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Ocular allergy is an exaggerated response of ocular tissues to a relatively harmless allergen. Most ocular allergies are IgE-mediated type I reactions (or have at least a type I component). Type I reactions are characterized by involvement of specific humoral factors: IgE and histamine release from mast cells.
The vast majority of ocular allergies affect the conjunctiva, the mucous membrane of the eye; however, use of the term allergic conjunctivitis as a synonym for all ocular allergic diseases is discouraged. Rather, according to the current understanding of the pathophysiology and clinical course of ocular allergic diseases, a more precise classification has been proposed. This classification scheme encompasses the wide spectrum of severity of ocular allergies. Some ocular allergic diseases, such as vernal keratoconjunctivitis and atopic keratoconjunctivitis, may affect the cornea and are, therefore, potentially blinding. This discussion will focus on more common conditions, seasonal and perennial allergic conjunctivitis, which although uncomfortable and irritating, do not threaten sight.
Seasonal Allergic Conjunctivitis
Seasonal allergic conjunctivitis, often referred to as acute allergic conjunctivitis, affects an estimated 15% to 20% of the general population and accounts for as much as 90% of all ocular allergies, making it the most frequently encountered type of ocular allergy.
Seasonal allergic conjunctivitis is the ocular manifestation of hay fever (allergic rhinitis) and often can be the predominant symptom in patients with hay fever. It occurs seasonally due mostly to exposure of the eye to pollen from grasses, trees, and/or weeds.
Like other allergic diseases, seasonal allergic conjunctivitis is a type I hypersensitivity response that occurs when a person encounters an antigen to which they are sensitized. Accordingly, symptoms associated with seasonal allergic conjunctivitis are consistent with an early-phase type I IgE reaction. However, studies have found occasional evidence of a late-phase reaction as well, which is characterized by an infiltration of inflammatory cells into the conjunctival mucosa. This can lead to continuous symptoms and persistent inflammation. The changes that occur in many type I allergic reactions are transient and resolve when the antigen is removed (ie, the reaction is self-limiting).
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Perennial Allergic Conjunctivitis
Perennial allergic conjunctivitis, also referred to as chronic allergic conjunctivitis because it persists throughout the year, affects an estimated 3% of the population; most patients with perennial allergic conjunctivitis will experience seasonal exacerbation. It results mostly from exposure of the eye to constantly present household allergens such as animal dander and dust mite feces. Seasonal exacerbations can occur in either spring or fall, as a result of increased exposure to mold and grass, but they appear most commonly in the fall when exposure to dust mites and fungal allergens is greatest.
Perennial allergic conjunctivitis, like acute or seasonal allergic conjunctivitis, is a type I hypersensitivity reaction. Because of its chronic nature, it most likely involves both an early- and a late-phase response.
Effects on Quality of Life
Ocular allergy symptoms can seriously impair a patient's quality of life. Disruption of activities due to active pollen avoidance and lack of sleep as well as the resulting fatigue and lack of concentration can have a profound negative effect on the allergy sufferer's workplace productivity. Similarly, impairments in quality of life may lead to absenteeism of school children and may thus disrupt education progress. The economic costs and personal suffering should be expected to escalate because allergic diseases appear to be increasing, mainly due to environmental pollution.
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