National Eye Institute ]]
, sometimes called '''near-sightedness''' or '''short-sightedness''', 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 is 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
Ophthalmologist s and
Optometrist s most commonly correct myopia through the use of
Corrective Lens es, such as
Glasses or
Contact Lens es. 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.
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:
- , also known as ''night myopia'' or ''twilight myopia'', is a condition in which the eye has a greater difficulty seeing in low illumination areas, even though its daytime vision is normal. Essentially, the eye's far point of an individual's focus varies with the level of light. Night myopia is believed to be caused by pupils dilating to let more light in, which adds aberrations resulting in becoming more nearsighted. A stronger prescription for myopic night drivers is often needed. Younger people are more likely to be affected by night myopia than the elderlyThe Eyecare Trust. '' Night Driving - The Facts. '' January 26, 2005..
Myopia has also been classfied by degree or severity. The strength or
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Floater s, shadow-like shapes which appear singly or in clusters in the
Field Of Vision .
A recent Australian study found that less than 1 in 10 (8.4%) children between the ages of 4 and 12 were myopic
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According to an American study published in ''Archives of Ophthalmology'', nearly 1 in 10 children between the ages of 5 and 17 have myopia
and a recent Brazilian study found that nearly 1 in 8 (13.3%) of the students in one city were myopic [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=16059562&query_hl=7 .
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
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The prevalence of myopia in adults in the United States has been estimated to be approximately 25%
{Link without Title} , however, a study of Jordanian adults aged 17 to 40 found that over half (53.7%) were myopic
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- — Among mainstream researchers and Eye Care Professional s, myopia is thought to be a combination of genetic and environmental factorsMorgan I, Rose K. "How genetic is school myopia?" ''Prog Retin Eye Res.'' 2005 Jan;24(1):1-38. PMID 15555525.. In China, myopia is more common in those with higher education backgroundXu L, Li J, Cui T, Hu A, Fan G, Zhang R, Yang H, Sun B, Jonas JB. "Refractive error in urban and rural adult Chinese in Beijing." ''Ophthalmology.'' 2005 Oct;112(10):1676-83. PMID 16111755. and some studies suggesting that nearwork may exacerbate a genetic predisposition to develop myopiaWolffsohn JS, Gilmartin B, Li RW, Edwards MH, Chat SW, Lew JK, Yu BS.
"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.
- — A widely held theory of the cause of myopia is that it is mainly ed 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.
- — 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 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 without Title} .
- 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 without Title} .
- 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 .
- 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 [http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowAbstract&ProduktNr=223858&Ausgabe=228159&ArtikelNr=55551 .
- A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creaton of myopia {Link without Title} .
- A recent study found that the encircling band used in the repair of retinal detachments may elongate the eye and considerably increase myopia {Link without Title} .
- 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 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 without Title} .
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
Pleiotropic ally 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). It has been speculated that myopes have until recently required either greater intelligence or assistance in order to survive to sexual maturity, and that as a result, they have evolved to be smarter.
Many people with myopia are able to read comfortably without eyeglasses. Myopes considering
Refractive Surgery are advised that this ability may be an advantage after the age of 40 when the eyes become
Presbyopic and lose their ability to
Accommodate or change focus.
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 .
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 , however, the efficacy of these practices are disputed by mainstream eye care practioners.
