COMMON REFRACTIVE CONDITIONS

What is a Refractive Error?  Refractive error means that the shape of your eye does not bend light correctly, resulting in a blurred image. The main types of refractive errors are myopia (nearsightedness), hyperopia (farsightedness), presbyopia (loss of near vision with age), and astigmatism. Together these vision conditions are referred to as refractive errors because they affect how the eyes bend or “refract” light. 

ASTIGMATISM

What Is Astigmatism? 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.

The specific cause of astigmatism is unknown. It can be hereditary and is usually present from birth. It can change as a child grows and may decrease or worsen over time.The symptoms described above may not necessarily mean that you have astigmatism. However, if you experience one or more of these symptoms, contact your eye doctor for a complete exam.Symptoms

•    Distortion or blurring of images at all distances 
•    Headache and fatigue 
•    Squinting and eye discomfort or irritationWhat causes astigmatism?

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. Normally, the cornea is smooth and equally curved in all directions and light entering the cornea is focused equally on all planes or in all directions. 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.How is astigmatism diagnosed?

Astigmatism can be diagnosed through a comprehensive eye examination. Testing for astigmatism measures how the eyes focus light and determines the power of any optical lenses needed to compensate for reduced vision. This examination may include:

• Visual acuity—As part of the testing, you’ll be asked to read letters on a distance chart. This test measures visual acuity, which is written as a fraction such as 20/40. The t op number is the standard distance at which testing is done, twenty feet. The bottom number is the smallest letter size you were able to read. A person with 20/40 visual acuity would have to get within 20 feet of a letter that should be seen at forty feet in order to see it clearly. Normal distance visual acuity is 20/20.
• Keratometry—A keratometer is the primary instrument used to measure the curvature of the cornea. By focusing a circle of light on the cornea and measuring its reflection, it is possible to determine the exact curvature of the cornea’s surface. This measurement is particularly critical in determining the proper fit for contact lenses. A more sophisticated procedure called corneal topography may be performed in some cases to provide even more detail of the shape of the cornea.
• Refraction—Using an instrument called a phoropter, your optometrist places a series of lenses in front of your eyes and measures how they focus light. This is performed using a hand held lighted instrument called a retinoscope or 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.

Using the information obtained from these tests, your optometrist can determine if you have astigmatism. These findings, combined with those of other tests performed, will allow the optometrist to determine the power of any lens correction needed to provide clear, comfortable vision, and discuss options for treatment.How is astigmatism treated?

Persons with astigmatism have several options available to regain clear vision. They include:

• Eyeglasses
• Contact lenses
• Orthokeratology
• Laser and other refractive surgery procedures

Eyeglasses are the primary choice of correction for persons with astigmatism. They will contain a special cylindrical lens prescription to compensate for the astigmatism. This provides for additional lens power in only specific meridians of the lens. An example of a prescription for astigmatism for one eye would be -1.00 -1.25 X 180. The middle number (-1.25) is the lens power for correction of the astigmatism. The “X 180” designates the placement (axis) of the lens power. The first number (-1.00) indicates that this prescription also includes a correction for nearsightedness in addition to astigmatism.

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.

Soft contact lenses conform to the shape of the eye, therefore standard soft lenses may not be effective in correcting astigmatism. However, special toric soft contact lenses are available to provide a correction for many types of astigmatism. Because rigid gas permeable contact lenses maintain their regular shape while on the cornea, they offer an effective way to compensate for the cornea’s irregular shape and improve vision for persons with astigmatism and other refractive errors.

Orthokeratology (Ortho-K) involves the fitting of a series of rigid contact lenses to reshape the cornea, the front outer cover of the eye. The contact lenses are worn for limited periods, such as overnight, and then removed. Persons with moderate amounts of astigmatism may be able to temporarily obtain clear vision without lenses for most of their daily activities. Orthokeratology does not permanently improve vision and if you stop wearing the retainer lenses, your vision may return to its original condition.

Astigmatism can also be corrected by reshaping the cornea using a highly focused laser beam of light. Two commonly used procedures are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

Individuals with astigmatism have a wide range 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.

HYPEROPIA (FARSIGHTEDNESS)

What Is Hyperopia? 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.

Symptoms

• Eye strain, fatigue and/or headaches after close work
• Difficulty concentrating and maintaining clear focus on near objects
• Aching or burning eyes
• Crossing of the eyes in children (esotropia)

Tests and Diagnosis

Hyperopia can be diagnosed by a careful refraction by an eye doctor. Common vision screenings, often done in schools, are generally ineffective in detecting farsightedness.

For children and young adults, a cycloplegic refraction may be necessary to diagnose the correct amount of hyperopia. The focusing mechanism of the eye can mask the full amount of hyperopia but, during a cycloplegic refraction, eye drops are given to paralyze this mechanism so the true amount of hyperopia can be measured and corrected if needed.  Treatment

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.  Refractive surgery may be able to correct some forms of hyperopia.

MYOPIA (NEARSIGHTEDNESS)

What Is Myopia? 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.

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.

Symptoms

• Blurred vision
• Difficulty seeing distant objects like a movie or TV screen or the chalkboard in school.

What causes nearsightedness?

The exact cause of nearsightedness is unknown, but two factors may be primarily responsible for its development:

• Heredity
• 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 whom do an excessive amount of near vision work may experience false or “pseudo” myopia. Their blurred distance vision is caused by over use 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.
• Some genetic disorders, such as Marfan syndrome, have a high association with myopia.  Retinopathy of prematurity (ROP) is also a risk factor for development of myopia later in life.

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.

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.How is nearsightedness treated?

Persons with nearsightedness have several options available to regain clear distance vision. They include:

• Eyeglasses
• Contact lenses
• Orthokeratology
• Laser and other refractive surgery procedures
• Vision therapy for persons with stress-related nearsightedness

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.

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), 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.

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.

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 you visual and lifestyle needs.

PRESBYOPIA

What Is Presbyopia?  Presbyopia is the result of the normal aging of the lens inside your eye.  The crystalline lens of your eye loses its flexibility, which makes it difficult for you to focus on close objects. Presbyopia may seem to occur suddenly, but the actual loss of flexibility takes place over a number of years. It is a natural part of the aging process of the eye, not a disease.

The condition commonly occurs after age 40, when the lens of the eye becomes more rigid and does not flex as easily. As a result, it is more difficult to read at close range. Presbyopia can also occur along with myopia, hyperopia, or astigmatism.

Symptoms

• Near objects appear blurred
• Difficulty seeing objects up close
• Holding reading materials at arms length
• Eye fatigue and headaches when doing close work

How is Presbyopia treated?

To help you compensate for presbyopia, your optometrist can prescribe reading glasses, bifocals, trifocals or contact lenses. Presbyopia can complicate other common vision conditions like nearsightedness, farsightedness and astigmatism. Your optometrist will determine the specific lenses to allow you to see clearly and comfortably at both near and far. You may only need to wear your glasses for close work like reading, but you may find that wearing them all the time is more convenient and beneficial for your vision needs.

Because the effects of presbyopia continue to change the ability of the crystalline lens to focus properly, periodic changes in your eyewear may be necessary to maintain clear and comfortable vision.