How does atrial fibrillation contribute to the development of a CVA? 

Case study:

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  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 histories. 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?