Conjunctiva, cornea, and external ocular disease
The external disease domain covers the ocular surface as a system: tear film, lids, conjunctiva, cornea and their immune responses. Questions are often a described red eye, and expect a differential plus a decision about steroids and referral.
The red eye
Bacterial conjunctivitis is purulent and sticky; viral is watery with a preauricular node and often starts unilaterally; allergic is itchy with papillae. Photophobia, reduced acuity, ciliary flush and corneal staining mean the disease is not simple conjunctivitis. Neonatal conjunctivitis: day 1 chemical, days 2 to 5 gonococcal and urgent, days 5 to 14 chlamydial.
Cornea
Epithelial healing needs an intact basement membrane, and recurrent erosion follows failed adhesion. For an ulcer, scrape or swab before treatment, take a contact lens history, and never start a steroid without antimicrobial cover. Herpes simplex dendrites have terminal bulbs; steroid alone is contraindicated. Acanthamoeba occurs with contact lenses and water, with pain out of proportion and a ring infiltrate. Fungal ulcer follows agricultural trauma with feathery margins and satellites. Keratoconus shows inferior steepening, Vogt striae and a Fleischer ring; cross-linking is used when it progresses.
Surface disease
Dry eye is tear film instability with hyperosmolarity and inflammation; treat with lubricants, lid hygiene, punctal plugs, cyclosporine or lifitegrast. Blepharitis is the commonest cause of evaporative dry eye. Trachoma is graded by the WHO simplified system (TF, TI, TS, TT, CO) and controlled with the SAFE strategy. Ocular surface squamous neoplasia mimics pterygium; pterygium is fibrovascular and recurrent.
Worked example
A contact lens wearer has a painful red eye, a white corneal infiltrate and a hypopyon. Reasoning: this is microbial keratitis until proven otherwise. Remove the lens, culture the scrape, start fortified antibiotics hourly and admit if the ulcer is central or drop instillation is unreliable; steroids only after infection is controlled.
Traps
Treating a dendritic ulcer with steroid alone; calling a ring infiltrate sterile in a lens wearer; ignoring pterygium asymmetry; prescribing topical anaesthetics for pain relief.