Eyeglasses or contact lenses alter the way light enters the eye to correct astigmatism.
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Astigmatism is a condition in which the eye does not focus images appropriately, often reducing the clarity of vision. There is a strong genetic component to astigmatism, but the condition can also be caused by injury or eye surgery. Astigmatism becomes more prevalent with age and is more common in people with nearsightedness or farsightedness. Astigmatism that is severe enough to affect visual clarity can be corrected with eyeglasses, contact lenses, refractive surgery or lenses implanted during cataract surgery.
Understanding the anatomy of the eye is helpful in understanding the effect of astigmatism on vision. The cornea is a clear dome at the front of the eye that allows light to pass to the inside of the eye. It converges or bends the light waves to help focus the image on the retina -- a layer of vision-perceiving cells at the back of the eye. The cornea accounts for two-thirds of the eye's refractive power, or ability to focus visual input. The lens is a clear, elastic membrane that sits behind the pupil -- the dark circle in the middle of the colored iris. It also helps focus images onto the retina and accounts for the remaining third of the eyes' refractive power. The macula is a small area of the retina with the best vision.
Focusing of Light
Ideally, the cornea and lens are spherical, or shaped like a perfect ball. This allows light waves to be focused on a single point in the macula so vision is clear. In the case of astigmatism, the cornea, lens or both are cylindrical or tonic, meaning they are more oblong in shape, like a football. This causes the light waves to spread out across the macula or other areas of the retina. The result is blurry, distorted vision. The more oblong the cornea or lens are, the more severe the astigmatism. Eye-care professionals measure the amount of distortion in diopters to determine whether correction is required and, if so, the level of correction needed. Mild astigmatism may not require correction, as it has little effect on visual clarity. According to the authors of a January 2014 article in the "Journal of Cataract and Refractive Surgery," astigmatism less than 0.5 diopters typically does not degrate visual clarity significantly and usually does not require correction.
Types of Astigmatism
In regular astigmatism, the cornea or lens may be cylindrical along different axes or directions, which will have different effects on vision. If the cylindrical shape occurs horizontally -- like a football lying down -- vision is more blurry in the vertical direction. If the cylindrical shape occurs vertically -- like a football standing on end -- vision is more blurry in the horizontal direction. The cylindrical shape may also occur in an oblique direction between the horizontal and vertical axis, causing general blurriness to vision. All of these can result in difficulty seeing fine detail. For example, it may be difficult to distinguish similar letters and numbers, such as B and 8 or P and F. Irregular astigmatism occurs when scarring on the cornea or a defect in the lens causes scattering of light. Special lenses are required for correction of irregular astigmatism, because it is not a simple correction of the curvature of the cornea or lens.
Other Signs and Symptoms
In addition to blurred vision, other signs and symptoms may occur with astigmatism. Eye stress or strain, squinting, poor night vision, eye irritation, headaches, fatigue and difficulty focusing while reading are common complaints. Symptoms are often worse when looking at high contrast items, such as black text on a white background. These symptoms can also be associated with other conditions, so check with an optometrist or ophthalmologist to determine if astigmatism is the cause and whether treatment is required.