EyeWorld India December 2019 Issue

EWAP DECEMBER 2019 55 CORNEA P resenting a review on how the cornea reacts to insult in a session at the 23rd European Society of Cataract and Refractive Surgeons Winter Meeting, Marie-Jose Tassignon, MD, detailed the mechanisms of corneal pathologic change. Corneal basics The cornea is a highly refractive surface that covers two-thirds of the eye. It is transparent and protects intraocular structures. The cornea is a highly metabolically active, avascular ̈ÃÃÕi] VœÛiÀi` LÞ > Ìi>À w“ layer, which contains diverse cytokines that are continuously secreted by the conjunctiva and the cornea. “ The cornea is composed œv >««ÀœÝˆ“>ÌiÞ wÛi >ÞiÀÃ] but for the purposes of the pathological response, I will reduce it to four: the epithelium, subepithelial zone (basement membrane, Bowman’s layer, and anterior stroma), stroma, and the endothelium and Descemet’s membrane,” Dr. Tassignon said. The structures of the cornea can be affected by infection, ˆ˜y>““>̈œ˜]…Þ«œÝˆ>] ˆÃV…i“ˆ>] trauma (accidental and surgical), chemical burns, drug toxicity, primary dystrophies, systemic metabolic diseases, secondary degenerations, genetic diseases, growth disorders such as hyperplasia and neoplasia, immunopathology, desiccation, and aging. Like most tissues, however, it manifests only a limited number of responses to a wide variety of diseases and insults, including responses ˆŽi `iviVÌÃ] wLÀœÃˆÃ >˜` vascularization, edema and VÞÃÌÃ] `i«œÃˆÌÃ] ˆ˜y>““>̜ÀÞ and immune responses, and proliferation. Defects and deposits Corneal defects can be limited to the epithelium and tend to recover within 24–48 hours, or affect the stroma, typically after infections, and tend to scar with or without vascularization. Defects also occur at all layers of the cornea and are associated with a partial or complete loss of corneal tissue and may be acute, recurrent, chronic, or persistent. Keratoconus is an example of a corneal defect affecting all layers. When endothelial cells are damaged, the cornea develops edema that can evolve into bullae. Cysts are a focal collection œv yՈ` ̅>Ì >««i>À Vœ““œ˜Þ ˆ˜ the corneal epithelium but rarely in the other layers. “ We know that we need a certain number of endothelial cells in order to have a proper metabolism of the cornea. If you have fewer than 500 cells, you will have edema and loss of transparency of the cornea,” Dr. Tassignon explained. The cornea is susceptible to all four types of immune responses: IgE-mediated atopic or anaphylactic reactions such as in vernal keratoconjunctivitis; antibody-mediated cytotoxic reactions like with Mooren’s ulcer; immune complex deposits as with herpes simplex stromal keratitis, fungi, or other infections keratitis; and foreign body-related delayed cell- mediated responses, such as those seen with corneal grafts. More than 50 known drugs may leave deposits in the cornea, such as epinephrine and amiodarone in the epithelium. Ocular or systemic diseases can leave deposits of non- metabolizable material, like copper in Wilson’s disease, melanin in Descemet’s, and pigment in the endothelium in pigment dispersion syndrome. Reviewing corneal pathologic responses by Stefanie Petrou Binder, MD EyeWorld Contributing Writer Contact information Tassignon: marie-jose.tassignon@uza.be This article originally appeared in the September 2019 issue of EyeWorld . +V JCU DGGP UNKIJVN[ OQFKƂGF CPF appears here with permission from the ASCRS Ophthalmic Services Corp. The four corneal layers relevant to the pathologic response: (1) epithelium, (2) subepithelial zone (basement membrane, Bowman’s layer, and anterior stroma), (3) stroma), and (4) endothelium and Descemet’s membrane. Corneal defects can be limited to the epithelium and tend to recover within 24–48 hours. Stromal corneal defects typically occur after infections and tend to scar with or without vascularization.