Myopia

Near-sightedness, also known as short-sightedness and myopia, is a condition of the eye where light focuses in front of, instead of on, the retina.[1] This causes distant objects to be blurry while close objects appear normal.[1] Other symptoms may include headaches and eye strain.[1] Severe near-sightedness increases the risk of retinal detachment, cataracts, and glaucoma.[2]

The underlying cause is believed to be a combination of genetic and environmental factors.[2] Risk factors include doing work that involves focusing on close objects, greater time spent indoors, and a family history of the condition.[2][3] It is also associated with a high socioeconomic class.[2] The underlying mechanism involves the length of the eyeball growing too long or less commonly the lens being too strong.[1][6] It is a type of refractive error.[1] Diagnosis is by eye examination.[1]

There is tentative evidence that the risk of near-sightedness can be decreased by having young children spend more time outside.[4][7] This may be related to natural light exposure.[8] Near-sightedness can be corrected with eyeglasses, contact lenses, or surgery.[1] Eyeglasses are the easiest and safest method of correction.[1] Contact lenses can provide a wider field of vision; however are associated with a risk of infection.[1] Refractive surgery permanently changes the shape of the cornea.[1]

Near-sightedness is the most common eye problem and is estimated to affect 1.5 billion people (22% of the population).[2][5] Rates vary significantly in different areas of the world.[2] Rates among adults are between 15 and 49%.[3][9] Rates are similar in males and females.[9] Among children it affects 1% of rural Nepalese, 4% of South Africans, 12% of Americans, and 37% in some large Chinese cities.[2][3] Rates have increased since the 1950s.[9] Uncorrected near-sightedness is one of the most common causes of vision impairment globally along with cataracts, macular degeneration, and vitamin A deficiency.[9]

Normally eye development is largely genetically controlled, but it has been shown that the visual environment is 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 unclear if it is 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. 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.[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 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.[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]

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]

•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]

A diagnosis of myopia is typically made by an eye care professional, usually an optometrist or ophthalmologist. During a refraction, an autorefractor or retinoscope

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