Gastrointestinal system additional information | Anatomy

SMA syndrome: Superior mesenteric artery syndrome or SMAS occurs when the duodenum is compressed between the aorta and superior mesenteric artery resulting in partial or complete obstruction of the third part of the duodenum. It presents as crampy abdominal pain that may be relieved by prone or knee chest position or by lying on the left side, fullness, nausea, vomiting of partially digested bile like fluid, weight loss , bloating etc. It’s pathology is interesting. The SMA branches at an acute angle from the aorta. The duodenum sits in this angle. Normally, the duodenum is safeguarded from compression by a fat pad in this area. When this fat pad is lost due to weight loss, surgery especially for scoliosis, psychological disorders causing weight loss etc., there is resulting SMAS. Untreated cases may lead to gastric or portal vein pneumatosis (gas in the walls), bezoar formation, gastric perforation and electrolyte imbalances. Dilated first and second part of duodenum may be seen on imaging.
Box for Mc burney’s point: It was first described by Charles McBurney in 1891. The McBurney’s point lies at two-thirds of the distance from the umbilicus, of a line joining the umbilicus to the right anterior superior iliac spine. Tenderness is elicited upon deep palpation at this point in acute appendicitis. McBurney’s point tenderness may be absent in patients with a retrocaecal or some cases with pelvic appendix. Psoas sign or obturator sign may be positive in such cases.
How to identify bowel obstruction on imaging: Remember the 3/6/9 rule i.e. the bowel is considered distended if it measures more than 3 cms for the small intestine, 6 cms for the colon and 9 cms for the caecum. Small intestine can be identified by the valvulae conniventes which cross the entire width of the lumen. Colon is identified by haustrations which bulge out from the wall and span only about a third of the diameter of the colon. Small bowel loops tend to be located centrally while large bowel loops are more peripheral. Large bowel typically has a mottled appearance from fecal material. Distension is more severe with large bowel obstructions. Massive dilation of the colon, especially the cecum or sigmoid colon is suggestive of volvulus. 'Bent inner tube” or coffee bean sign is seen in sigmoid volvulus. Strangulation will be seen as thumbprinting sign, thickened loops, pmeumatosis intestinalis i.e. air in bowel wall, free air is seen on perforation, pneumobilia or air in the biliary system is seen in gallstone ileus. Fixed bowel loops which do not change position with changing patient positions is suggestive of adhesions.
Box for Couinaud segments of the liver: Claude Couinaud divided the liver into eight functional segments from I to VIII. Each segment has its independent artery, bile duct and tributaries of the hepatic and portal veins. In Couinaud segments, the hepatic veins are situated peripherally. This classification is helpful during hepatic surgery like segmental resection. Segments I to IV make up the left hemiliver while segments V to VIII make up the right hemiliver. Segment I corresponds to the caudate lobe which is unique in that it receives both left and right hepatic artery branches and portal vein , and it is drained by both right and left biliary duct tributaries.