GI Imaging, “Answers”

1. When do you get abdominal plain films before CT in suspected SBO?
2. How do plain films guide your management in patients with suspected intraperitoneal free air?

With advances in radiologic technology and the increased availability of CT, ultrasound, and MRI, the contemporary use of plain abdominal radiographs (AXR) in the evaluation of acute abdominal pain is poorly defined (Hampson, 2010). A broad spectrum of indications are listed by the American College of Radiology. However, even for these, the accuracy of AXR is notoriously low, and it is rarely ever the ideal first-line imaging study. A prospective study of patients with non-traumatic abdominal pain presenting to the emergency department estimated the overall sensitivity, specificity and accuracy of AXR series for all pathology to be 30%, 87.8%, and 56%, respectively (MacKersie, 2005). A retrospective review by Kellow, et al. showed 72% of “normal” AXRs and 78% of “nonspecific” AXRs were actually found to have pathology on follow up imaging (Kellow, 2008).

EML GI Imaging AnswersSeveral recent studies have looked at the utility of obtaining AXR, including appropriate uses, diagnostic significance, and whether this imaging modality affects management. One study estimated that only 3% of AXRs obtained in 861 patients, significantly impacted management (Kellow, 2008).

Two diagnoses for which abdominal radiograph is still commonly used are bowel obstruction (SBO) and pneumoperitoneum.

1. Small bowel obstruction

Despite being one of the few abdominal pathologies with distinct plain film abnormalities, findings of obstruction on AXR are difficult to interpret. Markus, et al. found inter-observer agreement based on kappa values between radiologists for diagnosis of SBO was only “fair to good.” Additionally, this study showed that agreement was only “poor to fair” for determining large bowel obstruction, and location or completeness of SBO (Markus, 1989).

Several studies estimate AXR sensitivities between 45-90% and specificity of approximately 50% in diagnosing SBO. CT, in comparison, has a reported sensitivity of 93% and specificity of 100% (Suri, 1999; Frager, 1994). In patients with suspected SBO, Maglinte quotes AXR to yield accurate diagnoses in 50-60%, indifferent or nonspecific findings in 20-30%, and misleading reads in 10-20%. Identification of partial SBO lowers the sensitivity to 30% for AXR (Frager, 1994), whereas it is around 60% for CT (Maglinte, 1996).

Despite this data, bowel obstruction remains one of the most common indications for AXR ordered in the ED evaluation of abdominal pain. It is important to understand if, when, and how these images should affect patient management (Kellow, 2008).

A large study found that the addition of AXR to clinical assessment in ED evaluation of abdominal pain significantly increased the sensitivity of clinical diagnosis from 57% to 74%; the positive predictive value, however, was not significantly changed. Moreover, the addition of radiographs in suspected obstruction did not significantly change ED physicians’ initial diagnosis or confidence in their diagnosis (Van Randen, 2011).

In addition to poor diagnostic accuracy, AXR (unlike CT) lacks the ability to distinguish partial from complete obstruction, determine a transition point, or identify cause–information vital to clinical management and surgical planning. Thus, despite increasing diagnostic sensitivity, AXR is not likely sufficient to preclude further imaging. In a  retrospective study, the majority (53%) of patients with dilated loops of bowel on AXR deemed “significant” proceeded to CT scan (Jackson, 2001). Of these 47 patients, 9 had CTs without evidence of obstruction contributing to the authors’ conclusion that the yield of initial AXR in SBO was low (Jackson, 2001).  In another review, only 5% of AXRs performed to evaluate obstruction confirmed the diagnosis and were managed without obtaining further imaging (Kellow, 2008). Again, the majority of all abnormal AXRs underwent subsequent CT. In one study of patients with suspected acute SBO, CT corrected erroneous diagnoses and management in 21% of cases (Taourel, 1995).

MacKersie found nonenhanced CT scan to be more sensitive and specific compared with a three view AXR series. In equipped facilities, the time to obtain a noncontrast CT should be comparable to a 3 view AXR, suggesting that if time is critical, the test of choice is the one with diagnostic superiority (MacKersie, 2005). Given that the majority of suspected obstructions, regardless of AXR outcome, are followed by CT then radiation is rarely spared. Instead the patients ends up greater with a greater exposure than if the more definitive test was used initially.

The reasoning behind initial AXR for suspected SBO likely falls into one of two clinical scenarios. First is the situation of low clinical suspicion for obstruction and AXR acts to confirm or support a negative diagnosis.The second is one in which there is a high clinical suspicion for bowel obstruction, AXR is obtained in hopes of expediting disposition (surgical consult, operative intervention, etc.), while saving the patient time and radiation.

In both scenarios AXR is not ideal. In the first, AXR may provide a false sense of security, as its sensitivity is too low to comfortably rule out obstruction, especially early or partial. In the second scenario, AXR may support clinical suspicion, but rarely provides enough evidence to dictate management, and may actually delay treatment. Our surgical colleagues typically still want a CT even with a markedly positive AXR.

Bottom Line: AXR has a limited role in bowel obstruction. It may be useful in patients with recent surgery or known bowel adhesions, who are likely to be taken to OR for obstruction with an already known etiology,or who are too unstable to go to CT (Jackson, 2001).

