Evaluation of the Keratoconic Cornea After Corneal Collagen Cross Linking.

Keratoconus is a bilateral, asymmetric, progressive, non-inflammatory corneal ectatic disorder that is characterized by progressive thinning, steepening and potential scaring. Usually it affects the inferior or central cornea that becomes thinner and bulges forward in a cone-shaped fashion, inducing irregular astigmatism and myopia and reducing the quality of vision. Approximately 50% of clinically normal fellow eyes will progress to KC within 16 years. The greatest risk is during the first 6 years of the onset.

Annual incidence of KC also varies greatly from 0.002% , to 0.23% of 100,000 population per year. Most of the western studies support the lower figure of 0.002% , while in the Middle-East it is about 0.02 per year. In the middle-East, there is about ten-fold higher incidence (0.02% compared to 0.002%), and ten-fold higher prevalence (2.34% compared to 0.23%), as compared to Western counries.

Management of keratoconus depends on a variety of factors including visual acuity, the degree of corneal thinning and steepening. Rigid gas permeable contact lenses (RGPs) have been tried to correct corneal irregularity and astigmatism in keratoconus but they don't stop keratoconus progression. Corneal collagen cross linking (CXL) is now considered as the treatment of choice in mild to moderate cases of keratoconus and is proven to halt the disease progression. The implantation of intrastromal corneal ring segments (e.g. INTACS, Ferrara \& Keraring) has been indicated for cases with moderate keratoconus to flatten the steep irregular corneas. Advanced cases of keratoconus with marked deterioration of vision or corneal scarring may be good candidates for deep anterior lamellar or penetrating Keratoplasty (DALK or PKP).

Collagen cross-linking (CXL) is a relatively new conservative approach for progressive corneal ectasia, which is able to strengthen corneal tissue reforming new covalent bonds. This strategy is based on the underlying pathology of the disease. Corneal collagen cross linking (CXL) idea was based on the fact that a photosensitizer substance like riboflavin (vitamin B2) can interact with ultraviolet irradiation (Ultraviolet-A) to strengthen the corneal tissue inter and intrafibrillar collagen bonds thus preventing further thinning, corneal protrusion and reduces corneal irregular astigmatism.

Epithelial debridement enhances riboflavin corneal penetration that allows absorption of wide range of light spectrum wave lengths including ultra violet A.

The idea of trans-epithelial delivery (Epi-on technique) of riboflavin into the corneal tissue was hindered by the fact that riboflavin can't penetrate intact corneal epithelium. The addition of certain molecules such as trometamol allows penetration of riboflavin into the corneal stroma that markedly reduces the possible complications of removing of the corneal epithelium (Epi-off technique) such as persistent epithelial defects, scarring and serious infectious keratitis. Another advantage of trans-epithelial CXL that it reduces the cytotoxic effects of ultraviolet irradiation on corneal endothelium and intraocular structures especially in thin corneas less than 400 um.

Recently, CXL techniques were developed to minimize ultraviolet exposure and shorten the time of the procedure on basis of photochemical reciprocity in which increased irradiation intensity with reduced intervals achieve the same effect of the conventional cross linking techniques.

Corneal collagen cross linking induces stromal collagen fiber shrinkage. Ultraviolet A exposure enhances covalent bond formation between collagen fibers especially in the anterior stroma where 65% of ultraviolet irradiation is absorbed within first 250 um thus a hyperrefelctive transitional area can be detected between the anterior cross linked and the posterior untreated corneal stromal tissue referred to as a demarcation line that is usually evident 1 - 6 months after CXL procedure.

A comprehensive slit lamp examination could detect the demarcation line; however anterior segment ocular coherence tomography (AS-OCT) is a more sensitive tool to assess the extent and depth of a stromal demarcation line that is deeper centrally than peripherally due to the natural corneal curvature.

Several studies confirm the effectiveness and safety of conventional cross-linking procedure, which is also known as "Dresden protocol", in which the interaction between 0.1% riboflavin molecules absorbed in corneal tissue and UV-A rays delivered at 3 mW/cm2 for 30 minutes (5.4 J/cm2 energy dose) releases reactive oxygen species that promote the formation of "molecular bridges" between and within collagen fibers.

Corneal cross-linking causes a dose-dependent keratocytes damage. Wollensak et al. described cellular apoptosis to a depth of 300 µm radiating with UV- A at 3 mW/cm2. Histopathological studies showed an already complete keratocyte apoptosis limited to the anterior stroma within 24 hours. Some authors characterized the corneal stromal DL as a clinical sign to evaluate the depth of the CXL treatment.

Some studies hypothesize the role of the DL after CXL depth as representative of CXL effectiveness. Recently, the essential debate focused on whether the depth of the corneal stromal DL is indeed a true indicator of CXL efficacy. The main question is whether "the deeper, the better" principle can be applied to CXL.

In recent years, anterior segment optical coherence tomography (AS-OCT) and confocal microscopy have been used as tools to assess the depth of DL and consequently the depth of the cross-linking effect. By using the AS-OCT, the stromal DL is detected within an enhanced image of the cornea in the horizontal meridian. The image is captured when the corneal reflex is visible, and the depth of DL is measured using the caliper tool provided by the manufacturer. Doors et al described the best visibility of corneal stromal DL using AS-OCT at 1 month after CXL treatment, with an average DL depth of 313 µm; Yam et al measured the depth of DL at 6 months highlighting that the severity of ectasia and age may cause a worse DL visibility.