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Conjunctivitis is a very common cause for an uncomfortable, red eye.1,2 There are a number of distinct causes for conjunctivitis, which include bacteria, viruses and, importantly, allergies.
Allergic conjunctivitis affects the conjunctiva of the eye. The conjunctiva refers to the thin, translucent mucous membrane that extends from the limbus of the eye over the anterior surface of the eyeball and then continues over to cover the posterior surface of the eyelids. There are three parts to the conjunctiva: The bulbar conjunctiva covers the anterior part of the eye, the palpebral conjunctiva lines the eyelids and conjunctival fornix represents the space between the previous two parts.3,4

There are several main subtypes within the spectrum of allergic conjunctivitis. These include:4

  • Simple allergic conjunctivitis (SAC) comprising both seasonal and perennial allergic conjunctivitis;
  • Vernal keratoconjunctivitis (VKC);
  • Atopic keratoconjunctivitis (AKC); and
  • Giant papillary conjunctivitis (GPC).

Allergic conjunctivitis, as opposed to other forms of conjunctivitis, refers to the process by which the conjunctiva becomes inflamed due to allergic reactions, also known as hypersensitivity reactions.4 Hypersensitivity reactions are due to an acquired or innate over-sensitivity to an external allergen. There are four different types of hypersensitivity reactions.5 Type 1 (immediate) hypersensitivity reactions cause mast cell degranulation and the release of chemical mediators, such as histamine, which increase the permeability of the surrounding blood vessels.4,5 Further to this, chemotactic factors result in the migration of eosinophil and neutrophil cells.3,4 SAC is solely mediated by Type 1 hypersensitivity reactions.5 VKC, AKC and GPC are partly attributable to Type 1 reactions and partly due to other types of hypersensitivity, such as Type 4.5 The clinical signs and symptoms of allergic conjunctivitis arise from the chemical and inflammatory cascade that results from the hypersensitivity reaction.

Simple allergic conjunctivitis (SAC)

SAC refers to both seasonal allergic conjunctivitis (SAC) and perennial allergic conjunctivitis (PAC).

For more information, see Simple Allergic Conjunctivitis.

Vernal keratoconjunctivitis (VKC)

VKC represents a more chronic and severe form of allergic conjunctivitis, which is strongly linked to a personal or family history of other atopic diseases.

Atopic keratoconjunctivitis (AKC)

AKC refers to inflammation of the conjunctiva and eyelids, which is most commonly bilateral and strongly linked to atopic dermatitis.

Giant papillary conjunctivitis (GPC)

GPC refers to a conjunctival reaction in response to a foreign body causing prolonged mechanical irritation. Most commonly, contact lenses are implicated.


Allergic conjunctivitis (of any type) is extremely common and has been demonstrated to affect up to 40% of the population.6 The incidence of allergic conjunctivitis appears to be increasing, with some attributing this rise to increasing environmental exposure, such as air pollution.5

Predisposing Factors

There are a number of factors that may predispose an individual to developing allergic conjunctivitis, although these vary according to the specific type of conjunctivitis.
Family and personal histories are important factors in all types of allergic conjunctivitis. It is well reported that a personal or family history of atopic disease, including allergic rhinitis, asthma and atopic dermatitis, increases the risk of an individual developing allergic conjunctivitis.1,3,4,7,8
Other predisposing factors for allergic conjunctivitis include the environment, age, gender and ocular factors (for more information, see the specific sub-types).

Natural History

The clinical course and natural history of disease are variables. In simple allergic conjunctivitis, the condition is often self-limiting, although it may progress to chronic forms in some cases.9
The other types of allergic conjunctivitis often become inactive later in life. However, treatment is important as complications can occur.

Clinical History

A detailed clinical history is vital in all individuals presenting with ocular symptoms. It is important to rule out other more serious conditions that may cause an uncomfortable red eye, and also to rule out other causes of conjunctivitis, as the treatment and management varies markedly.
Close attention should be paid to several aspects of the clinical history. In particular:

  • Whether there has been any recent exposure to other people with conjunctivitis or an upper respiratory tract infection. This may point more towards a diagnosis of viral conjunctivitis;3
  • A detailed sexual history should be taken, particularly if there is a history of Chlamydia or Gonorrhoea infection, as these can both cause conjunctival eye disease;3
  • It is also important to enquire about the use of prescription and over-the-counter medications, as several topical medications can produce an inflammatory reaction;2,3
  • The past medical history needs to be elicited, particularly if there is a previous history of any eye diseases, contact lens use, or systemic diseases such as rheumatoid arthritis that may be associated with eye conditions;3
  • Attention should also be paid to aspects of the family history, in particular if there is a history of atopic diseases including allergic rhinitis, asthma and dermatitis.3

Those affected by allergic conjunctivitis will often describe a typical history and set of symptoms, including:3

  • Itching;
  • Tearing with a watery discharge;
  • Burning;
  • Stinging; and
  • Mild photophobia.

