What are the differential diagnoses for a patient presenting with headache and a red eye?

Differential Diagnosis for Headache with Red Eye

The differential diagnosis for headache with red eye must immediately distinguish between benign primary headache disorders with conjunctival injection (cluster headache, migraine) and sight-threatening or life-threatening secondary causes (acute angle-closure glaucoma, carotid-cavernous fistula, cavernous sinus pathology, orbital inflammation, and infectious/inflammatory conditions).

Algorithmic Approach to Headache with Red Eye

Step 1: Identify Red-Flag Features Requiring Urgent Evaluation

Historical red flags:

• Thunderclap onset (sudden, severe headache) suggests subarachnoid hemorrhage, ruptured carotid-cavernous aneurysm, or cerebral venous thrombosis 1, 2
• Recent head trauma warrants imaging for subdural hematoma 1
• Progressive headache over days to weeks indicates possible intracranial mass or cerebral venous thrombosis 1, 2
• Sudden onset with vomiting followed by rapid eye swelling suggests ruptured cavernous carotid aneurysm 3

Physical examination red flags:

• Fever with neck stiffness indicates meningitis 1, 2
• Focal neurological deficits (hemiparesis, aphasia, sensory loss) suggest stroke, mass lesion, or arterial dissection 1, 2
• Pupillary abnormalities (mid-dilated, unreactive, or afferent pupillary defect) may indicate acute angle-closure glaucoma, posterior communicating artery aneurysm, or optic nerve disease 4, 5, 6
• Severe chemosis, proptosis, or limited eye movements suggest carotid-cavernous fistula or orbital compartment syndrome 3
• Papilledema indicates raised intracranial pressure 2

Step 2: Characterize the Headache Pattern

Duration and timing:

• 15–180 minutes per attack suggests cluster headache 1, 2
• 4–72 hours indicates migraine 1, 2
• Rapid progression overnight from headache to swollen red eye suggests ruptured cavernous carotid aneurysm 3

Pain characteristics:

• Strictly unilateral, severe-to-very-severe orbital/periorbital pain with restlessness points to cluster headache 1, 2
• Unilateral or bilateral, pulsating, moderate-to-severe pain aggravated by activity suggests migraine 1, 2
• Sudden severe pain with blurred vision in a hyperopic patient over age 50 indicates acute angle-closure glaucoma 4

Step 3: Examine the Red Eye for Specific Patterns

Conjunctival injection with autonomic signs:

• Ipsilateral lacrimation, nasal congestion, ptosis, miosis, eyelid edema occurring during attacks (1–8 per day) confirms cluster headache 1, 2
• Bilateral conjunctival injection with photophobia and phonophobia during 4–72 hour attacks suggests migraine 1, 2

Corneal and anterior chamber findings:

• Mid-dilated, unreactive pupil with corneal edema and very high intraocular pressure indicates acute angle-closure glaucoma 4
• Shallow anterior chamber depth in a hyperopic patient supports angle-closure 4
• Corneal infiltrate or ulcer with purulent discharge suggests infectious keratitis or gonococcal conjunctivitis 7

Orbital and periocular signs:

• Rapid progression to severe chemosis, periorbital edema, and ophthalmoplegia within 8 hours indicates carotid-cavernous fistula requiring emergent canthotomy 3
• Proptosis with pain on eye movement suggests orbital inflammation, Tolosa-Hunt syndrome, or cavernous sinus pathology 8, 5, 6

Step 4: Apply Diagnosis-Specific Criteria

Cluster headache (primary headache):

• Strictly unilateral severe pain lasting 15–180 minutes 1, 2
• Ipsilateral conjunctival injection, lacrimation, nasal congestion, ptosis, miosis 1, 2
• Frequency of 1–8 attacks per day during cluster periods 2
• No visual loss, no pupillary abnormalities, no ophthalmoplegia 1, 2

Migraine (primary headache):

• Attacks lasting 4–72 hours with moderate-to-severe intensity 1, 2
• Photophobia and phonophobia (may cause conjunctival injection from light sensitivity) 1, 2
• Nausea/vomiting, aggravation by routine activity 1, 2
• No autonomic signs, no fixed pupillary abnormalities 1, 2

