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Anatomy, Physiology & Pathology of the Human Eye

The Cornea

The cornea is the clear covering of the front of the eye which bends (or refracts) light rays as they enter the eye. For clear vision to occur, the cornea must have the correct shape and power to focus incoming light rays precisely on the retina at the back of the eye. If the cornea is too steep, flat or irregular in shape, it cannot bend light at the angle needed to focus on the retina. As a result, eyeglasses or contact lenses may be needed to refract the incoming light rays at the angle needed for clear vision. In a nearsighted eye, for example, the cornea's shape causes incoming light rays to focus in front of the retina.

All corneal modification procedures are intended to alter the curvature of the cornea so that incoming light is refracted at an angle that allows it to focus precisely on the retina. This may eliminate the need for eyeglasses or contact lenses, or reduce the power of prescription lenses needed.

Those interested in any of the corneal modifications should first have a comprehensive eye examination to evaluate their eye health and determine their vision needs.

In the nearsighted eye, the cornea's shape causes incoming light rays to focus at a point in front of the retina.

A cornea of the appropriate shape bends (or refracts) incoming light rays so they focus precisely on the retina at the back of the eye.

Refractive Errors

The eye is like a camera. In a camera, light passes through a lens system back onto the film. The cornea and lens are at the front of the eye (anterior chamber) and are similar to a camera's lens system. The retina is located at the back of the eye (posterior chamber) and is similar to the camera's film. In the normal eye, light rays of an image pass through the cornea and the lens and are focused directly on the retina to produce a clear image.

Myopia (Nearsightedness)

In a myopic eye, the light rays are passed through the cornea and lens but the point at which they converge (focus) is in front of the retina, not on the retina. This configuration allows clear images of near objects but not those that are far away. Non-surgical treatment options for myopia include glasses and contact lenses. Surgical treatment options include ALK, clear lens extraction, LASEK, LASIK, epi-LASIK, phakic IOL and RK. While there are numerous surgical options available, not all individuals are good candidates for specific procedures. Patients should review these options in depth with their physicians prior to making any final decisions.

Read some of Dr. Chynn's articles on Myopia:

• Re-treatment one year after photorefractive keratectomy (PRK) for myopia
• How many myopes will have refractive surgery?

Hyperopia (Farsightedness)

In a hyperopic eye, the light rays do not converge or focus by the time they reach the retina. Hyperopic patients can focus on more distant objects, but not images that are close at hand. Non-surgical treatment options include glasses and contact lenses. Surgical treatment options include clear lens extraction, CK, LASIK, LTK, phakic IOL and PRK. While there are numerous surgical options available, not all individuals are good candidates for specific procedures. Patients should review these options in depth with their physicians prior to making any final decisions.

Read some of Dr. Chynn's articles on Hyperopia and treatments:

• Hyperopic LASIK
• Hyperopic PRK
• Contact Holmium Laser Thermokeratoplasty (LTK) for Hyperopia Surgery: 30 months follow-up
• Photorefractive keratectomy for the correction of hyperopia using the excimer laser VISX STAR
• Hyperopic LASIK: a comparison of the Nidek EC-5000, Chiron PlanoScan, and Kera Isobeam

Astigmatism

In the normal eye (without astigmatism), the cornea is curved the same in the horizontal and vertical directions, like a baseball. Without astigmatism, when light rays hit the cornea, they focus at a single point. With astigmatism, the curve of the cornea is not the same in the horizontal and vertical directions. The cornea looks like a football, with a steep curve on one side and a flat surface on the other. As a result, light rays entering the cornea do not focus at a single point, instead causing distorted vision. Many people with myopia or hyperopia have some degree of astigmatism.

Presbyopia

Presbyopia is a condition which is often confused with nearsightedness and farsightedness. This is a normal change in the eye in which people have increasing difficulty reading as they get older. Presbyopia affects people as they enter their 40s. Caused by changes within the eye's crystalline lens as one ages, presbyopia affects everyone -- eventually everyone will need to wear reading glasses or bifocals. As we get older, the lens of the eye becomes less flexible, preventing focusing for near vision. Presbyopia is not affected by the laser treatment.

Useful Links:

• www.eyecancer.com

Disclaimer: New York City eye surgeon Dr. Chynn's website is not intended to provide medical advice. It offers information to patients in Long Island, Manhattan and surrounding areas on Lasik eye surgery.