Red eye accounts for approximately 2 to 3% of all UK emergency department attendances and represents one of the most diagnostically stratified presentations a clinician will encounter on a busy shift. The vast majority are benign; a small but critical minority are sight-threatening or, rarely, life-threatening. That diagnostic tension, between the routine and the catastrophic, makes this topic a perennial favourite in FRCEM SBA practice questions, where single best answer stems are carefully constructed to test whether candidates can apply a structured clinical framework under time pressure. Whether you are working through an FRCEM SBA mock exam or refining your approach to ophthalmological emergencies, a rigorous understanding of red eye pathophysiology, examination findings, and management thresholds is non-negotiable.
FRCEM SBA practice questions: Key Points: Red Eye in the ED
- Acute primary angle-closure glaucoma (APACG) is the single most important sight-threatening diagnosis to exclude in any adult presenting with a painful, red eye and reduced visual acuity.
- The pattern of injection, conjunctival (peripheral, blanching) versus ciliary (perilimbal, non-blanching), is the most discriminating examination finding before a slit lamp is available.
- Intraocular pressure measurement and visual acuity testing are mandatory in any red eye presentation where a serious cause is suspected.
- Herpes simplex keratitis and herpes zoster ophthalmicus require urgent antiviral therapy and same-day ophthalmology review; topical steroids are contraindicated without specialist input.
- Endophthalmitis following intraocular surgery is an ophthalmic emergency requiring immediate specialist contact; any post-operative red eye with pain and visual loss must be treated as endophthalmitis until proved otherwise.
- FRCEM SBA clinical scenarios frequently hinge on a single discriminating feature, the shape of the pupil, the laterality of headache, or the timing relative to surgery, so train yourself to extract the decisive detail from each stem.
Epidemiology and Clinical Context
Red eye presentations span the full age spectrum but the aetiological mix shifts substantially with age. Conjunctivitis, both bacterial and viral, predominates in children and young adults and represents the most common cause overall. In working-age adults, corneal pathology, including traumatic abrasion and contact lens-related keratitis, is especially frequent, particularly in those in manual occupations. Anterior uveitis clusters in the 20 to 50 age group and is frequently associated with HLA-B27-related systemic conditions such as ankylosing spondylitis and inflammatory bowel disease. Acute primary angle-closure glaucoma (APACG) is most prevalent in older, hypermetropic women and carries the highest immediate threat to vision of all ED red eye presentations. NICE Clinical Knowledge Summaries provide a useful overview of the aetiology and primary care management of acute red eye, and candidates preparing FRCEM SBA revision should be familiar with the thresholds at which escalation to secondary care is mandated.
Risk factors for sight-threatening pathology that must be elicited in the history include: contact lens wear (keratitis, acanthamoeba), recent intraocular surgery (endophthalmitis), immunosuppression (atypical infections, viral keratitis), significant blunt or penetrating trauma (hyphaema, globe rupture, corneal perforation), systemic autoimmune disease (scleritis, peripheral ulcerative keratitis), and acute onset of pain with haloes in a patient who is hypermetropic (APACG).
Pathophysiology: Understanding the Pattern of Injection
The red appearance of the eye results from dilatation and engorgement of ocular surface vasculature. Two anatomically and clinically distinct patterns exist, and differentiating them is a core FRCEM SBA clinical skill.
Conjunctival (superficial) injection involves the anterior conjunctival vessels. These vessels are most prominent in the peripheral fornices and move freely with the conjunctiva. They blanch with topical phenylephrine 2.5%. This pattern is seen in bacterial and viral conjunctivitis, episcleritis, allergic eye disease, subconjunctival haemorrhage, and dry eye. In conjunctivitis, vessels are engorged throughout the tarsal and bulbar conjunctiva; in episcleritis, the inflammation is sectoral and the deeper episcleral vessels are uninvolved.
Ciliary (deep, perilimbal) injection involves the anterior ciliary vessels and produces a violet-red discolouration that is maximal at the limbus (the corneoscleral junction) and fades peripherally. These vessels do not move with the conjunctiva and do not blanch with phenylephrine. This pattern indicates intraocular or corneal disease, most importantly APACG, anterior uveitis, keratitis, and scleritis. Its recognition should immediately raise the clinical alert level and prompt formal visual acuity measurement and intraocular pressure (IOP) estimation.
In scleritis, a third, deeper injection pattern involves the scleral vessels themselves, producing a characteristic bluish-red discolouration. Scleritis is associated with systemic autoimmune disease in up to 50% of cases and carries a risk of scleral thinning and perforation; it is genuinely painful, in contrast to the relative comfort of episcleritis.
ED Assessment and Management
History
A focused history should establish: onset and duration, laterality, associated pain (and its character, dull ache versus severe, versus photophobia-associated), visual disturbance, discharge (purulent, watery, or mucoid), contact lens use, recent ocular surgery or trauma, systemic conditions, and current medications including topical steroids. Haloes around lights in a patient with sudden-onset pain, nausea, vomiting, and a hard eye are pathognomonic of APACG until proved otherwise.
