Abdominal pain is one of the most common complaints encountered in the acute care setting. Undifferentiated abdominal pain can be one of the most challenging conditions that a physician encounters on a daily basis. The decision regarding which labs or radiological tests to order can be frustrating and often unrevealing. Particularly in elderly patients, a difficult history and often inaccurate and changing physical examination can complicate the decision-making process.

There are many conditions that cause abdominal pain, but not all of them are best evaluated with ultrasound. However, there are certain situations in which bedside ultrasound is ideal. In any patient who presents with undifferentiated abdominal pain and is hemodynamically unstable, a quick bedside ultrasound can help to rule out free fluid or possible surgical causes of the pain, such as an abdominal aortic aneurysm. Other etiologies that bedside ultrasound can be useful for are biliary colic or acute cholecystitis, renal colic, bowel obstruction, and appendicitis.

The first distinction that should be made is whether or not the pain is diffuse or focal. This guides the physician in which abdominal areas should be evaluated first. The bedside ultrasound should begin with the evaluation of the most likely organ system to be causing the pain.

Diffuse Abdominal Pain

If the pain is diffuse, it is not always easy to pinpoint which organ system first became affected. Abdominal pain that is generalized is usually of higher concern for the physician as it indicates the progression of the disease. The concerning causes for diffuse abdominal pain include a perforated viscous, a bowel obstruction, the presence of new free fluid from a ruptured structure, or peritonitis in a patient with known ascites.

Perforated Viscous

There are multiple causes of a perforated viscous, including a ruptured appendix, a diverticulitis, a perforated ulcer, a prolonged bowel obstruction, or a ruptured esophagus from excessive vomiting. The presence of fluid in the abdomen in a patient without liver disease or known ascites should raise the suspicion of an abdominal catastrophe, including perforated viscous. Free air is a specific finding for a perforated viscous, and ultrasound may be able to detect free air. When present, free air is best seen at the edge of the liver and will be seen as hyperechoic areas with dirty comet tails that are not within the bowel. Plain radiographs may also detect free air, but a CT scan is much more sensitive than either ultrasound or x-ray.

Free Fluid

If a moderate to large amount of fluid is seen with a cirrhotic-looking liver (shrunken, irregular-shaped), then spontaneous bacterial peritonitis should be considered. Free fluid in a female with abdominal pain should lead to a search for gynecologic origin, such as a ruptured ovarian cyst, ectopic pregnancy with bleeding, or the extension of a pelvic infection. An elderly patient with diffuse abdominal pain and free fluid on their ultrasound should prompt search for a ruptured viscous or bowel obstruction. The abdominal aorta should be evaluated down to its bifurcation into the iliacs for aneurysm or dissection. Ultrasound is not sensitive for aneurysm rupture as this typically occurs retroperitoneally. Its branches should also be evaluated, if possible, including the celiac, splenic, and renal arteries. The spleen should also be evaluated in the presence of free fluid. It may rupture from even mild trauma when enlarged from a viral, congestive, or malignant source.

Small Bowel Obstruction

Small bowel obstruction (SBO) is a common cause of generalized abdominal pain. The most likely cause is previous abdominal surgery, but is also associated with hernias or bowel malignancies. The abdomen can be examined at the bedside, looking for signs of dilated bowel. Small bowel loops greater than 3 cm in diameter on ultrasound is consistent with a SBO. Small bowel can be differentiated from large bowel by the presence of plicae ciculares, folds that extend completely around the inner portion of the bowel, as opposed to haustra, which only partially encircle the interior of the large bowel. In addition to dilated bowel, a bowel obstruction may show a characteristic to-and-fro motion of bowel contents as well as free fluid around the dilated loops. Ultrasound is particularly helpful for detecting bowel obstruction when there is a paucity of air in the bowel on plain radiographs.

Focal Abdominal Pain

Localized abdominal pain should prompt the physician to investigate the organs most likely to be causing the discomfort. The abdomen is divided into quadrants based on the location of different structures.

Right Upper Quadrant Pain

Right upper quadrant (RUQ) pain can have multiple causes. The most common etiologies include biliary tract disease, hepatitis, liver cysts, masses or infections, and renal colic.

The evaluation of RUQ pain should begin with an assessment of the gallbladder and liver. The gallbladder should be interrogated for the presence of stones or sludge and for the additional signs of acute cholecystitis or choledocholithiasis. If gallstones are seen and there is a positive “sonographic Murphy sign” with maximal tenderness over the fundus of the gallbladder, then the pain is most likely biliary in origin. An enlarged common bile duct and hepatic ducts also can be seen on ultrasound and indicate biliary obstruction. The liver should be examined fully looking for cysts, masses, or abscesses. In addition, signs of acute or chronic hepatitis may be identified. The liver may be enlarged and edematous appearing in acute hepatitis, or shrunken and irregular in the chronic state.