lesser sac

The lesser sac (omental bursa) is a peritoneal recess situated posterior to the stomach, lesser omentum, and caudate lobe of the liver, and anterior to structures like the pancreas, left kidney, and splenic vessels. It communicates with the greater peritoneal sac (main peritoneal cavity) through the epiploic foramen (foramen of Winslow), bounded by the hepatoduodenal ligament anteriorly, IVC posteriorly, caudate lobe superiorly, and duodenum inferiorly.

Embryologically, the lesser sac develops from the rotation of the stomach during development, as the dorsal mesogastrium rotates and creates this potential space.

Clinical significance: - Pancreatitis: Pancreatic pseudocysts often form in the lesser sac since the pancreas is a primary posterior boundary; fluid collections and enzymatic leakage can accumulate here. - Gastric cancer/surgery: Surgeons access the lesser sac during procedures like distal pancreatectomy or to assess posterior gastric wall tumors. - Perforated posterior gastric ulcers: Can lead to localized collections within the lesser sac rather than diffuse peritonitis. - Omental bursa fluid collections: Can be identified on CT/ultrasound, aiding diagnosis of pancreatic or gastric pathology. - The epiploic foramen is a potential site for internal hernias (rare but classic exam topic).

Understanding this anatomy is important for interpreting abdominal imaging, understanding the spread of pancreatic fluid collections, and performing certain surgical approaches (e.g., accessing the pancreas via the gastrocolic ligament).