What are the key differential diagnoses for a patient presenting with acute red eye in primary care?

When a patient presents with an acute red eye in primary care, the key differential diagnoses to consider include a spectrum of conditions ranging from benign to sight-threatening. Immediate identification of serious causes requiring urgent referral is critical.

• Infective conjunctivitis (bacterial or viral) is common and presents with redness, discharge, and irritation; urgent referral is needed if there are red flags such as corneal ulceration, pseudomembrane, or suspected ophthalmia neonatorum ¹,³.
• Allergic conjunctivitis typically presents with itching and redness; severe or resistant cases, or those with corneal involvement, require specialist assessment ¹,².
• Corneal pathology such as corneal abrasion, corneal ulcer, or contact lens-related keratitis must be considered, especially if there is pain, photophobia, or visual disturbance; these require same-day ophthalmology referral ¹.
• Acute glaucoma presents with a red eye, pain, blurred vision, and a mid-dilated pupil; it is a sight-threatening emergency requiring immediate referral ¹,⁴.
• Anterior uveitis and scleritis cause red eye with pain and photophobia; these require urgent referral within 24 hours ¹.
• Subconjunctival haemorrhage is a benign cause presenting as a sharply demarcated red patch without pain or discharge; reassurance and blood pressure check are appropriate ¹.
• Episcleritis causes mild discomfort and redness, usually self-limiting and managed conservatively in primary care ¹.
• Trauma including corneal foreign bodies, lacerations, or chemical injuries require immediate irrigation and urgent referral ¹.
• Blepharitis, dry eye, and eyelid malpositions such as ectropion or entropion can cause redness and irritation but are generally non-urgent ¹.

In primary care, clinical assessment should focus on identifying red flags such as severe pain, photophobia, reduced vision, corneal opacity, or history of trauma/contact lens use to guide urgency of referral ¹. The literature emphasizes that a low threshold for referral is prudent to avoid missing sight-threatening conditions (Wirbelauer, 2006). Overall, the differential diagnosis of acute red eye is broad, but prioritizing potentially serious causes ensures timely specialist management and preservation of vision [1, (Wirbelauer, 2006)].