Allergic conjunctivitis is inflammation of the conjunctiva (the membrane covering the white part of the eye) due to allergy. Although allergens differ between patients, the most common cause is hay fever. Symptoms consist of redness (mainly due to vasodilation of the peripheral small blood vessels), oedema of the conjunctiva, itching and increased lacrimation (production of tears). If this is combined with rhinitis, the condition is termed allergic rhinoconjunctivitis.

The symptoms are due to release of histamine and other active substances by mast cells, which stimulate dilation of blood vessels, irritate nerve endings and increase secretion of tears.

Treatment of allergic conjunctivitis is by avoiding the allergen (e.g. avoiding grass in bloom during the "hay fever season") and treatment with antihistamines, either topical (in the form of eye drops), or systemic (in the form of tablets). Cromoglicate is sometimes used topically.

Signs and symptoms

The conjunctiva is a thin membrane that covers the eye. When an allergen irritates the conjunctiva, common symptoms that occur in the eye include: ocular itching, eye lid swelling, tearing, photophobia, watery discharge, and foreign body sensation.

Itching is the most typical symptom of ocular allergy and more than 75% of patients report this symptom when seeking treatment.

Symptoms are usually worse for patients when the weather is warm and dry, whereas cooler temperatures and rain tend to assuage symptoms.

A study by Klein et al. showed that in addition to the physical discomfort allergic conjunctivitis causes, it also alters patients’ routines, with patients limiting certain activities such as going outdoors, reading, sleeping, and driving. Therefore, treating patients with allergic conjunctivitis can improve their everyday quality of life.

Pathophysiology

The ocular allergic response is a cascade of events that is coordinated by mast cells. Beta chemokines such as eotaxin and MIP-1 alpha have been implicated in the priming and activation of mast cells in the ocular surface. When a particular allergen is present, sensitization takes place and prepares the system to launch an antigen specific response. TH2 differentiated T cells release cytokines, which promote the production of antigen specific immunoglobulin E (IgE). IgE then binds to IgE receptors on the surface of mast cells. Then, mast cells release histamine, which then leads to the release of cytokines, prostaglandins, and platelet activating factor. Mast cell intermediaries cause an allergic inflammation and symptoms through the activation of inflammatory cells.

When histamine is released from mast cells, it binds to H1 receptors on nerve endings and causes the ocular symptom of itching. Histamine also binds to H1 and H2 receptors of the conjunctival vasculature and causes vasodilatation. Mast cell derived cytokines such as chemokine interleukine IL-8 are involved in recruitment of neutrophils. TH2 cytokines such as IL-5 recruit eosinophils and IL-4, IL-6, and IL-13 which promote increased sensitivity. Immediate symptoms are due to the molecular cascade. Encountering the allergen a patient is sensitive to leads to increased sensitation of the system and more powerful reactions. Advanced cases can progress to a state of chronic allergic inflammation.

Types

SAC and PAC

Both Seasonal Allergic Conjunctivitis (SAC) and Perennial Allergic Conjunctivitis (PAC) are two acute allergic conjunctival disorders. SAC is the most common ocular allergy. Symptoms of the aforementioned ocular diseases include itching and pink to reddish eye(s). These two eye conditions are mediated by mast cells. Non specific measures to ameliorate symptoms include: cold compresses, eyewashes with tear substitutes, and avoidance of allergens. Treatment consists of antihistamine mast cell stabilizers, dual mechanism anti-allergin agents, or topical antihistamines. Corticosteroids are another option, but considering the side effects of cataracts and increased intraocular pressure, corticosteroids are reserved for more severe forms of allergic conjunctivitis such as vernal keratoconjunctivitis (VKC) and atopic keratoconjunctivitis (AKC).

VKC and AKC

Both Vernal Keratoconjunctivitis (VKC) and Atopic Keratoconjunctivitis (AKC) are chronic allergic diseases where eosinophils, conjunctival fibroblasts, epithelial cells, mast cells, TH2 lymphocytes aggravate the biochemistry and histology of the conjunctiva. VKC is a disease of childhood and is prevalent in males living in warm climates. AKC is frequently observed in males between the ages of 30 and 50. VKC and AKC can be treated by medications used to combat allergic conjunctivitis or the use of steroids.

GPC

Giant Papillary Conjunctivitis (GPC) is not a true ocular allergic reaction and is caused by repeated mechanical irritation of the conjunctiva. Repeated contact with the conjunctival surface caused by the use of contact lenses is associated with GPC.

Treatment

A detailed history allows physicians to determine whether the presenting symptoms are due to an allergen or another source. Diagnostic tests such as conjunctival scrapings to look for eosinophils are helpful in determining the cause of the allergic response. Antihistamines, medication that stabilizes mast cells, and non-steroidal anti-inflammatory drugs (NSAIDs) are safe and usually effective. Corticosteroids are reserved for more severe cases of ocular allergy disease and their use should be monitored by an ophthalmologist. When an allergen is identified, the patient should avoid the allergen as much as possible.

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