Corneas

The cornea is the transparent, anterior portion of the eye that covers the iris, pupil, and anterior chamber. It contributes about two-thirds of the eye's optical power and is essential for focusing light onto the retina. The cornea is avascular, nourished by tears and the aqueous humor, and highly innervated by sensory nerves from the trigeminal nerve, which provide strong sensitivity to touch and pain.

The cornea consists of five layers: epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The epithelium regenerates rapidly after injury; the stroma provides most of the thickness and shape; Descemet's membrane is a thin, strong layer; the endothelium maintains corneal clarity by pumping excess fluid out of the stroma. Endothelial cells have limited regenerative capacity, and damage can cause edema and vision loss. Corneal nerves contribute to surface health through reflex tearing and sensation.

Clinically, keratoconus, corneal ulcers, dystrophies such as Fuchs' endothelial dystrophy, and corneal edema are common problems. Diagnosis uses slit-lamp examination, corneal topography, pachymetry, and specular microscopy, among other imaging methods.

Treatment ranges from lubricants and contact lenses to collagen cross-linking for keratoconus. When replacement is needed, transplantation is performed as penetrating keratoplasty (full-thickness) or lamellar keratoplasty that preserves parts of the cornea; endothelial disorders may be treated with Descemet's membrane endothelial keratoplasty (DMEK) or DSEK. Graft survival has improved, and many patients achieve good vision.