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Although transcatheter closure of patent foramen ovale (PFO) and atrial septal defect (ASD) has become a commonly performed intervention, the incidence of cerebral embolism with or without neurological deficits during such procedures has not been studied.We monitored the middle cerebral artery in two different depths (48 mm and 53 mm) by continuous transcranial Doppler ultrasound during transcatheter PFO closure in 35 consecutive patients (F/M 20/15, mean age 47 ± 11 years) and during ASD closure in 8 patients (F/M 7/1, mean age 45 ± 5 years). All automatically detected high intensity transient signals (HITS) were manually reviewed to eliminate artifacts.HITS were detected in 33 of 35 patients (96%) with a median rate of 8 (interquartile range 4-19, range 2-29) HITS. The highest rates were observed when the septum was crossed with the guide wire (median 2; IQR 0-12; range 0-25) and when the left atrial disc was deployed (median 2; IQR 1-4; range 0-13). Despite this high rate of cerebral microembolism no clinically apparent neurological or neuropsychological deficit was observed.Silent cerebral embolism frequently occurs during transcatheter PFO and ASD closure. The peak of HITS at the time of crossing the septum with the guide wire may support the hypothesis that cerebral emboli in patients with PFO may originate from the septum itself. This may represent an alternative mechanism to the generally assumed paradoxical embolism.