2. Pneumoperitoneum

Intraabdominal free air as seen on XR has been used to dictate surgical intervention for decades. Several studies have looked at the accuracy of AXR in determination of free intraperitoneal air secondary to perforated viscus, with varied results. Sensitivities ranging from 15-83% have been reported (Gans, 2012).Among common causes of this variability are the adequacy of films, amount of air present, and the use of proper positioning techniques.

Miller and Nelson’s 1971 paper demonstrated the importance of patient positioning and compared different radiographic views. The study aimed to find the best technique to detect extraluminal air by injecting subjects with small volumes of air intraperitoneally at McBurney’s point, followed by radiographic evaluation. They found the highest sensitivity with the following sequence:First, 10-20 minutes of left lateral decubitus positioning, followed by AXR. Second, careful placement into an upright position for 10 minutes, then CXR (AP or PA) and upright AXR. Third, recumbency and AXR in supine position (Miller, 1971). This technique theoretically supports the movement of air to below the right hemidiaphragm, avoiding the superimposed gastric bubble on the left. Using this technique, it was reported “possible to consistently demonstrate as little as 1cc [of air] under the right hemidiaphragm” (Markus, 1989).

For a patient with peritonitis, transport and positioning is difficult. Utilizing the imaging views with the best diagnostic yield is crucial. One study of free air determination in various AXR views, reports the accuracy of left lateral decubitus, upright, and supine to be 96%, 60%, and 56%, respectively (Roh, 1983).Despite the lower accuracy of supine films, they are often the easiest to obtain in an unstable patient. Detection of pneumoperitoneum on supine films requires the presence of significantly more extraluminal air than other views; the most frequent findings include Rigler’s Sign (gas on both sides of the bowel wall) and linear or triangular right upper quadrant gas (Levine, 1991).

Upright CXR has been repeatedly demonstrated to be superior in free air detection to upright AXR (Flak, 1993), (Miller, 1971). Sensitivity of 85% has been reported (Gans, 2012). Although AP or PA CXR is commonly utilized, upright lateral CXR may have better sensitivity, as noted in a small retrospective review (Markowitz, 1986). Field, et al. questioned the utility of the erect AXR, claiming it added nothing to the upright CXR and supine AXR. CXR has the additional benefit of identifying diagnostically significant chest pathology (Field, 1985).

CT has revolutionized evaluation of the acute abdomen. CT has several advantages over plain film including better sensitivity and higher accuracy. In a study of trauma patients status post introduction of intraperitoneal air by diagnostic peritoneal lavage, upright CXR was only 38% sensitive, missing all patients with minimal air and most with moderate free air. CT within 24 hours of DPL was 100% senstitive for free air (Stapakis, 1992). CT has the ability to identify contained perforation and to localize the site of perforation in a majority of cases, guiding management and surgical intervention (Mindelzun, 1997).

 A recent study looking at the the value of plain radiographs in abdominal pain found that of those with confirmed perforated viscus, the sensitivity of initial AXR was determined to be only15% (Van Randen, 2011).Of thirteen perforations, four were contained and were not visible on AXR. The addition of AXR to clinical assessment did not significantly increase the sensitivity or positive predictive value, nor did it significantly change the suspected diagnosis.

Bottom Line: While AXR performs better in detecting free air than it does for detecting other pathologies, its diagnostic use is technique-dependent and is insufficient to rule out perforated viscous in patients with a moderate to high clinical suspicion. For patients too unstable to be taken to CT, upright CXR should be the test of choice for emergent determination of free intraperitoneal air. Supine or left lateral decubitus AXR may be of limited benefit.

3. Who do you CT scan in the work up of pancreatitis?

In acute pancreatitis (AP) the diagnosis of disease, identification of a treatable cause, and determination of disease severity are important parts of evaluation. CT scanning can theoretically aid in all of these.

AP is most commonly diagnosed by the presence of at least two of the following three criteria: characteristic abdominal pain (constant upper abdominal pain with radiation to the back), elevated amylase/lipase levels (> 3 times the upper limit of normal), and consistent findings on imaging (Tenner, 2013). When history and labs clearly indicate AP, CT is unlikely to add important information. However, abdominal pain and symptoms of AP may be atypical. Amylase and lipase have limited sensitivity and specificity for AP: both may be elevated in other causes of abdominal pain such appendicitis, cholecystitis, and bowel ischemia. Contrast-enhanced CT has been shown to have greater than 90% sensitivity and specificity for diagnosis of AP (Balthazar, 2002). Additionally, it provides the advantage of simultaneously ruling out other causes of abdominal pain.

Identifying the cause of pancreatitis may be crucial in guiding management. Gallstones are the leading cause of pancreatitis, and abdominal biliary ultrasound is recommended for all patients with undifferentiated AP to evaluate for gallstones (Tenner, 2013). However, ultrasound is limited in its evaluation of distal stones. Contrast CT can visualize evidence of obstruction such as biliary dilatation, however, it is only moderately sensitive for detecting gallstones and biliary stones (Anderson, 2006; Anderson, 2008).While contrast CT and MRI are comparable studies for use in early assessment of AP, MR adds sensitivity in detecting choledocolithiasis and pancreatic duct disruption (Macari, 2010). MRCP, endoscopic ultrasound, or ERCP should be considered when biliary obstruction is strongly suspected.

