Astigmatism is a refractive error impacting the curvature of the cornea (the clear round dome covering the iris and pupil) or the shape of the lens (the transparent, flexible tissue located directly behind the iris and the pupil). This common refractive error affects one in three people and may occur in combination with near- or farsightedness. Both the cornea and lens help focus light rays sharply onto the retina in the back of the eye. When a person has astigmatism, the cornea or lens isn’t smooth or uniformly curved, thereby causing light rays to refract improperly. In sports terms, a normal cornea is curved like a basketball with the same degree of roundness everywhere. One with astigmatism is curved more like a football, with some areas that are steeper or rounder.
Astigmatism causes images to focus in front of and beyond the retina, resulting in both near and far objects appearing out of focus or distorted. It’s almost akin to looking into a fun house mirror in which a person sees themselves and others distorted – appearing too tall, wide, or thin. It is possible for astigmatism to affect the cornea and lens (mixed astigmatism). In such cases, the unequal curvature of the cornea and the lens can cause one meridian of the eye to be farsighted and a second meridian (perpendicular to the first) to be nearsighted.
Signs and Symptoms of Astigmatism
In addition to distorted or blurry vision, uncorrected astigmatism can lead to headaches, fatigue, squinting, pain in the muscles around the eye, and difficulty with night vision. When astigmatism is mild, it may not produce noticeable symptoms or require correction. Astigmatism can gradually worsen with age, most likely due to a combination of hereditary and environmental factors.
A 2011 study analyzed the prevalence of astigmatism by severity in more than 11,600 eyeglass wearers who were being fitted for soft contact lenses. The prevalence of patients with astigmatism of 0.75 and 1.00 diopters or greater in at least one eye was 47.4% and 31.8% respectively, and in both eyes, 24.1% and 15.0%, respectively. The prevalence of astigmatism of 0.75 diopters or greater was almost double in people with nearsightedness compared to those with farsightedness: 31.7% vs. 15.7%. This study provided insightful data on how many people would require special toric contact lenses for correction.
In an experimental study, uncorrected astigmatism was found to aggravate eye discomfort associated with reading text on computer screens. A study analyzing neck/shoulder discomfort related to computer use found many cases were temporary, however, some eye conditions exacerbated this discomfort. During visually demanding, cumulative computer work, researchers found astigmatism, extent of accommodation, and concurrent internal eye discomfort symptoms (e.g. dry eye, ache or strain) all increased neck/shoulder discomfort.
A previous study on 12 youngsters with normal vision analyzed the impact of induced astigmatism on reading a computer screen. The presence of induced astigmatism produced a significant increase in post-task symptoms, however, reading rates and the number of reading errors were unaffected. Researchers concluded the correction of small astigmatic refractive errors may optimize comfort for computer users.
Astigmatism Measurements and Correction
Astigmatism is measured in diopters, therefore an eye without any traces of astigmatism is measured at 0 diopters. Most people have 0.5 to 0.75 diopters of astigmatism. The last two numbers on an eyeglass prescription refer to astigmatism. Cylinder is the measurement indicating the extent of astigmatism and flatness or irregularity of the cornea shape. Axis is the measurement in degrees referring to the location on the cornea where the astigmatism occurs. Axis numbers range from 0 to 180.
People with a measurement of 1.5 diopters or more typically need eyeglasses or contact lenses to correct the imperfection in order to see clearly. Many people assume they cannot wear contacts if they have astigmatism.
In astigmatism, varying degrees of nearsightedness or farsightedness require toric lenses rather than regular soft contacts. Toric lenses have different meridians to correct refractive variances and a special design feature that enables the lens to rotate to the proper orientation on the cornea. This allows the power meridians of the lens to align with the appropriate meridians of the eye. To better understand meridians, envision the front of the eye as a clock face. A line connecting the 12 and 6 is one meridian; a line connecting the 3 and 9 is another. The steepest and flattest meridians of an eye with astigmatism are called principal meridians.
Testing Astigmatism at Home
Two types of tests can be performed with the EyeQue Personal Vision Tracker. The Practice Test is comprised of three measurements per eye, with the EyeQue Miniscope detached and attached on your smartphone. The Full Test measures astigmatism and generates EyeGlass Numbers that are the same professional measurements used for refractive correction. For each eye, nine measurements are conducted, each one corresponding to a different angle. Once you receive your vision data, you can use your EyeGlass Numbers to order glasses online.