Autoimmune pancreatitis (AIP) is a subtype of chronic pancreatitis with its own specific histological findings and clinical course, and, usefully, it is treatable. Two forms are recognised.
Type 1 AIP
Type 1 AIP is the pancreatic manifestation of a multi-systemic disorder called IgG4-related disease. Along with the pancreas, it classically involves bilateral enlargement of the submandibular glands, renal involvement, peritoneal involvement, and stricturing of the supra-pancreatic biliary tree. Histologically it is characterised by a lymphoplasmocytic infiltrate, storiform fibrosis and abundant IgG4-positive plasma cells.
Type 2 AIP
Type 2 AIP, also called idiopathic duct-centric chronic pancreatitis (IDCP), is characterised histologically by the absence of IgG4-positive cells and by granulocytic epithelial lesions (GEL) — accumulations of granulocytic cells in the epithelial wall of ductal portions. It is associated with inflammatory bowel disease: about a third of patients with IDCP also have IBD, most often ulcerative colitis.
Presentation and diagnosis
The common manifestations of autoimmune pancreatitis are jaundice, new-onset diabetes and weight loss. Positivity for IgG4 antibodies in serum supports type 1 AIP in about two thirds of patients, but a negative result does not exclude the condition.
On CT, the main findings in AIP are focal or diffuse enlargement of the pancreas and a hypo-echoic rim, or capsule sign, which is the accumulation of inflammatory cells and oedema around the pancreas. The capsule sign is highly specific, but not sensitive, for AIP.
Treatment
AIP responds dramatically to high-dose steroid therapy: over a course of 2–4 weeks, symptoms improve and the enlarged pancreas shrinks significantly. A poor response to glucocorticoid therapy should raise the suspicion of another disorder, such as pancreatic cancer.