Basilar Artery Territory Lesion
The most likely lesion location is the basilar artery (Option A), given this constellation of brainstem findings including extraocular movement weakness, vertical nystagmus, reactive miosis, dysarthria, and ataxia.
Clinical Localization
This symptom complex localizes to the posterior circulation, specifically the brainstem structures supplied by the basilar artery 1. The combination of findings points to involvement of multiple brainstem levels:
Key Localizing Features
Extraocular movement weakness and vertical nystagmus indicate midbrain or pontine involvement, structures directly supplied by basilar artery branches 1
Dysarthria and ataxia are classic posterior circulation symptoms that occur with brainstem and cerebellar ischemia from basilar artery territory involvement 1
Reactive miosis suggests involvement of descending sympathetic pathways in the brainstem, consistent with basilar artery territory lesions 1
Vertical nystagmus specifically suggests involvement of the midbrain or pontomedullary junction, areas supplied by the basilar artery 1
Why Not Other Vessels?
Anterior Cerebral Artery (Option B)
The anterior cerebral artery supplies the medial frontal lobes and would cause contralateral leg weakness, behavioral changes, and urinary incontinence—none of which match this presentation 1.
Middle Cerebral Artery (Option C)
Middle cerebral artery strokes produce contralateral hemiparesis, hemisensory loss, aphasia (dominant hemisphere), or neglect (non-dominant hemisphere)—not brainstem or cerebellar signs 1.
Posterior Cerebral Artery (Option D)
While the posterior cerebral artery is part of the posterior circulation, it primarily supplies the occipital lobes and medial temporal structures, causing visual field defects and memory impairment rather than the brainstem signs described here 2, 3.
Basilar Artery Syndrome Characteristics
The basilar artery supplies critical brainstem structures including the pons, midbrain, and cerebellum through its branches 1. The clinical presentation described represents a classic basilar artery territory syndrome with:
- Multiple cranial nerve involvement (extraocular movements) 1
- Cerebellar signs (ataxia) 1, 4
- Brainstem motor pathway involvement (dysarthria) 1
- Autonomic dysfunction (miosis) 1
Common Pitfalls
Posterior circulation strokes can be missed or delayed in diagnosis because symptoms may be non-specific initially, and the NIHSS underemphasizes cranial nerve and cerebellar findings 1. Maintain high clinical suspicion when patients present with vertigo, ataxia, diplopia, dysarthria, and nystagmus—even with relatively low NIHSS scores 1.
Basilar artery occlusions carry high mortality (45-86%) and require urgent recognition and imaging 1. The combination of multiple brainstem signs at different levels strongly suggests basilar artery involvement rather than a single branch vessel 5, 2.