REFRACTIVE ERRORS IS THE TERM USED to refer to disorders that are related to how your eye focuses light. The word refraction is used to describe the way light is focused by your eye. The three factors that influence how your eye refracts (bends) rays of light are the curvature of your cornea, the power of your lens, and the length of your eye.

Normal eyesight is present when light rays strike
the retina, causing a clear image to form

Myopia (nearsightedness)

Myopia occurs when the curvature of the cornea is too steep and/or the length of the eye is too long. As the light rays pass through the steeper central cornea, they are refracted too much and come to a point of focus in front of the retina. This creates a blurred image. Patients with myopia experience blurred vision at a distance, but may see well close up.

The curvature of the cornea and the axial length of the eyeball are responsible for myopia. With accelerated orthokeratology, contact lenses are used to reduce the corneal curvature and thus the myopia, which improves unaided visual acuity.

Myopia is measured in terms of diopters of lens correction. Minus correction lenses are used to correct this condition. The rays entering a myopic eye are refracted too much and the light focuses in front of the retina. The minus lenses allow the light to be refracted less, causing the point of focus to move back to the retina. The greater the myopia, the higher the minus lens power necessary to achieve a clear focus on the retina. For example, -1.00 D= low myopia, -3.00 D= moderate myopia, -8.00 D= high myopia. A patient’s unaided visual acuity is directly related to the degree of myopia.

With myopia (nearsightedness), rays of light focus
in front of the retina instead of upon it, causing
distant objects to appear blurred

With hyperopia (farsightedness), rays of light focus behind the retiana

Astigmatism
Many patients with myopia or hyperopia have some degree of astigmatism. This means that your cornea, rather than being completely round in shape, similar to a ball, it is slightly oval and shaped more like a football. People with astigmatism experience blurred vision and sometimes distortion or tilting of images due to unequal bending of the rays of light entering their eyes. High degrees of astigmatism will cause blurred vision for both far and near objects. For example, the image observed on the backside of a spoon is quite distorted and demonstrates a very high degree of astigmatism. Astigmatism often causes eye strain and headaches as well as blurred vision.

With astigmatism, light entering the eye focuses
in multiple areas rather than in one location

Many people have at least a small amount of astigmatism. For many patients, their astigmatism has very little effect on their vision. Unfortunately, the word "astigmatism" is one of those words in the English language that almost everyone remembers and is frequently concerned about in other connotations. The good news is that many people with mild to moderate amounts of astigmatism can dramatically improve their vision with Orthokeratology.

Presbyopia
Presbyopia is a normal aging process. It occurs as the protein composition of the lens changes, making it rigid and less flexible. As the lens loses its ability to flex and becomes more rigid, it can no longer refract the light adequately to focus on near objects. The so-called near-point is then blurred. Though some patients show signs of this in their late thirties, Presbyopia typically occurs between age of 40 and 55 years, and continues to worsen through age sixty. When this occurs, people who already wear glasses may need bifocals, and those who have never worn glasses may require reading glasses.

The accelerated orthokeratology procedure has no effect on your eye’s focusing system, caused by this natural aging process due to presbyopia. Therefore, if you only need glasses for reading, refractive surgery or accelerated ortho-K is not likely to help you unless you choose to have correction for monovision (mentioned on the following page). Additionally, if you are mildly nearsighted (myopic) and in your forties, you may notice that while you cannot read clearly with your glasses on, you can read well without them. One advantage of mild myopia is the ability to remove your glasses and see well to read, even after the onset of presbyopia.

If you are nearsighted and obtain excellent distance vision after undergoing accelerated Orthokeratology, you will typically lose some detail in your near vision after forty with the onset of presbyopia. Like other normal sighted individuals, you will require reading glasses for small print.

One way to counteract the effects of presbyopia is to keep your non-dominant eye nearsighted after Orhokeratology. With your dominant eye, corrected for distance vision.The non-dominant eye is left with myopia so you may read well without lenses. Your brain will automatically use the non-dominant eye for reading and the fully corrected dominant eye for distance. This is called monovision. You should discuss this possibility with your eye care professional if you are in your late thirties or older.

