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Allergic conjunctivitis

26 January 2017

What it is

Conjunctivitis is an eye disease characterised by an inflammatory state of the conjunctiva, the mucous membrane that covers part of the eyeball and the inner side of the eyelids.
The health of the conjunctiva is essential for the proper production and functioning of the tear film. In addition, conjunctival diseases can alter the homeostasis of the entire ocular surface causing alterations of various other structures, from the eyelid edge to the cornea.
Conjunctivitis can have various causes, primarily infectious and allergic ones.
Allergic conjunctivitis are inflammatory diseases caused by an exaggerated response of the immune system (the natural defence system) to certain substances, called allergens, which in non-allergic people are harmless, such as pollen, grasses, dust or animal hair. These forms of immunological hypersensitivity are characterised by an increase in Immunoglobulin E (IgE) in the tears and in the blood circulating in the capillaries of the conjunctiva and by an increase in circulating inflammatory cells (eosinophils) which trigger the inflammatory reaction by releasing inflammatory mediators, in particular histamine, produced by mast cells.
Allergic conjunctivitis is very frequent (affecting 25-35% of the population) and there are several clinical forms. The most common, possibly associated with rhinitis or airway involvement, is seasonal and perennial allergic conjunctivitis. The most severe and rarest forms, with corneal involvement and potential permanent visual impairment, are atopic keratoconjunctivitis and vernal keratoconjunctivitis.
Seasonal allergic conjunctivitis (SAC) accounts for more than 50% of allergic conjunctivitis and is caused by environmental allergens (pollen) that vary with the seasons. Perennial forms, on the other hand, can be triggered by allergens perpetually present in the environment, such as dust or animal hair.

What it entails


The main symptoms of allergic conjunctivitis are redness of the eyes, ocular and periocular itching, tearing, light sensitivity (photophobia), burning, sensation of a foreign body. They are usually associated with characteristic signs, such as: swelling and accumulation of fluid (oedema) in the eyelids, conjunctival redness (hyperaemia), enlargement of cells (papillary hypertrophy) at the level of the eyelid conjunctiva, initially clear/transparent secretion (acute phase) and subsequently dense/fibrous secretion (chronic phase). In subjects who have a complicated allergy, these symptoms are also associated with the nasal mucosa, therefore there is allergic rhinitis and rhinoconjunctivitis with sneezing, itchy nose, post-nasal drip, etc. The symptomatology of the perennial forms is less pronounced than the seasonal ones, but always characterised by itching, mild conjunctival hyperaemia, burning and sensation of a foreign body.

Diagnosis

Diagnosis is based on the patient's medical history (allergic history, patient's living and working context) and on clinical evidence (presence of symptoms and signs that are typical of allergic conjunctivitis). Some laboratory tests are useful for diagnosis, such as: the Prick test, which consists of the superficial cutaneous injection of a small amount of suspected allergens: the test is positive if a skin reaction with redness (rush) appears; conjunctival provocation tests that are carried out by placing a small amount of the suspected allergen in contact with the conjunctiva: if it is the right one, the redness and burning reappear or worsen; the examination of the tear fluid, which allows to identify the presence of specific IgE directed against a particular allergen.
Clinical diagnosis can be very complex due to the overlap of other ocular surface diseases such as infectious forms and dry eye syndrome.

Treatment

Treatment for allergic conjunctivitis primarily involves avoiding or minimising contact with the allergen responsible for the uncontrolled immune response in the "sensitive" subject, including some types of contact lenses and cosmetics. Secondly, the therapy involves the administration, on prescription by the ophthalmologist, of antiallergic drugs (of different categories) and steroidal and non-steroidal anti-inflammatory drugs, which reduce the signs and symptoms of the inflammatory reaction.
Antihistamine eye drops work by blocking the action of histamine, the main chemical mediator responsible for the inflammatory phenomena of the allergic reaction. By doing so, antihistamines prevent or greatly reduce the symptoms of the allergic reaction. However, they must be used before the spread of pollen that trigger the allergic reaction, with a prophylactic action. Some anti-allergic drugs also have an anti-inflammatory action and therefore are able to reduce the signs and symptoms of the allergic reaction even when it has already been triggered.
Other preparations used in the treatment of allergic conjunctivitis are vasoconstrictors and decongestants, which rapidly reduce conjunctival redness and swelling, but should be used in moderation and for short periods, as they do not treat the mechanism responsible for the pathology and can worsen the inflammation at the end of their effect (rebound effect).

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