27.1 The Acute Red Eye & Sudden Painless Visual Loss

Medical ophthalmology encompasses ocular emergencies and the ocular manifestations of multisystem medical diseases. The MRCP(UK) Part 1 examination rigorously tests the differential diagnosis of the acute red eye, the clinical approach to sudden visual loss, retinal manifestations of vascular disease, and ophthalmic complications of systemic autoimmune disorders.

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1. Differential Diagnosis of the Acute Painful Red Eye

Evaluating the acute painful red eye requires distinguishing between benign external ocular disease and vision-threatening intraocular conditions.

Acute Angle-Closure Glaucoma (AACG)

• Pathophysiology: Occurs in anatomically predisposed eyes with a shallow anterior chamber (hyperopes, elderly with thickening cataracts). In mid-dilation, the peripheral iris apposes the anterior lens surface (pupillary block), causing pressure in the posterior chamber to rise, which bows the peripheral iris forward (iris bombé) against the trabecular meshwork, completely obstructing aqueous outflow.
• Signs: Extremely firm, stony-hard globe on digital palpation through closed eyelids; intraocular pressure (IOP) markedly elevated (40–70 mmHg; normal 10–21 mmHg); "steamy" cornea with microcystic epithelial oedema; fixed, mid-dilated, vertically oval pupil.
• Emergency Management Protocol:
  1. Intravenous Acetazolamide 500 mg stat: Carbonic anhydrase inhibitor that rapidly reduces aqueous humour production by the ciliary body epithelium.
  2. Topical Beta-Blocker: Timolol 0.5% drops (decreases aqueous production).
  3. Topical Alpha-2 Agonist: Apraclonidine 1% drops (reduces aqueous production and enhances uveoscleral outflow).
  4. Topical Pilocarpine 2%: Muscarinic agonist that stimulates the pupillary sphincter, inducing miosis to pull the peripheral iris root away from the trabecular meshwork (ineffective when IOP > 40–50 mmHg due to pressure-induced iris ischaemia; instil once IOP begins to fall).
  5. Hyperosmotic Agent: Intravenous mannitol 20% (1–2 g/kg over 45 minutes) if IOP remains refractory.
  6. Definitive Treatment: Bilateral Nd:YAG laser peripheral iridotomy (LPI). Creates a full-thickness bypass hole in the peripheral iris, equalizing anterior and posterior chamber pressures. Prophylactic iridotomy of the unaffected contralateral eye is mandatory because anatomical predisposition is bilateral.

Anterior Uveitis (Iritis)

• Clinical Manifestations: Circumcorneal injection (ciliary flush), severe aching pain, pronounced photophobia (direct and consensual pupillary constriction elicits excruciating pain due to spasm of the inflamed ciliary muscle and iris sphincter). Slit-lamp biomicroscopy reveals inflammatory leukocytes (cells) and protein exudate (flare) circulating in the anterior chamber, keratic precipitates on the corneal endothelium, and occasionally a gravitational layer of leukocytes (hypopyon).
• Posterior Synechiae: Adhesions between the inflamed pupillary margin of the iris and the anterior lens capsule produce an irregular, scalloped, fixed pupil and can precipitate secondary angle-closure glaucoma.
• Treatment: Topical corticosteroids (dexamethasone 0.1% or prednisolone acetate 1% drops) to suppress uveal inflammation, combined with a topical cycloplegic/mydriatic agent (cyclopentolate 1% or atropine 1%). Cycloplegics paralyze the ciliary muscle to relieve excruciating photophobic ciliary spasm and maintain pupillary dilation, breaking and preventing posterior synechiae.

Scleritis vs. Episcleritis: The Phenylephrine Blanching Test

• Scleritis involves destructive inflammation of the deep episcleral and scleral vascular plexuses. Instillation of topical phenylephrine 2.5% drops constricts the superficial conjunctival and episcleral vessels (causing prompt blanching in benign episcleritis), but fails to blanch the deep scleral vessels in scleritis.
• Scleritis can cause scleral necrosis and globe perforation (scleromalacia perforans, classical in rheumatoid arthritis). Topical steroids are ineffective for anterior scleritis; it mandates oral NSAIDs or high-dose systemic corticosteroids / biologic immunosuppression.

Microbial Keratitis & Herpes Simplex Keratitis

• Microbial Keratitis: Contact lens wearers are prone to aggressive Gram-negative infection by Pseudomonas aeruginosa, which secretes elastases and proteases causing rapid corneal liquefaction ("melting") and perforation within 24–48 hours; treated with intensive hourly fluoroquinolone monotherapy (e.g. ofloxacin 0.3%) or fortified cephalosporin/aminoglycoside drops.
• Herpes Simplex Virus (HSV) Keratitis: Presents with foreign body sensation, redness, and photophobia. Staining with 1% fluorescein under cobalt-blue illumination reveals a pathognomonic dendritic ulcer (branching epithelial lesion with terminal bulbs) associated with decreased corneal sensation (tested with a fine wisp of cotton wool). Treated with topical aciclovir 3% eye ointment 5 times daily. CRITICAL EXAM WARNING: Topical corticosteroids are absolutely contraindicated in dendritic ulcers, as steroids suppress local viral immunity, transforming a linear ulcer into an extensive, destructive "geographic" or "amoeboid" ulcer with imminent corneal perforation.

