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Autoimmune pancreatitis (AIP) is an increasingly recognized type of chronic pancreatitis that can be difficult to distinguish from pancreatic carcinoma but which responds to treatment with corticosteroids, particularly prednisone. There are two categories of AIP: Type 1 and Type 2, each with distinct clinical profiles.
Type 1 AIP is now regarded as a manifestation of IgG4-related disease, and those affected have tended to be older and to have a high relapse rate. Patients with Type 2 AIP do not experience relapse and tend to be younger. AIP does not affect long-term survival.
Signs and symptoms
AIP is relatively uncommon and is characterized by the following features:
- Scleral Icterus (yellow eyes), jaundice (yellow skin) which is usually painless, usually without acute attacks of pancreatitis.
- Relatively mild symptoms, such as minimal weight loss or nausea.
- Increased serum levels of gamma globulins, immunoglobulin G (IgG) or IgG4.
- The presence of serum autoantibodies such as anti-nuclear antibody (ANA), anti-lactoferrin antibody, anti-carbonic anhydrase II antibody, and rheumatoid factor (RF).
- Contrast-enhanced CT demonstrates a diffusely enlarged (sausage-shaped) pancreas.
- Diffuse irregular narrowing of the main pancreatic duct, and stenosis of the intrapancreatic bile duct on endoscopic retrograde cholangiopancreatography (ERCP).
- Rare pancreatic calcification or cyst formation.
- Marked responsiveness to treatment with corticosteroids.
AIP is sometimes associated with other autoimmune disorders, most commonly Sjögren's syndrome, primary sclerosing cholangitis (PSC), and inflammatory bowel disease. AIP occurring in association with an autoimmune disorder has been referred to as "secondary" or "syndromic" AIP. Two-thirds of patients present with either obstructive jaundice or a "mass" in the head of the pancreas mimicking carcinoma. It is mandatory to rule out carcinoma prior to making a diagnosis of AIP.
Most recently the fourteenth Congress of the International Association of Pancreatology developed the International Consensus Diagnostic Criteria (ICDC) for AIP. The ICDC emphasizes five cardinal features of AIP which includes the imaging appearance of pancreatic parenchyma and the pancreatic duct, serum IgG4 level, other organ involvement with IgG4-related disease, pancreatic histology and response to steroid therapy. (http://www.ajronline.org/doi/abs/10.2214/AJR.13.11247) .
In 2002, the Japanese Pancreas Society proposed the following diagnostic criteria for autoimmune pancreatitis:
- I. Pancreatic imaging studies show diffuse narrowing of the main pancreatic duct with irregular wall (more than 1/3 of length of the entire pancreas).
- II. Laboratory data demonstrate abnormally elevated levels of serum gamma globulin and/or IgG, or the presence of autoantibodies.
- III. Histopathologic examination of the pancreas shows fibrotic changes with lymphocyte and plasma cell infiltrate.
For diagnosis, criterion I (pancreatic imaging) must be present with criterion II (laboratory data) and/or III (histopathologic findings).
Computed tomography (CT) findings in AIP include a diffusely enlarged hypodense pancreas or a focal mass that may be mistaken for a pancreatic malignancy. A low-density, capsule-like rim on CT (possibly corresponding to an inflammatory process involving peripancreatic tissues) is thought to be an additional characteristic feature (thus the mnemonic: sausage-shaped). Magnetic resonance imaging (MRI) reveals a diffusely decreased signal intensity and delayed enhancement on dynamic scanning. The characteristic ERCP finding is segmental or diffuse irregular narrowing of the main pancreatic duct, usually accompanied by an extrinsic-appearing stricture of the distal bile duct. Changes in the extrapancreatic bile duct similar to those of primary sclerosing cholangitis (PSC) have been reported.
The role of endoscopic ultrasound (EUS) and EUS-guided fine-needle aspiration (EUS-FNA) in the diagnosis of AIP is not well described, and EUS findings have been described in only a small number of patients. In one study, EUS revealed a diffusely swollen and hypoechoic pancreas in 8 of the 14 (57%) patients, and a solitary, focal, irregular mass was observed in 6 (46%) patients. Whereas EUS-FNA is sensitive and specific for the diagnosis of pancreatic malignancy, its role in the diagnosis of AIP remains unclear.
Histopathologic examination of the pancreas reveals a characteristic lymphoplasmacytic infiltrate of CD4- or CD8-positive lymphocytes and IgG4-positive plasma cells, and exhibits interstitial fibrosis and acinar cell atrophy in later stages. at the initial stages, typically, there is a cuff of lymphoplasma cells surrounding the ducts but also more diffuse infiltration in the lobular parenchyma. However, localization and the degree of duct wall infiltration are variable. Whereas histopathologic examination remains the primary method for differentiation of AIP from acute and chronic pancreatitis, lymphoma, and cancer. By Fine Needle Aspiration (FNA) the diagnosis can be made if adequate tissue is obtained. In such cases, lymphoplasmacytic infiltration of the lobules are the key finding. Rarely, granulomatous reaction could be observed. It has been proposed that a cytologic smear primarily composed of acini rich in chronic inflammatory cells (lymphocytes, plasma cells), with rare ductal epithelial cells lacking atypia, favors the diagnosis of AIP. The sensitivity and the specificity of these criteria for differentiating AIP from neoplasia are unknown. In cases of systemic manifestation of AIP, the pathologic features would be similar in other organs.
AIP often completely resolves with steroid treatment. The failure to differentiate AIP from malignancy may lead to unnecessary pancreatic resection, and the characteristic lymphoplasmacytic infiltrate of AIP has been found in up to 23% of patients undergoing pancreatic resection for suspected malignancy who are ultimately found to have benign disease. In this subset of patients, a trial of steroid therapy may have prevented a Whipple procedure or complete pancreatectomy for a benign disease which responds well to medical therapy. "This benign disease resembles pancreatic carcinoma both clinically and radiographically. The diagnosis of autoimmune pancreatitis is challenging to make. However, accurate and timely diagnosis may preempt the misdiagnosis of cancer and decrease the number of unnecessary pancreatic resections." Autoimmune pancreatitis responds dramatically to corticosteroid treatment.
If relapse occurs after corticosteroid treatment or corticosteroid treatment is not tolerated, immunomodulators may be used. Immunomodulators such as azathioprine, and 6-mercaptopurine have been shown to extend remission of autoimmune pancreatitis after corticosteroid treatment. If corticosteroid and immunomodulator treatments are not sufficient, rituximab may also be used. Rituximab has been shown to induce and maintain remission.
Controversies in nomenclature
As the number of published cases of AIP has increased, efforts have been focused on defining AIP as a distinct clinical and pathologic entity and toward developing some generally agreed upon diagnostic criteria and nomenclature. Terms frequently encountered are autoimmune or autoimmune-related pancreatitis, lymphoplasmacytic sclerosing pancreatitis, idiopathic tumefactive chronic pancreatitis, idiopathic pancreatitis with focal irregular narrowing of the main pancreatic duct, and non-alcoholic duct destructive chronic pancreatitis. There are also a large number of case reports employing descriptive terminology such as pancreatitis associated with Sjögren’s syndrome, primary sclerosing cholangitis, or inflammatory bowel disease. Some of the earliest cases were reported as pancreatic pseudotumor or pseudolymphoma.
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