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.
First principles
The disease is superficial inflammation of the conjunctiva, not the deeper eye
The conjunctiva is a thin, vascular membrane covering the sclera and lining the inner eyelids. Infective agents (viral, most commonly adenovirus, or bacterial, such as Staphylococcus or Streptococcus species) or allergens trigger vasodilation and an inflammatory exudate confined to this superficial layer. Because the cornea, anterior chamber and pupil are untouched, vision remains essentially normal and the pupil reacts normally, which is precisely what separates conjunctivitis from the sight-threatening causes of a red eye.
Educational content pending clinical review. Not medical advice.