Conjunctivitis. Differential diagnosis

  Conjunctivitis symptoms and signs are relatively non-specific. Even after biomicrosopy, laboratory tests are often necessary if proof of aetiology is needed.

  Prominent itch and pale watery swelling (edema) of the conjunctiva or eyelids suggests allergy.

  A purulent discharge strongly suggests bacterial cause, unless there is known exposure to toxins. Infection with Neisseria gonorrhoeae should be suspected if the discharge is particularly thick and copious.

  A diffuse, less 'injected' conjunctivitis (looking pink rather than red) suggests a viral cause, especially if numerous follicles are present on the lower tarsal conjunctiva on biomicroscopy.

  Scarring of the tarsal conjunctiva suggests trachoma, especially if seen in endemic areas, if the scarring is linear (von Arlt's line), or if there is also corneal vascularisation.

  Clinical tests for lagophthalmos, dry eye (Schirmer test) and unstable tear film may help distinguish the various types of dry eye.

  Other symptoms including pain, blurring of vision and photophobia should not be prominent in conjunctivitis. Fluctuating blurring is common, due to tearing and mucoid discharge. Mild photophobia is common. However, if any of these symptoms are prominent, it is important to exclude other diseases such as glaucoma, uveitis, keratitis and even meningitis or carotico-cavernous fistula.