Ocular allergy, classify and treat by subtype

Separates the two mild, mast-cell-driven forms of ocular allergy (seasonal and perennial allergic conjunctivitis, together roughly 90% of cases) from the three that are not mild: vernal and atopic keratoconjunctivitis, which are T-cell driven and can threaten sight through corneal damage, and giant papillary conjunctivitis, which is mechanical rather than truly allergic. Gates on red flags first, because pain, photophobia, and reduced acuity are not features of simple allergic conjunctivitis.

Evidence tier: Expert consensus. Reflects expert-consensus criteria, not a prospectively validated instrument. Apply clinical judgment and local policy.

Decision points

Any red flag for non-allergic or sight-threatening disease?
Which pattern of ocular allergy?
Vernal or atopic keratoconjunctivitis?

Do-not-miss pitfalls

Itch is the cardinal symptom. A red eye that hurts rather than itches is usually not allergic conjunctivitis, and a strictly unilateral red eye almost never is.
VKC and AKC are not "severe hay fever eyes". They are T-cell driven, can scar the cornea, and cause permanent visual loss, they need ophthalmology, not another antihistamine drop.
A shield ulcer in VKC is a sight-threatening emergency. Giant upper tarsal papillae abrading the cornea is the mechanism, and it will not respond to escalating topical antihistamines.
Never start a topical corticosteroid empirically in an undiagnosed red eye. If it is herpes simplex keratitis, the steroid can produce a sight-threatening dendritic ulcer, and even when correctly indicated steroids carry cataract, glaucoma, and infection risk.
Topical vasoconstrictors (naphazoline, tetrahydrozoline) treat nothing and cause rebound hyperaemia and conjunctivitis medicamentosa with continued use. Whiter is not better.
Eye rubbing degranulates mast cells mechanically, so it amplifies the itch it is meant to relieve, and chronic rubbing is associated with keratoconus, particularly in AKC.
GPC is mechanical. No drop resolves it while the contact lens, prosthesis, or exposed suture responsible is still in place.
Oral antihistamines are less effective for ocular symptoms than topical agents and can worsen the ocular surface by drying it. Treat the eye with an eye drop.
Dupilumab-associated conjunctivitis is a recognised class effect, worth anticipating in exactly the atopic dermatitis population that also gets AKC.

Evidence & citations

Clinician decision support. Verify against the cited source. Not a substitute for clinical judgment. 100% on-device; no patient data is stored or transmitted.