Astigmatism is a common eye condition. It is a type of refractive error that is diagnosed by the eye doctor and can be corrected with glasses, contacts and, occasionally, with surgery. The cause of astigmatism is an irregularly shaped cornea. The oblong shape of the cornea makes the retina focus on 2 different places, creating a blurred image. People with astigmatism often complain of distorted or blurry vision, headaches and the need to squint to focus on images. The condition is diagnosed by an optometrist or ophthalmologist through a series of tests that measure the shape of the cornea. This article will explain how your eye doctor uses these tests to measure astigmatism during an eye exam.
Visual Acuity Test – Look at a chart of numbers and letters when prompted by your doctor. Read the numbers on the distance chart of numbers and letters. This is called a visual acuity test. It tests for myopia (near-sightedness) and hyperopia (far-sightedness) as well as astigmatism. Astigmatism accompanies myopia or hyperopia 50 percent of the time. Blurry vision could be caused by astigmatism or one of these other common conditions.The results will be listed as a fraction, such as 20/20 or 6/6 in metric conversion, which is the visual acuity fraction for normal vision. The first number or numerator is the distance between you and the chart, usually 20 feet or 6 meters. The second number or denominator is the size of the letters on the last line you can read compared to normal visual acuity. A person with 20/40 vision has half the visual acuity of a person with 20/20 vision.
Keratometry Test – Take a test given by your doctor to measure the anterior surface of the cornea. This can be done by either a keratometer or a corneal topographer. Both pieces of equipment are carried by doctor’s offices.
Choose a doctor who has a corneal topographer if you want the most exact measurement of your cornea’s surface. The doctor will ask you to sit at a table and look into a bowl. This bowl’s inner surface is covered in a pattern of concentric rings. A digital camera sits at the base of the bowl and charts the cornea’s surface by measuring the reflected pattern in the eye. The data that the camera gathers can be constructed into a topographic map, with concentric circles in different colors showing the changes of elevation of the cornea. A large difference in shape and height could indicate an oblong cornea and astigmatism.Most corneal topography tests measure 8,000 to 10,000 points on the cornea, making it the most accurate keratometry test.
Refraction Test – Sit up straight and look forward, while your doctor places a retinoscope in front of your eyes. The retinoscope is a small, handheld instrument that measures light refraction on the eye. The doctor will move a large machine with lenses in front of your eyes that either clarify or obscure your vision. The doctor will keep the retinoscope trained on your eyes and it measures how your eyes focus light.If you have myopia, the light is refracted in a different direction than the light beam is traveling, and if you have hyperopia, the light is refracted in the same direction that the light beam is traveling. The refraction test helps to establish the placement of the irregular shape of the cornea. This will be called the “axis” in a corrective lens prescription.