Abstract: AIM: This study investigated the causes and mechanism for headache observed after atrial septal defect(ASD) occlusion by studying the correlation between headache and the geometric characteristics of the occluder.METHODS: One hundred eighty three patients(66 male,117 female) with secundum ASD were included with ages of 6-72(30.1±18.5) years.The maximum size of the defect was in the range of 8-35 mm(19.8±7.5) mm,measured by transthoracic echocardiography(TTE).Sizes of the occluders used were 10-44 mm(25.9±8.9).RESULTS: Sixteen patients(8.7% of all patients) reported a postoperative headache after 3 months.The group with reported headache had significantly greater ASD defect sizes(27.3±6.2) mm compared with the defect sizes in the group without headache (17.1±7.2) mm,P<0.01.In the group with headaches,both the diameter and thickness(measured by ultrasound) of the occluder were significantly larger than those used in the group without headaches(P<0.01).The difference between the occluder diameter and the defect size,as well as the difference between the original and postoperative diameters of the occluder,were significantly larger in the group with headaches compared to the group without headaches(P<0.05 and P<0.01,respectively).Correlation analysis demonstrated that the incidence rate of a headache was significantly related to the thickness of the occluder(P<0.01) but independent of age,defect size,occluder diameter,and other morphometric indicators.CONCLUSION: The higher incidence of headache after ASD occlusion positively correlates with the thickness of the occluder.We speculate that headache may be caused by the incomplete endothelialization of the surface of the occluding device or to prolonged endothelialization process.