Myopia
Myopia, also known as near-sightedness and short-sightedness, is an eye condition where light from distant objects 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 myopia is associated with an increased risk of macular degeneration, retinal detachment, cataracts, and glaucoma.
Myopia results from 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 the use of cycloplegics during eye examination.
Myopia is less common in people who spent more time outside during childhood. This lower risk may be due to greater exposure to sunlight. Myopia can be corrected with eyeglasses, contact lenses, or by refractive surgery. Eyeglasses are the simplest and safest method of correction. Contact lenses can provide a relatively wider corrected field of vision, but are associated with an increased risk of infection. Refractive surgeries such as LASIK and PRK permanently change the shape of the cornea. Other procedures include implantable collamer lens placement inside the anterior chamber in front of the natural eye lens. ICL does not affect the cornea.
Myopia is the most common eye problem and is estimated to affect 1.5 billion people. Rates vary significantly in different areas of the world. Rates among adults are between 15% and 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 myopia is one of the most common causes of vision impairment globally along with cataracts, macular degeneration, and vitamin A deficiency.
Signs and symptoms
A person with myopia can see clearly out to a certain distance, 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.
Myopic individuals have larger pupils than far-sighted and emmetropic individuals, likely due to requiring less accommodation.
Causes
The underlying cause of myopia 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.
The role of corrective lenses interfering with emmetropization has also been suggested.
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 that increase the risk of myopia include insufficient light exposure, low physical activity, near work, and increased years 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.
Myopia is also more common in children with diabetes, childhood arthritis, uveitis, and systemic lupus erythematosus.
Other factors
Research indicates a relationship between body mass index and myopia, with both low and high BMI associated with an increased risk of developing myopia. A nationwide study of 1.3 million Israeli adolescents found that individuals with underweight status had higher chances of mild-to-moderate and high myopia compared to those with low-normal BMI.Similarly, a study involving Korean young adult men reported that those who were of average or shorter height and lean had a higher prevalence of high myopia.
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.
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. This is by refracting the eye with the use of cycloplegics such as atropine with responses recorded when accommodation is relaxed. Diagnosis of progressive myopia requires regular eye examination using the same method.Types
Myopia can be classified into two major types; anatomical and clinical. The types of myopia based on anatomical features are axial, curvature, index and displacement of refractive element. Congenital, simple and pathological myopia are the clinical types of myopia.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 : 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, sometimes, results from various medications, increases in glucose levels, nuclear sclerosis, oxygen toxicity 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 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.