Astigmatism is a vision condition that causes blurred vision due either to the irregular shape of the cornea, the clear front cover of the eye, or sometimes the curvature of the lens inside the eye. An irregular shaped cornea or lens prevents light from focusing properly on the retina, the light-sensitive surface at the back of the eye. As a result, vision becomes blurred at any distance.
Astigmatism is a very common vision condition. Most people have some degree of astigmatism. Slight amounts of astigmatism usually don’t affect vision and don’t require treatment. However, larger amounts cause distorted or blurred vision, eye discomfort, and headaches.
What Causes Astigmatism?
When the cornea or lens of an eye is irregularly shaped, vision may be out of focus at any distance.
Astigmatism occurs due to the irregular shape of the cornea or the lens inside the eye. The cornea and lens are primarily responsible for properly focusing light entering your eyes allowing you to see things clearly.
The curvature of the cornea and lens causes light entering the eye to be bent in order to focus it precisely on the retina at the back of the eye. In astigmatism, the surface of the cornea or lens has a somewhat different curvature in one direction than another. In the case of the cornea, instead of having a round shape like a basketball, the surface of the cornea is more like a football. As a result, the eye is unable to focus light rays to a single point causing vision to be out of focus at any distance.
Sometimes astigmatism may develop following an eye injury or eye surgery. There is also a relatively rare condition called keratoconus where the cornea becomes progressively thinner and cone-shaped. This results in a large amount of astigmatism resulting in poor vision that cannot be clearly corrected with spectacles. Keratoconus usually requires contact lenses for clear vision, and it may eventually progress to a point where a corneal transplant is necessary.
Farsightedness, or hyperopia, as it is medically termed, is a vision condition in which distant objects are usually seen clearly, but close ones do not come into proper focus. Farsightedness occurs if your eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly.
Common signs of farsightedness include:
- Difficulty in concentrating and maintaining a clear focus on near objects
- Eye strain
- Fatigue and/or headaches after close work
- Aching or burning eyes
- Irritability or nervousness after sustained concentration.
Common vision screenings, often done in schools, are generally ineffective in detecting farsightedness. A comprehensive optometric examination will include testing for farsightedness. In mild cases of farsightedness, your eyes may be able to compensate without corrective lenses. In other cases, your optometrist can prescribe eyeglasses or contact lenses to optically correct farsightedness by altering the way the light enters your eyes.
Nearsightedness, or myopia, as it is medically termed, is a vision condition in which close objects are seen clearly, but objects farther away appear blurred. Nearsightedness occurs if the eyeball is too long or the cornea, the clear front cover of the eye, has too much curvature. As a result, the light entering the eye isn’t focused correctly and distant objects look blurred.
Nearsightedness is a very common vision condition affecting nearly 30 percent of the U.S. population. Some research supports the theory that nearsightedness is hereditary. There is also growing evidence that it is influenced by the visual stress of too much close work.
Generally, nearsightedness first occurs in school-age children. Because the eye continues to grow during childhood, it typically progresses until about age 20. However, nearsightedness may also develop in adults due to visual stress or health conditions such as diabetes.
What causes nearsightedness?
If one or both parents are nearsighted, there is an increased chance their children will be nearsighted.The exact cause of nearsightedness is unknown, but two factors may be primarily responsible for its development:
- visual stress
There is significant evidence that many people inherit nearsightedness, or at least the tendency to develop nearsightedness. If one or both parents are nearsighted, there is an increased chance their children will be nearsighted. Even though the tendency to develop nearsightedness may be inherited, its actual development may be affected by how a person uses his or her eyes. Individuals who spend considerable time reading, working at a computer, or doing other intense close visual work may be more likely to develop nearsightedness.
Nearsightedness may also occur due to environmental factors or other health problems:
- Some people may experience blurred distance vision only at night. This “night myopia” may be due to the low level of light making it difficult for the eyes to focus properly or the increased pupil size during dark conditions, allowing more peripheral, unfocused light rays to enter the eye.
- People who do an excessive amount of near vision work may experience a false or “pseudo” myopia. Their blurred distance vision is caused by overuse of the eyes’ focusing mechanism. After long periods of near work, their eyes are unable to refocus to see clearly in the distance. The symptoms are usually temporary and clear distance vision may return after resting the eyes. However, over time constant visual stress may lead to a permanent reduction in distance vision.
- Symptoms of nearsightedness may also be a sign of variations in blood sugar levels in persons with diabetes or an early indication of a developing cataract.
How is nearsightedness diagnosed?
Testing for nearsightedness may use several procedures in order to measure how the eyes focus light and to determine the power of any optical lenses needed to correct the reduced vision.
