Myopia pathophysiology
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2] Associate Editor(s)-in-Chief: Saumya Easaw, M.B.B.S.[3]
Overview
There are currently two basic mechanisms believed to cause myopia: form deprivation (also known as pattern deprivation[1]) and optical defocus.[2] Form deprivation occurs when the image quality on the retina is reduced; optical defocus occurs when light focus in front of or behind the retina. Numerous experiments with animals have shown that myopia can be artificially generated by inducing either of these conditions. In animal models wearing negative spectacle lenses, axial myopia has been shown to occur as the eye elongates to compensate for optical defocus.[2] The exact mechanism of this image-controlled elongation of the eye is still unknown. It has been suggested that accommodative lag leads to blur (i.e. optical defocus) which in turn stimulates axial elongation and myopia.[3]
Pathophysiology
Theories
- Combination of genetic and environmental factors — In China, myopia is more common in those with higher education background[4]and some studies suggest that near work may exacerbate a genetic predisposition to develop myopia.[5] Other studies have shown that near work (reading, computer games) may not be associated with myopic progression, however[6]. A "genetic susceptibility" to environmental factors has been postulated as one explanation for the varying degrees of myopia among individuals or populations,[7] but there exists some difference of opinion as to whether it exists.[8] 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 slightly 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 — The wide variability of the prevalence of myopia in different ethnic groups has been reported as additional evidence supporting the role of genetics in the development of myopia.[9] 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 [4]. 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. 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. Some recommend a variety of eye exercises to strengthen the ciliary muscle. 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.One Austrian study confirmed that the axial length of the eye does mildly increase while reading, but attributed this elongation due to contraction of the ciliary muscle during accommodation (the process by which the eye increases optical power to maintain a clear image focus), not “squeezing” of the extraocular muscles.[10]Near work and nightlight exposure in childhood have been hypothesized as environmental risk factors for myopia.[11] Although one initial study indicated a strong association between myopia and nightlight exposure,[12] recent research has found none.[11][13][14][15]
- Near work. Near work has been implicated as a contributing factor to myopia in some studies, but refuted in others[16]. One recent study suggested that students exposed to extensive "near work" may be at a higher risk of developing myopia, whereas extended breaks from near work during summer or winter vacations may retard myopic progression [5]. Near work in certain cultures (e.g. Vanuatu) does not result in greater myopia[6][7][8][9]. It has been hypothesized that this outcome may be a results of genetics or environmental factors such as diet or over-illumination, changes in which seem to occur in Asian, Vanuatu and Inuit cultures acclamating to intensive early studies[10].
- 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.
- Stress has been postulated as a factor in the development of myopia.[17]
Relevant Research
- A Turkish study found that accommodative convergence, rather than accommodation, may be a factor in the onset and progression of myopia in adults.[18]
- A recent Polish study revealed that "with-the-rule astigmatism" may lead to the creation of myopia.[19]
References
- ↑ http://arapaho.nsuok.edu/~salmonto/VSIII_2006/Lecture27.pdf
- ↑ 2.0 2.1 Saw SM, Gazzard G, Au Eong KG, Tan DT. "Myopia: attempts to arrest progression." Br J Ophthalmol. 2002 Nov;86(11):1306-11. PMID 12386095.
- ↑ Schor C. "The influence of interactions between accommodation and convergence on the lag of accommodation." Ophthalmic Physiol Opt. 1999 Mar;19(2):134-50.PMID 10615449.
- ↑ Xu 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.
- ↑ Wolffsohn 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.
- ↑ Saw S, Tong L, Chua W, Chia K, Koh D, Tan D, Katz J (2005). "Incidence and progression of myopia in Singaporean school children". Invest Ophthalmol Vis Sci. 46 (1): 51–7. PMID 15623754.
- ↑ Hammond 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
- ↑ 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
- ↑ Saw SM, Katz J, Schein OD, Chew SJ, Chan TK. "Epidemiology of myopia." Epidemiol Rev. 1996;18(2):175-87. PMID 9021311.
- ↑ Drexler W, Findl O, Schmetterer L, Hitzenberger CK, Fercher AF. "Eye elongation during accommodation in humans: differences between emmetropes and myopes."Invest Ophthalmol Vis Sci. 1998 Oct;39(11):2140-7. PMID 9761293.
- ↑ 11.0 11.1 Saw SM, Wu HM, Hong CY, Chua WH, Chia KS, Tan D. "Myopia and night lighting in children in Singapore." Br J Ophthalmol. 2001 May;85(5):527-8. PMID 11316706.
- ↑ Quinn GE, Shin CH, Maguire MG, Stone RA. "Myopia and ambient lighting at night." Nature. 1999 May 13;399(6732):113-4. PMID 10335839.
- ↑ Zadnik K, Jones LA, Irvin BC, Kleinstein RN, Manny RE, Shin JA, Mutti DO. "Myopia and ambient night-time lighting. CLEERE Study Group. Collaborative Longitudinal Evaluation of Ethnicity and Refractive Error." Nature. 2000 Mar 9;404(6774):143-4. PMID 10724157.
- ↑ Gwiazda J, Ong E, Held R, Thorn F. "Myopia and ambient night-time lighting." Nature.2000 Mar 9;404(6774):144. PMID 10724158.
- ↑ Guggenheim JA, Hill C, Yam TF. "Myopia, genetics, and ambient lighting at night in a UK sample." Br J Ophthalmol. 2003 May;87(5):580-2. PMID 12714399.
- ↑ Saw S, Tong L, Chua W, Chia K, Koh D, Tan D, Katz J (2005). "Incidence and progression of myopia in Singaporean school children". Invest Ophthalmol Vis Sci. 46 (1): 51–7. PMID 15623754.
- ↑ Bowan M. "Stress and Eye: New Speculations on Refractive Error." J. Behavioral Optom. 7(5)115-22, 1996.
- ↑ Bayramlar H, Cekic O, Hepsen IF. "Does convergence, not accommodation, cause axial-length elongation at near? A biometric study in teens." Ophthalmic Res. 1999;31(4):304-8. PMID 10325546.
- ↑ Czepita D, Filipiak D. [The effect of the type of astigmatism on the incidence of myopia]. Klin Oczna. 2005;107(1-3):73-4. PMID 16052807.