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Astigmatism is an affliction of the eye, where vision is blurred by an irregularly shaped cornea or eye which causes distortions in the light reaching the retina. The cornea, instead of being shaped like a sphere, is more oval-like and reduces the cornea's ability to focus light. Astigmatism is a refractive error of the eye in which there is a difference in degree of refraction in different meridians. So, rather than the surface topology of the eye being spherical, it has two curves, one steeper than the other, in a similar way to the shape of an oval. The condition is typically characterized by an aspherical, non-figure of revolution cornea in which the corneal profile slope and refractive power in one meridian is greater than that of the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can often be corrected by glasses with a cylindrical lens (i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.
Astigmatism occurs when either the cornea or the lens of the eye is not perfectly smooth or round. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retina in the horizontal (sagittal) plane, but not in front of the retina in the vertical (tangential) plane.
In some cases vertical lines (e.g., walls) may appear to the patient to be leaning over like the tower of Pisa.
Types of astigmatismEdit
Based on asymmetry of structureEdit
- Corneal astigmatism - astigmatism due to an irregularly shaped cornea (like an American football or rugby ball instead of a soccer ball)
- Lenticular astigmatism - astigmatism due to an irregularly shaped lens
Based on Axis of the Principal MeridiansEdit
- Regular astigmatism
- Against-the-rule astigmatism
- With-the-rule astigmatism
- Oblique astigmatism
- Irregular astigmatism
Based on focus of the principal meridiansEdit
- Simple astigmatism
- Simple hyperopict astigmatism
- Simple myopic astigmatism
- Compound astigmatism
- Compound hyperopic astigmatism
- Compound myopic astigmatism
- Mixed astigmatism
According to an American study published in Archives of Ophthalmology, nearly 3 in 10 children between the ages of 5 and 17 have astigmatism . A recent Brazilian study found that 34% of the students in one city were astigmatic . Regarding the prevalence in adults, a recent study in Bangladesh found that nearly 1 in 3 (32.4%) of those over the age of 30 had astigmatism.
A number of studies have found that the prevalence of astigmatism increases with age.
Signs and testsEdit
There are a number of tests used by ophthalmologists and optometrists during eye examinations to determine the presence of astigmatism and to quantify the amount and axis of the astigmatism. A Snellen chart or other eye chart may initially reveal reduced visual acuity. A keratometer may be used to measure the curvature of the steepest and flattest meridians in the cornea's front surface. A corneal topographer may also be used to obtain a more accurate representation of the cornea's shape. An autorefractor or retinoscopy may provide an objective estimate of the eye's refractive error and the use of Jackson cross cylinders in a phoropter may be used to subjectively refine those measurements  . An alternative technique with the phoropter requires the use of a "clock dial" or "sunburst" chart to determine the astigmatic axis and power.
Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another. In those with keratoconus, rigid gas permeable contact lenses often enable patients to achieve better visual acuities than eyeglasses. If the astigmatism is caused by a problem such as deformation of the eyeball due to a Chalazion, treating the underlying cause will resolve the astigmatism.
- ↑ http://www.eyetopics.com/articles/45/1/Astigmatism
- ↑ http://www.medicinenet.com/astigmatism/article.htm
- ↑ http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20symptoms.htm
- ↑ http://www.hipusa.com/eTools/webmd/A-Z_Encyclopedia/astigmatism%20treatment.htm
- ↑ http://www.stlukeseye.com/eyeq/Keratometry.asp
- ↑ http://www.emedicine.com/OPH/topic711.htm
- ↑ http://www.quantumoptical.com/onlinecourses/nysso/brp/slide1.asp?courses=19
- ↑ http://www.nova.edu/hpd/otm/nbeo/refract1.htm
See also Edit
- MedlinePlus article
- AllAboutVision.com article
- VisionSimulations.com | What the world looks like to people with various diseases and conditions of the eye
- Medical Info on Astigmatism
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