Conjunctivitis
Inflammation of the conjunctiva from infection, allergy or irritation causes a red, gritty eye with discharge but preserved vision and a normal pupil: the mechanism that distinguishes it from sight-threatening causes of the red eye.
In a nutshell
Conjunctivitis is superficial inflammation of the conjunctiva, sparing the cornea, anterior chamber and pupil. This is why vision and pupil reaction stay normal, and why diffuse, movable redness with discharge, not pain or a fixed pupil, is the expected pattern.
Classic presentation
A red, gritty eye with discharge, normal vision and a normal pupil: purulent and initially unilateral in bacterial disease, watery and bilateral with a preceding URTI in viral disease, or watery and itchy with atopy in allergic disease.
Key points
- Preserved visual acuity and a normal, reactive pupil are the key features that separate conjunctivitis from sight-threatening causes of a red eye.
- Discharge character predicts the cause: purulent and sticky in bacterial disease, watery with preauricular lymphadenopathy in viral disease, watery and itchy in allergic disease.
- The redness is diffuse and superficial, moves with conjunctival manipulation and blanches with vasoconstrictors, unlike fixed circumcorneal injection.
- Contact lens wearers with a red eye need careful assessment to exclude microbial keratitis, which can mimic conjunctivitis but threatens vision.
- Most conjunctivitis, especially viral, is self-limiting; antibiotics are for bacterial cases and mainly shorten duration and infectivity.
First-line investigation
None routinely: conjunctivitis is a clinical diagnosis. Visual acuity testing and fluorescein staining confirm the diagnosis by excluding sight-threatening differentials.
First-line management
Conservative care with lid hygiene and reassurance for viral or mild cases, topical antibiotics for troublesome bacterial cases, and allergen avoidance with antihistamine drops for allergic cases.
Exam traps
- Pain, photophobia or reduced acuity should make you reconsider the diagnosis: these are not features of simple conjunctivitis.
- A contact lens wearer with a painful red eye is a corneal ulcer or keratitis until proven otherwise, not conjunctivitis.
- Neonatal conjunctivitis (ophthalmia neonatorum) needs urgent assessment for gonococcal or chlamydial infection, unlike typical adult conjunctivitis.
Educational content pending clinical review. Not medical advice.