Myopia ( Nearsightedness )
Myopia – also known as “shortsightedness” – is the ability to see things clearly up close, but not at a distance. It is the opposite of hyperopia (farsighted), which is another refractive vision disorder, along with astigmatism (distorted vision). Myopia is the most common vision disorder in the world, and an estimated one quarter of the U.S. population lives with poor distance vision.
How Nearsightedness Happens
Myopic people usually start to notice poor distance vision by the age of ten to fifteen. This inability to see objects at a distance is due to a mismatch between the eyeball’s length and its refractive power.
Each eye has a fixed length, front-to-back. This is the distance between the cornea and the retina. The lens and cornea both refract incoming light and between them should focus it on the retina.
When light comes from far objects, it needs less refraction to focus on the retina
When light comes from near objects, it needs more refraction to focus on the retina
In a myopic eye the cornea has a steep curvature, which makes it refract light at sharper angles. No cornea can change its own curvature as a lens can, so the myopic cornea is refracting light from far objects too much, given the fixed eyeball length. This focuses those images before the light arrives at the retina, and gives blurry distance vision.
But the steep curvature is good for near vision, and the myopic cornea is focusing those close-up images right on the retina and giving crisp near vision.
Correction for Shortsightedness
There are three main options for myopic patients:
- Glasses – which add a third refractive body in front of the cornea, designed to correct for the refractive error of each individual’s myopia
- Contacts – which also add a third refractive body, resting right on the corneal surface and functioning the same way as glasses
- Refractive Surgery – a reshaping of the corneal curvature to make it flatter so that it will refract light less sharply
Both techniques use the excimer laser to reshape the corneal curvature. The difference is that LASIK creates a surface flap on the cornea, folds it back out of the way, and replaces it afterwards. PRK creates no flap, but removes surface tissue altogether, and it must grow back afterwards. Both techniques work on the same corneal layer however – the stroma – which is a more stable layer than the surface epithelium. Each has its advantages and disadvantages.
Ophthalmologists may suggest PRK for a variety of reasons. Not everyone is a good LASIK candidate. You should discuss your options with an experienced eye surgeon to determine if LASIK or PRK surgery is right for you as a treatment for myopia.