The red eye: a case-based approach

The presentation

Imagine an adult who wakes with one red, gritty eye and a little sticky discharge. Their vision is normal, the eye is not painful, and they can look at bright light without discomfort. This picture — redness with no pain, no vision change and no light sensitivity — is typical of conjunctivitis, the most common and least worrying cause of a red eye. Now imagine a different person: one eye is deeply red around the coloured part, aches badly, the vision is blurred and bright light hurts. That second picture is completely different and points towards something inside the eye. The art of assessing a red eye is separating these two stories quickly, because the safe ones can be reassured while the dangerous ones must be seen urgently.

The questions that matter

Three questions do most of the work. First, is the vision affected? A drop in vision is a serious sign and should never be dismissed. Second, is there real pain, as opposed to mild grittiness or itching? Deep, aching pain suggests the problem is inside the eye rather than on its surface. Third, does bright light hurt the eye — this is called photophobia? Painful light sensitivity points towards inflammation inside the eye, such as uveitis. Other useful clues include whether one or both eyes are involved, whether the person wears contact lenses, and whether they have a headache, feel sick or are seeing haloes around lights. These answers steer everything that follows.

The sight-threatening causes

A handful of causes can damage vision and must not be missed. Acute angle-closure glaucoma causes a painful, red eye with blurred vision, haloes around lights, headache and sometimes vomiting; the eye may feel hard and the pupil mid-sized and fixed. Anterior uveitis (iritis) causes a painful red eye with light sensitivity and a small or irregular pupil. A corneal ulcer — especially in contact-lens wearers — causes a painful red eye with a white spot on the clear front of the eye and needs same-day care. Scleritis causes a deep, boring pain. Each of these is a reason to arrange urgent assessment, ideally by an eye service, rather than treating at home.

The reassuring causes

Most red eyes are not dangerous. Conjunctivitis — whether caused by a virus, bacteria or an allergy — makes the eye red and gritty with discharge or itching, but vision stays normal and there is no significant pain or light sensitivity. Viral conjunctivitis usually clears by itself over one to two weeks and is very contagious, so hand washing and not sharing towels matter. A subconjunctival haemorrhage looks alarming — a bright red patch of blood on the white of the eye — but is painless, harmless and fades over a couple of weeks. Dry eye and blepharitis cause chronic grittiness and redness. These conditions can be managed in the community and rarely need emergency care.

The safe pathway

The practical rule is simple: a red eye with normal vision, no significant pain and no light sensitivity is usually safe to manage in primary care or with a community pharmacist or optometrist. A red eye with any of the danger signs — reduced or blurred vision, marked pain, painful light sensitivity, haloes around lights, a contact lens in a painful eye, or a foreign body or chemical splash — needs urgent, same-day assessment. In the UK, community optometrists often run Minor Eye Conditions Services, and eye casualty departments can see urgent problems. When in doubt, it is always safer to have an eye examined than to assume redness alone is harmless.