[2] A reduction in visual acuity in a 'red eye' is indicative of serious ocular disease,[3] such as keratitis, iridocyclitis, and glaucoma, and never occurs in simple conjunctivitis without accompanying corneal involvement.

Whichever, it is always potentially serious and sometimes necessitates urgent treatment and corneal opacities are the fourth leading cause of blindness.

Corneal epithelial disruptions may be detected with fluorescein staining of the eye, and careful observation with cobalt-blue light.

With acute angle-closure glaucoma, the pupil is generally fixed in mid-position, oval, and responds sluggishly to light, if at all.

Shallow anterior chamber depth may indicate a predisposition to one form of glaucoma (narrow angle) but requires slit-lamp examination or other special techniques to determine it.

Those with conjunctivitis may report mild irritation or scratchiness, but never extreme pain, which is an indicator of more serious disease such as keratitis, corneal ulceration, iridocyclitis, or acute glaucoma.