|Allergic eye disease|
|Classification and external resources|
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 (swelling) of the conjunctiva, itching and increased lacrimation (production of tears). If this is combined with rhinitis, the condition is termed allergic rhinoconjunctivitis.
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). Antihistamines, medication that stabilizes mast cells, and non-steroidal anti-inflammatory drugs (NSAIDs) are safe and usually effective.
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, eyelid swelling, tearing, photophobia, watery discharge, and foreign body sensation (with pain).
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 may improve their everyday "quality of life."
The cause of allergic conjunctivitis is an allergic reaction of the body's immune system to an allergen. Allergic conjunctivitis is common in people who have other signs of allergic disease such as hay fever, asthma and eczema.
Among the most common allergens that cause conjunctivitis are:
- Pollen from trees, grass and ragweed
- Animal skin and secretions such as saliva
- Skin medicines
- Air pollution
- Dust mites
- Eye drops
Most cases of seasonal conjunctivitis are due to pollen and occur in the hay fever season, grass pollens in early summer and various other pollens and moulds may cause symptoms later in the summer.
Perennial conjunctivitis is commonly due to an allergy to house dust mite (a tiny insect-like creature that lives in every home).
Giant papillary conjunctivitis is a very rare condition that is mainly caused by an allergic reaction to "debris". Surgery may also cause this type of allergic conjunctivitis.
Contact dermatoconjunctivitis is caused by the rest of the allergens that conjunctiva may come into contact with: cosmetics, medications and so on.
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 interleukin 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.
SAC and PAC 
Both seasonal allergic conjunctivitis and perennial allergic conjunctivitis 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. Nonspecific 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-allergen 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 and atopic keratoconjunctivitis are chronic allergic diseases wherein eosinophils, conjunctival fibroblasts, epithelial cells, mast cells, and 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.
Giant papillary conjunctivitis 
Giant papillary conjunctivitis 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.
Phlyctenular keratoconjunctivitis 
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 eye care physician due to possible side-effects. When an allergen is identified, the patient should avoid the allergen as much as possible.
If the allergen is encountered and the symptoms are mild, a cold compress can be used to provide relief. It is a quick and easy solution without using any medications. The cold temperature of the water will help to bring down swelling, as it would in a bruise or burn. In addition, there are many antihistamine medications available for purchase.
Mast cell stabilizers can help curing patients with allergic conjunctivitis when cold compress are no longer effective. They tend to have delayed results, but they have fewer side-effects than the other treatments and last much longer than those of antihistamines. Some patients are given an antihistamine at the same time so that there is some relief of symptoms before the mast cell stabilizers becomes effective. Doctors commonly prescribe lodoxamide and nedocromil as mast cell stabilizers, which come as eye drops.
A mast cell stabilizer is a class of non-steroid controller medicine that reduces the release of inflammation-causing chemicals from mast cells. They block a calcium channel essential for mast cell degranulation, stabilizing the cell, thus preventing the release of histamine. Decongestants may also be prescribed. Another common mast cell stabilizer that is used for treating allergic conjunctivitis is sodium cromoglicate.
Antihistamines such as diphenhydramine and chlorpheniramine are commonly used as treatment. Patients treated with H1 antihistamines exhibit reduced production of histamine and leukotrienes as well as downregulation of adhesion molecule expression on the vasculature which in turn attenuates allergic symptoms by 40–50%.
Dual-action medications are also prescribed frequently. Olopatadine (Patanol) and Ketotifen Fumarate (Alaway or Zaditor) both provide protection by acting as an antihistamine and a mast cell stabilizer together. Patanol is a prescription medication, whereas Ketotifen Fumarate is not.
Many of the eye drops can cause burning and stinging, and have side-effects. Proper eye hygiene can improve symptoms, especially with contact lenses. Avoiding precipitants, such as pollen or mold can be preventative. Desensitization allergen immunotherapy with subcutaneous introduction of allergens can be performed in atopic individuals as well. Sublingual administration has also shown effect, although multiple allergens are more difficult to combine with this approach.
Experimental research has targeted adhesion molecules known as selectins on epithelial cells. These molecules initiate the early capturing and margination of leukocytes from circulation. Selectin antagonists have been examined in preclinical studies, including cutaneous inflammation, allergy and ischemia-reperfusion injury. There are four classes of selectin blocking agents: (i) carbohydrate based inhibitors targeting all P-, E-, and L-selectins, (ii) antihuman selectin antibodies, (iii) a recombinant truncated form of PSGL-1 immunoglobulin fusion protein, and (iv) small-molecule inhibitors of selectins. Most selectin blockers have failed phase II/III clinical trials, or the studies were ceased due to their unfavorable pharmacokinetics or prohibitive cost. Sphingolipids, present in yeast like Saccharomyces cerevisiae and plants, have also shown mitigative effects in animal models of gene knockout mice.
Allergic conjunctivitis occurs more frequently among those with allergic conditions, with the symptoms having a seasonal correlation.
Allergic conjunctivitis is a frequent condition as it is estimated to affect 20 percent of the population on an annual basis and approximately one-half of these people have a personal or family history of atopy.
Giant papillary conjunctivitis accounts for 0.5–1.0% of eye disease in most countries.
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