In a patient with an acute cerebrovascular accident who develops a new unilateral constricted pupil, what is the most likely diagnosis and what immediate work‑up and treatment are recommended?

Acute CVA with Unilateral Constricted Pupil: Diagnosis and Management

A unilateral constricted (miotic) pupil in the setting of acute stroke most likely indicates ipsilateral Horner syndrome from lateral medullary (Wallenberg) syndrome or other brainstem/cervical sympathetic pathway involvement, though in the context of cerebellar infarction with brainstem compression, pinpoint pupils may indicate pontine compression. 1

Most Likely Diagnoses

Lateral Medullary Syndrome (Wallenberg Syndrome)

• Ipsilateral Horner syndrome (ptosis, miosis, anhidrosis) results from disruption of the descending sympathetic pathway in the lateral medulla 2, 3
• Associated findings typically include ipsilateral facial pain/temperature loss, contralateral body pain/temperature loss, ataxia, dysphagia, and vertigo
• The miotic pupil remains reactive to light, distinguishing it from third nerve palsy 3

Cerebellar Infarction with Brainstem Compression

• Bilateral pinpoint pupils (not unilateral miosis) are the classic finding when cerebellar swelling causes pontine compression 1
• Accompanied by pupillary anisocoria, loss of oculocephalic responses, and deteriorating consciousness 1
• This represents a neurosurgical emergency requiring immediate decompression

Carotid or Vertebral Artery Dissection

• Can cause both stroke and ipsilateral Horner syndrome from involvement of pericarotid sympathetic fibers 2
• This is a critical diagnosis not to miss, as it may require anticoagulation or endovascular intervention

Critical Distinction: Horner Syndrome vs. Third Nerve Palsy

The key differentiating feature is pupil size:

• Horner syndrome = constricted (miotic) pupil with ptosis 2, 3
• Third nerve palsy = dilated (mydriatic) pupil with ptosis 1

In supratentorial hemispheric stroke with deterioration, the classic finding is ipsilateral pupillary dilation (mydriasis), not constriction, indicating uncal herniation 1

Immediate Work-Up Required

Neuroimaging Protocol

• Urgent MRI brain with and without gadolinium contrast plus MRA to evaluate the posterior circulation, brainstem, and cerebellum 4, 5
• Dedicated MR venography (MRV) or CT venography if any concern for cerebral venous thrombosis 5
• If MRI unavailable, CT angiography (CTA) of head and neck to evaluate for arterial dissection 5

Clinical Monitoring

• Frequent assessment of level of consciousness and additional brainstem signs (Class I recommendation) 1
• Monitor for progression to bilateral pinpoint pupils, loss of oculocephalic reflexes, or respiratory pattern changes indicating pontine/medullary compression 1
• Serial neurological examinations every 1-2 hours in the acute phase

Additional Diagnostic Studies

• Pharmacologic testing with apracolidine or cocaine eye drops can confirm Horner syndrome if diagnosis uncertain 3
• Measure redilatation lag during light reflex with pupillometry if available (70% sensitivity, 95% specificity for Horner syndrome) 6

Immediate Treatment Considerations

If Cerebellar Infarction with Deterioration

• Osmotic therapy (mannitol or hypertonic saline) is reasonable for clinical deterioration from cerebral swelling (Class IIa recommendation) 1
• Elevate head of bed to 30 degrees 1
• Urgent neurosurgical consultation for possible suboccipital decompressive craniectomy if signs of brainstem compression 1
• External ventricular drain if obstructive hydrocephalus present 1

If Lateral Medullary Syndrome

• Standard acute ischemic stroke management
• Antiplatelet therapy (aspirin may be continued) 1
• Subcutaneous heparin for DVT prophylaxis 1
• No specific treatment needed for Horner syndrome itself 2, 3

If Arterial Dissection Suspected

• Anticoagulation or antiplatelet therapy per institutional protocol
• Consider endovascular intervention if progressive symptoms

Critical Pitfalls to Avoid

• Do not assume unilateral miosis indicates supratentorial herniation—this causes mydriasis, not miosis 1
• Do not delay vascular imaging if any concern for dissection or posterior circulation stroke 5
• Do not use corticosteroids, barbiturates, or hypothermia for ischemic stroke with swelling (Class III recommendation—insufficient data and not recommended) 1
• Do not miss bilateral Horner syndrome in autonomic neuropathy—measure redilatation lag bilaterally 6
• Recognize that normal initial MRI does not exclude serious pathology—dedicated vascular imaging is essential 5