Cornea

The cornea is the eye's clear, dome-shaped outermost layer, responsible for 65-75% of the eye's total focusing power -- it has no blood vessels, drawing nourishment instead from tears and the fluid behind it, and is built from five distinct layers (epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium) that each play a role in clarity, strength, and keeping the cornea from swelling. A wide range of conditions can affect it: injuries and scratches (usually self-healing, but deeper ones can scar and impair vision), allergies, keratitis (corneal inflammation, often infectious), dry eye, corneal dystrophies such as keratoconus (progressive thinning and bulging, usually starting in the teens or 20s) and Fuchs' dystrophy (gradual endothelial cell loss causing swelling and blurred vision, usually noticed in the 50s-60s), Herpes Zoster and ocular herpes (viral infections that can recur and scar the cornea), Iridocorneal Endothelial Syndrome, pterygium (a pinkish tissue growth, more common with sun exposure), and Stevens-Johnson Syndrome (a severe drug-reaction disorder that can cause serious corneal damage). Treatment ranges from Phototherapeutic Keratectomy (PTK, a laser resurfacing technique that can delay or avoid a full transplant) to corneal transplant surgery -- either a full-thickness transplant (penetrating keratoplasty) or, more commonly today, a partial-thickness (lamellar) transplant that replaces only the diseased layer, leaving healthy tissue in place for fewer complications and better visual recovery. For patients who aren't good candidates for donor tissue, an artificial cornea (keratoprosthesis, most commonly the Boston type-1 KPro) is an option.