Definition

They are eye disorders that do not allow the distinct vision of images of the surrounding world. There are various refractive errors: myopia, hyperopia, astigmatism and presbyopia. The cause of refractive errors lies in anatomical imperfections of the eyeball or, as in presbyopia, in a reduction in the function of the lens.  The blurring of the images depends on the fact that the light rays that pass through the lenses of the eye (cornea, lens) are not projected on the retina.

Symptomatology

Myopia is the most frequent refractive error and involves blurred vision at a distance. The myopic eye is longer than normal and light rays fall in front of the retina.
In hypermetropia, where the eye is shorter than normal, vision can remain sharp to a certain extent thanks to the accommodation of the crystalline lens. People with a high hyperopia, however, will see poorly at a distance and worse up close
Astigmatism is a condition in which the focus of the rays that pass through the lenses of the eye, due to a particular conformation of the eyeball, instead of projecting on a single point project on two different planes.
Presbyopia is the defect that “forces” many people over the age of 45 to wear a pair of glasses to be able to read. The eye, with the passage of age, is no longer able to focus images closely for the reduction of the accommodative activity of the lens.

Diagnosis

Refractive defects can be diagnosed with a complete specialist eye examination. If necessary, the patient can have a visual acuity and refraction test.

Therapy

In general, refractive defects can be corrected using glasses, contact lenses or refractive surgery. The reasons that can cause the myopic, hypermetropic or astigmatic to request surgical correction of the defect are multiple, first of all the need for an optical aid (the glasses or contact lenses) to be able to move or do anything.
The clinical reasons include 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 the treatment of photorefractive corneal surgery 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 error 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 pathology such as keratoconus, glaucoma, macular degeneration; the patient must also not be affected by systemic pathologies or immune pathogenesis.

 Surgical techniques to treat refractive defects are:

• PRK (photorefractive keratectomy) which 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.
• LASIK (Laser In Situ Keratomileusis) in which 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;
• SMILE (Small Incision Lenticule Extraction) which is an innovative technique that consists in the use of a single Femtosecond laser for 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 defect 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;
• LASEK and EPI-LASIK, similar to PRK, but with a different mode of de-epithelialization.

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 must consider the lifestyle and visual expectation of the patient by directing him to the technique most indicated case by case.