Cryptogenic Stroke

After a gym workout, a 48-year-old man had sudden ataxia, nausea, and diplopia, followed by persistent inability to see the upper left quadrant of space with either the left or right eye. He did not have neck pain. His medical history included hypertension and migraines with aura. Magnetic resonance imaging (MRI) showed a right occipitotemporal and thalamic infarct. Magnetic resonance angiography showed an abrupt cutoff of a distal segment of the right posterior cerebral artery. The complete blood count, prothrombin time, and partial-thromboplastin time were normal. Transthoracic echocardiographic results suggested a possible right-to-left shunt. Cardiac telemetry during the first 2 inpatient days revealed no dysrhythmias. How should this case be further evaluated?