Keratoconus (KC) is one of the most prevalent corneal ectatic disorders characterized by progressive, non-inflammatory changes in stromal collagen structure and usually results in protrusion and alteration of the central and paracentral cornea. The etiology of this condition remains unknown; however, several ocular and systemic associations exist, like Leber congenital amaurosis, atopy, Down syndrome, and the connective tissue disorders of Ehlers-Danlos and Marfan syndromes. Presentation is typically in the second or third decade of life with features of progressive myopia and astigmatism. The initial presentation is unilateral; however, both eyes eventually become involved.
On examination, several eponymous clinical signs may increase the suspicion for KC. Munson’s sign is a V shape bulging of the lower eyelid on downgaze. A slit-lamp examination may reveal 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 can demonstrate a characteristic scissoring reflex.
Placido-disc topography, Scheimpflug imaging, and optical coherence tomography allow for detecting subtle changes in corneal topography, tomography, and epithelium changes associated with KC. A well-known classification system is an Amsler-Krumeich system which uses the patient’s refractive error, central keratometry readings, central corneal thickness, and the presence or absence of scarring. Notably, the Amsler-Krumeich system does not utilize corneal topographic values. Various topographic indices have been proposed for diagnosing preclinical KC (forme fruste keratoconus) 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. The newer Scheimpflug imaging-based 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.
Treatment of early keratoconus involves prescribing spectacles to improve vision, but as the disease progresses, rigid gas-permeable contact lenses are required. In a small but significant proportion of patients, disease progression may require eventual corneal transplantation. Several new therapeutic options have emerged, including refractive, optical, and lamellar surgery, which slow the progression of the disease and/or delay more intensive treatment. Collagen crosslinking (CXL) with ultraviolet A (UV-A) light and riboflavin (vitamin B2) is a relatively new treatment that reportedly slows the advancement of the disease in its early stages.
CXL was introduced to clinical practice in the late 1990s and has since completely modified conservative management of progressive corneal ectasia. CXL utilizes riboflavin as a photosensitizer, which, when exposed to longer wavelength UV-A, induces chemical reactions in the corneal stroma and ultimately results in the formation of covalent bonds between the collagen molecules. This collagen crosslinking increases the tensile strength and rigidity of the cornea, preventing further thinning and ectasia.
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