Where is the lesion in a patient presenting with extraocular muscle (EOM) weakness, vertical nystagmus, reactive miosis, dysarthria, and ataxia?

Basilar Artery Territory Lesion

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

Anatomical Localization

The combination of these specific clinical findings points definitively to basilar artery involvement:

• The basilar artery supplies critical brainstem structures including the pons, midbrain, and cerebellum through its branches, making it the only vascular territory that can produce this complete symptom complex. 1

• Vertical nystagmus specifically indicates involvement of the midbrain or pontomedullary junction—areas directly supplied by the basilar artery. 1

• Reactive miosis suggests involvement of descending sympathetic pathways in the brainstem, which is 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

Why Other Territories Are Excluded

Anterior cerebral artery (Answer B) is incorrect because:

• Anterior cerebral artery strokes cause contralateral leg weakness, behavioral changes, and urinary incontinence—none of which match this presentation. 1

• This territory does not produce brainstem or cerebellar signs. 1

Middle cerebral artery (Answer C) is incorrect because:

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

Posterior cerebral artery (Answer D) is incorrect because:

• 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

• This territory cannot account for the combination of cranial nerve involvement, cerebellar signs, and autonomic dysfunction seen in this patient. 1

Clinical Significance

This presentation represents basilar artery syndrome with multiple concerning features:

• The clinical presentation includes multiple cranial nerve involvement, cerebellar signs, brainstem motor pathway involvement, and autonomic dysfunction—all hallmarks of basilar artery territory syndrome. 1

• Basilar artery occlusions carry high mortality (45-86%) and require urgent recognition and imaging. 1

• 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

• Gait ataxia is almost always present in posterior circulation stroke, regardless of the specific lesion site within the territory. 2