A suspected case of serum IgG4-negative type 1 autoimmune pancreatitis detected due to localized pancreatic duct narrowing: a case report.

A 50-year-old woman was referred to our hospital with elevated serum amylase levels. Physical examination revealed no jaundice or abdominal tenderness. Serum IgG4 was negative. Computed tomography revealed a localized pancreatic duct narrowing in the pancreatic head, with caudal pancreatic duct dilation and an intraductal papillary mucinous neoplasm. Pancreatic enlargement was not observed. Endoscopic ultrasonography (EUS) showed a small hypoechoic mass. Although EUS-guided, fine-needle aspiration was performed, no diagnosis was established. Endoscopic retrograde pancreatography showed a localized narrowing in the main pancreatic duct of the pancreatic head. A biopsy of the narrowing was performed through the minor papilla because of difficult access from the major papilla. The specimen showed the infiltration of numerous IgG4-positive plasma cells, suggesting type 1 autoimmune pancreatitis (AIP). Six months later, magnetic resonance cholangiopancreatography revealed improvement in the narrowing without specific treatment. The patient presented with localized narrowing of the pancreatic duct and caudal duct dilation, which was distinct from pancreatic cancer. Diagnostic difficulties arose from negative serum IgG4 results, the lack of typical imaging characteristics of AIP, and failure to meet the AIP criteria according to the relevant Japanese and international guidelines. However, AIP was suspected and surgery was successfully avoided through a biopsy.