Refractive Surgery

Understanding Refractive Surgery
Refractive errors of the eye are caused by an imbalance in the light-bending power of the cornea (the window of the eye) and the natural lens inside the eye with respect to the length of the globe (the eyeball). When light is focused either too strongly or weakly, a clear image cannot fall upon the retina located on the inside back wall of the eye. The retina is the nervous tissue that transforms light energy into neural signals, and when the retina receives an unfocused image, it must send signals about an unfocused picture to the brain. The retina is much like the film in a camera; a healthy retina receiving poorly focused images is about as useless as a camera with expensive film but an out-of-focus lens; both provide only poor quality images. In recent years, our understanding of the optical properties of the cornea and dramatic improvements in both micro-surgical blades and lasers have permitted the development of painless, bloodless procedures lasting only minutes and designed to successfully reshape the cornea. This improves its natural abilities to focus light onto the retina without additional help from spectacles or contact lenses.

Who Might Benefit from Refractive Surgery
Patients who want neither spectacles nor contact lenses but have a refractive error might very much enjoy the freedom to see well without either device. Patients with moderately large refractive errors would probably notice the improvement and convenience the most, especially those with astigmatism.

Photorefractive Keratectomy
Photorefractive keratectomy, or PRK is a procedure that reshapes the cornea by removing the anterior surface with a laser so that the cornea becomes a weaker lens, thereby enabling the myopic patient to see more clearly without spectacles. In this procedure, the anterior surface epithelium of the cornea is removed either chemically or mechanically, and a special laser vaporizes the front of the stroma (the thickest layer corneal sublayer) until it reaches a desired, pre-calculated thickness. The procedure is bloodless and performed in the outpatient setting under local anesthesia. It is crucial that the refractive surgeon, a specially trained ophthalmologist, carefully measures the patient's cornea and evaluates the patient's pre-existing refractive error to ensure that the correct amount of stroma is removed. The patient is usually given an eyepatch after surgery along with eyedrops so that the corneal epithelium can regrow with minimal discomfort. Patients undergoing PRK usually can expect to see better than they ever had before without glasses within a week post-operatively. Some patients who have undergone PRK have also reported some glare and shifting of refraction over the long term. On the other hand, there is no evidence that the structural integrity of the globe with respect to later trauma is reduced.

Radial Keratotomy
Radial keratotomy, or RK, is a procedure where a microsurgery blade is used to cut radial or circumferential cuts into the cornea, thereby allowing the natural stresses on the cornea to be relieved and secondarily lessen the power of the cornea to bend light. This procedure is particularly useful for myopic and astigmatic patients. The blade cuts to 90% of the corneal thickness and is prevented from cutting through the entire thickness of the cornea by its special design. The surgery can be performed in an outpatient setting under local anesthesia and is essentially bloodless. The refractive surgeon will evaluate the patient before the surgery to explain the risks and benefits of the procedure and will make a series of careful measurements with a number of devices to ensure that the cuts are made in the correct locations on the cornea. One of these measurements is corneal topography, during which a map of the cornea is plotted to show exactly where and how much the cornea is curved. Most patients undergoing RK can expect to see better within hours after surgery than they had ever seen before without spectacles. Of course, there are some risks to this procedure. The eye may be more susceptible to infection, especially at the incision sites, and some patients complain of glare after surgery. The eye may also be more susceptible to rupturing from trauma. Finally, some studies have indicated the new refraction of the cornea may not be stable for at least one year but instead shifts gradually toward overcorrection.

LASIK
LASIK is an acronym for Laser Assisted In-Situ Keratomileusis, a procedure for improving vision in nearsighted and astigmatic individuals by removing a portion of their corneal stroma, the thick central layer of the cornea, without damaging the corneal epithelium. The laser used is a special new machine called the Excimer Laser; it actually vaporizes the stroma with incredible precision. The anterior surface of the cornea, the epithelium, is first removed by a mechanical knife that literally rolls across the cornea slicing the top layer off and leaving it attached by a hinge. After the laser removes the desired amount of stroma, the corneal epithelium is simply flipped back into place and adheres well. The recovery period for this procedure is shorter and more comfortable than that for PRK. However, the preparations that the refractive surgeon makes are as detailed as with any other procedure. Patients seem to do rather well with this procedure and the complications seem to be similar to those seen in other refractive procedures.

Epilasik
Epilasik is a variant of standard LASIK. The difference is in how one removes the top layer, (called the epithelium), of the cornea prior to reshaping the underlying cornea. In LASIK this is accomplished using a micro-keratome which removes the top surface of the cornea with a partial thickness cut using a razor sharp blade. Some people cannot have this done due to their cornea being too thin. In those cases, Epilasik can be tried. This typically uses a chemical, such as alcohol, to remove the epithelium, which allows for a much thinner section of cornea to be removed. Therefore, there is more underlying cornea on which to perform the actual refractive part of the surgery. This alternative to standard LASIK surgery typically has similar long term results to standard LASIK, but can have more postoperative discomfort and a longer healing time.

Intraocular contact lenses (ICLs)
Intraocular contact lenses (ICLs) are essentially contact lenses that are designed to be implanted in your eye rather than the normal placement of a lens on the cornea. These are designed to correct your refractive error without removing any normal corneal tissue which occurs in LASIK. However, since it is intraocular surgery (the eye is being penetrated full thickness), there are potentially more risks associated with ICLs than with normal contact lenses.

Who Might Reconsider Refractive Surgery
Refractive surgery is not for everybody. Those people with mild to moderate myopia might want to avoid this procedure, especially if they are in their 30's, because they would soon realize the need for reading glasses if they were corrected, where they might have only had to remove their distance glasses for reading if they remained uncorrected. Corneas with stable scars may benefit greatly from the laser procedures because there is evidence that some scars can be polished away. On the other hand, patients with chronic corneal disease may be putting themselves at risk for infection, ruptured globe, and at the least, unpredictable outcomes.