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2. Sudden Painless Loss of Vision

                      Sudden Painless Loss of Vision Algorithms
                                         |
     +-------------------+---------------+-------------------+-------------------+
     |                   |                                   |                   |
CRAO (Artery)        CRVO (Vein)                         AION (Optic Nerve)  Retinal Detachment
- Embolic stroke     - Thrombosis / Hyperviscosity       - Arteritic (GCA)   - Rhegmatogenous
- RAPD present       - RAPD absent/mild                    vs Non-arteritic  - Photopsia / Floaters
- Cherry-red spot    - 'Blood & thunder' fundus          - Altitudinal field - 'Dark curtain' defect
- Pale milky retina  - Risk of 100-day glaucoma            defect            - Elevated grey retina
- Exclude GCA        - Anti-VEGF / PRP                   - High ESR/CRP (GCA)  with folds

Central Retinal Artery Occlusion (CRAO)

• Pathophysiology: Ophthalmic equivalent of an acute ischemic cerebral stroke; thromboembolism lodged in the central retinal artery (arising from internal carotid atheroma or cardiac sources: atrial fibrillation, valvular vegetations).
• Presentation: Catastrophic, sudden, painless, profound monocular visual loss (acuity dropped to counting fingers or hand movements).
• Physical Signs: Marked Relative Afferent Pupillary Defect (RAPD / Marcus Gunn pupil).
• Fundoscopy:
  • Diffusely pale, opaque, milky-white edematous retina (intracellular cloudy swelling of ischemic retinal ganglion cells).
  • Pathognomonic "Cherry-Red Spot" at the Fovea: The fovea centralis is extremely thin and devoid of overlying ganglion cell layers; it is nourished directly by the underlying choroidal circulation, allowing the vivid reddish choroid to shine through the surrounding opaque retina.
  • Severe retinal arteriolar attenuation with segmenting of the blood column ("box-carring" / cattle-trucking).
• Emergency Management: Retinal tissue suffers irreversible infarction within 90 to 100 minutes. Immediate maneuvers (ocular massage, anterior chamber paracentesis, sublingual isosorbide dinitrate) aim to dislodge the embolus into peripheral arterioles. Urgent systemic workup: Carotid Doppler ultrasonography, echocardiogram, Holter ECG, and immediate ESR/CRP to exclude Giant Cell Arteritis (GCA).

Central Retinal Vein Occlusion (CRVO)

• Pathophysiology: Thrombosis of the central retinal vein at or posterior to the lamina cribrosa.
• Risk Factors: Advancing age, systemic hypertension, diabetes mellitus, primary open-angle glaucoma, and hyperviscosity syndromes (polycythaemia vera, multiple myeloma, Waldenström's macroglobulinaemia, antiphospholipid syndrome).
• Presentation: Sudden or subacute, painless visual blurring or moderate-to-severe visual impairment.
• Fundoscopy: Classic "stormy sunset" or "blood and thunder" appearance:
  • Widespread, dramatic flame-shaped and blot retinal haemorrhages throughout all four quadrants;
  • Markedly engorged, tortuous, dilated retinal veins;
  • Widespread cotton wool spots (microinfarcts of the nerve fibre layer) and optic disc oedema.
• Late Complication: Ischaemic CRVO releases vascular endothelial growth factor (VEGF), stimulating abnormal neovascularization of the iris (rubeosis iridis) and filtration angle, precipitating intractable, painful neovascular glaucoma, classically occurring around 90 to 100 days post-occlusion ("100-day glaucoma").
• Management: Intravitreal anti-VEGF agents (aflibercept, ranibizumab) or intravitreal dexamethasone implants for macular oedema; panretinal photocoagulation (PRP) laser for anterior segment neovascularization.

Anterior Ischaemic Optic Neuropathy (AION)

• Presentation: Sudden, painless monocular visual loss with a characteristic altitudinal visual field defect (sharp loss of either the superior or inferior hemifield respecting the horizontal meridian).
• Arteritic AION (A-AION): Secondary to Giant Cell Arteritis (GCA / Temporal Arteritis) in patients > 50 years. Occlusion of the posterior ciliary arteries causes transmural necrotizing vasculitis. Accompanied by temporal headache, scalp tenderness, jaw claudication, and constitutional symptoms. Fundoscopy demonstrates a chalky-white, pale swollen optic disc. ESR (> 50–100 mm/h) and CRP are markedly elevated. Management: Immediate IV methylprednisolone (500 mg – 1 g daily for 3 days) followed by high-dose oral prednisolone (60 mg daily) to prevent irreversible contralateral visual loss (which occurs in up to 50% within days). Never delay corticosteroids for temporal artery biopsy!
• Non-Arteritic AION (NA-AION): Nocturnal hypotension and microvascular hypoperfusion in patients with small, crowded optic discs ("disc at risk", absent physiological cup). Normal ESR/CRP.

Retinal Detachment

• Pathophysiology: Separation of the neurosensory retina from the underlying retinal pigment epithelium (RPE), most commonly rhegmatogenous (vitreous traction creates a full-thickness retinal tear in myopic eyes or after cataract surgery).
• Classic Symptom Cascade:
  1. Photopsia: Flashes of light in the peripheral visual field (mechanical vitreoretinal traction);
  2. Floaters: Sudden burst of black specks or spiderwebs (vitreous hemorrhage or pigment release);
  3. Progressive Field Defect: A dense, dark "curtain" or "shadow" descending or ascending across the visual field;
  4. Central Vision Loss: Sudden catastrophic loss of visual acuity occurs when the detachment involves the macula ("macula off").
• Fundoscopy: Elevated, folded, mobile, grey-white opalescent retina with loss of choroidal vascular details.

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