Conjunctivitis, commonly called pink eye, is one of the most frequently encountered eye conditions in primary and specialist eye care. It refers to inflammation of the conjunctiva: the thin, transparent membrane that lines the inner surface of the eyelids and covers the white part of the eyeball. When inflamed, the blood vessels within the conjunctiva become dilated and visible, producing the characteristic redness that gives the condition its popular name.
Despite being common and often self-limiting, conjunctivitis deserves careful attention for two important reasons. First, the correct treatment depends entirely on identifying the type -- viral, bacterial, or allergic -- and each type requires a different approach. Using the wrong treatment (such as antibiotic drops for a viral infection) is ineffective and contributes to antibiotic resistance. Second, some serious eye conditions that can cause permanent vision loss -- including corneal ulcer, uveitis, and acute glaucoma -- can initially present with a red eye that resembles conjunctivitis. Knowing the warning signs that distinguish these conditions from straightforward conjunctivitis is clinically important.
Viral conjunctivitis is the most common of the three types and is highly contagious. It typically presents with a watery or slightly mucoid clear discharge rather than the thick pus associated with bacterial infection. Itching is mild and may not be a prominent feature, whereas redness is often quite marked.
The condition often begins in one eye before spreading to the other over a day or two -- a pattern that is characteristic of the viral form. It is closely associated with upper respiratory tract infections: patients frequently have a concurrent cold, sore throat, or recent exposure to someone with similar symptoms.
Viral conjunctivitis is not treated with antibiotics, which have no effect on viruses. Management is supportive: artificial tears, cool compresses, and strict hand hygiene to prevent spread. The exception is herpetic conjunctivitis, which requires specific antiviral treatment. Most cases resolve within 7 to 14 days, though epidemic keratoconjunctivitis caused by adenovirus can persist for up to three weeks and may involve the cornea, temporarily affecting vision.
Bacterial conjunctivitis is distinguished most reliably by its discharge, which is mucopurulent -- thick, yellow or green, and copious enough to glue the eyelids together on waking. This is the feature patients most often describe as "eyes stuck shut in the morning," and it is strongly suggestive of a bacterial rather than viral or allergic cause. Unlike viral conjunctivitis, itching is generally mild or absent. The condition often begins in one eye, though it can spread to the other.
Bacterial conjunctivitis responds to a short course of topical antibiotic eye drops -- typically chloramphenicol, fusidic acid, or tobramycin -- over five to seven days. Symptoms should begin improving within 48 to 72 hours of starting treatment. Gonococcal conjunctivitis is an exception that requires same-day ophthalmologist assessment and systemic antibiotics given the risk of rapid corneal damage.
Allergic conjunctivitis is the only type that is not contagious and the only type in which itch is the dominant symptom. It is often described by patients as intense and difficult to resist rubbing, which in turn worsens the condition by releasing more histamine from mast cells. The discharge is watery and clear, similar to the viral form, and both eyes are almost always affected simultaneously from the outset, distinguishing it from the sequential pattern of the infectious types.
The condition is not associated with a preceding cold or infection. Instead, it is closely linked with other atopic conditions, hay fever, asthma, and eczema, and triggered by airborne allergens such as house dust mite (particularly relevant in Singapore's humid climate), pollen, pet dander, and mould. Unlike the infectious types, allergic conjunctivitis persists for as long as allergen exposure continues and can be a chronic, recurrent condition rather than a self-resolving episode.
Treatment focuses on allergen avoidance, antihistamine or mast cell stabiliser eye drops (or dual-action agents that do both), and artificial tears to flush allergens from the ocular surface. Topical corticosteroids may be considered in severe cases but only under ophthalmologist supervision.
Contact lens wearers who develop conjunctivitis require specific attention because of the significantly higher risk of contact-lens-associated keratitis (corneal infection) compared to non-wearers. Pseudomonas aeruginosa is a particularly virulent pathogen in contact lens-related keratitis and can cause rapid, severe corneal ulceration with risk of permanent scarring and vision loss.
Most conjunctivitis can be diagnosed clinically based on history and examination. Diagnostic tests are reserved for cases where the cause is uncertain, the infection is severe, or treatment has failed.
Prevention strategies differ by type. The principles below apply most directly to viral and bacterial conjunctivitis, the contagious forms.
