Autoimmune pancreatitis: with special reference to a localized variant

In 2006, the Japan Pancreas Society revised the diagnostic criteria for autoimmune pancreatitis (AIP) so as to more clearly define its morphological, pathological, and immunological features, as follows: (1) diffuse or segmental narrowing of the main pancreatic duct with an irregular wall and diffuse or localized enlargement of the pancreas recognized by imaging studies; (2) high serum gamma globulin, IgG, or IgG4 levels, or the presence of autoantibodies; and (3) marked interlobular fibrosis and prominent infiltration of lymphocytes and plasma cells in the periductal area, occasionally with lymphoid follicles in the pancreas. Establishing a diagnosis of AIP has become easier with knowledge of its immunological abnormalities, including serum IgG4 levels. However, the localized form of AIP sometimes mimics pancreatic cancer. The rate of focal mass formation in patients with AIP is reportedly 24%-43%; however, there have been few reports on the histological findings of localized AIP, in contrast to mass-forming pancreatitis (MFP). Our review of patients who had undergone resection due to a preoperative diagnosis of MFP with possible cancer revealed 72% to be patients with localized AIP. For the discrimination of these conditions, it is important to recognize the characteristic ultrasonographic findings of AIP, i.e., (1) diffuse or localized enlargement and hypoechogenicity of the pancreas; (2) rarity of calcification, cystic lesions, and peripancreatic fluid collection; (3) thickened layer structure of the bile duct wall; (4) iso/hypervascularity in the swollen portion of the pancreas; (5) attenuation of pancreatic swelling and bile duct wall thickening after steroid therapy; and (6) multiple hypoechoic masses in various organs, including the pancreas. Contrast-enhanced endoscopic ultrasonography is potentially a useful tool in the differential diagnosis and for assessment of the efficacy of steroid therapy by enabling evaluation of the vascularity of the lesions. Along with the presence of IgG4-positive plasma cells, verification of obliterative phlebitis is highly specific for the histological diagnosis of AIP.