An adult female case of autoimmune pancreatitis with gastric outlet obstruction complicated by annular pancreas.

A 65-year-old woman presented with epigastric pain persisting for more than 3 months. She was diagnosed with autoimmune pancreatitis (AIP), based on high serum IgG4 levels (981 mg/dL) and diffuse pancreatic enlargement with a capsule-like rim on computed tomography (CT). Additionally, the main pancreatic duct was indistinct on magnetic resonance cholangiopancreatography. CT, esophagogastroduodenoscopy, and upper gastrointestinal radiography revealed stenosis with gastric outlet obstruction (GOO) in the second part of the duodenum. Prednisolone administration was initiated as treatment; on day 3 of treatment, the patient's symptoms improved. After 2 weeks, CT and endoscopic ultrasonography of the duodenal bulbs revealed improvement of the enlarged pancreas. The second part of the duodenum ran into the pancreatic head, and no malignant lesions were observed. Based on the above findings, we suspect that she developed AIP in the annular pancreas (AnnP), where duodenal stenosis worsened with diffuse pancreatic enlargement, resulting in GOO. She is currently under careful observation with tapering of prednisolone-without surgical treatment for AnnP. The pathogenesis of GOO caused by AIP without malignancy is rare. One case of GOO caused by AIP, wherein AIP developed in the AnnP (similar to the present case), has been reported, highlighting the novelty of our report.