Sphincter of Oddi Dysfunction · What Is It, Causes, Signs, Symptoms, and More

Sphincter of Oddi dysfunction (SOD) occurs when the sphincter of Oddi cannot contract or relax normally, which obstructs outflow of bile and pancreatic secretions. The sphincter of Oddi (SO) is located at the ampulla of Vater, a junction where the common bile duct and pancreatic duct meet the small intestine. The sphincter itself is a three-layered muscular valve that regulates the flow of bile from the common bile duct (CBD) and pancreatic juices from the pancreatic duct into the duodenum while also preventing backflow of intestinal content into the biliary system and allowing the gallbladder (if present) to fill with bile. Cholecystokinin (CCK) is a hormone that promotes relaxation of the muscular sphincter, while somatostatin is a hormone that tends to increase contraction.

The prevalence of SOD is about 1.5% among the general population; however, it is much higher amongst individuals who have undergone cholecystectomy and those assigned female at birth between the age of 20 to 50.

The cause of sphincter of Oddi dysfunction is largely unknown; however, it is thought to be caused by trauma, congenital hypertrophic sphincter, or an unknown neurological or hormonal response.

Sphincter of Oddi dysfunction encompasses two separate categories of dysfunction: sphincter stenosis and sphincter dyskinesia. Stenosis represents an abnormal tightening of the sphincter space, which may be caused by an inflammatory condition, trauma from gallstones stones or microcrystals, or a neoplastic process. Conversely, sphincter dyskinesia refers to a motility dysfunction that may be related to dysregulated hormonal response of hormones like CCK, somatostatin, motilin, and secretin (which have a role in contraction and relaxation of the sphincter). In addition, other proposals for the etiology of sphincter of Oddi dysfunction include neurological pathways in the parasympathetic nervous system—specifically, the vagus nerve, which when stimulated, has an excitatory effect on the gastrointestinal tract. 

Risk factors for the development of SOD include cholecystectomy, cholelithiasis, gallstone lithotripsy, gallbladder agenesis, increased alcohol use, and hypothyroidism.

SOD is evaluated through history and physical examination, blood tests, and imaging studies and is confirmed using sphincter of Oddi manometry (SOM) and endoscopic retrograde cholangiopancreatography (ERCP). Initially, blood tests are typically drawn and include a complete metabolic panel to assess liver enzymes and pancreatic enzymes, such as amylase and lipase, to assess for pancreatitis. Sometimes in SOD, liver enzymes, such as alanine aminotransferase (ALT), aspartate aminotransferase (AST), and bilirubin, will be elevated and indicate biliary tree and liver involvement. Pancreatic enzymes may also be elevated if the pancreas becomes significantly damaged; however, they may also be within normal limits even during episodes of pancreatitis. 

Imaging studies such as an ultrasound or computed tomography (CT) scan may then be ordered to further evaluate the underlying cause. A CT may reveal common bile duct and/or pancreatic duct dilation. A magnetic resonance cholangiopancreatography (MRCP) may also be useful to visualize both the pancreatic and biliary ducts. Another nuclear imaging study, the hepatobiliary iminodiacetic (HIDA) scan may also be useful to evaluate the outflow of the biliary system. In addition, fatty meal sonography, which is an ultrasound taken before and after ingesting fatty foods, may also be used. This imaging study evaluates the response of the sphincter of Oddi to CCK, which is released after food intake. While the sensitivity of fatty meal sonography is 21%, the specificity is 97%. 

Once initial imaging studies have been done, other methods may be used to confirm the diagnosis. A sphincter of Oddi manometry (SOM) is the gold standard to diagnose SOD and can be done during an endoscopic retrograde cholangiopancreatography (ERCP): a surgical procedure that allows the healthcare provider to visualize the sphincter of Oddi and surrounding structures using endoscopy and fluoroscopy. SOM uses a catheter to measure sphincter pressure. When the pressure is greater than 40 mmHg, a diagnosis of SOD can be made.