Conjunctivitis

Conjunctivitis is inflammation of the conjunctiva, the thin, transparent membrane covering the white of the eye and the inner eyelids. Commonly called 'pink eye', it is one of the most frequent reasons for primary-care and emergency eye consultation and may be infectious (viral or bacterial) or non-infectious (allergic or irritant).

Definition

Conjunctivitis is inflammation of the conjunctiva, presenting with conjunctival redness and discharge, and classified by cause into infectious (most often viral or bacterial) and non-infectious (allergic, toxic, or mechanical) forms.

Scope

This entry covers the concept and classification of conjunctivitis, the distinction between viral, bacterial, and allergic forms, the underlying inflammatory and allergic mechanisms, and the condition's place as a high-frequency, usually self-limited eye disease. It is a reference overview and does not provide treatment instructions.

Key concepts

• Viral conjunctivitis (notably adenoviral)
• Bacterial conjunctivitis
• Allergic conjunctivitis
• Conjunctival hyperaemia and discharge
• Contagiousness and transmission
• Distinguishing conjunctivitis from sight-threatening red eye

Mechanisms

In infectious conjunctivitis, viruses (commonly adenovirus) or bacteria infect the conjunctival epithelium, triggering a local inflammatory response with vascular dilation (hyperaemia), increased vascular permeability, and discharge, watery in viral disease and more purulent in bacterial disease. Allergic conjunctivitis is an IgE-mediated type-I hypersensitivity reaction in which allergen exposure cross-links mast-cell-bound IgE, releasing histamine and other mediators that cause itching, redness, and chemosis. The shared endpoint, conjunctival inflammation, explains the overlapping signs across subtypes, while the pattern of discharge, itch, and contagiousness helps separate them.

Clinical relevance

Conjunctivitis is extremely common and usually benign and self-limited, but it must be distinguished from more serious causes of a red eye such as keratitis, uveitis, or acute glaucoma; understanding its types and mechanisms supports that triage. This entry is descriptive and for reference only, not a basis for diagnosis or treatment.

Epidemiology

Conjunctivitis is among the commonest eye complaints in primary and emergency care. Viral conjunctivitis, chiefly adenoviral, is the most frequent infectious cause in adults and is highly contagious, while bacterial conjunctivitis is relatively more common in children; allergic conjunctivitis is widespread and tracks with the prevalence of atopic disease such as hay fever.

Debates

Overuse of topical antibiotics for presumed bacterial conjunctivitis

Because most acute infectious conjunctivitis is viral and self-limited, frequent prescription of topical antibiotics for undifferentiated 'pink eye' is questioned on grounds of limited benefit and antimicrobial stewardship.

Seminal works

• azari-2013

Frequently asked questions

What are the main types of conjunctivitis?

The principal types are viral (most commonly adenoviral and highly contagious), bacterial, and allergic. Less commonly it can be caused by chemical or mechanical irritation. The cause influences the pattern of discharge, itching, and contagiousness.

How can conjunctivitis be told apart from a more serious red eye?

Conjunctivitis typically causes redness and discharge without severe pain, marked light sensitivity, or vision loss. Features such as significant pain, photophobia, reduced vision, or a fixed pupil suggest sight-threatening conditions like keratitis, uveitis, or acute glaucoma and require clinical assessment.