Autoimmune Pancreatitis

Autoimmune pancreatitis is caused by periductal infiltration with IgG4-positive plasma cells, which leads to periductal and interlobular fibrosis, causing diffuse narrowing of the pancreatic duct and acinar atrophy. Autoimmune pancreatitis may be responsible for up to 10% of cases of chronic pancreatitis. Men are more commonly affected. Patients typically present with fluctuating obstructive jaundice, abdominal pain, weight loss, steatorrhea, and diabetes (~50% of patients).

The Mayo Clinic diagnostic criteria require one or more of the following findings:

• Histologic findings diagnostic of autoimmune pancreatitis
• Characteristic CT and pancreatographic findings with elevated serum IgG4 levels
• Response to steroid therapy

The role of the radiologist is in identifying the characteristic imaging findings and differentiating them from those of acute pancreatitis and pancreatic carcinoma (up to ~10% of pancreatectomy patients for presumed carcinoma actually have autoimmune pancreatitis).

The affected area(s) of the pancreas are typically hypoechoic, hypoattenuating, T1-hypointense, and mildly T2-hyperintense. Decreased enhancement is usually seen during early dynamic imaging, and moderate enhancement is seen in the late phases. A capsule-like rim or halo of low attenuation or intensity is common and is thought to represent fluid, phlegmon, or fibrous tissue. ERCP or MRCP typically reveals a narrow and irregular pancreatic duct in the affected portion(s) of the pancreas.

Three patterns of autoimmune pancreatitis have been recognized

• Diffuse: Most common type. The pancreas is sausage-like: diffusely enlarged with a sharp margin and loss of the normal lobular contour and pancreatic clefts.
  
  This form can mimic the appearance of acute pancreatitis. Unlike acute pancreatitis, there is minimal or no peripancreatic stranding and no peripancreatic fat necrosis.
• Focal: Looks like a relatively well-demarcated focal mass, often involving the pancreatic head.
  
  This form Can mimic pancreatic carcinoma, but upstream dilatation of the main pancreatic duct is typically milder than in patients with carcinoma (usually smaller than 5 mm). In addition, while the peripancreatic veins may be involved in patients with focal autoimmune pancreatitis and in those with pancreatic cancer, involvement of the peripancreatic arteries is unlikely in the former.
  
  On ERCP patients with focal autoimmune pancreatitis usually have a stenosed main pancreatic duct longer than 30 mm and an upstream main pancreatic duct that is smaller than 6 mm in diameter.
• Multifocal: Self explanatory.

The pancreas and pancreatic duct usually return to normal on imaging within 4–6 weeks after initiation of steroid therapy. However, there is atrophy of the affected areas of the pancreas in the burnt-out phase of the disease in about half of patients. Spontaneous regression is can also occur. Diabetes may also resolve following treatment.

Two cases of autoimmune pancreatitis are shown above. The left image has a sausage-like pancreas that is diffusely enlarged with a sharp margin and loss of the normal lobular contour and pancreatic clefts. The pancreas in the right image has a low-attenuation peri-pancreatic halo. Peripancreatic stranding is minimal to absent in both cases.

References

• Vlachou PA, Khalili K, Jang HJ, Fischer S, Hirschfield GM, Kim TK. IgG4-related sclerosing disease: autoimmune pancreatitis and extrapancreatic manifestations. Radiographics. 2011 Sep-Oct;31(5):1379-402.
• Yang DH, Kim KW, Kim TK, Park SH, Kim SH, Kim MH, Lee SK, Kim AY, Kim PN, Ha HK, Lee MG. Autoimmune pancreatitis: radiologic findings in 20 patients. Abdom Imaging. 2006 Jan-Feb;31(1):94-102.