1-2-3 rule — culture if ≥ 1+ AC cell, ≥ 2 mm infiltrate, or ≤ 3 mm from the visual axis (also if atypical or not responding).

• Bacterial: fluoroquinolone hourly; fortified vancomycin + tobramycin for severe/central ulcers.
• Fungal (trauma with vegetable matter): feathery margins, satellite lesions → natamycin 5% (filamentous), voriconazole.
• Acanthamoeba (contact lenses + water): pain out of proportion, ring infiltrate, radial perineuritis → PHMB/chlorhexidine.
• HSV: dendrite with terminal bulbs; reduced sensation → antivirals; no steroids for epithelial disease.

SCUT: adjunctive topical steroids after 48 h of antibiotics may help non-Nocardia bacterial ulcers with severe vision loss.