Teaching Case

A 12-year-old boy with a previous diagnosis of ulcerative colitis presents with recurrent abdominal pain. Laboratory examinations revealed normal faecal calprotectin, an increase of faecal fat (8.7 g/24h, normal <6), and very low faecal elastase (<15 mcg/g), indicating severe exocrine pancreatic insufficiency.

The ultrasound examination revealed a hypoechoic mass in the pancreatic head (Figure 1), without Doppler signal or biliary tract dilation. No abnormalities were appreciated on the intestine examination.

On MRI, the lesion was hyperintense with a hypointense capsule-like rim on T2-weighted images (Figure 2), and restricted diffusion (high DWI signal and low ADC signal) was evident. The cholangiopancreatography sequences (MRCP) showed a smooth narrowing of the main pancreatic duct passing through the pancreatic mass, without obstruction (duct penetrating sign, Figure 3). The common bile duct had a normal calibre. After gadolinium administration, the pancreatic mass became hypoenhancing compared to the rest of the pancreatic parenchyma (Figure 4). Peripancreatic inflammation or adenopathies were not observed.

Fine needle aspirate guided by endoscopic ultrasound demonstrated chronic inflammation.

With steroid therapy, the symptoms disappeared, and the lesion reduced in size on follow-up ultrasound (Figure 5).

Background

Autoimmune pancreatitis (AIP) is a rare form of chronic pancreatitis, particularly in children. The underlying pathophysiology is poorly understood, likely due to an autoimmune mechanism [1,2].

Although there are no established guidelines for children, a diagnosis based on typical clinical symptoms and imaging findings has been proposed, without biopsy requirement [3].

In adults, AIP is classified into two types (type 1, lymphoplasmacytic sclerosing, and type 2, idiopathic duct-centric). Although paediatric AIP has a distinct presentation, most cases have features similar to adult type 2, as a positive IgG4 serology is not usual in children (which is a diagnostic criterion for type 1), and concomitance with inflammatory bowel disease is more frequent (which is more associated with type 2) [3]. However, it is unclear if the adult diagnostic criteria apply to children [4], so paediatric AIP may not fit into either of these categories.

Clinical Perspective

Children with AIP may experience acute abdominal pain, jaundice, fatigue, and/or weight loss. Patients can also develop exocrine pancreatic insufficiency or diabetes [2,3]. Amylase and lipase may be normal due to a common sub-acute presentation[2].

Imaging Perspective

AIP can manifest as focal, segmental, or global pancreas enlargement, or even a pancreatic mass lesion [2,3,5]. Among these, pancreatic head enlargement or non-encapsulated mass lesions are the most frequently reported findings [3].

Ultrasound should be the first line imaging modality, to evaluate pancreatic enlargement/mass, its echogenicity (usually hypoechoic in AIP), and to rule out other pathologies[2].

MRI would be the next step, as CT should be avoided for pancreatitis assessment in children. Main MRI findings include hypointense pancreas on T1-weighted images, hypointense capsule-like rim on T2-weighted images, capsule-like rim enhancement, main pancreatic duct irregularities or stricture, and dilatation of the common bile duct (which tapers toward an enlarged pancreatic head) [2,3].

These features are not specific to AIP, but the presence of more than one should raise suspicion [2]. Diagnosis should also be considered if dilatation of bile or pancreatic ducts is present without a pancreatic mass.

Outcome

Treatment typically involves a high dose of oral steroids for 15 to 50 days, followed by a progressive taper over 2 to 24 months [3].

Post-treatment imaging (ultrasound or MRI) is important for assessing the improvement or resolution of pancreatic manifestations, which supports AIP diagnosis [5].

Take Home Message / Teaching Points

• Paediatric AIP diagnosis is based on typical clinical symptoms and imaging findings.
• Pancreas enlargement / pancreatic mass with pancreatic duct irregularities or stricture are the main MRI manifestations.
•  Paediatric AIP has a distinct presentation with features similar to type 2 AIP in adults.

All patient data have been completely anonymised throughout the entire manuscript and related files.