The most characteristic feature of allergic conjunctivitis is itching. If this is not present, then other diagnoses should be carefully considered as allergic conjunctivitis is unlikely.10 The itching is most often mild but may be intense and intolerable, although this is less common. The eyes are generally uncomfortable, but are not normally particularly painful. The individual may report exacerbations with warmer weather and describe nasal symptoms that accompany their eye symptoms.3
If unremitting ocular pain and reduced vision are reported, the individual requires urgent referral to an ophthalmologist. These symptoms are not characteristic of allergic conjunctivitis and may point to an alternative and more severe condition, such as uveitis, acute glaucoma or an ocular infection.10

Clinical Examination

Clinical examination of an individual with suspected allergic conjunctivitis should begin with observation of the external eye and eyelids, in particular looking for any signs of eyelid involvement such as dermatitis. A close examination of the conjunctiva is also required, looking at both the bulbar conjunctiva and the palpebral conjunctiva. The bulbar conjunctiva can be examined by retracting the eyelid and asking the individual to look up and down. The palpebral conjunctiva can be examined by everting the eyelid. In eyelid eversion, the individual is required to look downwards while a cotton swab (Q-tip) is placed horizontally on the upper eyelid. The clinician performing the eversion will then gently grasp the upper lid eyelashes and pull the eyelid out and up, gently rolling it over the cotton swab in order to visualise the inside surface of the eyelid.1,10
Fundoscopy of the eye should be performed using an ophthalmoscope. Slit-lamp microscopy is also useful for clearly visualising the conjunctival blood vessels.3

General Investigation

In most cases, no general investigations are required, as the diagnosis of allergic conjunctivitis can be made on the basis of the clinical history and examination.

Specific Investigations

In select cases, conjunctival scrapings, levels of inflammatory mediators in the tears, and skin allergy testing may all help to establish the diagnosis and provide further information about the condition.1, 3
Conjunctival scraping involves dropping a small amount of topical local anaesthetic into the eye, after which the inner surface of the eyelid is carefully scraped and the contents examined microscopically. Typically, eosinophilic cells are seen, which are not normally found in the conjunctiva of non-allergic individuals.3


In many cases the symptoms of allergic conjunctivitis are mild and the affected individual does not require long-term treatment. In these cases, the prognosis is favourable. In more complicated cases, the prognosis is dependent on the type of allergic conjunctivitis present and the available treatment.3

For more information about allergic conjunctivitis and its subtypes, see Allergic Conjunctivitis.


  1. Khaw PT, Shah P, Elkington AR. ABC of Eyes (4th edition). London: BMJ Publishing Group; 2004.
  2. Leibowitz HM. The red eye. N Engl J Med. 2000;343(5):345-51.
  3. Bielory L, Friedlaender MH. Allergic conjunctivitis. Immunol Allergy Clin North Am. 2008;28(1):43-58.
  4. Khurana AK. Comprehensive Ophthalmology (4th edition). New Delhi: New Age International Publishers; 2007.
  5. Schmid KL, Schmid LM. Ocular allergy: Causes and therapeutic options. Clin Exp Optom. 2000;83(5):257-270.
  6. Singh K, Bielory L. Ocular allergy: A national epidemiologic study. J Allergy Clin Immunol. 2007; 119(1 Suppl): S154.
  7. Bonini S. Atopic keratoconjunctivitis. Allergy. 2004;59 Suppl 78:71-3.
  8. Ono SJ, Abelson MB. Allergic conjunctivitis: Update on pathophysiology and prospects for future treatment. J Allergy Clin Immunol. 2005;115(1):118-22.
  9. Abelson MB, Granet D. Ocular allergy in pediatric practice. Curr Allergy Asthma Rep. 2006;6(4):306-11.
  10. Moloney G, McCluskey PJ. Classifying and managing allergic conjunctivitis. Medicine Today. 2007; 8(11): 16-21.
  11. Bielory L, Mongia A. Current opinion of immunotherapy for ocular allergy. Curr Opin Allergy Clin Immunol. 2002;2(5):447-52.

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