near-sightedness near-sightedness near-sightedness, also known as myopia and short-sightedness, is an eye disease where light focuses in front of, instead of on, the retina. as a result, distant objects appear blurry while close objects appear normal. other symptoms may include headaches and eye strain. severe near-sightedness is associated with an increased risk of retinal detachment, cataracts, and glaucoma. near-sightednessother namesshort-sightedness, myopiadiagram showing changes in the eye with near-sightednessspecialtyophthalmology, optometrysymptomsdistant objects appear blurry, close objects appear normal, headaches, eye straincomplicationsretinal detachment, cataracts, glaucomacausescombination of genetic and environmental factorsrisk factorsnear work, greater time spent indoors, family historydiagnostic methodeye examinationpreventionunknowntreatmenteyeglasses, contact lenses, surgeryfrequency1.5 billion people (22%) the underlying mechanism involves the length of the eyeball growing too long or less commonly the lens being too strong. it is a type of refractive error. diagnosis is by eye examination. tentative evidence indicates that the risk of near-sightedness can be decreased by having young children spend more time outside. this decrease in risk may be related to natural light exposure. near-sightedness can be corrected with eyeglasses, contact lenses, or a refractive surgery. eyeglasses are the easiest and safest method of correction. contact lenses can provide a wider field of vision, but are associated with a risk of infection. refractive surgery permanently changes the shape of the cornea. near-sightedness is the most common eye problem and is estimated to affect 1.5 billion people (22% of the world population). rates vary significantly in different areas of the world. rates among adults are between 15% to 49%. among children, it affects 1% of rural nepalese, 4% of south africans, 12% of people in the us, and 37% in some large chinese cities. in china the proportion of girls is slightly higher than boys. rates have increased since the 1950s. uncorrected near-sightedness is one of the most common causes of vision impairment globally along with cataracts, macular degeneration, and vitamin a deficiency. etymology the term myopia is of koine greek origin: μυωπία myōpia 'short-sight' and μυωπίασις (myōpiasis) 'short-sight-ness'. it is derived from the 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)). the opposite of myopia in english is hyperopia, or far-sightedness. signs and symptoms near-sighted vision (top/left), normal vision (bottom/right) a near-sighted individual can see clearly out to a certain distance (the far point of the eye), but objects placed beyond this distance appear blurred. 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. onset is often in school children, with worsening between the ages of 8 and 15. causes the underlying cause is believed to be a combination of genetic and environmental factors. risk factors include doing work that involves focusing on close objects, greater time spent indoors, urbanization, and a family history of the condition. it is also associated with a high socioeconomic class and higher level of education. a 2012 review could not find strong evidence for any single cause, although many theories have been discredited. twin studies indicate that at least some genetic factors are involved. myopia has been increasing rapidly throughout the developed world, suggesting environmental factors are involved. a single-author literature review in 2021 proposed that myopia is the result of corrective lenses interfering with emmetropization. genetics a risk for myopia may be inherited from one's parents. genetic 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. 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. aberrant genetic recombination and gene splicing in the opnlw1 and opnmw1 genes that code for two retinal cone photopigment proteins can produce high myopia by interfering with refractive development of the eye. human population studies suggest that contribution of genetic factors accounts for 60–90% of variance in refraction. 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. environmental factors environmental factors which increase the risk of nearsightedness include insufficient light exposure, low physical activity, near work, and increased year of education. one hypothesis is that a lack of normal visual stimuli causes improper development of the eyeball. under this hypothesis, "normal" refers to the environmental stimuli that the eyeball evolved to. modern humans who spend most of their time indoors, in dimly or fluorescently lit buildings may be at risk of development of myopia. people, and children especially, who spend more time doing physical exercise and outdoor play have lower rates of myopia, 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 long hours of exposure to daylight on the production and the release of retinal dopamine. myopia can be induced with minus spherical lenses, and overminus in prescription lenses can induce myopia progression. overminus during refraction can be avoided through various techniques and tests, such as fogging, plus to blur, and the duochrome test. the near work hypothesis, also referred to as the "use-abuse theory" states that spending time involved in near work strains the intraocular and extraocular muscles. some studies support the hypothesis, while other studies do not. while an association is present, it is not clearly causal. nearsightedness is also more common in children with diabetes, childhood arthritis, uveitis, and systemic lupus erythematosus. mechanism 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: 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. 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. under rare conditions, edema of the ciliary body can cause an anterior displacement of the lens, inducing a myopia shift in refractive error. diagnosis a diagnosis of myopia is typically made by an eye care professional, usually an optometrist or ophthalmologist. during a refraction, 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. other types of refractive error are hyperopia, astigmatism, and presbyopia. types various forms of myopia have been described by their clinical appearance: simple myopia: myopia in an otherwise normal eye, typically less than 4.00 to 6.00 diopters. this is the most common form of 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. pseudomyopia is the blurring of distance vision brought about by spasm of the accommodation system. nocturnal myopia: without adequate stimulus for accurate accommodation, the accommodation system partially engages, pushing distance objects out of focus. nearwork-induced transient myopia (nitm): 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. instrument myopia: over-accommodation when looking into an instrument such as a microscope. induced myopia, also known as acquired myopia, results from various medications, increases in glucose levels, nuclear sclerosis, oxygen toxicity (e.g., from diving or from oxygen and hyperbaric therapy) or other anomalous conditions. sulphonamide therapy can cause ciliary body edema, resulting in anterior displacement of the lens, pushing the eye out of focus. elevation of blood-glucose levels can also cause edema (swelling) of the crystalline lens as a result of sorbitol accumulating in the lens. this edema often causes temporary myopia. scleral buckles, 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 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. degree the degree of myopia is described in terms of the power of the ideal correction, which is measured in diopters: myopia between −0.00 and −0.50 diopters is usually classified as emmetropia. low myopia usually describes myopia between −0.50 and −3.00 diopters. moderate 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 more. 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 in the field of vision. in addition to this, high myopia is linked to macular degeneration, cataracts, and significant visual impairment. age at onset myopia is sometimes classified by the age at onset: congenital myopia, also known as infantile myopia, is present at birth and persists through infancy. youth onset myopia occurs in 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. 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. a 2004–2015 singapore–sydney study of children of chinese descent found that time spent on outdoor activities was a factor. adult onset myopia early adult onset myopia occurs between ages 20 and 40. late adult onset myopia occurs after age 40. prevention various methods have been employed in an attempt to decrease the progression of myopia, although studies show mixed results. many myopia treatment studies have a number of design drawbacks: small numbers, lack of adequate control group, and failure to mask examiners from knowledge of treatments used. among myopia specialists, mydriatic eyedrops are the most favored approach, applied by almost 75% in north america and more than 80% in australia. a 2015 review suggested that increased outdoor time protects young children from myopia. a 2020 study of global practice patterns used by paediatric ophthalmologists to decrease the progression