Examination
Visual acuity must be measured and recorded in every patient with a red eye in whom a serious cause is possible. It is the single most important examination finding for triaging urgency. Additional mandatory elements include: pupillary assessment (a mid-dilated, oval, non-reactive pupil in APACG; a small, irregular, sluggish pupil in anterior uveitis), pattern of injection as described above, corneal clarity (corneal oedema in APACG; dendritic ulcer in herpes simplex keratitis visible with fluorescein staining and blue light), anterior chamber depth, and IOP estimation. The normal IOP is 10 to 21 mmHg; APACG typically produces pressures of 40 to 70 mmHg.
Investigation
In most red eye presentations, investigations are limited in the acute ED setting. Fluorescein staining and cobalt blue light examination of the cornea is essential to identify epithelial defects, dendritic ulcers, and foreign bodies. IOP measurement using a calibrated tonometer (Tonopen or Goldmann in the ophthalmology suite) is mandatory where glaucoma is suspected. Swabs are of limited immediate clinical value in most infective conjunctivitis presentations but are indicated in neonatal conjunctivitis (gonococcal or chlamydial infection), contact lens wearers with keratitis, and immunocompromised patients.
Management Priorities by Diagnosis
Bacterial conjunctivitis: Typically self-limiting within 7 to 10 days. Topical chloramphenicol remains first-line in the UK for patients in whom treatment is warranted. Contact lens wearers should be treated as keratitis and referred. NHS guidance supports a no-treatment or delayed-prescribing approach for uncomplicated cases in immunocompetent adults.
Corneal abrasion: Topical anaesthesia for examination, fluorescein staining to confirm, topical chloramphenicol, cycloplegic analgesia if uveitis coexists, and avoidance of eye patching (which delays healing and is not evidence-supported). Contact lens wearers require ophthalmology review given the risk of Pseudomonas keratitis.
Acute primary angle-closure glaucoma: This is a time-critical ophthalmic emergency. Immediate management includes: supine positioning (avoid prone), topical pilocarpine 2% to both eyes (to constrict the pupil and open the drainage angle), topical beta-blocker (timolol 0.5%), topical carbonic anhydrase inhibitor (dorzolamide), and systemic acetazolamide 500 mg intravenously if tolerated. Analgesia and antiemetics as required. Immediate emergency ophthalmology referral is mandatory; definitive treatment is laser peripheral iridotomy. The RCEM Learning platform provides detailed guidance on acute glaucoma management in the ED context.
Anterior uveitis (iritis): Characterised by perilimbal injection, photophobia, a small irregular pupil, and anterior chamber cells and flare on slit lamp. Treatment is topical corticosteroids and a cycloplegic agent (cyclopentolate 1%), with same-day ophthalmology review. Underlying systemic cause must be considered.
Herpes simplex keratitis: Dendritic ulcer on fluorescein staining is diagnostic. Treatment is topical aciclovir 3% five times daily or oral aciclovir 400 mg five times daily. Topical steroids are absolutely contraindicated without specialist direction as they can cause catastrophic corneal melting. Same-day ophthalmology review is required.
Herpes zoster ophthalmicus: Shingles involving the nasociliary branch of V1 (Hutchinson’s sign: vesicles on the tip of the nose) predicts ocular involvement. Systemic aciclovir or valaciclovir should be started within 72 hours of rash onset. Same-day ophthalmology referral.
Endophthalmitis: Post-operative red eye with pain, visual loss, and hypopyon (pus layering in the anterior chamber) constitutes an ophthalmic emergency. Immediate ophthalmology contact is required; management includes intravitreal antibiotics. The BMJ has published evidence reviews on the management of infectious endophthalmitis that are relevant to FRCEM SBA exam preparation.
How the FRCEM SBA Exam Tests This Topic
The RCEM curriculum for the FRCEM SBA examination maps red eye to the domain of presentations by symptoms and the ophthalmology subspecialty section. Exam stems are almost invariably structured as clinical vignettes, and red eye questions typically test one of three discriminatory skills: identifying the most likely diagnosis from a combination of examination findings, selecting the single most appropriate initial intervention, or identifying the correct referral threshold and urgency.
Common FRCEM SBA clinical scenarios involving red eye include:
- A 65-year-old hypermetropic woman presenting with sudden-onset right-sided headache, nausea, blurred vision, and a hard right eye with a mid-dilated oval pupil, testing recognition of APACG and correct immediate management.
- A contact lens wearer presenting with a unilateral painful red eye and a central corneal infiltrate, testing recognition of infective keratitis and the need for urgent ophthalmology review rather than routine chloramphenicol prescription.
- A patient two days post-cataract surgery presenting with pain, visual loss, and a hypopyon, testing recognition of endophthalmitis.
- A young man with ankylosing spondylitis presenting with photophobia, perilimbal injection, and a small irregular pupil, testing diagnosis of anterior uveitis and correct initial management.
