An annular pancreas is a congenital anomaly in which the pancreas either partially or completely encircles the duodenum, resulting in the obstruction of the gastrointestinal tract or the biliary tree. Epigastric discomfort, and nausea, followed by relief after vomiting are the main symptoms. The diagnosis is made through the use of imaging studies and invasive upper gastrointestinal endoscopy.
Annular pancreas, defined as encircling of the duodenum by the pancreas due to improper embryonal development, was initially considered to be a rare finding in clinical practice, with an incidence rate of approximately 1 in 20,000 newborn babies  . However, it is recognized as the second most common congenital anomaly of the pancreas (after pancreas divisum), and studies show that up to 1 in 250 individuals have this congenital abnormality, but not all patients develop symptoms . It is diagnosed in two patient populations - in newborns and adults   . In newborns, severe obstruction of the duodenum with vomiting is the main presenting symptom  . Down syndrome (trisomy 21) has been strongly associated with annular pancreas . On the other hand, the condition may be asymptomatic until adulthood, and patients between 20-60 years of age (more specifically within the 30-50 year range period) can develop a sudden onset of epigastric pain, nausea, relief after vomiting and post-prandial fullness, while hematemesis is seen in about 10% of cases  . In addition, an increased incidence of peptic ulcer disease (PUD), acute pancreatitis, biliary obstruction, and jaundice, as well as pancreatic carcinoma is noted in this patient population  . It is not uncommon for annular pancreas to be detected incidentally  .
Because certain reports have identified its surprisingly common occurrence, annular pancreas must be included in the differential diagnosis of unexplained gastrointestinal complaints. In neonates and in adults, the role of a detailed patient history revealing the course and progression of symptoms is of pivotal importance, whereas a carefully obtained physical examination is equally important in the assessment of such complaints. Once valid clinical suspicion is raised about a gastrointestinal pathology, imaging studies should be employed. Plain radiography, although being of little benefit in the evaluation of the GI tract, can detect air in the dilated stomach and the duodenal bulb (known as the double-bubble sign)  . Abdominal ultrasonography is of limited use in evaluating hollow organs, and so radiological techniques like computed tomography (CT) and magnetic resonance imaging (MRI) are favored  . The presence of pancreatic tissue completely or partially surrounding the duodenum is a cardinal feature of annular pancreas . The anomaly may not be large enough to be confirmed through non-invasive imaging studies, however, which is why more direct methods are often necessary to determine the cause of GI symptoms. Magnetic resonance cholangiopancreatography (MRCP), endoscopic retrograde cholangiopancreatography (ERCP), and endoscopic ultrasonography (EUS) are recommended procedures definitive diagnosis     .