A 75-year-old man who had a small right parietal infarction 8 months earlier underwent baseline and acetazolamide enhanced Tc-99m hexamethylpropylene amineoxime brain SPECT imaging. The acetazolamide study demonstrated a bilaterally symmetric perfusion deficit posteriorly near the midline. The baseline study was essentially normal. This finding was felt to represent watershed ischemia at the junction of the anterior circulations (anterior cerebral and middle cerebral arteries) and the posterior circulation (posterior cerebral artery). Carotid arteriography subsequently demonstrated left subclavian steal syndrome with retrograde flow through the left vertebral artery.
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