Conjunctivitis !
Conjunctivitis is inflammation of the thin membrane covering the white of the eye and inner eyelids; it is the commonest cause of red eye.
Updated 2026-10-29
Structures involved
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Microorganisms that cause it
In histology
Why it happens
What it is
Inflammation of the conjunctiva, the thin membrane that lines the eyelids and covers the white of the eye. It is the commonest cause of red eye and is usually viral, bacterial or allergic.
Who gets it
Very common at all ages. Viral conjunctivitis spreads in outbreaks in schools and workplaces; bacterial forms are more frequent in children; allergic conjunctivitis often accompanies allergic rhinitis.
Causes
- Viral: adenovirus most often; also herpes simplex and herpes zoster
- Bacterial: Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella
- Hyperacute: Neisseria gonorrhoeae; chronic or inclusion: Chlamydia trachomatis
- Allergic: seasonal or perennial allergens, vernal and atopic forms
- Irritant, toxic or chemical
Risk factors
- Contact with an infected person; crowded settings
- Contact lens use
- Atopy (allergic rhinitis, asthma, eczema)
- Newborn of a mother with gonococcal or chlamydial infection
- Sexually transmitted infection in adults
Pathophysiology
Infection or allergen exposure triggers dilation of conjunctival vessels, oedema (chemosis) and discharge. In allergy, IgE-mediated mast cell release of histamine causes itching. Viruses cause a watery discharge and lymphoid follicles; bacteria cause purulent discharge. The cornea is usually spared, so vision is normal.
Classification
- Infectious: viral, bacterial (acute, hyperacute, chronic), chlamydial
- Non-infectious: allergic, irritant or toxic
- Neonatal conjunctivitis (ophthalmia neonatorum)
- Acute (under 4 weeks) or chronic
Symptoms and signs
- Red eye, often both eyes
- Gritty or burning sensation
- Discharge: watery (viral), purulent (bacterial), stringy (allergic)
- Eyelids stuck together in the morning
- Intense itching (allergic)
- Swollen preauricular lymph node (viral)
- Normal or nearly normal vision
Diagnosis
- Clinical
- Mostly clinical, from the type of discharge, itching, laterality, preauricular nodes and history. Fluorescein staining looks for corneal involvement. Visual acuity is always checked.
- Tests
- Culture and Gram stain for hyperacute, neonatal, severe or persistent cases; PCR for chlamydia, gonococcus, adenovirus or herpes when suspected. Rapid adenovirus tests exist.
- Imaging
- Not needed. Orbital imaging only if orbital cellulitis is suspected.
Differential diagnosis
- Keratitis
- Acute angle-closure glaucoma
- Anterior uveitis
- Scleritis or episcleritis
- Subconjunctival haemorrhage
- Preseptal or orbital cellulitis
Goal of treatment
Relieve symptoms, limit spread, use antibiotics only where they help, and detect sight-threatening causes of red eye.
Treatment
- Conservative
- Cold compresses, lubricating drops and lid hygiene; viral conjunctivitis settles on its own over one to a few weeks. Contact lenses are left out until it resolves.
- Medical
- Topical broad-spectrum antibiotics shorten bacterial conjunctivitis; many mild cases resolve without them.
- Medical
- Gonococcal or chlamydial conjunctivitis: systemic antibiotics, treatment of sexual partners and, in newborns, paediatric assessment.
- Medical
- Allergic conjunctivitis: topical antihistamines and mast cell stabilisers; topical corticosteroids only under ophthalmology supervision.
- Lifestyle
- Hand washing and not sharing towels or eye drops to reduce spread.
Drugs for this condition
Complications
- Keratitis and corneal ulcer
- Subepithelial corneal infiltrates after adenovirus
- Corneal perforation in gonococcal infection
- Scarring in chronic chlamydial infection (trachoma)
- Spread to contacts
Prognosis
Most viral and bacterial conjunctivitis resolves without lasting harm. Hyperacute gonococcal and neonatal infections can damage the cornea quickly without urgent treatment.
Prevention
- Hand hygiene and not sharing personal items
- Neonatal eye prophylaxis and prenatal screening for sexually transmitted infections
- Contact lens hygiene
- Allergen avoidance in allergic forms
Test yourself
Think of the answer, then tap the question to see it.
Which type of discharge suggests bacterial rather than viral conjunctivitis?
A thick purulent discharge; viral conjunctivitis usually causes watery discharge.
Which symptom is most characteristic of allergic conjunctivitis?
Itching, usually in both eyes, often with allergic rhinitis.
Which virus causes most infectious conjunctivitis?
Adenovirus, often with a tender preauricular lymph node and outbreaks in close contacts.
What features in a red eye suggest something more serious than conjunctivitis?
Reduced vision, significant pain, photophobia, a fixed or irregular pupil or a cloudy cornea.
Which organism causes hyperacute conjunctivitis with copious purulent discharge?
Neisseria gonorrhoeae, which can perforate the cornea and needs systemic antibiotics.
Related conditions
- Acute otitis media
- Uveitis
- Acute angle-closure glaucoma
- Chalazion and hordeolum (stye)
- Allergic rhinitis
- Gonorrhoea
- Chlamydia infection
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Conjunctivitis Overview (StatPearls (NCBI Bookshelf), 2026).
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