Myopia overview
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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
Myopia (from Greek: μυωπία myopia "near-sightedness"[1]), also called near-or short-sightedness, is a refractive defect of the eye in which collimated light produces image focus in front of theretina when accommodation is relaxed.
Those with myopia see nearby objects clearly but distant objects appear blurred. With myopia, the eyeball is too long, or thecornea is too steep, so images are focused in the vitreous inside the eye rather than on the retina at the back of the eye. The opposite defect of myopia ishyperopia or "farsightedness" or "long-sightedness" — this is where the cornea is too flat or the eye is too short.
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 optical power (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye. In some cases, pinhole glasses are used by patients with low-level myopia. These work by reducing the blur circle formed on the retina.
Pathohysiology
There are currently two basic mechanisms believed to cause myopia: form deprivation (also known as pattern deprivation[2]) and optical defocus.[3]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.[3] 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.[4]
References
- ↑ http://www.etymonline.com/index.php?term=myopia
- ↑ http://arapaho.nsuok.edu/~salmonto/VSIII_2006/Lecture27.pdf
- ↑ 3.0 3.1 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.