Distinguishing Acute Angle-Closure Glaucoma from Cluster Headache
The critical distinguishing feature is measuring intraocular pressure (IOP) and examining the anterior chamber—acute angle-closure glaucoma presents with markedly elevated IOP (often >40 mmHg), a mid-dilated fixed pupil, corneal edema, and a closed anterior chamber angle on examination, while cluster headache has normal IOP, a reactive pupil, and a clear cornea.
Key Clinical Differentiators
Acute Angle-Closure Crisis (AACC) Presentation 1
Ocular findings that definitively indicate AACC:
- Mid-dilated, fixed pupil (typically 4-6mm, non-reactive to light)
- Corneal edema causing blurred vision and halos around lights
- Conjunctival and episcleral injection (red eye)
- Shallow anterior chamber on penlight examination
- Markedly elevated IOP (typically >40 mmHg, palpably rock-hard globe)
Systemic symptoms:
- Severe eye pain radiating to forehead/temple
- Headache (often frontal or temporal)
- Nausea and vomiting
- Visual disturbances (blurred vision, halos)
Cluster Headache Presentation 2
Distinguishing features:
- Normal IOP (critical differentiator)
- Reactive pupil (may have Horner's syndrome with miosis and ptosis on affected side)
- Clear cornea without edema
- Normal anterior chamber depth
- Lacrimation and nasal congestion on affected side
- Attacks last 20-120 minutes
- Episodic pattern: daily attacks for weeks/months, then remission for months/years
- Restlessness during attacks (unlike glaucoma patients who prefer to remain still)
Diagnostic Algorithm
Immediate Bedside Assessment
Palpate the globe: A rock-hard eye suggests markedly elevated IOP 3, 4
Penlight examination:
- Check pupil size and reactivity (mid-dilated fixed = AACC; reactive or miotic = cluster headache)
- Assess corneal clarity (hazy/edematous = AACC; clear = cluster headache)
- Evaluate anterior chamber depth (shallow = AACC; normal = cluster headache)
Measure IOP: This is the definitive test
- IOP >40 mmHg = AACC
- Normal IOP (10-21 mmHg) = not AACC, consider cluster headache
Gonioscopy (by ophthalmologist): Reveals closed angle in AACC 1
Critical Pitfall to Avoid
Patients with shallow anterior chambers and optic disc cupping can have cluster headaches 2. The anatomical predisposition to angle closure does not mean every headache is glaucoma. Always measure IOP—this single test prevents misdiagnosis.
When Both Conditions Can Mimic Each Other
The episodic nature of both conditions creates diagnostic confusion 2, 5:
- Both occur unilaterally
- Both cause severe pain involving the eye and head
- Both tend to occur at night
- Both can present with nausea/vomiting
However, subacute angle closure can present primarily as headache without significant ocular symptoms 5, making IOP measurement mandatory in adult-onset headaches with any ocular component.
Management Implications
If AACC is Confirmed 1
Immediate treatment is required to prevent permanent vision loss:
- Topical beta-blockers and alpha-2 agonists
- Topical/oral/IV carbonic anhydrase inhibitors
- Parasympathomimetics (pilocarpine) if IOP not extremely elevated
- Oral/IV osmotic agents (mannitol) for rapid IOP reduction
- Definitive treatment: Laser peripheral iridotomy within hours to days
- Prophylactic iridotomy in fellow eye (50% risk of AACC within 5 years) 1
If Cluster Headache is Confirmed
Standard cluster headache management (oxygen, triptans, preventive therapy)—not an ophthalmologic emergency.
Red Flags Requiring Urgent Ophthalmology Consultation 3, 4, 6
- Any patient presenting with headache + red eye + pupil abnormality
- Headache with visual disturbances and nausea
- Unilateral eye pain with systemic symptoms
- 28% of AACC patients initially present to non-ophthalmologists 6, leading to unnecessary CT scans and lumbar punctures
The single most important action: Measure IOP in any patient with unilateral headache and ocular symptoms. This simple test prevents both misdiagnosis and unnecessary invasive procedures 7, 6.