Annular pancreas

Annular pancreas is a morphological anomaly that results in pancreatic tissue completely or incompletely encircling the duodenum. This condition can cause duodenal obstruction and is therefore important to recognize, as radiologists are frequently the first to make the diagnosis.

Epidemiology

The incidence is probably 1 in 250, however its incidence is not accurately reported ¹. It is equally common in children and adults. Detection usually occurs during the 3^rd to 6^th decade of life.

Clinical presentation

The presentation in adults and children can be different ².

About 25-33% of cases of annular pancreas in adults are asymptomatic and is an incidental finding on imaging. However, it can cause pancreatitis, duodenal obstruction and rarely biliary obstruction in adults. More common symptoms in adults include abdominal pain, post-prandial fullness, vomiting and gastrointestinal bleed from peptic ulcer disease. In children, an annular pancreas may be associated with other congenital anomalies or cause duodenal obstruction.

Pathology 

Embryology

The pancreas develops from a single dorsal and two ventral buds, which appear as outgrowths of primitive foregut at 5 weeks of gestation. The ventral buds fuse rapidly. In the 7^th week of gestation, the duodenum expands, and rotates the ventral bud from right to left, and fuses with the dorsal bud. The ventral bud forms the inferior part of uncinate process and inferior head of pancreas while the dorsal bud gives rise to tail and body of pancreas.

Annular pancreas develops due to failure of the ventral bud to rotate with duodenum, causing encasement of duodenum.

Classification

Annular pancreas can be either complete or incomplete ¹:

• complete annular pancreas: pancreatic parenchyma or annular duct is seen to completely surround the 2^nd part of duodenum
• incomplete annular pancreas: annulus does not surround the duodenum completely, giving a 'crocodile jaw' appearance

Associations

• Down syndrome
• pancreas divisum
• pancreatitis
• pancreatic cancer
• intraductal papillary mucinous neoplasm (IPMN)

Radiographic features

CT

Pancreatic tissue is seen to completely or incompletely surround the 2^nd part of the duodenum. Associated duodenal narrowing and dilatation of proximal duodenum may also be seen. In adults, it is frequently associated with pancreatitis.

MRI/MRCP

Apart from annular pancreas features, pancreatic ductal anatomy can be well assessed with MR imaging. Annular duct usually joins the main pancreatic duct or accessory duct (duct of Santorini).