Because mortality increases significantly based on severity, early prediction of severe disease is important for proper management and disposition, but may be difficult on initial presentation to the ED. Severity scoring systems such as Ranson’s criteria are generally less accurate within the first 48 hours of disease, and have been routinely debunked by intensivists. Even APACHE II is only 75% sensitive on presentation (Osvaldt, 2001). AP severity is now separated into three categories after the 2012 revision of the Atlanta classifications (Banks, 2012). Mild AP is the absence of organ failure or local complications, and has expected improvement within 48 hours. Moderately severe AP includes local complications and/or <48 hours of organ failure. Severe AP is defined only by persistent organ failure >48 hours.

Two phases of disease are recognized as peaks of mortality: early (<1 week from symptom onset) and late (>1week). The early phase is characterized by systemic inflammatory response syndrome (SIRS). Morbidity and mortality reflects the presence of end-organ failure (defined as SBP<90, creatinine >2, PaO2<60%, or GI bleeding >500cc/24hr). In this phase, management is based on disease presentation and not imaging as findings on contrast CT often underestimate disease severity and rarely prompt urgent intervention. The later phase connotes development of local complications (including peripancreatic fluid collections, necrosis, and pseudocysts) and/or persistent SIRS. Infected pancreatic necrosis is associated with significant morbidity and may, like other local complications, require intervention. Contrast CT evaluation is an effective technique to detect and characterize these complications. After four days from symptom onset and with proper technique contrast CT is reported to identify necrosis with 87% accuracy, 50-100% sensitivity (depending if the patient has minor or extended areas of necrosis, respectively), and specificity nearing 100% (Balthazar, 2002).

Most cases of AP are mild, and will clinically improve with supportive care by 48 hours. For more severe cases, local complications, including necrosis, typically will not be present on initial presentation and likely not clinically important in the first week of symptoms. Given this, recent guidelines published in the American Journal of Gastroenterology regarding management of AP state that contrast CT is not recommended as part of the routine evaluation of AP.  These guidelines recommend contrast CT or MRI be limited to those in whom diagnosis is unclear on initial presentation, those who fail to improve after 2-3 days, or exhibit acute decline, in order to evaluate for local complications (Tenner, 2013).

Bottom Line: Contrast CT on initial presentation of acute pancreatitis does not routinely contribute to management and should be reserved for those who have not improved after 2-3 days or have decompensated.

Bonus: Do you use dextrose containing IV fluids in the resuscitation of kids with vomiting?

Gastroenteritis in children is a frequent cause of ED visits. Dehydration and carbohydrate depletion from vomiting, diarrhea and poor oral intake leads to decreased tissue perfusion, anaerobic metabolism, and glucagon release. Glucagon, in turn, promotes breakdown of glycogen with resultant ketogenesis. Ketones contribute to metabolic acidosis, which has been associated with oral intolerance and shown to be predictive of hospital admission (Friedman, 2005; Gorelick, 2013). Ketones themselves are postulated to be associated with persistent nausea, vomiting, and anorexia (Wheless, 2001). The idea of using dextrose-containing fluids in these dehydrated, ketotic patients makes physiologic sense. By giving carbohydrate, insulin production is stimulated, glucagon suppressed, lipolysis stops, ketosis resolves, and oral intake improves. Several studies have looked at the effects of dextrose containing solutions versus fluids without dextrose in relatively small sample populations. Two small studies showed, as one might expect, increased blood glucose in the treatment arm, but no significant clinical benefits (Juca, 2005). Rahman, et al., in a randomized control trial of 67 children, showed no adverse outcomes with dextrose-containing solution, noting comparable urine output, suggesting osmotic diuresis did not occur in the treatment arm (Rahman, 1988).

Levy and Bachur performed a non-blinded retrospective case control study in which they found a significant inverse association between the amount of IV dextrose received on initial visit for acute gastroenteritis with dehydration and return visits with admission (Levy, 2007).

Levy, et al.,in a double-blind randomized controlled trial, looked at the effects of an initial bolus of normal saline (NS) versus D5NS in children with dehydration from gastroenteritis (Levy, 2013). A significantly greater decrease in serum ketones was seen in the treatment group at 1 and 2 hours. A trend towards a lower admission rate was seen (9% absolute risk reduction in the treatment arm), however, no statistically significant decrease in hospitalization was found. Of the discharged patients reached by phone for follow up, a trend towards more unscheduled medical care was seen in the normal saline control group. This trend was exaggerated when analyzing only the discharged patients who had had an acidosis. Further research to increase the power of this study may help better determine if these trends are meaningful, and should influence general practice.

Bottom Line: Despite the logic behind the addition of dextrose to IVF in gastroenteritis-associated dehydration with ketosis, no compelling evidence exists yet to show its association with an improvement in clinically significant outcomes.

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4 Responses to GI Imaging, “Answers”

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