Where is the most likely lesion in a patient presenting with Extraocular movement (EOM) weakness, vertical nystagmus, reactive miosis, dysarthria, and ataxia?

Basilar Artery Territory Lesion

The most likely lesion location is the basilar artery (Option A), as this constellation of extraocular movement weakness, vertical nystagmus, reactive miosis, dysarthria, and ataxia specifically localizes to brainstem structures supplied by the posterior circulation. 1

Anatomical Localization

The clinical presentation points definitively to the basilar artery territory through the following reasoning:

• Vertical nystagmus specifically indicates involvement of the midbrain or pontomedullary junction, which are supplied by the basilar artery 1, 2
• Reactive miosis suggests involvement of descending sympathetic pathways in the brainstem, consistent with basilar artery territory lesions 1
• Dysarthria and ataxia are classic posterior circulation symptoms that occur with brainstem and cerebellar ischemia from basilar artery territory involvement 1
• Extraocular movement weakness combined with the other findings indicates multiple cranial nerve involvement characteristic of basilar artery syndrome 1

Why Other Options Are Incorrect

Anterior Cerebral Artery (Option B)

• Anterior cerebral artery strokes cause contralateral leg weakness, behavioral changes, and urinary incontinence—none of which match this presentation 1
• This territory does not supply brainstem or cerebellar structures 1

Middle Cerebral Artery (Option C)

• Middle cerebral artery strokes produce contralateral hemiparesis, hemisensory loss, aphasia, or neglect—not brainstem or cerebellar signs 1
• The absence of cortical signs and presence of cranial nerve findings exclude this territory 1

Posterior Cerebral Artery (Option D)

• Posterior cerebral artery strokes primarily supply the occipital lobes and medial temporal structures, causing visual field defects and memory impairment rather than brainstem signs 1
• While part of the posterior circulation, PCA territory does not explain the combination of cranial nerve palsies, nystagmus, and ataxia seen here 1

Clinical Significance of Basilar Artery Syndrome

The basilar artery supplies critical brainstem structures including the pons, midbrain, and cerebellum through its branches 1. The clinical presentation of basilar artery territory syndrome characteristically includes:

• Multiple cranial nerve involvement 1
• Cerebellar signs (ataxia) 1
• Brainstem motor pathway involvement 1
• Autonomic dysfunction (miosis) 1

Critical Clinical Pitfall

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. Basilar artery occlusions carry high mortality (45-86%) and require urgent recognition and imaging 1. The gradual onset pattern is typical, with warning signs present for up to two months before the final stage in half of patients 3.