- A child with bilateral purulent discharge and conjunctival injection where the distractors include viral conjunctivitis and allergic conjunctivitis, testing the candidate’s ability to identify bacterial conjunctivitis and appropriate prescribing.
A common exam pitfall is selecting topical steroids in a patient with a dendritic corneal ulcer. Another is failing to recognise that a contact lens wearer with a corneal abrasion should not simply receive chloramphenicol and be discharged. The RCEM curriculum explicitly mandates competency in recognising and escalating sight-threatening presentations, so questions testing referral threshold are particularly high-yield in FRCEM SBA exam questions.
Revision Pearls: High-Yield Facts for the FRCEM SBA
- APACG produces a mid-dilated, oval, non-reactive pupil because the pupil is fixed by the ischaemic sphincter. Anterior uveitis produces a small, irregular pupil due to posterior synechiae. These pupillary findings are classic discriminators in single best answer stems.
- Conjunctival injection blanches with phenylephrine; ciliary injection does not. This distinction maps directly to the likelihood of intraocular pathology.
- Topical steroids must never be initiated in the ED without ophthalmology input. In herpes simplex keratitis they are absolutely contraindicated and can cause progressive corneal melting and perforation.
- A dendritic ulcer on fluorescein staining is pathognomonic of herpes simplex keratitis; a geographic ulcer (expanded dendritic) indicates more advanced disease or previous inappropriate steroid use.
- Scleritis is genuinely painful and wakes patients at night; episcleritis is relatively comfortable. This clinical distinction appears frequently in MRCEM SBA and FRCEM SBA exam questions because the management and urgency differ substantially.
- Gonococcal conjunctivitis produces a hyperacute purulent discharge within 24 hours of onset and can perforate the cornea rapidly. It requires systemic ceftriaxone and immediate ophthalmology review.
- Orbital cellulitis is distinguished from preseptal (periorbital) cellulitis by the presence of proptosis, ophthalmoplegia, pain on eye movement, and reduced visual acuity. Orbital cellulitis requires urgent CT orbits, IV antibiotics, and emergency surgical review.
- Subconjunctival haemorrhage is alarming in appearance but benign if there is no associated trauma, vision loss, or high IOP. In the context of significant head injury, it may indicate an orbital floor fracture (posterior limit not visible).
Common Pitfalls: Where Candidates Lose Marks
- Prescribing topical chloramphenicol for a contact lens wearer with a painful red eye without recognising the high risk of Pseudomonas aeruginosa keratitis requiring urgent review.
- Failing to measure visual acuity before any intervention, which is both a clinical safety error and a reliable way to lose marks in an FRCEM SBA clinical scenario.
- Confusing the management of anterior uveitis (cycloplegics plus topical steroids under ophthalmology guidance) with that of infective conjunctivitis (topical antibiotics).
- Missing the diagnosis of APACG because the question stem emphasises nausea and vomiting, leading candidates toward a gastrointestinal or neurological answer.
- Applying topical steroids to a red eye with a dendritic ulcer, a characteristically dangerous distractor in herpes simplex keratitis questions.
- Not recognising Hutchinson’s sign (vesicles on the tip of the nose in herpes zoster ophthalmicus) as an indication for same-day ophthalmology referral regardless of whether ocular symptoms are present at the time.
How EM Learning Centre Supports Your FRCEM Single Best Answer Revision
Ophthalmological emergencies including red eye are covered in depth within the FRCEM Single Best Answer revision course at EM Learning Centre. The course is structured around the RCEM curriculum and written by practising UK emergency physicians, ensuring that clinical detail, management algorithms, and examination technique all reflect contemporary UK practice. Questions are accompanied by detailed explanatory rationales that go beyond the correct answer to explain why each distractor is incorrect, which is the most efficient way to consolidate understanding rather than simply accumulating correct responses.
If you are building your FRCEM SBA exam preparation plan, ophthalmology is a topic area where systematic revision pays dividends. The discriminating features between diagnoses are specific and learnable, and the management decisions are well-defined enough to be tested reliably in the SBA format. Working through structured FRCEM SBA practice questions with high-quality explanatory feedback is the most evidence-informed revision strategy for this format, and the EM Learning Centre homepage provides access to question banks, lesson content, and mock examinations aligned to the current RCEM curriculum.
The Royal College of Emergency Medicine publishes the FRCEM SBA curriculum and exam guidance, including the breadth of clinical topics assessed. Candidates should cross-reference their revision plan against the published curriculum to ensure comprehensive coverage, with ophthalmology representing a predictable and high-yield subset of the examination.
References
- Royal College of Emergency Medicine. FRCEM Examination Curriculum and Guidance. Available at: rcem.ac.uk
- NICE Clinical Knowledge Summaries. Red Eye. Available at: cks.nice.org.uk
- RCEM Learning. Ophthalmological Emergencies. Available at: rcemlearning.com
- BMJ. Evidence reviews in infectious endophthalmitis and ocular emergency management. Available at: bmj.com
- NHS. Red Eye: Assessment and Management. Available at: nhs.uk
