Cornea and External Eye Disease
Cornea and external eye disease tests whether you can separate the common causes of a red, painful eye with a corneal lesion - bacterial, fungal, herpetic and acanthamoebic - and whether you know when a steroid is a mistake.
Core rules
- Bacterial keratitis: contact lens wear and trauma dominate. Pseudomonas is the classic contact lens organism; Staphylococcus and Streptococcus follow ocular surface disease. Scrape before treating, start fortified antibiotics (cefazolin 5% plus tobramycin 1.5%, or a fluoroquinolone), and taper only after the epithelium closes.
- Fungal keratitis: agricultural trauma, vegetative matter, slow onset, feathery margins, satellite lesions and an endothelial plaque. Culture on Sabouraud agar; natamycin 5% is first line for filamentous fungi and amphotericin B for yeasts.
- Herpes simplex keratitis: dendritic ulcer with terminal bulbs and reduced corneal sensation. Treat with topical or oral antiviral; never use a steroid alone. Disciform keratitis is immune-mediated and needs steroid with antiviral cover.
- Acanthamoeba: contact lens wear with poor hygiene, pain out of proportion to signs, radial keratoneuritis and a ring infiltrate. Confocal microscopy and culture on non-nutrient agar with Escherichia coli.
- Ocular surface: tear film breakup time and Schirmer testing; limbal stem cell deficiency shows conjunctivalisation and a dull reflex.
- Ectasia: keratoconus shows inferior steepening, Vogt striae, a Fleischer ring and scissoring on retinoscopy; offer cross-linking when progression is documented.
Worked example
A 22-year-old soft lens wearer has severe pain, photophobia and a paracentral ring infiltrate after swimming in a pool. Antibiotics for 48 hours have changed nothing. The combination of pain disproportionate to signs, contact lens history and a ring infiltrate points to Acanthamoeba. You send the lens and corneal scrape for confocal microscopy and non-nutrient agar culture, stop the antibiotic, and start hourly polyhexamethylene biguanide plus propamidine. Steroids are withheld until the infection is controlled.
Traps
Giving a steroid to a dendrite, missing a hypopyon and calling it sterile, treating fungal keratitis with antibiotics alone, and forgetting that herpetic disease can present with reduced sensation and a negative stain.