Background: To further clarify the clinico-epidemiological features of autoimmune pancreatitis (AIP) in Japan, we conducted the fourth nationwide epidemiological survey. Methods: This study consisted of two stage surveys; the number of AIP patients was estimated by the first survey and their clinical features were assessed by the second survey. We surveyed the AIP patients who had visited hospitals in 2016. Results: The estimated number of AIP patients in 2016 was 13,436, with an overall prevalence rate of 10.1 per 100,000 persons. The estimated number of newly diagnosed patients was 3984, with an annual incidence rate of 3.1 per 100,000 persons. Compared to the 2011 survey, both numbers more than doubled. We obtained detailed clinical information of 1474 AIP patients. The male-to-female sex ratio was 2.94, the mean age was 68.1, and mean age at diagnosis was 64.8. At diagnosis, 63% patients were symptomatic and nearly half of them presented jaundice. Pancreatic cysts were found in 9% of the patients and calcifications in 6%. Histopathological examination was performed in 64%, mainly by endoscopic ultrasonography-guided fine needle aspiration. Extra-pancreatic lesions were detected in 60% of the patients. Eighty-four % patients received the initial steroid therapy, and 85% received maintenance steroid therapy. Kaplan–Meier analysis revealed that the relapsed survival was 14% at 3 years, 25% at 5 years, 40% at 10 years, and 50% at 15 years. Mortality was favorable, but pancreatic cancer accounted for death in one quarter of fatal cases. Conclusion: We clarified the current status of AIP in Japan.
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