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Red Eye in PLAB 2: A Structured Approach to Differentials and Red Flags


Red eye is a common PLAB 2 presentation. While many causes are benign, some are ophthalmic emergencies. The key is to systematically rule out serious causes before diagnosing a benign condition.


Initial Approach to Red Eye


Important Questions and What They May Suggest

  • Painful or painless?

    • Painless → conjunctivitis, subconjunctival haemorrhage

    • Painful → keratitis, uveitis, glaucoma, scleritis

  • Any reduced vision?

    • Reduced vision suggests more serious pathology such as keratitis, uveitis, acute glaucoma, or scleritis

  • Photophobia?

    • Suggests corneal disease or inflammation such as keratitis or anterior uveitis

  • Discharge?

    • Purulent discharge → bacterial conjunctivitis

    • Watery discharge → viral conjunctivitis

    • Minimal/no discharge with pain → keratitis or uveitis

  • Sudden or gradual onset?

    • Sudden → subconjunctival haemorrhage, corneal abrasion, acute glaucoma

    • Gradual → conjunctivitis, episcleritis

  • Contact lens use?

    • Raises concern for keratitis/corneal ulcer

  • Trauma or foreign body?

    • Suggests corneal abrasion, foreign body injury, or subconjunctival haemorrhage

  • Headache, nausea, vomiting?

    • Suggestive of acute angle-closure glaucoma

  • Painful eye movements or fever?

    • Raises concern for orbital cellulitis



Red Flags Requiring Urgent Assessment

  • Reduced visual acuity

  • Severe pain

  • Photophobia

  • Corneal opacity

  • Contact lens wearer with pain

  • Fixed mid-dilated pupil

  • Trauma

  • Headache with nausea/vomiting

  • Painful eye movements



Normal Values

  • Normal Visual Acuity: 6/6

  • Normal Intraocular Pressure (IOP): 10–21 mmHg



Important Differentials to Rule Out


Conjunctivitis

Consider if there is:

  • Diffuse redness

  • Discharge

  • Gritty/itchy sensation

  • Normal vision

Risk factors:

  • Viral illness

  • Allergy

  • Contact lens use


Keratitis / Corneal Ulcer

Must be ruled out if:

  • Pain

  • Photophobia

  • Reduced vision

  • Foreign body sensation

Major risk factor:

  • Contact lens use

  • Trauma

  • Diabetes

  • Immunosuppression

This is an ophthalmic emergency.


Anterior Uveitis

Consider if there is:

  • Photophobia

  • Deep aching pain

  • Blurred vision

Associated with:

  • Autoimmune disease:

    • Reactive arthritis

    • Psoriatic arthritis

    • Inflammatory bowel disease

    • Juvenile idiopathic arthritis

    • Sarcoidosis

    • Behçet disease

    • Ankylosing spondylitis


Acute Angle-Closure Glaucoma

Must be excluded in patients with:

  • Severe eye pain

  • Reduced vision

  • Halos around lights

  • Headache/nausea

Risk factors:

  • Older age

  • Hypermetropia

  • Asian ethnicity

This is an emergency.


Episcleritis

Consider if there is:

  • Mild discomfort

  • Sectoral redness

  • Normal vision

May be associated with autoimmune disease such as

  • Rheumatoid arthritis

  • Systemic lupus erythematosus (SLE)

  • Inflammatory bowel disease

  • Psoriatic arthritis

  • Ankylosing spondylitis

Sometimes idiopathic.


Scleritis

Must be considered if:

  • Severe deep pain

  • Pain with eye movement

  • Reduced vision

Often associated with autoimmune conditions such as:

  • Rheumatoid arthritis

  • Granulomatosis with polyangiitis (Wegener’s)

  • Systemic lupus erythematosus (SLE)

  • Relapsing polychondritis

  • Polyarteritis nodosa

  • Inflammatory bowel disease



Subconjunctival Haemorrhage


Typical Features

Diagnosis becomes more likely when there is:

  • Bright red well-demarcated patch

  • No pain

  • Normal vision

  • No discharge

  • No photophobia


Risk Factors

  • Coughing/sneezing

  • Straining/heavy lifting

  • Eye rubbing

  • Minor trauma

  • Hypertension

  • Blood thinners

  • Recent eye procedures

Sometimes it occurs spontaneously without a clear cause (idiopathic/spontaneous).



Differentiating Episcleritis vs Subconjunctival Haemorrhage

Feature

Episcleritis

Subconjunctival Haemorrhage

Appearance

Inflamed superficial vessels

Bright red blood patch

Pain

Mild discomfort

Painless

Vision

Normal

Normal

Cause

Inflammation

Bleeding


How to Explain Subconjunctival Haemorrhage to a Patient

“This happens when a tiny blood vessel on the surface of the eye bursts, causing a red patch to appear. Although it can look alarming, it is usually harmless and settles on its own.”

Investigations

Usually no tests are needed for a first isolated episode.

Consider investigations if:

  • Recurrent episodes

  • Bleeding elsewhere

  • Anticoagulant use

  • Suspected bleeding disorder

Possible tests:

  • Blood pressure

  • FBC

  • Clotting profile

  • Blood glucose


Safety Netting Advice

Advise urgent review if:

  • Pain develops

  • Vision worsens

  • Photophobia occurs

  • Discharge/swelling develops

  • Recurrent episodes occur


High-Yield PLAB 2 Tip

Pain + photophobia + reduced vision should always make you think of a serious ocular condition until proven otherwise.

References

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