| The diagnosis of ocular allergy is based on a clinical, environmental and occupational history, combined with the physical examination and laboratory tests. A careful ophthalmologic examination is crucial for the correct diagnosis, since many clinical conditions present as red eye and mimic ocular allergy. Such conditions include chlamydial, bacterial and viral conjunctivitis, superior limbic keratoconjunctivitis, phlyctenular conjunctivitis, rosacea-associated conjunctivitis, dry eye, erythema multiforme, eoiscleritis/scleritis, and ocular cicatricial pemphigoid.
Ophthalmologic examination includes the observation of external ocular surfaces, namely the eyelid and their margins, the limbus, the cornea, and the bulbar and tarsal conjunctiva. Tarsal conjunctiva observation may provide some findings to direct the differential diagnosis. Thus, the presence of follicles (lymphoid aggregates), characteristically pale and round, surrounded by blood vessels, suggests a non-allergic disease, such as viral, chlamydial or toxic conjunctivitis. Otherwise, the papillae, pinkish and with a central vessel, are characteristic of ocular allergy when >0.3 mm (macropapillae) or >1 mm (giant papillae). Micropapillae (<0.3 mm of diameter) are present in 80% of the normal population. The characteristics of the conjunctival secretion, serous, watery or mucopurulent, may also help the differential diagnosis of a conjunctivitis.
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Confirmation of a suspected allergic sensitization by allergy diagnostic tests also plays a fundamental role. A skin prick test for the diagnosis of immediate hypersensitivity is the most sensitive, fastest and cheapest method to confirm an allergic sensitization. However, it carries a small but significant risk of systemic anaphylaxis. Challenge tests are the only way to relate the allergen to the triggering of ocular symptoms.
Total serum IgE measurement may support the diagnosis of allergy, especially when higher than 200 to 300 kU/l. However, the measurement of specific IgE is much more useful in the setting of an etiologic diagnosis of ocular allergy. The search for allergen-specific IgE, by the radioallergosorbent test (RAST) method, is theoretically attractive: a simple blood sample replaces many skin tests, with no risks of anaphylaxis. In spite of its good specificity, in practice RAST is less sensitive and more expensive than cutaneous tests, especially for the diagnosis of multiple sensitization.
Conjunctival scrapings and tear cytology performed after topical ocular allergen challenge to sensitized subjects have shown significant increases in neutrophils and eosinophils, and their presence can be a positive diagnostic criterion.
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