Visual Acuity
You may hear your vision referred to as 20/20 or 20/40 or even 20/400. This is a measurement of your visual acuity using a Snellen Chart. The notation 20/40 means you can see at twenty feet what a normal sighted person sees at forty feet (see diagram). There are other variables that affect an interpretation of the eye chart, such as squinting, guessing at the letters, and room light.

Refractive Error
The lens power necessary to correct your refractive error is measured in diopters. This is the number used to determine your refractive treatment. An example prescription:

Sphere -1.50 D

Astigmatism -1.00 D

Axis X180 degrees

The first number in your prescription is called the sphere power. This represents the amount of nearsightedness ( - number) or the amount of farsightedness (+) In the example above -1.50 D is the amount of myopia.

The more nearsighted or farsighted you are, the greater the blur, and the higher your prescription is in diopters.

People who are nearsighted (myopic), focus their images in front of the retina located at the back of the eye. They require Concave, minus power lenses, either glasses or contact lenses that diverge the light rays farther back, resulting in a clearer focus on the retina. Myopic refractive errors and corrective lenses are designated by a minus sign in front of the lens. Example = -2.00 Diopters.

People who are farsighted (hyperopic) are lacking refractive or light bending power in their eyes. This lack of refractive power focuses their image behind the retina. They require convex positive lenses to help converge light. Farsighted refractive errors and corrective lenses are designated by a plus sign in front of the lens. Diopter power for example = + 2.00 Diopters.

Your prescription for glasses may be written in three numbers. Let’s take the prescription

-3.00 -1.25 X 90

The first number (-3.00) identifies your degree of nearsightedness or farsightedness. The sign is an indication of whether you are nearsighted (-) or farsighted (+).

The second number (-1.25) represents your amount of astigmatism. This can be written as either + or - astigmatism form.

The third number, 90 degrees, refers to the exact axis or orientation of the astigmatism. Your particular prescription is used by your doctor to help determine your prognosis (results) and prescribe your glasses or contact lenses.

Refractive Error & Visual Acuity

Refractive Error Myopia in Diopters

Approximage Unaided Visual Acuity

—.50 D —1.00 D —2.00 D —3.00 D —4.00 D —5.00 D —6.00 D —7.00 D —8.00 D —9.00 D —10.00 D

20/50 20/100 20/200 20/300 20/400 20/500 20/600 20/700 20/800 20/900 20/1000

These approximations apply to nearsighted (myopia) patients wearing glasses. After Orthokeratology—the unaided visual acuities are better than expected.

Non surgical Vision Correction Options
Before considering surgical vision correction, you should review the variety of different non surgical ways refractive disorders can be corrected. All have benefits and drawbacks.

Glasses

Glasses have been used to corrected refractive errors for hundreds of years. They are affordable, easy to maintain, and versatile. Glasses may, however, restrict peripheral vision and create a smaller or larger image size. They create fogging in certain weather conditions, causing a number of visual aberrations. They may interfere with certain occupations and recreational activities, and some people don’t like the cosmetic impact on their appearance.

Contact Lenses

Contact lenses are another common solution for the correction of refractive visual problems. Advantages include natural vision, no change in your cosmetic appearance, more freedom in recreational activities, and better peripheral vision. Rigid contact lenses have proven to slow the progression of myopia. On the other hand, contacts require more maintenance than glasses. Poorly maintained contact lenses may carry an increased risk of eye infection and a variety of comfort problems.

There are many types of contact lenses available today, As a result, the vast majority of people may be fit comfortably, regardless of their level of nearsightedness, farsightedness, or astigmatism.

Orthokeratology (reshaping the cornea). This procedure uses a series of therapeutic rigid, gas permeable contact lenses to reshape the cornea toward less curvature. The result is reduced myopia and astigmatism, and improved unaided visual acuity.