Definition

Corneal laser refractive surgery is aimed at modifying the optical properties of the eye by reshaping the profile of the corneal tissue.

The cornea is the transparent tissue that makes up the front of the eyeball and consists of three overlapping layers: the epithelium, the stroma and the endothelium, the innermost one. The excimer laser acts on the central optical zone of the corneal stroma, redesigning the profile and modifying the refractive power in such a way as to focus the light rays on the retina, without the aid of additional optical lenses.

Symptomatology

The reasons that can induce the myopic, the hyperopic or the astigmatic to request the surgical correction of the defect are multiple, first the need for an optical aid (the glasses or contact lenses) to be able to move or do anything.

Among the clinical reasons is anisometropia, a condition in which the two eyes have a marked difference in refractive defect that cannot be adequately corrected with glasses. As with any other medical-surgical intervention, the patient who wants to undergo photorefractive corneal surgery treatment must meet certain requirements.
The refractive error must be stable for at least one year and since it is common experience of the ophthalmologist to observe a modification of the defect up to 21 years, wait at least this age. The corneal thickness of the optical zone must be greater than 450 µm (average normal values 500-520 µm).
The patient must be free of any acute or chronic ocular pathologies: such as keratoconus, glaucoma, macular degeneration; the patient must not be affected by systemic pathologies or immune pathogenesis.

Therapy

The application of the excimer laser can be carried out by different techniques:

• PRK (photorefractive keratectomy) is based on the ablation of the anterior stroma after removal of the corneal epithelium, a layer that regenerates in the first days of the post-operative course. The intervention lasts a few minutes, including the preparation of the patient, that is the anesthesia of the corneal surface through eye drops and corneal deepithelialization. At the end of the intervention, a therapeutic contact lens is applied to the corneal surface, with the dual purpose of promoting epithelial regeneration and reducing the painful symptoms of the first days. The lens will be removed after three to five days.
• In Laser In Situ Keratomileusis (LASIK) the excimer laser acts on a deeper layer of stroma, after the surgeon has created, through the use of a mechanical microblade, a lamella of superficial tissue that is overturned on itself to allow the exposure of the tissue in depth. At the end of the treatment the lamella will be repositioned and will be able to adhere perfectly to the underlying tissue in a short period of time. LASIK also requires topical anesthesia, but it has a slightly longer duration than PRK; Currently, a Femtosecond laser is used tocut the lamella, for which we speak of Femtolasik or I-Lasik, which therefore involves the use of two different lasers, an excimer laser and a Femtosecond laser.
• SMILE (Small Incision Lenticule Extraction) is an innovative technique that consists in the use of a single Femtosecond laserfor the correction of the refractive defect.
  The procedure consists in the sculpting by the laser of a lentil of adequate thickness, diameter and geometric characteristics to correct the myopic or astigmatic refractive error of the patient that is subsequently extracted by the surgeon. This technique allows to reduce the time of the intervention, minimizes the extension of the corneal incisions and allows a very rapid visual recovery.
• LASEKand EPI-LASIK are like PRK, the difference is in the mode of de-epithelialization. In LASEK an epithelial flap is created by applying an alcohol solution, in EPI-LASIK, the epithelium is removed with a blunt microblade of plastic material. Once laser ablation has been performed on the anterior stroma, the epithelial flap is resurfaced on the corneal tissue and protected with a therapeutic contact lens.

The choice of surgical technique must be implemented considering the anatomical characteristics of the eyeball and the extent of the visual defect to be corrected. In addition, the surgeon will have to consider the lifestyle and visual expectation of the patients by directing them to the most appropriate technique on a case-by-case basis.