Annular pancreas

Annular pancreas is a morphological anomaly which can cause duodenal obstruction. This condition is important to recognise, as radiologists are frequently the first to make the diagnosis.

Epidemiology

The incidence is probably 1 in 250, however incidence is not accurately reported ¹. It is as common in adults as among children. Age of detection however usually corresponds to 3^rd-6^th decade of life.

Clinical presentation

Presentation in adults and children can be different ². About 25-33% of cases in adults are asymptomatic and incidental finding in radiology. In children, it may be associated with other congenital anomalies or cause duodenal obstruction, while in adults, pancreatitis is the usual presentation. However, it can cause duodenal obstruction in adults. More common symptoms in adults also include abdominal pain, post-prandial fullness, vomiting, GI bleed from peptic ulcers. In rare cases, biliary obstruction may also be seen.

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. Ventral buds fuse rapidly. In 7^th week of gestation, duodenum expands, and rotates the ventral bud from right to left, and causes fusion with 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 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 duodenum. Associated duodenal narrowing and dilatation of proximal duodenum may also be seen. In adults, it is frequently presented 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).