Ophthalmic Consultants
  Laser Vision Correction Center
    at the New York Eye and Ear Infirmary

How the eye works

General Information

Normal Vision
Normal Vision

The normal eye has the ability to receive pictures in the form of light and transmit those images to a part of the brain called the visual cortex, creating the picture that you see. The eye is composed of three major layers: the outside, or corneo-scleral layer; the blood vessel or vascular layer called the uvea; and the inner nerve layer called the retina. The cornea is transparent and is often called the window of the eye.

Understanding how a normal eye works will help you understand how an eye works which requires glasses or contact lenses to see clearly. A normal eye is also called emmetropic, meaning no correction with glasses or contact lenses is necessary. In this case, light goes through the cornea, into focus directly on the retina. The image is very clear and sharp; no blurriness is noted.

There are four major refractive conditions that can be responsible for an image not coming into focus on the retina, or being blurry: myopia or nearsightedness, hyperopia or farsightedness, astigmatism and presbyopia; of the conditions listed, the first three are correctable through refractive surgery.

Myopia

Myopia -- Nearsightedness
Myopia -- Nearsightedness

Myopia or nearsightedness is a condition in which images come into focus in front of the retina, resulting in a blurred image on the retina. The more severe the nearsightedness, the farther the image is from the retina thus resulting in blurrier vision in the distance.

What causes myopia? There are three ways for an eye to be myopic. One, the front surface of the eye, the cornea, is too curved and therefore too powerful. Two, the eye is too long. Three, a combination of both, the cornea is too curved and the eye is too long. The result is all the same, the image if focused in front of the retina and blurry distance vision.

To correct myopia, the excimer laser removes small amounts of cornea tissue with each pulse, flattening it centrally, allowing light rays entering the eye to focus sharply on the retina.

Hyperopia

Hyperopia -- Farsightedness
Hyperopia -- Farsightedness

Hyperopia means to be farsighted. What causes the eye to be hyperopic? If the cornea is too flat and therefore not strong enough, or if the eye is smaller than average, the image is going to come into focus "behind" the retina. Using the camera analogy: the optical system of the camera, our eye, is failing to make the picture come into focus on the film, our retina. Unlike myopia, an eye with this refractive condition usually is stable before the age of twenty.

Certain laser technologies such as holmium lasers and hyperopic LASIK may correct this condition.

The excimer laser corrects hyperopia by removing small amounts of cornea tissue with each pulse in the mid-periphery of the cornea, causing a relative steepening or strengthening of the central cornea. This allows light rays entering the eye to focus sharply on the retina.

Astigmatism

Astigmatism
Astigmatism

Astigmatism means that the cornea or window of the eye is not round or spherical. A cornea without astigmatism is shaped like the surface of a round ball; its surface is a sphere. A cornea with astigmatism is shaped more like a football, with two different curvatures, one steeper than the other. The flatter curve is weaker and therefore doesn't focus the image as much as the steeper, more powerful curve. The result is an eye with two different points of focus, giving you blurry vision. The image may not only be blurred, but may be seen as a doubled or overlapping image. This effect is also termed a ghost image.

Astigmatism can be found in combination with myopia or hyperopia and can be corrected through refractive surgery.

The excimer laser corrects astigmatism by removing small amounts of corneal tissue with each pulse in the areas of the cornea that are irregular, leaving a smooth, symmetrical corneal surface. This allows light rays entering the eye to focus sharply on the retina.

Presbyopia

Eye Section
Eye Section

Presbyopia is a condition, which comes with age, whereby a person who had previously been able to see near objects clearly, becomes progressively dependant on spectacles to maintain sharp near vision. This is a result of a loss of strength and elasticity of the muscles and natural lens inside the eye.

We are currently studying methods to strengthen the muscles / lens to overcome presbyopia, but unfortunately, the current excimer lasers are not able to correct this condition.

However, some patients find a treatment plan called monovision, in which one eye is corrected by a laser for distance and the other eye is corrected for near vision. How monovision will work for an individual patient is demonstrated at the laser vision correction evaluation.

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