Collagen Cross Linking for Keratoconus

Introduction

Keratoconus is an Eye condition in which cornea gets thinner, conical and gradually bulging outwards into conical shape. This can cause blurred vision and discomfort in light. It is one of the most common corneal ectasia, characterized by progressive, non-inflammatory changes in corneal stromal collagen and manifests as the protrusion and alteration of the central and paracentral cornea (1). The progression of the disease usually increasing in the first three decades of life and if not treated on time can result in a decreased visual acuity. The Keratoconus management is decided by many factors such as age, stage of keratoconus, corneal topography and visual acuity. There are many management options available include spectacles, Hard contact lenses, corneal collagen cross-linking with riboflavin, Intacs, penetrating keratoplasty, and deep anterior lamellar keratoplasty. Cross Linking is a method of making the corneal tissue strong by using riboflavin and ultraviolet-A light to stop the progression of keratoconus. In Cross Linking, riboflavin dye 

act as a photosensitizing agent and UVA light improves the formation of intra and interfibrillar covalent bonds by oxidative photosensitization. This procedure explained the roll of collagen cross-linking in managing keratoconus.

Keratoconus has variable prevalence in the Middle East. Keratoconus can be high as five percent in gulf region who has refractive error. Rubbing, environmental and genetic factors can be responsible for the keratoconus.

The etiology of this condition is unknown. However, there are several ocular and systemic association exist like connective tissue disorders of Ehlers-Danlos and Marfan syndromes, Leber congenital amaurosis, atopy and Down syndrome. It can usually occur in the second or third decade of life with progressive myopia and astigmatism. It can be unilateral or bilateral. It is initially unilateral and latter both eyes are involved (2).

On examination, there are several clinical signs. Munson’s sign is a V shape bulging of the lower eyelid on downgaze. A slit-lamp examination can show Vogt striae: fine, vertical, stromal stress lines, and a Fleischer ring: a ring-like configuration of epithelial iron deposits. Distant direct ophthalmoscopy reveals a characteristic “oil-droplet” reflex, and retinoscopy shows an irregular scissor reflex.

Corneal topography and optical coherence tomography shows minor changes in corneal topography and tomography. Amsler-Krumeich system is known classification system which uses the patient’s refractive error, central keratometry readings, central corneal thickness and the presence or absence of scarring. Amsler-Krumeich system does not utilize corneal topographic reading. Various topographic indices have been proposed for diagnosing forme fruste keratoconus / preclinical and clinical KC. Rabinowitz suggests the following topographical characteristics of KC: increased areas of keratometric readings surrounded by areas of reduced corneal power, inferior-superior symmetry and skewed radial axes (3). Pentacam system (Oculus GmbH, Wetzlar, Germany) utilizes the Belin/Ambrósio Enhanced Ectasia Display (BAD) to screen for KC using maximal keratometry, anterior and posterior elevation, and tomographic thickness data (4).

Treatment of early keratoconus is spectacles to improve vision, but as the disease progresses, rigid gas-permeable contact lenses are required. Advanced keratoconus may need eventual corneal transplantation. Collagen cross linking with ultraviolet A (UV-A) light and riboflavin (vitamin B2) is a relatively new treatment that slows the advancement of the disease in its early stages (5).

Corneal cross linking was invented in the 1990s. Corneal cross linking utilizes riboflavin as a photosensitizer which is exposed to UV-A light to induce chemical reactions in the corneal stroma which finally results in the formation of covalent bonds between the collagen molecules. Collagen cross linking increases the tensile strength and rigidity of the cornea to prevent further thinning of the cornea.