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

Anatomic Localization

The clinical presentation points definitively to posterior circulation involvement:

• Vertical nystagmus specifically indicates midbrain or pontomedullary junction pathology, areas directly supplied by the basilar artery 1
• Reactive miosis suggests involvement of descending sympathetic pathways in the brainstem, consistent with basilar artery territory lesions 1
• The combination of dysarthria and ataxia represents classic posterior circulation symptoms occurring with brainstem and cerebellar ischemia from basilar artery territory involvement 1
• Extraocular movement weakness reflects cranial nerve nuclei dysfunction in the brainstem, which the basilar artery supplies through its branches 1

Why Other Arterial Territories Are Excluded

Anterior Cerebral Artery (Option B)

• Anterior cerebral artery strokes cause contralateral leg weakness, behavioral changes, and urinary incontinence—none of which match this patient's brainstem and cerebellar presentation 1

Middle Cerebral Artery (Option C)

• Middle cerebral artery strokes produce contralateral hemiparesis, hemisensory loss, aphasia, or neglect—not brainstem or cerebellar signs 1

Posterior Cerebral Artery (Option D)

• Posterior cerebral artery strokes primarily supply occipital lobes and medial temporal structures, causing visual field defects and memory impairment rather than the brainstem signs present in this patient 1

Basilar Artery Syndrome Characteristics

• The basilar artery supplies critical brainstem structures including the pons, midbrain, and cerebellum through its branches 1
• Clinical presentation of basilar artery territory syndrome includes multiple cranial nerve involvement, cerebellar signs, brainstem motor pathway involvement, and autonomic dysfunction 1—all of which are present in this patient

Critical Clinical 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
• Basilar artery occlusions carry high mortality (45-86%) and require urgent recognition and imaging 1
• Most patients with basilar artery occlusion present with gradual onset, with warning signs present for up to two months before the final stage in half of cases 2