Cornea and External Disease
Core concepts
Know the layers of the cornea and which layer each dystrophy or ulceration affects. Epithelial disease (recurrent erosion, EBMD), stromal disease (macular, granular, lattice), and endothelial disease (Fuchs, posterior polymorphous) behave differently and are tested differently. Understand tear film anatomy and the aqueous-deficient versus evaporative distinction in dry eye. Recognize infectious keratitis patterns: bacterial (suppurative, focal), fungal (feathery margins, satellite lesions), acanthamoeba (ring infiltrate, severe pain out of proportion, contact lens history), and viral (dendrite with terminal bulbs).
Traps
Questions love to test the first management step and the safest step, not the definitive one. For a corneal ulcer, scraping/culture before antibiotics in severe cases; for contact-lens keratitis, do not reflexively start steroids. Distinguish epithelial basement membrane dystrophy from recurrent herpetic disease. Remember that topical anesthetics are never therapeutic. Another trap: confusing pterygium (conjunctival, crosses limbus) with pinguecula (does not cross).
Rule of thumb
A dendrite that stains with fluorescein and has terminal bulbs = HSV; a dendrite-like lesion that stains with rose bengal but not fluorescein and lacks bulbs = acanthamoeba or healing abrasion. When in doubt about a nonhealing epithelial defect, ask about contact lens wear.
Worked example
A soft contact lens wearer with severe pain, a ring-shaped stromal infiltrate, and no response to broad-spectrum antibacterials. Best next step: corneal scrape for culture and start anti-acanthamoeba therapy (polyhexamethylene biguanide or chlorhexidine), stop lens wear.