Autoimmune pancreatitis (AIP) is a form of chronic pancreatitis whose main clinical manifestation is painless obstructive jaundice.^1,2 It is a rare condition, with a prevalence of approximately 1 case per 100 000 people in the general population, accounting for 5-6% of all chronic pancreatitis cases.^3,4 Based on its clinical manifestations and histologic pattern, AIP can be classified into two subtypes, namely, type 1 or lymphoplasmacytic sclerosing pancreatitis and type 2 or idiopathic ducto-centric pancreatitis^4,5

In order to diagnose AIP, different aspects should be considered, among them: imaging findings -through computed axial tomography (CT) or magnetic resonance imaging (MRI)- of an enlarged diffuse pancreatic enlargement and loss of lobularity; serological findings such as elevated immunoglobulins; and histological findings such as lymphoplasmacytic infiltration without granulocytic acinar inflammation, storiform fibrosis, obliterative phlebitis, and presence of immunoglobulin G4 (IgG4), as well as involvement of other organs and positive response to corticosteroids.^6-8 AIP does not require surgery and has a good prognosis with medical treatment.⁶

The following is the case of a patient with AIP diagnosed due to the presence of IgG4 and successfully treated with corticosteroid therapy. This case report was written following the CARE guideline criteria.⁹

AIP is a form of chronic pancreatitis characterized by clinical manifestations such as obstructive jaundice and histological changes of the pancreas (lymphoplasmacytic infiltration and fibrosis).^1,2 Unlike other forms of chronic pancreatitis, this condition usually responds to corticosteroid therapy and is completely reversible in most cases.^1,2 It was first described in 1961 when Sarles et al.¹⁰ reported a form of idiopathic chronic pancreatitis that was associated with jaundice and hypergammaglobulinemia, raising suspicion of an autoimmune process underlying this manifestation.^1,2 However, it was not until 1995 when Yoshida et al.¹¹ coined the term "autoimmune pancreatitis", which is still used worldwide to refer to this form of chronic pancreatitis^3,4

AIP is considered to be a rare disease, with a prevalence of approximately 1 case per 100 000 people in the general population,¹² accounting for 5-6% of all chronic pancreatitis cases.^3,4 In clinical terms, AIP is characterized by painless obstructive jaundice, but its clinical spectrum varies widely, including manifestations of acute pancreatitis, onset of diabetes, weight loss, asthenia, and adynamia.^4,5 To make a diagnosis, in 2010, the International Association of Pancreatology developed the International consensus diagnostic criteria (ICDC) for AIP,¹³ which established that aspects such as imaging, serologic and histologic findings, involvement of other organs, and response to corticosteroids should all be considered together.^5,7

Regarding imaging findings, it has been reported that CT and MRI scans in these patients show a diffuse enlargement of the pancreas (sausage-shaped appearance) associated with a loss of loburality, as well as a hypoattenuated peripheral halo sign on CT and hypointensity on MRI. During ERCP, it is common to see a long stricture (>1/3 of the pancreatic duct) and multiple or focal strictures without proximal dilatation of the system.^5,7 Serologic findings involve elevated levels of immunoglobulins, especially IgG4, antinuclear antibodies, and rheumatoid factor; however, it should be noted that, in some cases of AIP, immunoglobulin levels are not elevated.⁸

In their study, Cai & Tan⁷ state that AIP may cause extrapancreatic lesions due to the involvement of other organs, including lacrimal and salivary gland lesions, pulmonary lesions (involving hilar lymphadenopathy), sclerosing cholangitis, tubulointerstitial nephritis, and retroperitoneal fibrosis. Likewise, it has been established that histological findings include lymphoplasmacytic infiltration with or without granulocytes, storiform fibrosis, obliterative phlebitis, and presence of IgG4.^6-8

Regarding the criterion of response to corticosteroids, O'Reilly et al. ¹ state that the initiation of corticosteroid therapy has shown resolution of both imaging and clinical manifestations in patients with AIP. Consequently, response to treatment is also considered as a diagnostic criterion.

In the present case, the patient was diagnosed with AIP since both CT and MRI scans showed a diffuse alteration of the pancreatic morphology, an enlargement of the pancreas and an obstruction of the bile duct and pancreatic duct (suggesting narrowing of these organs), with no findings of proximal dilatation, which are characteristic imaging findings of this condition.^1,2

With respect to serological criteria, the literature cites multiple alterations, including elevated IgG levels,^4,5 which is key to diagnose AIP and to exclude other differential diagnoses. It has also been established that IgG4 levels >135 mg/dL can differentiate AIP from pancreatic cancer, with a limited sensitivity ranging from 79% to 93%. In addition, IgG4 levels >280 mg/dL are considered to have a specificity of 99% for AIP,^1,2 so the serological criterion was met in the present case since the patient's IgG4 level was 518 mg/dL.

Regarding the criterion of involvement of other organs, it has been concluded that the most affected organs are the biliary tree, the lymph nodes, and the salivary and lacrimal glands.^1,4 In the present case, this parameter was not observed; however, it is considered that its occurrence cannot be ruled out because, according to the literature, extrapancreatic manifestations can be prior, simultaneous or even subsequent to AIP.^2,4,5

In order to describe the histological parameter, it is necessary to point out that, according to the presentation of the diagnostic criteria for AIP established by the ICDC, it can be classified into two subtypes: type 1 AIP and type 2 AIP. Type 1, or lymphoplasmacytic sclerosing pancreatitis due to its histologic pattern, occurs more frequently in men, with a peak incidence during the sixth decade of life, and is currently recognized as the pancreatic manifestation of IgG4-related disease because it causes abundant plasma cell infiltration (IgG4 >10 cells/field), stromal fibrosis, obliterative phlebitis with pancreatic inflammation, and elevated serum IgG4 levels, and it also has an excellent response to corticosteroid therapy.^5-7 On the other hand, type 2, or idiopathic duct-centric pancreatitis, does not cause elevated serum IgG4 levels and its histopathologic features include granulocytic epithelial lesions with little or no plasma cell infiltration (IgG4 <10 cells/field).^2,4 In the present case, the biopsy result is striking as it reported 8 IgG4-positive cells, a characteristic finding of type 2 AIP, contrary to type 1 AIP where IgG4-positive cells are usually >10.

Finally, given that, as mentioned above, corticosteroids are not only used to treat this disease, but a positive response to them is also a diagnostic criterion of AIP,^6,7 it is recommended to administer prednisone or prednisolone at 0.6 mg/kg per day for 4 to 6 weeks.⁶ In the present case, given the high suspicion of AIP, prednisolone was administered at a dose of 40mg per day for 6 weeks, which resulted in an improvement, so it is considered that the patient met the criteria of responding to corticosteroid therapy. Thus, in association with the criteria of the symptoms mentioned above, it can be concluded that the patient presented type 1 AIP, since the observed findings and his clinical course are characteristic of this subtype, despite the fact that the biopsy showed only 8 IgG4-positive cells.

AIP is a disease that every general practitioner and specialist should be aware of and consider as a differential diagnosis in patients with symptoms of painless obstructive jaundice and imaging findings of a diffusely enlarged pancreas and loss of lobularity, thus avoiding misdiagnosis and unnecessary interventions.