Autoimmune pancreatitis complicated with inflammatory bowel disease and comparative study of type 1 and type 2 autoimmune pancreatitis

Two types of autoimmune pancreatitis (AIP) have been reported, lymphoplasmacytic sclerosing pancreatitis and idiopathic duct-centric chronic pancreatitis (IDCP), which are now recognized as type 1 and type 2 AIP, respectively. Since the clinical features of type 2 AIP have not been fully elucidated and this condition is frequently accompanied by inflammatory bowel disease (IBD), we performed a nationwide survey of patients with AIP complicated with IBD to precisely characterize this disease entity. We collected 138 cases of pancreatitis with complicating IBD from affiliated institutes specializing in AIP or IBD, and comparative study between the IDCP groups and type 1 AIP was performed. Histological examination revealed 15 AIP cases to be IDCP of institutional diagnosis, among which 11 cases were upgraded to IDCP of central diagnosis by an expert pathologist. The IDCP group exhibited younger onset age, no gender bias, frequent abdominal pain, and normal IgG4 value, similar to those of type 2 AIP reported previously. We also witnessed a lower prevalence of jaundice in type 2 AIP than in type 1 AIP that corresponded to imaging findings of less frequent pancreatic head swelling and scarce bile duct stenosis. A characteristic feature of type 2 AIP compared with type 1 AIP is a low frequency of obstructive jaundice that is related to rare lower bile duct stricture due to lower prevalence of pancreatic head swelling. Contrary to type 1 AIP, lower bile duct stricture in this condition has no apparent relation to sclerosing cholangitis.