A guide to understanding laser eye surgery

Thanks to laser eye surgery, people too blind to see their alarm clocks in the morning can walk into an ophthalmologist's office and have 20/20 vision 10 minutes later. And with recent advances in technology, the surgical doors have opened to people who were previously turned away due to large pupil size or extremely bad eyesight. Here's a guide to help you understand exactly what's involved.

Yes. Studies show that Lasik corrects vision impairments to 20/20 or better for most patients, with few risks.

• Lasik is the acronym for laser-assisted in situ keratomileusis, a type of surgery for correcting nearsightedness, farsightedness and astigmatism.
• After administering anesthetic eyedrops, the surgeon slices a flap in the outer layer of the cornea, leaving just enough tissue to act as a hinge. He uses a laser to reshape the cornea, then folds the flap back in place.
• Surveys show that more than 90 percent of patients are satisfied with the results of the procedure.
• In a recent study, post-operative vision correction was 20/20 in 51 percent of eyes and 20/40 or better in 97 percent of eyes. And one year after treatment, 93 percent of patients' vision was within the intended level of vision correction.
• The procedure appears to be quite safe: a study of more than 32,000 US Armed Forces members who underwent laser surgery found a vision loss of one line on an eye chart in one in 1,250 people. And recent data showed no cases of vision loss greater than two lines after 18,000 surgeries.
• Flap complications occur in about 0.2 percent of surgeries. A study conducted in Tokyo found that among 3,751 eyes, 15 required retreatment with a new flap during the six-month study period.

Some people experience side effects such as seeing starbursts or halos at night or not being able to see well in dim light. But the most common complication is dry eyes.

• In a study of 190 eyes, 20 percent were diagnosed with chronic dry eyes that persisted for six months or more.
• People who need the most correction are most likely to experience dry eyes.

Yes. Lasik surgery continues to become more refined — and effective. In wave-front guided Lasik, also called custom Lasik, the ophthalmologist takes several three-dimensional measurements of how your eye processes images, creating a customized map that will guide the laser in reshaping your eye.

• Each bump, pock and imperfection in your cornea is reflected on the map, and your cornea is reshaped to correct these irregularities.
• As a result, the custom surgery poses a lower risk of complications, such as halos and difficulty seeing in low light, while achieving better vision correction.
• A study found that the surgery produced 20/20 or better correction in 74 of 84 eyes treated.

One review of Lasik procedures found that results were influenced by the humidity in the room and the humidity and temperature outside.

• Surgeries performed in the summer were more likely to require second surgeries to tweak the outcome. Your surgical room should feel like an icebox. If you have the option, have the procedure on a cold winter day.

No. A review of several large studies found a greater risk of vision loss with contact lenses (thanks to infections, corneal scratches and so on) than with laser correction surgery.

Lasik surgery is an excellent alternative to glasses and contact lenses. Consider this guide before deciding if laser eye surgery is right for you.