Unilateral Pupillary Dilation with Red Eye: Differential Diagnosis
The most critical diagnosis to consider in a patient with unilateral pupillary dilation and red eye is acute angle-closure crisis (AACC), which requires immediate treatment to prevent permanent vision loss. 1
Primary Diagnostic Considerations
Acute Angle-Closure Crisis (AACC)
This is the most vision-threatening cause and presents with:
- Mid-dilated, poorly reactive or nonreactive pupil on the affected side 1
- Conjunctival and episcleral hyperemia (red eye)
- Corneal edema causing blurred vision and halos around lights
- Markedly elevated intraocular pressure (IOP)
- Eye pain, headache, nausea/vomiting 1
The mid-dilated pupil in AACC occurs because pupillary block causes iris bombé, crowding the anterior chamber angle and triggering the acute attack. 1 The pupil becomes fixed in mid-dilation due to iris ischemia from the elevated IOP.
Herpes Simplex Keratouveitis
This presents with:
- Iris atrophy leading to persistent pupillary dilation
- Anterior uveitis with red eye
- 9 of 13 patients in one series demonstrated pupillary dilation on the affected side despite not using dilating drops 2
- The iris atrophy and dilation pattern is characteristic enough that it suggests HSV as the diagnosis in unexplained anterior uveitis
Pharmacologic Mydriasis
Accidental or intentional exposure to anticholinergic agents:
- Ipratropium bromide nebulizer can cause unilateral fixed dilated pupil if mist contacts the eye 3, 4
- Topical antihistamines, atropine, scopolamine, or tropane alkaloids (from Datura plants) 5
- Over-the-counter decongestants, cold/flu medications with anticholinergic properties 1
The diagnosis can be confirmed using pilocarpine eye drops: pharmacologically dilated pupils will not constrict, while neurologic causes typically will 4.
Algorithmic Approach
Step 1: Measure IOP immediately
- If IOP >21 mmHg with narrow angle on gonioscopy → AACC
- Initiate immediate medical treatment with aqueous suppressants, parasympathomimetics, and osmotic agents 6
Step 2: Examine pupil characteristics
- Mid-dilated (4-6mm), oval, poorly reactive → AACC
- Fully dilated (>7mm), completely nonreactive → Consider pharmacologic cause
- Irregular pupil with sectoral atrophy → HSV keratouveitis
Step 3: Slit-lamp examination
- Corneal edema + shallow anterior chamber + conjunctival injection → AACC
- Dendritic corneal lesions + anterior chamber inflammation → HSV
- Normal cornea with isolated mydriasis → Pharmacologic or neurologic cause
Step 4: Medication/exposure history
- Recent nebulizer use (ipratropium) 3, 4
- Topical eye medications or systemic anticholinergics 1
- Plant exposure (Datura species) 5
Step 5: Pilocarpine challenge if diagnosis unclear
- Apply 1% pilocarpine to both eyes
- Pharmacologic mydriasis will NOT constrict
- Neurologic causes typically will constrict (though may be sluggish) 4
Critical Pitfalls to Avoid
Do not delay treatment of AACC while pursuing other diagnoses - elevated IOP causes irreversible optic nerve damage. The combination of red eye, mid-dilated pupil, and elevated IOP is AACC until proven otherwise 1.
Do not assume third nerve palsy - in an alert patient with isolated pupillary dilation and red eye, third nerve palsy is unlikely. The pupil in third nerve palsy is typically fully dilated (not mid-dilated) and occurs without conjunctival injection 7.
Consider bilateral examination - AACC and primary angle-closure disease tend to be bilateral, though may present asymmetrically. A wide-open angle in the fellow eye suggests alternative diagnosis 6.
Recognize medication triggers - patients with narrow angles can develop AACC from systemic medications with anticholinergic or adrenergic effects, including common over-the-counter preparations 1.
Secondary Considerations
Other causes of unilateral pupillary dilation with potential red eye include:
- Uveitis with posterior synechiae causing iris bombé and secondary angle closure 6
- Trauma to iris sphincter (though history usually evident)
- Tonic pupil (Adie's) - but this typically lacks significant red eye and has light-near dissociation 7
The key distinguishing feature is that AACC presents with marked conjunctival injection, elevated IOP, and a mid-dilated pupil, while pharmacologic causes show a fully dilated pupil with minimal inflammation, and HSV shows iris atrophy with variable inflammation 1, 2.