What is conjunctivitis?
Conjunctivitis is inflammation of the conjunctiva of the eye.
The conjunctiva is the semi-transparent membrane that covers the white part of the eye (the sclera) and lines the inside of the eyelids.
When the conjunctiva is inflamed, the white of the eye appears red. Conjunctivitis is the most likely diagnosis when a patient has a red eye and discharge.
Allergic conjunctivitis is a non-infectious form of conjunctivitis and is due to a reaction to an allergen in contact with the surface of the eye.
What causes allergic conjunctivitis?
Allergic conjunctivitis can be caused by an immediate Type 1 hypersensitivity reaction, often to airborne allergen.
The allergen causes the rapid release of chemical mediators including histamine and others, resultin in acute or episodic symptoms within minutes of exposure.
Chronic allergic conjunctivitis is often more correctly called keratoconjunctivitis because it usually involves the cornea as well as the conjunctiva. The most common forms of allergic keratoconjunctivitis are vernal keratoconjunctivitis and atopic keratoconjunctivitis.
- Atopic forms of keratoconjunctivitis are associated with hay fever (allergic rhinitis), asthma and eczema.
- Symptoms may be seasonal, most often arising during spring from tree pollen or during summer from grass pollen; they can also be perennial (year-round) in reaction to house dust mite, pet dander or moulds.
- It mainly affects people under 20, but some people continue to suffer lifelong.
- It can cause corneal ulceration.
Conjunctivitis due to Type III hypersensitivity reaction arises as part of Stevens-Johnson syndrome-toxic epidermal necrolysis (SJS-TEN), a serious systemic reaction to a drug.
Conjunctivitis can also be due to delayed Type IV hypersensitivity reactions to allergens, which includes local reactions to preservatives or medicaments (eg neomycin) in eye drops. It may take some hours or days to start reacting to these.
What are the symptoms and signs of allergic conjunctivitis?
Conjunctivitis is a diagnosis of exclusion. All forms of conjunctivitis are characterised by:
- Redness and discharge in one or both eyes
- Diffuse redness around the globe including the underside of the upper and lower lids
- Adhesion of the upper and lower eyelids on waking
- Normal vision
- Absence of focal pathology (eg stye or blepharitis)
The following "red flags" point to other more serious diagnoses, such as keratitis (inflammation of the cornea), iritis, or angle closure glaucoma:
- Changes in visual acuity
- Ciliary flush: A pattern of injection in which the redness is most pronounced in a ring at the limbus (the border of the cornea and the sclera)
- Photophobia (light sensitivity)
- Severe foreign body sensation that prevents the patient from keeping the eye open
- Corneal opacity
- Fixed pupil
- Severe headache with nausea
Allergic conjunctivitis is distinguished by:
- Bilateral redness and watery discharge
- Itching (compared with the gritty, burning, irritation sensation of viral conjunctivitis)
- Assocation with hay fever, asthma and/or eczema
- Recent exposure to an allergen (not always identified)
- In some cases, marked chemosis (conjunctival oedema)
Vernal keratoconjunctivitis is a chronic form of allergic conjunctivitis. Atopic keratoconjunctivitis is characterised by bilateral eyelid dermatitis.
What are the complications of allergic conjunctivitis?
Most forms of allergic conjunctivitis are relatively mild, but in certain cases (eg SJS-TEN), severe complications may occur. Pain, loss of vision or photophobia require immediate referal to an ophthalmologist.
Complications from allergic conjunctivitis include corneal ulceration.
How is allergic conjunctivitis treated?
Treatment of allergic conjunctivitis or keratoconjunctivitis includes:
- Avoiding triggers for the allergy
- Treatment of allergic rhinitis using oral antihistamines, intranasal corticosteroids, saline eye drops, antihistamine eye drops (eg olapatidine) or cromoglycate eye drops
- Stopping using contact lenses until the conjunctivitis has cleared
- Avoiding use of eye drops containing preservatives
- Immunotherapy: testing for specific allergen sensitivities — this allows targeted immunotherapy
Nonsteroidal anti-inflammatory drugs, such as ketorolac, and steroid eyedrops or systemic glucocorticoids are sometimes used under specialist advice. Warning: glucocorticoids can cause sight-threatening complications, including corneal scarring and perforation, cataracts and glaucoma, when used inappropriately.