Daniel A. Feeney, DVM, MS, DACVR
Professor of Radiology, University of Minnesota College of Veterinary Medicine, St. Paul, MN, USA
Dogs usually have about 30–70% of their small bowel containing some gas, but with a variable degree of distension which is evenly distributed along the length of the small bowel whereas cats unless aged or stressed usually have gasless small bowel. Another helpful perspective is to think of the alimentary tract caudal to the gastroesophageal junction as a tube which has some areas (e.g., stomach) that may be larger than others, but there should be no area disproportionally dilated with either fluid or gas. The stomach, small intestine, colon, and the proximal aspect of the rectum are functionally intraperitoneal. While these organs are fully wrapped with peritoneum, anything that affects the regional peritoneum will affect these organs. These organs are normally visible, but there should not be air or fluid around them unless this is an immediate postoperative situation (note uncomplicated post-op fluid disappears within 24 hours, but the air can take 10–14 days). The loss of organ clarity may be the result of fat store depletion. Retroperitoneal vs. peritoneal contrast comparisons will clarify this (if the retroperitoneal fat is gone, the peritoneal will usually have poor contrast/clarity unless there is retroperitoneal fluid). Free fluid or trapped fluid may be classified as transudate, exudate, hemorrhage, bile, gut contents or urine. Ultrasound-guided abdominocentesis may be the key to the primary problem. Regional or "trapped" fluid may be the result of gut leak/perforation, adjacent organ inflammation (pancreatitis, peritonitis, mediastinitis), or abscess. Additional considerations germane to organ obscurity include adhesions and peritoneal tumor seeding (carcinomatosis).
Depending on the time since the last meal and the presence of physiologic or morphologic abnormalities, the stomach and small intestine may be dilated to varying degrees. Alimentary organ dysfunction often results in fluid or gas accumulation depending on the degree and extent of dysfunction. However, sudden onset distension with persistence is usually abnormal. Considerations for causes of gut dilation include obstruction, mural abnormalities (irritation/inflammation or infiltration/mass), ischemia and trauma (e.g., migrating FB). Depending on the degree of obstruction (e.g., partial vs. complete) the relative amounts of gas will vary. Complete obstructions are characterized by rapid accumulations of gas. Incomplete obstructions are characterized by persistent accumulation of fluid (with or without retained opacities that would otherwise not be visible) which may be generalized or regional depending on the cause.
Leaks (due to perforation, wall necrosis, ...) in the alimentary tract can result in a range of pathologic problems from sepsis to adhesions as well as the acute local/diffuse peritonitis. Anytime there is a local/regional accumulation of fluid, a leak must be considered as an etiologic possibility. Anytime there is unexplained free or seemingly localized/trapped air in the peritoneal space, the mediastinum, and possibly the retroperitoneal space, alimentary organ leakage must be considered and a risk assessment made. If the risk of alimentary leakage is high according to the history and physical examination, contrast radiographic follow-up is mandatory. Positive contrast studies with isotonic (nonionic) iodinated contrast medium is the safest approach, if leaks are suspected. Equivocal findings that might indicate a leak should be clarified with liquid barium (but be prepared to go immediately to surgery).
Organs in an improper orientation or location require in depth evaluation because of the need for immediate surgical intervention, the generally poor prognosis, or both. Any displacement of alimentary organs may have not only the problem of organ dysfunction (e.g., partial or complete obstruction, dysfunctional or absent peristalsis), but also the problem of ischemia. For displacements due to masses, the position of the stomach can provide clues to differentiate liver from non-liver masses. For gastric displacements (e.g., 180° volvulus), the right lateral and the sternally recumbent views are indicated. On the right lateral view, the pylorus will be dorsal/anterior to the fundic area and on the DV view the pylorus will be anterior or superimposed on the cardiofundic area. For suspected herniation, variable recumbency, horizontal-beam and contrast radiographic studies are indicated to determine of gut is entrapped and, if so, is it strangulated in the hernia. These varied views are quite useful for diaphragmatic hernias as well even if there is no suspicion of alimentary-related emergency.
