Cornea and External Eye Disease
This domain tests infectious and inflammatory disease of the ocular surface: dry eye, blepharitis, keratitis, keratoconus, pterygium, and ocular surface neoplasia. The exam favors pattern recognition and safe first-line management.
Core disease patterns
- Dry eye (TFOS DEWS II): tear film instability, hyperosmolarity, and inflammation. Symptoms often exceed signs; TBUT under 10 s, Schirmer testing, MMP-9. Treat with lubricants, lid hygiene, omega-3, cyclosporine, and punctal plugs.
- Bacterial keratitis: contact lens wear (Pseudomonas), trauma (Staphylococcus). Central infiltrate, pain, photophobia, hypopyon. Corneal scrape before hourly fluoroquinolone; do not patch.
- Fungal keratitis: vegetable trauma, Fusarium or Aspergillus; feathery margins, satellite lesions, endothelial plaque. Treat with natamycin 5% or voriconazole.
- Acanthamoeba: contact lens plus tap water; severe pain, ring infiltrate, radial keratoneuritis. Confocal microscopy; PHMB plus chlorhexidine.
- HSV epithelial keratitis: dendritic ulcer with terminal bulbs; debridement, topical ganciclovir, or oral acyclovir. Steroid alone worsens it.
- Keratoconus: inferior steepening, Vogt striae, Fleischer ring, Munson sign; cross-linking halts progression.
- Pterygium and OSSN: OSSN is malignant; impression cytology and topical 5-FU or interferon are options.
Worked example
A soft contact lens wearer presents with severe pain and a ring-shaped infiltrate after swimming in a pool. Reasoning: ring infiltrate plus lens wear plus water exposure points to Acanthamoeba. Perform corneal scrape and confocal microscopy, then start a biguanide plus diamidine. Bacterial keratitis usually produces a focal infiltrate without a ring.
Traps
Never patch a corneal ulcer. Steroids without antiviral can worsen HSV. Satellite lesions suggest fungus, not bacteria.