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Myopia

Encyclopedia : M : MY : MYO : Myopia

Normal vision. Courtesy NIH National Eye Institute
Normal vision. Courtesy NIH National Eye Institute

The same view when myopic.
The same view when myopic.

Myopia, sometimes called near-sightedness or short-sightednessTerm "short-sightedness" is more frequently used as "lack of [foresight]", is a refractive defect of the eye in which collimated light produces image focus in front of the retina when accommodation is relaxed. This may be caused by a defect of the cornea (generally where the cornea protrudes too far outward from the eye) or from elongation of the eye structure itself (generally the eye is too long). Those with myopia typically can see nearby objects clearly but distant objects appear blurred. The opposite defect of myopia is hyperopia or "far-sightedness" or "long-sightedness" — this is where the cornea is too flat or the eye is too short.

Worldwide, myopia is the most common eyesight problem. About one quarter of the adult population in the United States is myopic. In some countries, such as Japan, Singapore and Taiwan, up to 44% of the adult population is myopic.

Mainstream ophthalmologists and optometrists most commonly correct myopia through the use of corrective lenses, such as glasses or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.

Contents

Classification

Myopia has been classified in various mannersGrosvenor T. "A review and a suggested classification system for myopia on the basis of age-related prevalence and age of onset." Am J Optom Physiol Opt. 1987 Jul;64(7):545-54. PMID 3307441. Goss et. al described the various forms of myopia based on their clinical appearanceGoss DA, Eskridge JB. "Myopia." In: Amos, JF, e. Diagnosis and management in vision care. Boston: Butterworths, 1988:445:

Clinical entity

Degree

Myopia has also been classfied by degree or severity. The strength or optical power of a corrective lens that enables the eye to focus distant images on the retina is measured in diopters. Myopia of –6.00 diopters or more is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment [link][link] or glaucoma [link] [link]. They are also more likely to experience floaters, shadow-like shapes which appear singly or in clusters in the field of vision.

Prevalence

A recent Australian study found that less than 1 in 10 (8.4%) children between the ages of 4 and 12 were myopic [link]. According to an American study published in Archives of Ophthalmology, nearly 1 in 10 children between the ages of 5 and 17 have myopia [link], and a recent Brazilian study found that nearly 1 in 8 (13.3%) of the students in one city were myopic [link].

A recent study involving first-year undergraduate students in the United Kingdom found that 50% of British whites and 53.4% of British Asians were myopic[link].

The prevalence of myopia in adults in the United States has been estimated to be approximately 25%[link], however, a study of Jordanian adults aged 17 to 40 found that over half (53.7%) were myopic [link].

Myopia is more common in Asians than in Whites, and more common in Whites than in Blacks. It is also more common than average in Jews.Arthur R. Jensen, The g Factor: The Science of Mental Ability (1998) ISBN 0275961036.

Pathogenesis

Theories

["Nearwork-induced transient myopia in preadolescent Hong Kong Chinese."] Invest Ophthalmol Vis Sci. 2003 May;44(5):2284-9. PMID 12714672.. This "genetic susceptibility" to environmental factors has been postulated as one explanation for the varying degrees of myopia among individuals or populationsHammond CJ, Andrew T, Mak YT, Spector TD. "A susceptibility locus for myopia in the normal population is linked to the PAX6 gene region on chromosome 11: a genomewide scan of dizygotic twins." Am J Hum Genet. 2004 Aug;75(2):294-304. Epub 2004 Jun 24. PMID 15307048, but there exists some difference of opinion as to whether it exists.Morgan I, Megaw P. Using natural STOP growth signals to prevent excessive axial elongation and the development of myopia. Ann Acad Med Singapore. 2004 Jan;33(1):16-20. PMID 15008556. High heritability simply means that most of the variation in a particular population at a particular time is due to genetic differences. If the environment changes — as, for example, it has by the introduction of televisions and computers — the incidence of myopia can change as a result, even though heritability remains high. From a little bit different point of view it could be concluded that — determined by heritage — some people are at a higher risk to develop myopia when exposed to modern environmental conditions with a lot of extensive near work like reading. In other words, it is often not the myopia itself, which is inherited, but the reaction to specific environmental conditions — and this reaction can be the onset and the progression of myopia.

