myopia under similar circumstances, a hereditary factor was suspected.[12] However, a hereditary nature of myopia has been ruled out by observing the experience of ethnicities transitioning to a modern (industrial and urban) lifestyle. For instance, around the 1960s, while the older generation of the Inuit in Canada had nearly no cases of near-sightedness, between 10% and 25% of the next generation was myopic. This result would be nearly impossible if genetics were an important causal factor. A relationship between the length of time of exposure to sunlight (by being outdoors) and a lesser incidence of myopia has been observed, which would explain the growth in incidence as people begin spending more time indoors.[13] Myopic shifts seen during growth spurts of childhood and adolescence, as well as in acromegaly, indicate a relationship between the timing of myopic development and the release of human growth hormone. However, the lack of correlation between height and myopia seems to suggest the relationship between human growth hormone and myopia is complex. Myopia has been increasing rapidly throughout the developed world, suggesting environmental factors must be important.[14] Quite similarly, the mechanisms of emmetropization are still unclear. Emmetropization is the process by which a child's eye grows and changes to become less hyperopic. The same triggers and signals that cause this growth are thought to play a role in the eye growing beyond the point of emmetropia and into myopia.

Research Edit

Normally, eye development is largely genetically controlled, but the visual environment has been shown to be an important factor in determining ocular development.[15] Some research suggests that some cases of myopia may be inherited from one's parents.[16] Genetics Edit Genetically, linkage studies have identified 18 possible loci on 15 different chromosomes that are associated with myopia, but none of these loci is 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.[17] 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.[18] Human population studies suggest that contribution of genetic factors accounts for 60-90% of variance in refraction.[19][20][21][22] 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.[23] Visual environment Edit 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.[24] Form-deprived myopia (FDM) induced with a diffuser, like the goggles mentioned, shows significant myopic shifts.[25] 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.[26][27][28][29] Anatomically, the changes in axial length of the eye seem to be the major factor contributing to this type of myopia.[30] 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.[31] Chemically, daytime retinal dopamine levels drop about 30%.[32] 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.[33] 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.[34] “Near work” hypothesis Edit 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.[3] While an association is present, it is not clearly causal.[3] "Visual stimuli" hypothesis Edit 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. The 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.[35] 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.[35] Experiments in the 1970s and 1980s where animals such as kittens and monkeys had their eyes sewn shut for long periods of time also showed eyeball elongation, demonstrating that complete lack of stimuli also causes improper growth trajectories of the eyeball.[36][37] Further research shows that people, and children especially, who spend more time doing physical exercise and outdoor activity have lower rates of myopia,[35][38][39][40] suggesting the increased magnitude and complexity of the visual stimuli encountered during these types of activities decrease myopic progression. There is preliminary evidence that the protective effect of outdoor activities on the development of myopia is due, at least in part, to the effect of daylight on the production and the release of retinal dopamine.[41][42] Other risk factors Edit 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.[43] Long hours of exposure to daylight appears to be a protective factor.[14][44] Lack of outdoor play could be linked to myopia.[45] Other personal characteristics, such as value systems, school achievements, time spent in reading for pleasure, language abilities, and time spent in sport activities all correlated to the occurrence of myopia in studies.[43][46][47]

Mechanism Edit

Because myopia is a refractive error, the physical cause of myopia is comparable to any optical system that is out of focus. Borish and Duke-Elder classified myopia by these physical causes:[48][49] Axial myopia is attributed to an increase in the eye's axial length. [50]

Refractive myopia is attributed to the condition of the refractive elements of the eye.[50] Borish further subclassified refractive myopia:[48] Curvature myopia is attributed to excessive, or increased, curvature of one or more of the refractive surfaces of the eye, especially the cornea. [50] In those with Cohen syndrome, myopia appears to result from high corneal and lenticular power. [51]

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.[50] As with any optical system experiencing a defocus aberration, the effect can be exaggerated or masked by changing the aperture size. In the case of the eye, a large pupil emphasizes refractive error and a small pupil masks it. This phenomenon can cause a condition in which an individual has a greater difficulty seeing in low-illumination areas, even though there are no symptoms in bright light, such as daylight.[52] Under rare conditions, edema of the ciliary body can cause an anterior displacement of the lens, inducing a myopia shift in refractive error.[53]

