Nearsightedness or myopia is a “refractive error” of the eye caused by a misshaped cornea. LASIK and PRK can correct nearsightedness with billionth of an inch accuracy so light is focused on the retina instead of in front of it.

A patient from Thousand Oaks told me she knew she was nearsighted, but wasn’t sure exactly what that meant. Nearsightedness, or myopia, occurs due to a flaw in the curvature of the cornea. Nearsightedness, along with fasightedness and astigmatism, are “refractive errors” of the eye — all caused by different shape flaws in the curvature of the cornea. Refractive eye surgery is surgery to reshape the cornea to correct these shape flaws.

Curvature is the basic characteristic of lenses that helps them focus light. This is because clear, curved substances — such as a raindrop or a magnifying lens — will focus light as light passes through them. By contrast, clear, flat substances which are not curved — such as a window pane — do not focus light as the light passes through. In fact, if a window pane is very clean, you might not even notice its presence since the light passes through undisturbed. Because curvature causes focus, the more curvature there is in a lens, the more powerfully a lens will focus light as it passes through it.

The job of the cornea is to help light entering the eye focus exactly on the retina in the back of the eye at a point called the fovea in the macula of the retina (the center of the retina). In a “perfect” eye — in someone who sees 20/20 or better — the light rays entering the eye are all focused purely on the fovea in the retina. In order for this to happen, the cornea needs to have precisely the proper curvature to focus the light to this point — and no where else. If there is too much curvature, then the person is nearsighted. In the case of nearsightedness, this excessive corneal curvature focuses the light rays too abruptly and they come to a point of focus before they reach all the way to the retina in the back of the eye. They come to focus too soon due to too much focusing power stemming from too much corneal curvature. Another word for nearsightedness is “shortsightedness” because the light rays focus short of their intended goal on the retina.

From a practical standpoint, nearsighted or myopic people need to hold things close up to see them without glasses or contacts. Typically their near vision is quite good, but their distance vision is poor — or even terrible! The more nearsighted someone is, the closer they hold things to see them without glasses or contact lenses.

Glasses or contact lenses work by putting the opposite curvature flaw (in the case of nearsightedness, a lens with not enough curvature) in front of the eye so that light will flow first through one curvature error (in the corrective lens) and then through the opposite curvature error (in the cornea) so they neutralize each other and the light focuses properly on the retina in the back of the eye. The strength of this “reverse” shape flaw in the glasses is measured in units called “diopters.” Nearsightedness is measured in negative or “minus” diopters while farsightedness is measured in positive or “plus” diopters.

In contrast to the approach of glasses or contacts which simply places the opposite flaw in front of the eye, LASIK and PRK laser vision correction surgery go to the heart of the matter and actually reshape the cornea to correct its inherent shape flaw. For this reason, when LASIK and PRK are used to treat nearsightedness or myopia, the excimer laser is used to precisely make the cornea flatter by removing excess curvature with tolerances of a billionth of an inch. The final, resulting shape of the cornea accurately focuses light on the retina on its own, without the need of putting corrective lenses in front of the eye.

See Also

Nearsightedness
Discussion of nearsightedness.

Myopia (Nearsightedness)
American Optometric Association discussion of nearsightedness, or myopia.

Nearsightedness
All About Vision discussion of nearsightedness.

Nearsightedness: What is Myopia
American Academy of Ophthalmology discussion of myopia or nearsightedness.

Myopia
Medical journal article in the Lancet about myopia, or nearasightedness.

What is LASIK? (FDA)
FDA webpage discussing LASIK and other refractive eye surgery options for the correction of myopia.