Like HowStuffWorks on Facebook!

Lasik Surgery

Preoperative Visit
I gaze through the lenses of the phoreopter.
I gaze through the lenses of the phoreopter.

Before LASIK can be performed, you must have a thorough eye examination to ensure that you are an ideal candidate for the procedure. An ideal LASIK candidate meets the following criteria:

  • Vision correction - Your existing vision must fall within an acceptable correction range and must not have changed significantly within the last two years. The differences in vision are measured in diopters, which are degrees of prescription on a range that scales from -10.00 diopters, for severe myopia, to +4.00 diopters for severe hyperopia. A normal eye falls within the diopter range of - 0.50 to +0.50. Here are the diopter ranges that LASIK can treat: Myopia (-0.75 to -10.00) Hyperopia (+0.75 to +4.00) Astigmatism (+/- 0.75 to +/- 4.00)
  • Cornea thickness - The cornea must have a total thickness of 500 microns or greater, depending on ablation depth (how deep and how round the reshaping needs to be) and diopter range treated. The microkeratome creates a flap that is 160 microns thick and each diopter treated results in the removal of approximately 10 microns. To be considered a healthy treatment, the laser must leave about 250 to 300 microns of posterior (behind the flap) thickness after the procedure.
  • Pupil diameter - The diameter of the pupil ideally should be no more than 6.5 mm. However, advances in the laser technology can now work with diameters up to 8.5 mm.

In addition to the list above, certain conditions are considered too risky and can keep a person from being an ideal candidate:

  • Pregnancy - You are pregnant or attempting to become pregnant
  • Severe heart problems - Particularly if you must wear a pacemaker
  • Certain diseases - Auto-immune diseases (rheumatoid arthritis, lupus), vascular disease, eye diseases (severe glaucoma, cataracts, ocular herpes simplex), severe diabetes
  • Certain drugs - Acutane (acne), Imitrex (migraines), immune-system medications
  • Kerataconus - Condition characterized by a thinning of the cornea

It is very important that you meet the requirements for an ideal candidate. Otherwise, you greatly increase the chance for complications or poor results. Most reputable ophthalmologists will not perform the procedure on anyone who is not an ideal candidate. There are some unscrupulous doctors out there who will accept almost anyone as a LASIK patient. Many of these doctors perform LASIK for bargain rates, doing a hundred or more of the procedures a day! If you are interested in having LASIK done, be sure to research the ophthalmologist you select and check his or her success rate and patient references.

Your eyes are thoroughly checked during a scheduled pre-op (short for preoperative) visit. One thing that is done during your pre-op visit is a check to see what your current vision-correction prescription is. A device called a phoropter is used to check your prescription. Information about your existing prescription is entered into the phoropter, which combines lenses to match the prescription.

The doctor or an assistant will put together slightly different combinations of the lenses in the phoreopter while you are looking through the combined lenses at an eye chart. You will be asked which combination makes the image clearer. Then the doctor will change the combination of the one you select slightly and ask if it is better or worse. This process takes a few minutes but gives the doctor a very exact idea of what your level of vision correction is.

Another test that is performed is a manual check of the surface of the cornea for any discrepancies. Dr. Kelly uses Fluoracaine. This is a specially-formulated dye that is safe to use in the eye. A drop of Fluoracaine in the eye stains the cornea. When a blue light is shined into the eye, the Fluoracaine causes the cornea, normally clear, to glow. If there are any irregularities, a trained ophthalmologist can easily discern them.

Notice the blue glow of my eyes caused by the Fluoracaine.
Notice the blue glow of my eyes caused by the Fluoracaine.

Fluoracaine and the phoreopter and just a part of the pre-op process. On the next page, you will learn about the other amazing tools used to examine your eyes.