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BACKGROUND
Astigmatism is a type of refractive error which is commonly seen in people. It is due to imperfection in the curvature of the eyes which leads to blurred vision and near vision. The front surface of the eye is egg shaped causing blurred vision at all times. It may coexist with myopia or hypermetropia and can go undiagnosed. The astigmatism can be corneal, retinal or lenticular or can be regular, oblique or irregular astigmatism.1
The symptoms include blurred or distorted vision, discomfort in eyes or eye strain, recurrent headaches, night vision difficulty and squinting. The blurred vision can be either horizontally, vertically or diagonally.2
The aim of the experiment is to perform examination of astigmatism.
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REQUIREMENTS
Subjects: Human
Apparatus: Snellen chart
Keratometer
Phoropter
Jackson cross cylinder
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PROCEDURE
The subject is seated comfortably in an eye examination chair. The following examinations are done for the diagnosis of astimagtism.3
Visual acuity
This is tested using Snellen chart. The visual acuity is tested which is given as a fraction. The top number is standardized testing distance and the bottom number is the smallest letter read on the chart by the subject.3
Keratometry
Keratometer is the device used to measure the curvature of the cornea.3
Refraction
The optometrist places different kinds of lenses in front of the subject’s eyes using a phoropter and estimates the focus of light. The retinoscope is used to evaluate the focusing power of the eyes of the subject.3
Jackson cross cylinder test
The cylinders of equal strength and opposite signs are placed at right angle to each other. This is mounted on a handle. The cross cylinders used is ±0.25 and ± 0.50 and the sphere is adjusted to the most plus or least minus which gives the best visual acuity. Hence the astigmatism is diagnosed and refinement of axis and cylindrical power as well.4
Astigmatism fan test
These are the series of radiating lines spaced at 10 degree arranged in the form of sunrays. In the center, there are “v” and perpendicular blocks which are mutual to each other. These are rotated through 180 degree simultaneously. Record the best visual acuity and the subject is asked to identify the clear lines on fan chart and directed towards Maddox arrow. The both blocks to be equally cleared when directed towards blocks and by adding negative cylinder at appropriate axis.4
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CONCLUSION
The diagnosis of astigmatism is important, so it can be treated at earliest with eyeglasses, contact lenses or orthokeratology.
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REFERENCES
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https://nei.nih.gov/healthyeyes/astigmatism. Accessed on 28 March 2018.
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https://www.mayoclinic.og/diseases-condition/astigmatism/symptoms-causes/syc-2035835. Accessed on 28 March 2018.
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https:www.aoa.org/patients-and public/eye-and-vision-problems/glossary-of –eye-and-vision-conditions/astigmatism. Accessed on 28 March 2018.
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https://www.slideshare.net/mobile/RohitRao2/ASTIGMATISM-31822434. Accessed on 29 March 2018.
