Autoimmune Pancreatitis

A distinct form of chronic pancreatitis mediated by autoimmune mechanisms, responsive to corticosteroids, and divided into two subtypes with different pathology, associations and natural history.

Type 1: the pancreatic manifestation of IgG4-related disease, affecting older men predominantly, with lymphoplasmacytic sclerosing pancreatitis, storiform fibrosis, obliterative phlebitis and abundant IgG4-positive plasma cells. Frequently associated with other organ involvement (biliary, salivary, retroperitoneal). High relapse rate.

Type 2: idiopathic duct-centric pancreatitis, affects a younger population with no sex predominance, characterised by granulocytic epithelial lesions (neutrophilic infiltration of duct epithelium) without significant IgG4 tissue infiltration or elevated serum IgG4. Strongly associated with inflammatory bowel disease (up to 30%). Lower relapse rate, usually a single episode.

• Painless obstructive jaundice is the commonest presentation of both types, mimicking pancreatic cancer.
• Diffuse pancreatic swelling on imaging without the severe pain typical of ordinary pancreatitis.
• New-onset diabetes is common.
• Weight loss.
• Type 1: often has synchronous or metachronous extrapancreatic organ involvement (salivary swelling, retroperitoneal fibrosis, biliary strictures).
• Type 2: may present as acute pancreatitis; strongly linked to ulcerative colitis or Crohn disease, which may be diagnosed concurrently or subsequently.

• Imaging: diffusely enlarged "sausage-shaped" pancreas with a peripheral capsule-like rim (delayed enhancing halo) on CT/MRI; diffusely irregular narrowing of the main pancreatic duct on ERCP/MRCP without significant upstream dilation, distinguishing it from the abrupt cutoff typical of malignancy.
• Serum IgG4 elevated in type 1 (though not universally); normal in type 2.
• Other organ involvement supports type 1.
• Histology (core biopsy, ideally) is the reference standard where diagnosis is uncertain: lymphoplasmacytic infiltrate with storiform fibrosis and obliterative phlebitis with IgG4-positive plasma cells for type 1; granulocytic epithelial lesions for type 2.
• Serology to exclude other causes and to assess associated conditions (inflammatory bowel disease workup where type 2 is suspected).
• The critical diagnostic discipline: malignancy must be excluded before treating with steroids. A focal mass, upstream duct dilation, weight loss disproportionate to the picture, or diagnostic uncertainty after imaging and serology mandates tissue diagnosis before a steroid trial. A steroid trial with careful imaging follow-up at 2 weeks (expecting resolution) is an accepted diagnostic tool only once malignancy has been reasonably excluded.

• Corticosteroids: prednisolone 0.6 mg/kg/day (30-40mg) for 2-4 weeks with taper over 3-6 months. Response is typically rapid and near-universal in both types, supporting the diagnosis retrospectively.
• Type 1 relapses in up to 50%, particularly with biliary involvement; relapse is managed with repeat steroid course or steroid-sparing agents (azathioprine, rituximab).
• Type 2 rarely relapses once treated; a single course is usually sufficient.
• Biliary stenting for significant obstructive jaundice while awaiting steroid response, or where malignancy cannot be excluded.
• Manage new-onset diabetes and exocrine insufficiency as they arise; both may improve with steroid treatment but can persist, requiring insulin and pancreatic enzyme replacement respectively.
• Investigate and treat inflammatory bowel disease where type 2 is diagnosed.
• Long-term monitoring for relapse (imaging, IgG4, symptoms) and for pancreatic exocrine/endocrine insufficiency.