Acute angle-closure glaucoma (ophthalmic emergency):

• Sudden severe unilateral eye pain with blurred vision 4
• Mid-dilated, unreactive pupil with corneal edema 4
• Shallow anterior chamber depth, very high intraocular pressure 4
• Requires immediate ophthalmology referral for IOP-lowering treatment and laser peripheral iridotomy 4

Ruptured carotid-cavernous aneurysm with fistula (vascular emergency):

• Sudden onset headache followed by rapid progression to swollen, bloodshot eye within 24 hours 3
• Severe chemosis, periorbital edema, ophthalmoplegia, enlarged ophthalmic veins 3
• Requires emergent CT angiography and endovascular intervention 3

Infectious/inflammatory causes:

• Bacterial/gonococcal conjunctivitis: purulent discharge, preauricular lymphadenopathy, possible corneal ulcer 7
• Viral conjunctivitis: watery discharge, follicular reaction, may have systemic symptoms 7
• Episcleritis/scleritis: localized or diffuse redness, pain with eye movement 8, 6
• Uveitis: photophobia, blurred vision, anterior chamber inflammation on slit-lamp exam 8

Step 5: Determine Urgency of Ophthalmology Referral

Immediate referral (within hours):

• Any visual disturbance, blurred vision, or visual loss 8, 4
• Mid-dilated or unreactive pupil 4
• Severe chemosis, proptosis, or ophthalmoplegia 8, 3
• Corneal infiltrate or ulcer 7
• Suspected acute angle-closure glaucoma 4

Urgent referral (within 1–2 days):

• Persistent red eye with pain despite treatment 8
• Any grade of uveitis or episcleritis 8
• Eyelid swelling with pain and erythema (possible preseptal cellulitis) 8

Avoid starting corticosteroids before ophthalmology evaluation unless systemic steroids are indicated for non-ocular toxicity, as steroids may worsen infectious causes (herpetic keratitis) or mask accurate diagnosis 8

Step 6: Imaging Recommendations

Perform neuroimaging only when red-flag features are present 1, 2:

• CT angiography for suspected ruptured carotid-cavernous aneurysm (shows giant aneurysm, enlarged ophthalmic veins) 3
• MRI with MR venography for suspected cerebral venous thrombosis 2
• Non-contrast CT for thunderclap headache (98% sensitivity for subarachnoid hemorrhage) 2
• MRI preferred over CT for most secondary headache evaluations (higher resolution, no radiation) 1, 2

Common Pitfalls to Avoid

• Do not dismiss sudden headache with red eye as migraine without excluding carotid-cavernous fistula, especially if symptoms progress rapidly overnight 3
• Do not assume bilateral pain excludes migraine—approximately 40% of migraine patients report bilateral pain 1, 2
• Do not delay ophthalmology referral for any visual complaint in patients with red eye and headache, as conditions like angle-closure glaucoma require treatment within hours to prevent blindness 8, 4
• Do not start topical or systemic corticosteroids before ophthalmology examination unless indicated for non-ocular reasons, as this may worsen infectious causes or obscure diagnosis 8
• Do not routinely image primary headaches with normal neurological examination—imaging yield is only 0.2% in migraine and comparable to asymptomatic volunteers 2

Summary of Key Differential Diagnoses

Primary headache disorders (benign):

• Cluster headache: strictly unilateral severe pain 15–180 min with ipsilateral autonomic signs 1, 2
• Migraine: 4–72 hour attacks with photophobia/phonophobia, nausea 1, 2

Ophthalmic emergencies:

• Acute angle-closure glaucoma: mid-dilated pupil, corneal edema, very high IOP 4
• Corneal ulcer/infectious keratitis: infiltrate, purulent discharge 7

Vascular emergencies:

• Ruptured carotid-cavernous aneurysm: rapid progression to chemosis, ophthalmoplegia 3
• Cavernous sinus thrombosis: proptosis, ophthalmoplegia, fever 5, 6

Inflammatory/infectious:

• Uveitis: photophobia, anterior chamber inflammation 8
• Episcleritis/scleritis: localized redness, pain with movement 8, 6
• Orbital cellulitis: eyelid swelling, pain, fever 8
• Conjunctivitis (bacterial/viral): discharge, follicles 7