Abnormal Organ/Cavity Radiopacity
Air in the wall of the gut (e.g., pneumatosis coli, pneumatosis intestinalis) may be the result of gut wall ischemia, gas-forming bacterial penetration (translocation) through a compromised mucosal barrier, or severe immunosuppression. Radiopaque foreign bodies in/near the alimentary tract require clarification as to their effect on the bowel (e.g., penetration [from inside or outside the gut], circulatory compromise, future adhesions/strictures...). Horizontal-beam radiographs are quite sensitive to permit identification of free intraperitoneal air often indicative of a gut leak. Radiolucent foreign bodies pose an even more challenging problem in that they must be suspected based on their effects on the bowel contents (e.g., gas/fluid imbalance [e.g., uneven fluid/gas distribution, affected and unaffected segments]) as well as their direct effects on the bowel itself (e.g., distension, plication [linear foreign body]...). Survey radiographs are very useful here, if interpreted completely. In patients where the situation is not clear, follow-up upper GI studies are indicated. The choice of barium (no leak suspected) vs. nonionic iodinated (suspected leak) contrast medium must be made based on the history and survey radiographic findings. In the case of acute, nonobstructive disease with good peritoneal contrast, follow-up survey radiographs in 12–24 hours may be the best approach. In the case of gunshot wounds, one must weigh the likelihood of gut penetration based on the suspected path of the bullet/pellet.
Flocculation (agglomeration, precipitation) of barium sulfate into bead-like aggregates within the stomach (and particularly the small bowel of dogs and cats) indicates the presence of material with (+) ions in the lumen of the viscus or the presence of blood, protein or excess mucus. Ileus or bowel atony is one of the few possible reactions the stomach and bowel can exhibit. One form of ileus is adynamic (also referred to as paralytic) ileus. It is characterized by generalized or segmental/regional enlargement of the bowel. This is associated with pain, peritonitis (infections or chemical), electrolyte imbalance (e.g., hypocalcemia or hyperkalemia), surgical trauma (not in excess of 24 hours), primary alimentary organ inflammation (e.g., reflux esophagitis or viral enteritis), and parasympatholytic drugs including sedatives or general anesthetic agents. Dynamic (also referred to as obstructive) ileus characterized by massive segmental (or generalized if obstruction is in terminal alimentary segment) enlargement of the bowel proximal/orad to the site of the problem.
For ease of understanding, alimentary tract lesions are classified as extramural (outside the wall such as an enlarged prostate compresses the colorectal junction), mural (within the wall such as regional tumor, stricture or diffuse infiltrate including round cell neoplasia, the various forms of "IBD", and pyogranulomatous disease, and luminal (in the gut lumen such as a foreign body).
There are three other specific classifications of lesions related to the gut. First is the "plication", "pleating", and shortening of a bowel segment (usually small intestine) due to a linear foreign body. Next is the "coil spring" or "onion skin" (multilayered series of concentric rings or "ring" sign recognizable at ultrasonography) appearance of intussusceptions. This appearance is usually appreciated radiographically only if there is positive contrast media in the lesion area. Intussusceptions may also be recognized as tubular intraluminal filling defects if surrounded by air at radiography or merely as a complete obstruction (lots of gas, possibly two different populations of bowel [one dilated and one about normal]. Finally, there is "thumbprinting" which is the diffuse intramural infiltrate or irregular wall thickening that may be due to pyogranulomatous, neoplastic or inflammatory/traumatic etiologies.
When making the choice of a "next" procedure following survey radiography, the decision should be based on several factors including the individual's expertise (particularly with ultrasound), the lesion location (endoscopy not applicable for jejunal or ileal lesions), risks (barium aspiration or body cavity contamination) and predictability.