Clear vision of objects depends on the proper power of the eye’s optical system. The cornea and the crystalline lens constitute the two lenses of the eye’s optical system responsible for focusing the image on the retina. Ophthalmology specialists affirm that when the power of this system is adequate, the image is focused exactly on the retina, and a clear vision is obtained.
Myopia, hyperopia and astigmatism
When the power of the optical system of the cornea and the crystalline lens is greater than normal, the image is focused in front of the retina, and a blurred vision is obtained. This is the defect called myopia. The opposite problem, i.e., a less powerful optical system than normal, and therefore, an image formation behind the retina, occurs in hyperopia.
On other occasions, the problem lies in the fact that the cornea is not a perfectly spherical surface, like a soccer ball, presenting different curvatures on its anterior surface that makes it look more like a rugby ball. In these cases, a distorted image of objects is also obtained: this is the problem called astigmatism. Very often, this defect is associated with one of the previous ones, i.e., most myopic or hyperopic eyes also present some degree of astigmatism.
All these refractive problems can be effectively corrected with an appropriate prescription, both in glasses and contact lenses. But nowadays there is also the possibility of correcting them by means of refractive surgery.
Laser refractive surgery
Refractive surgery comprises various surgical techniques developed to reduce dependence on glasses or contact lenses. The purpose of all these techniques is to introduce a series of changes in the eyeball in order to improve the focusing of the image on the retina without the need for correction, not to improve visual acuity.
With a few exceptions, refractive surgery is not a medical indication, so the decision to perform the procedure is left to the patient. The doctor’s role is reduced to providing objective and truthful information about possible risks and benefits so that the patient can decide freely, and to indicate the most appropriate technique for each case, since each of them has its indications depending on the degree of myopia or astigmatism of the patient.
In order to have access to any of these techniques, the patient must be over 21 years of age and the refractive defect must be relatively stable, without variation in the last 1 year.
Laser refractive surgery technique
The Excimer Laser is a technique that allows modifying the anterior surface of the cornea through the action of a high energy invisible light. By eliminating part of the most superficial layers of the cornea with this light, the anterior curvature of the cornea is reduced. The end result is a flatter cornea and, consequently, lower optical power. The light rays now form a focus closer to the retina, thus reducing or eliminating the need for glasses or contact lenses to obtain useful vision.
The Excimer Laser is indicated for:
- Low and moderate myopia, i.e., less than 7 diopters.
- Astigmatism, up to 4 diopters.
- Hyperopia, less than 4 diopters.
The surgical procedure used is called LASIK. In this procedure, a horizontal cut is made on the cornea with a special laser called FEMTSECOND laser. With this laser the incomplete resection of a corneal lenticule is performed, so that the lenticule can be everted as if it were the covers of a book, acting as a hinge. On the remaining cornea, a tissue removal (photoablation) is performed with another type of Excimer Laser to correct the desired number of diopters, after which the lenticule is placed back in its original place, where it is firmly adhered after carefully drying its edges, without the need for sutures.
The procedure, which lasts about 10 minutes, is performed on an outpatient basis with topical anesthetic drops. After the operation there may be some blurred vision and foreign body sensation, the duration of which usually does not exceed 24-48 hours.
Generally, useful vision is recovered in 24-48 hours, allowing a practically normal life. However, the maximum achievable vision may take 1 to 3 months to be achieved, depending on the extent of the initial defect. Usually, distance vision recovers sooner than near vision.
Risks of laser refractive surgery
The possible complications that may occur are: abnormal cutting of the corneal lenticule, corneal infection, corneal inflammation, and unwanted hyper- or undercorrection. The most serious, although exceptional, is corneal infection. Such complications are rare.
Occasionally, in situations of low luminosity, when the pupil dilates, the size of the optical zone carved by the laser may be exceeded. This may cause the patient to experience glare or halos around the lights, more intense during the immediate postoperative period, which usually improve with the passage of time.