Cornea & Sclera

• Forms the anterior $1/5\text{th}$ of the outer eye coat, contributing $+43\text{D}$ ($70\%$) of the eye's refractive power.

• 6 Layers (anterior to posterior): 1) Epithelium (stratified, highly regenerative), 2) Bowman's layer (non-regenerative), 3) Stroma (thickest, parallel collagen bundles maintaining transparency), 4) Dua's layer, 5) Descemet's membrane (elastic), 6) Endothelium (single layer; Na+/K+ ATPase pumps maintain stromal deturgescence/hydration).

Diagnostic Stains

• Fluorescein Dye: Stains epithelial defects green under cobalt blue light (used for corneal abrasions, ulcers, Seidel test for perforations).

• Rose Bengal: Stains dead and degenerating epithelial cells.

• pseudomonas Aeruginosa:

• The most common pathogen in contact lens wearers 60%. Aggressive, featuring a ring ulcer, green-yellowish discharge, and hypopyon.

• Fungal Keratitis:

Associated with plant trauma or immunosuppression. Classic signs include gray-white infiltrate with feathery edges and satellite lesions.

• Herpes Simplex Virus (HSV):

Classic dendritic ulcer with branching morphology and terminal bulbs (stains with fluorescein centrally, rose bengal at margins), accompanied by reduced corneal sensation.

• Herpes Zoster Ophthalmicus (VZO):

Reactivation along the ophthalmic division ($V_1$) of the trigeminal nerve. Hutchinson's sign (vesicles on the tip of the nose) indicates nasociliary nerve involvement and high risk of ocular complications. Treated promptly with oral acyclovir.

• Acanthamoeba Keratitis:

Free-living protozoa linked to poor contact lens hygiene (tap water/swimming). Presents with severe disproportionate pain and ring-shaped stromal infiltrates.

Corneal Ectasia (Keratoconus)

• Progressive bilateral conical corneal thinning and steepening leading to irregular astigmatism.

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• Signs: Conical profile, Fleischer's ring (iron ring at the base of the cone), Vogt's striae (vertical fine lines in deep stroma), Munson's sign (V-shaped indentation of the lower lid on downgaze), and scissoring reflex on retinoscopy.

• Management: Rigid gas-permeable lenses, Corneal Collagen Cross-Linking (CXL) using riboflavin and UV-A to halt progression, and keratoplasty (DALK or PKP) for advanced scarring.