Left-Sided Weakness

A 72-year-old man is brought to the emergency department (ED) by paramedics after experiencing acute onset of left-sided weakness that started approximately 30 minutes before arrival to the ED. The patient has a medical history of hypertension and atrial fibrillation and was treated with atenolol and warfarin. The patient underwent successful electrical cardioversion 2 days prior to presentation after his international normalized ratio (INR) was confirmed to be within therapeutic range.

On physical examination, his initial vital signs include a temperature of 37.2°C, a blood pressure of 170/80 mm Hg, a heart rate of 54 beats per minute, a respiratory rate of 12 breaths per minute, and oxygen saturation of 99% on room air. His heart rhythm is regular, and no murmurs are appreciated. Findings on neck, lungs, abdomen, extremities, and skin examinations are all normal. The patient is awake and able to follow commands appropriately; however, he has a significant expressive aphasia. His eyes are deviated to the right, and his visual fields are absent on the left. His pupils are round and reactive to light bilaterally. The tongue is midline, the palate is elevated symmetrically, and the trapezius strength is 5/5 bilaterally. The patient has normal muscle bulk with flaccid tone on the left (upper and lower extremities). His strength is 5/5 on his right extremities and 0/5 on his left. Sensation to light touch is normal on the right and markedly decreased on the left. An electrocardiogram demonstrates sinus bradycardia with no acute ST-T wave changes. His serum glucose level is 105 mg/dL, and his INR is 2.2. The patient is taken emergently for a noncontrast head CT scan (see Image).

What is the cause of this patient's weakness?

Laboratory results, including the CBC, are within normal limits. A CT scan of the neck is ordered.

What is the diagnosis?

What is the diagnosis?
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This complication of cardioversion of atrial fibrillation is uncommon under therapeutic anticoagulation.