A phoropter and retinoscope are often used to determine the lenses that allow the clearest vision during a comprehensive eye exam. As part of the testing, letters on a distance chart are identified. This test measures visual acuity, which is written as a fraction such as 20/40. The top number of the fraction is the standard distance at which testing is performed, twenty feet. The bottom number is the smallest letter size read. A person with 20/40 visual acuity would have to get within 20 feet to identify a letter that could be seen clearly at forty feet in a “normal” eye. Normal distance visual acuity is 20/20, although many people have 20/15 (better) vision.
Using an instrument called a phoropter, an optometrist places a series of lenses in front of your eyes and measures how they focus light using a hand held lighted instrument called a retinoscope. The doctor may choose to use an automated instrument that automatically evaluates the focusing power of the eye. The power is then refined by patient’s responses to determine the lenses that allow the clearest vision.
This testing may be done without the use of eye drops to determine how the eyes respond under normal seeing conditions. In some cases, such as for patients who can’t respond verbally, or when some of the eye’s focusing power may be hidden, eye drops may be used. They temporarily keep the eyes from changing focus while testing is performed.
Using the information obtained from these tests, along with the results of other tests of eye focusing and eye teaming, your optometrist can determine if you have nearsightedness. He or she will also determine the power of any lens correction needed to provide clear vision. Once testing is complete, your optometrist can discuss options for treatment.
How is Nearsightedness Treated?
Persons with nearsightedness have several options available to regain clear distance vision.
- Eyeglasses. Eyeglasses are the primary choice of correction for persons with nearsightedness. Generally, a single vision lens is prescribed to provide clear vision at all distances. However, for patients over about age 40, or children and adults whose nearsightedness is due to the stress of near vision work, a bifocal or progressive addition lens may be needed. These multifocal lenses provide different powers or strengths throughout the lens to allow for clear vision in the distance and also clear vision up close. Eyeglasses are frequently used to correct myopia.A large selection of lens types and frame designs are now available for patients of all ages. Eyeglasses are no longer just a medical device that provides needed vision correction, but can also be a fashion statement. They are available in a wide variety of sizes, shapes, colors, and materials that not only correct for vision problems but also may enhance appearance.
- Contact Lenses. For some individuals, contact lenses can offer better vision than eyeglasses. They may provide clearer vision and a wider field of view. However, since contact lenses are worn directly on the eyes, they require regular cleaning and care to safeguard eye health.
- Orthokeratology (Ortho-k). Orthokeratology, also known as corneal refractive therapy, involves the fitting of a series of rigid contact lenses to reshape the cornea, the front outer surface of the eye. The contact lenses are worn daily for limited periods, such as overnight, and then removed. Persons with moderate amounts of nearsightedness may be able to temporarily obtain clear vision for most of their daily activities.
- LASIK. Nearsightedness can also be corrected by reshaping the cornea using a laser beam of light. Two commonly used procedures are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). In PRK, a laser is used to remove a thin layer of tissue from the surface of the cornea in order to change its shape and refocus light entering the eye. There is a limit to how much tissue can safely be removed and therefore the amount of nearsightedness that can be corrected. LASIK does not remove tissue from the surface of the cornea but from its inner layers. To do this, a section of the outer corneal surface is cut and folded back to expose the inner tissue. Then a laser is used to remove the precise amount of corneal tissue needed to reshape the eye, and then the flap of outer tissue is placed back in position to heal. The amount of nearsightedness that LASIK can correct is limited by the amount of corneal tissue that can be removed in a safe manner.
- Intraocular Lenses. People who are highly nearsighted or whose corneas are too thin to allow the use of laser procedures now have another option. They may be able to have their nearsightedness surgically corrected by implanting small lenses in their eyes. These intraocular lenses look like small contact lenses and they provide the needed optical correction directly inside the eye.
- Vision therapy is an option for people whose blurred distance vision is caused by a spasm of the muscles which control eye focusing. Various eye exercises can be used to improve poor eye focusing ability and regain clear distance vision.
People with nearsightedness have a variety of options to correct their vision problem. In consultation with your optometrist, you can select the treatment that best meets your visual and lifestyle needs.
Presbyopia is a condition in which your eyes gradually lose the ability to see things up close. It is not a disorder or disease but rather a natural aging process of the eye. Presbyopia literally means “old eye” in Greek.
In order for our eyes to be able to see, light rays must be bent, or refracted, by the cornea and the lens so they can focus on the retina, the layer of light-sensitive tissue lining the back of the eye. The retina receives the picture formed by these light rays and sends the image through the optic nerve to the brain.
When we are young, the lens in our eye is flexible enough to change shape easily, making it possible to focus on near or distant objects. As we age, our lens becomes less flexible, making it more difficult for the eye to focus clearly on close objects. Reading and performing other close-up tasks such as threading a needle become very difficult.