Scenario 13: Cerebral Artery Vascular Accident (CVA)

Scenario 13: Cerebral Artery Vascular Accident (CVA)
An 83-year-old man presents with a history of atrial fibrillation (AF), hypertension, and diabetes. His daughter, who accompanied the patient, states that yesterday the patient had a period when he could not speak or understand words, and that approximately 4 weeks prior he staggered against a wall and was unable to stand unaided because of weakness in his legs. She states that both instances lasted approximately a half-hour. She was unable to persuade her father to go to the emergency room either time. Today he suffered another episode of right sided weakness, dysarthria, and difficulty with speech. Past medical history: Hypertension for 15 years, well controlled; diabetes for the past 10 years, and hyperlipidemia. Medications: Diltiazem CD 300 mg daily; lisinopril 40 mg daily; metformin 500 mg twice daily; aspirin 81 mg daily and atorvastatin 20 mg po qhs.
Social history: reported former smoker with 40 pack year history. Alcohol -drinks one beer a day. Denies any other substance abuse. Review of systems: Denies dyspnea, dizziness, or syncope; complains that he cannot move or feel his right arm or leg. Difficulty with speech.
Physical exam: Vitals: height = 70 inches; weight = 185 pounds; body mass index = 26.5; BP = 134/82 mm Hg; heart rate = 88 bpm at rest, irregularly irregular pattern.
HEENT remarkable for expressive aphasia, eyes with contralateral homonymous hemianopsia.
No loss of sensation but unable to voluntarily move right arm or leg.
The patient was diagnosed with a right middle cerebral artery vascular accident (CVA) secondary to atrial fibrillation (AF)
Question:
How does atrial fibrillation contribute to the development of a CVA? 
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