Diagnosis Edit

Prevention Edit

Treatment Edit

Epidemiology Edit

Global refractive errors have been estimated to affect 800 million to 2.3 billion.[92] The incidence of myopia within sampled population often varies with age, country, sex, race, ethnicity, occupation, environment, and other factors.[93][94] Variability in testing and data collection methods makes comparisons of prevalence and progression difficult.[95] 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.[94] Myopia is about twice as common in Jews than in people of non-Jewish ethnicity.[96] Myopia is less common in African people and associated diaspora.[93] In Americans between the ages of 12 and 54, myopia has been found to affect African Americans less than Caucasians.[97] Asia Edit 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. [98]

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%, and in college is more than 80%. [99]

In some areas, such as China and Malaysia, up to 41% of the adult population is myopic to 1.00 dpt, [100] and up to 80% to 0.5 dpt. [101]

and up to 80% to 0.5 dpt. A study of Jordanian adults aged 17 to 40 found over half (54%) were myopic. [102]

Some research suggests the prevalence of myopia in India in the general population is only 7%.[103][104] Europe Edit In first-year undergraduate students in the United Kingdom found 50% of British whites and 53% of British Asians were myopic. [105]

In Greece, the prevalence of myopia among 15- to 18-year-old students was found to be 37%. [103]

A recent review found 27% of Western Europeans aged 40 or over have at least −1.00 diopters of myopia and 5% have at least −5.00 diopters.[106] North America Edit 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%. Using the same method, in 1999–2004, myopia prevalence was estimated to have climbed to 42%.[107] A study of 2,523 children in grades 1 to 8 (age, 5–17 years) found nearly one in 10 (9%) have at least −0.75 diopters of myopia.[108] In this study, 13% had at least +1.25 D hyperopia (farsightedness), and 28% had at least 1.00-D difference between the two principal meridians (cycloplegic autorefraction) of astigmatism. For myopia, Asians had the highest prevalence (19%), followed by Hispanics (13%). Caucasian children had the lowest prevalence of myopia (4%), which was not significantly different from African Americans (7%).[108] A recent review found 25% of Americans aged 40 or over have at least −1.00 diopters of myopia and 5% have at least −5.00 diopters.[106] Australia Edit In Australia, the overall prevalence of myopia (worse than −0.50 diopters) has been estimated to be 17%.[109] In one recent study, less than one in 10 (8%) Australian children between the ages of four and 12 were found to have myopia greater than −0.50 diopters.[110] A recent review found 16% of Australians aged 40 or over have at least −1.00 diopters of myopia and 3% have at least −5.00 diopters.[106] South America Edit In Brazil, a 2005 study estimated 6% of Brazilians between the ages of 12 and 59 had −1.00 diopter of myopia or more, compared with 3% of the indigenous people in northwestern Brazil.[111] Another found nearly 1 in 8 (13%) of the students in the city of Natal were myopic.[112]

History Edit

The difference between the near-sighted and far-sighted people was noted already by Aristotle.[113] Graeco-Roman physician Galen first used the term "myopia" for near-sightedness.[113] Johannes Kepler in his Clarification of Ophthalmic Dioptrics (1604) first demonstrated that near-sightedness was due to the incident light focusing in front of the retina. Kepler also showed that near-sightedness could be corrected by concave lenses.[113] In 1632, Vopiscus Fortunatus Plempius examined a myopic eye and confirmed that myopia was due to a lengthening of its axial diameter.[114]

Society and culture Edit

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.[115] 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.[116] Correlations Edit Numerous studies have found correlations between myopia, on the one hand, and intelligence and academic achievement, on the other;[citation needed] it is not clear whether there is a causal relationship.[117] Myopia is also correlated with increased microsaccade amplitude, suggesting that blurred vision from myopia might cause instability in fixational eye movements.[118][119]

Etymology Edit

The term myopia is of Koine Greek origin: μυωπία myōpia (or μυωπίασις myōpiasis) "short-sight(-ness)", from Ancient Greek μύωψ myōps "short-sighted (man), (man) with eyes getting shut", from μύειν myein "to shut the eyes" and ὤψ ōps "eye, look, sight" ( GEN ὠπός ōpos).[120][121][122][123][124] The opposite of myopia in English is hyperopia (long-sightedness).

See also Edit