Keratoconus is a degenerative disease of the cornea characterized by progressive thinning and bleeding of the corneal stroma. Etiology is unknown. The clinical onset, usually bilateral, occurs at a young age, in the second-third decade of life.
Keratoconus has an estimated average incidence of 1 case per 2000 people. It may be familiar in nature. An association of keratoconus with non-inflammatory connective tissue disorders, including Elhers-Danlos syndrome and osteogenesis imperfecta syndrome, has been observed. In patients with keratoconus, isolated eye diseases can be observed. A classic example is retinitis pigmentosa.
The first symptom is the reduction of Visus due to irregular astigmatism and high order optical aberrations induced by the apex of the cone. Photophobia, dazzling light sources, monocular diplopia and frequent eye redness may be symptoms. Biomicroscopic examination at an early stage reveals signs on the surface of stromal layers (Bowman membrane scars) or deep layers (Vogt strias) and, in late cases, ectatic protrusion of the cornea. If the apex of the cone is generally found at the visual axis, there is a significant reduction in the patient's visual performance. The diagnosis of keratoconus is performed through the objective eye examination and corneal topography. Topography allows computerized reconstruction of the corneal curvature and allows both early diagnosis, in the pre-clinical stage of keratoconus, and the study of evolution and staging of the pathology. Clinical examination also uses corneal pachymetry, through which corneal thickness measurement is possible. A corneal, central or paracentral thickness of less than 450 microns is considered pathological. Currently an examination that integrates topography and pachymetry is represented by corneal tomography that can be performed with instruments using a Scheimplfug camera or corneal OCT.
In the early stages, keratoconus treatment uses Visus correction by eyeglasses or contact lenses, which in any case never slow down the progression of the cone. Contact lenses allow better visual performance than eyeglasses, thanks to the fact that they are in contact with the corneal surface and can therefore better regulate the profile of the cone.
The only therapeutic option that allows us to slow down or even stop the progression of keratoconus is Corneal Cross-Linking. This technique aims to increase the mechanical stiffness of corneal tissue, weakened by corneal degeneration, through the generation of new chemical bonds between the collagen molecules of the corneal stroma, according to a chemical process called photopolymerization. There are two different types of treatment: the standard epi-off with which corneal epithelium is initially removed to facilitate riboflavin absorption and subsequent ultraviolet irradiation, and cross-linking transepithelial (epi-on) where absorption and irradiation take place without the removal of corneal epithelium; this technique is therefore more suitable for people with excessively thin cornea who could not be subjected to the previous classical procedure.
The epi-on treatment has proved more effective if riboflavin absorption is performed by creating a low-voltage electric field. Cross-linking will therefore be discussed through iontophoresis.
The cross-linking epi-off treatment (cross-linking standard, coded by the so-called Dresden protocol) consists of instilling drops of riboflavin (vit. B2) on the cornea, approximately every 2-3 minutes for 30 minutes and therefore of radiating the corneal tissue with a ray of UltraViolet light (wavelength: 370 nm, UV-A) for about 30 minutes, continuing to instill a drop of riboflavin every 2-3 minutes. The results six years after the introduction of cross-linking in clinical practice demonstrated the efficacy and safety of treatment. On average, a significant flattening of the keratoconus apex (average 1-2 diopters) has been observed, with a simultaneous improvement in visual acuity with contact lenses of one or two snellen lines. The surgical indication is addressed to the evolutionary keratocons at an early stage with a slight reduction of the best natural or corrected Visus with contact lenses and a corneal thickness of more than 400 microns. The surgery is performed in an outpatient procedure under topical anesthesia (with drops of anesthetic eye drops); at the end of the procedure a therapeutic contact lens is placed on the cornea which is removed 3-4 days after the surgery, as soon as the epithelium is intact. Post-operative medical therapy is topical and makes use of antibiotic and anti-inflammatory eye drops. During the first 2-3 days after surgery, there may be a sense of foreign body or ocular burning, due to the corneal recapitelization process. Once the epithelium has healed, any discomfort ceases and the patient may soon return to wearing the contact lens again, always at the ophthalmologist's request.
Iontophoresis treatment is quicker, with an absorption phase of about 5 minutes and a UV irradiation phase of 9 minutes. A clinical study carried out at the Bietti Foundation's clinical trial center has shown that treatment with iontophoresis is fairly effective. One year after treatment, in fact, all participants aged over 24 years presented a reduction in the curvature of the apex of the cone comparable to that obtained with traditional treatment. In addition, cross-linking with iontophoresis has led to an improvement in visual acuity in at least two-thirds of patients operated, compared to 20% of patients undergoing standard treatment.
Another method for improving the quality of vision in keratoconus patients is the insertion of intrastromal corneal rings. It is important to underline that this treatment is not intended to stop the progress of the pathology but only to improve vision in patients with keratoconus who are already stable.
Finally, in those patients with keratoconus who, despite cross-linking treatment, do not obtain a satisfactory stabilization or vision, cornea transplantation (cheratoplasty) is necessary. In the case of young patients with healthy corneal endothelium and in the absence of corneal scars, the treatment of choice is represented by deep anterior lamellar keratoplasty (Dalk), in which the corneal stroma is transplanted leaving the descemet membrane and endothelium of the recipient in situ (see cornea transplant).