autoimmune pancreatitis

**Definition:** A chronic, immune-mediated form of pancreatitis characterized by lymphoplasmacytic infiltration, fibrosis, and steroid responsiveness. Important mimic of pancreatic cancer.

**Two subtypes:** - **Type 1 (lymphoplasmacytic sclerosing pancreatitis):** Pancreatic manifestation of IgG4-related disease (IgG4-RD). Typically men >50 y. Elevated serum IgG4, IgG4+ plasma cell infiltrate (>10/HPF), storiform fibrosis, obliterative phlebitis. Frequent extrapancreatic involvement: sclerosing cholangitis (IgG4-related), sialadenitis/dacryoadenitis (Mikulicz), retroperitoneal fibrosis, tubulointerstitial nephritis, aortitis. Higher relapse rate. - **Type 2 (idiopathic duct-centric pancreatitis):** Younger patients, men and women equally affected. Granulocytic epithelial lesions (GELs) with neutrophils in the duct wall. Serum IgG4 usually normal. Association with inflammatory bowel disease (especially ulcerative colitis). Lower relapse rate. No systemic IgG4-RD.

**Clinical presentation:** - Painless obstructive jaundice (most common), often with mild or absent abdominal pain. - Weight loss, new-onset diabetes mellitus, steatorrhea (exocrine insufficiency). - Acute pancreatitis episodes are uncommon.

**Diagnosis (HISORt criteria / ICDC):** - **H**istology: lymphoplasmacytic infiltrate, storiform fibrosis, obliterative phlebitis. - **I**maging: diffuse "sausage-shaped" pancreatic enlargement with delayed enhancement and a hypoattenuating "capsule-like" rim on CT/MRI; ERCP shows diffuse or segmental main pancreatic duct narrowing ("beaded" or irregular, long strictures). - **S**erology: elevated IgG4 (a minority of cases of pancreatic cancer also have mildly elevated IgG4, so not specific). - **O**ther organ involvement. - **R**esponse to steroid therapy. - Must exclude pancreatic adenocarcinoma (CA 19-9 may be mildly elevated in AIP; EUS-guided biopsy is often needed).

**Treatment:** - First-line: glucocorticoids (prednisone 0.6–1 mg/kg/day for ~4 weeks, then taper over weeks to months). Rapid clinical and radiographic response. - Biliary stenting for severe jaundice if needed. - Relapse (especially type 1, proximal biliary involvement): immunomodulators (azathioprine, mycophenolate) or rituximab; maintenance low-dose steroid therapy.

**Complications and associations:** Pancreatic exocrine/endocrine insufficiency, biliary strictures, relapse. Possible slightly increased malignancy risk is debated.

**High-yield board points:** - Older man with painless jaundice, pancreatic mass/diffuse enlargement, and elevated IgG4 → think AIP, not just cancer. - Treatment = steroids (dramatic response). - Type 1 = IgG4-RD, multi-organ; Type 2 = IBD association, GELs, normal IgG4. - IgG4-RD histology: storiform fibrosis, obliterative phlebitis, dense lymphoplasmacytic infiltrate.