  • Genetic Factors — A widely held theory of the cause of myopia is that it is mainly hereditary[[Citing sources citation needed]]. Measures of the heritability of myopia have yielded figures as high as 89%, and recent research has identified genes that may be responsible: defective versions of the PAX6 gene seem to be associated with myopia in twin studies [link]. Under this theory, the eye is slightly elongated front to back as a result of faults during development, causing images to be focused in front of the retina rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescence and then levels off as a person reaches adulthood. Genetic factors can work in various biochemical ways to cause myopia, a weak or degraded connective tissue is a very essential one. Genetic factors include an inherited, increased susceptibility for environmental influences like excessive near work, and the fact that some people do not develop myopia in spite of very adverse conditions is a clear indication that heredity is involved somehow in any case.
  • Environmental Factors — It has been suggested that a genetic susceptibility to myopia does not exist.["Lifestyle causes myopia not genes." New Scientist. 2004] A high heritability of myopia (as for any other condition) does not mean that environmental factors and lifestyle have no effect on the development of the condition.   Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak.  This theory in its pure form has been questioned since near work in certain cultures (e.g. Vanuatu) does not result in greater myopia[link][link][link][link]. This outcome suggests an environmental link to some other cause such as diet or over-illumination, changes in which seem to occur in Asian, Vanuatu and Inuit cultures acclamating to intensive early studies[link].  Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. Other theories suggest that the eyes become strained by the constant extra work involved in "nearwork" and get stuck in the near position, and eye exercises can help loosen the muscles up thereby freeing it for far vision. These primarily mechanical models appear to be in contrast to research results, which show that the myopic elongation of the eye can be caused by the image quality, with biochemical processes as the actuator. Common to both views is, however, that extensive near work and corresponding accommodation can be essential for the onset and the progression of myopia. A variation of this theory was touted by William Bates in the early 1900s. Bates claimed that with nearwork and other "stresses", the extraocular muscles would squeeze the eyeball causing it to elongate.
    • *Near work. Near work has been implicated as a contributing factor to myopia in many studies. New research from NSU College of Optometry shows that students exposed to extensive "near work" are at a higher risk of developing myopia, whereas taking summer or winter vacations (which amount to extended breaks from near work) will either reduce or stop myopic progression [link].
    

    *Abnormal breathing pattern. An abnormal breathing pattern causing lower than normal pressure pushing on the rear of the eyeball has been put forward as the cause of myopia.  An abnormal breathing pattern may arise as a result of stress or excessive near work.  This new theory, which holds that the respiratory system directly influences the visual system, is known as Scanlan's General Theory of Myopia.  [link].
    

    *Diet and nutrition - One 2002 [article] suggested that myopia may be caused by over-consumption of bread in childhood, or in general by diets too rich in carbohydrates, which can lead to chronic hyperinsulinemia. Various other components of the diet, however, were made responsible for contributing to myopia as well, as summarized in a [documentation].
    


    Relevant research
    

    • One Austrian study ("Eye elongation during accommodation in humans: differences between emmetropes and myopes" by Drexler et al) suggests that there is eye elongation during accommodation (the process by which the eye increases optical power to maintain a clear image focus.), but that it is caused by “accommodation-induced contraction of the ciliary muscle”, not “squeezing” of the extraocular muscles.
    • Numerous experiments with animals showed that myopia can be artificially generated either by reducing the image quality on the retina, or by applying minus glasses. The exact mechanism of this image-controlled elongation of the eye is still unknown.
    • A Turkish study found that accommodative convergence, rather than accommodation, may be a factor in the onset and progression of myopia in adults[link] [link].
    • A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creation of myopia[link].
    • A recent study found that the encircling band used in the repair of retinal detachments may elongate the eye and considerably increase myopia[link].
    • Children who slept with nightlights, small electrical light sources typically used during the night, during infancy are theorized to be at a significant higher risk of developing myopia, according to a University of Pennsylvania study [link]. Subsequent studies from other sources, including Ohio State University and New England College of Optometry in Boston have contradicted this conclusion. They claim that the original study did not control for hereditary factors (myopic parents are both more likely to have myopic children, and more likely to use night lights) [link].
    

    Myopia and IQ
    
Many studies have shown a relationship between myopia and IQ.  According to Arthur Jensen, myopes average 7-8 IQ points higher than non-myopes.  The relationship also holds within families, and siblings with a higher degree of refraction error average higher IQs than siblings with less refraction error. Jensen believes that this indicates myopia and IQ are pleiotropically related (both myopia and IQ are caused by the same genes). The mechanism that has caused a relationship between myopia and IQ is not yet known with certainty (Jensen, 1998).
    


    Presbyopia and the 'payoff' for the short-sighted
    
Many people with myopia are able to read comfortably without eyeglasses. Myopes considering refractive surgery are advised that this may be an advantage after the age of 40 when the eyes become presbyopic and lose their ability to accommodate or change focus.
    


    Diagnosis
    
A diagnosis of myopia is typically confirmed during an eye examination by an ophthalmologist or an optometrist. 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.
    


    Treatment, management, and prevention
    
Eyeglasses, contact lenses, and refractive surgery are the primary options to treat the visual symptoms of those with myopia. Orthokeratology is the practice of using special rigid contact lenses to flatten the cornea to reduce myopia.
    

Practitioners and advocates of alternative therapies often recommend eye exercises and relaxation techniques such as the Bates method[Eye Exercises for Myopia][Chinese Eye Exercises], however, the efficacy of these practices are disputed by mainstream eye care practitioners.
    

There is no universally accepted method of preventing myopia.American Optometric Association. [Optometric Clinical Practice Guideline: Care of the patient with myopia.] 1997. Some clinicians and researchers recommend plus power lenses in the form of single vision reading lenses or bifocals[The International Myopia Prevention Association]; however, a recent Malaysian study suggested that undercorrection of myopia caused more rapid progression of myopia.Chung K, Mohidin N, O'Leary DJ. "Undercorrection of myopia enhances rather than inhibits myopia progression." Vision Res. 2002 Oct;42(22):2555-9. PMID 12445849.
    


    See also
    


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