Ultrasound
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Dx: The No-CT Appy

The surgeon is right beside you as you pull up the images. “Let’s forget the CT. That sure looks like a swollen appendix to me,” he says. An hour later he walks back through the ED and pats you on the back. “You were right,” he says, then adds, “Hey, am I allowed to use that machine in the future?”

Tips & Tricks for
IMAGING THE APPENDIX

01 Benefits of Ultrasound: Using ultrasound to diagnose acute appendicitis has many benefits. It avoids both radiation and IV contrast and their small, but real, associated risks. In addition, ultrasound may be faster than CT as there is no need to wait for BUN & creatinine results or for oral contrast to move distally, if you are still one of those people who do full contrast CTs. If you do the ultrasound yourself it is even faster!

02 Sensitivity & Specificity: Both the sensitivity & specificity of ultrasound for appendicitis are less than that of CT. In pediatrics the values are about 88% and 94% respectively, and in adults about 83% and 93%. (These numbers may vary depending on the experience of the ultrasonographer.) There are studies from Europe and Israel where they have used the “ultrasound first” approach for many, many years that show even better test characteristics. These values are actually not that bad when compared to CT scan whose sensitivity and specificity are around 94% and 95% respectively. Remember, however, that the performance characteristics for ultrasound can be significantly worse in overweight patients or those with overlying bowel gas. In addition, if the appendix is retrocecal or is lying in a difficult anatomical plane, the study will be more challenging.  Unfortunately, you may still have to do a CT scan if your ultrasound is non-diagnostic and your clinical suspicion is moderate to high, but the strategy of ultrasound first would likely decrease CTs by about 50%.

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03 Patient Selection: Ultrasound is an excellent initial imaging modality for the appendix in thin individuals, especially children and young adults.  The lower amount of interfering subcutaneous fat and heightened concern over unnecessary radiation in this population makes them optimal candidates for ultrasound instead of, or at least before, CT. In the pregnant patient, ultrasound is the initial study of choice to evaluate right-lower-quadrant pain and can be performed simultaneously with a pelvic scan to look for a cyst, mass, free fluid, or ectopic pregnancy.

04 Probe Selection: In most patients, use a 7.5-10 MHz linear array probe. If the appendix is superficial in a thin patient, you may be surprised at just how easy it is to find. For deeper imaging, you may need to obtain images with the 3-5 MHz curvilinear probe. Consider this probe in patients with increased subcutaneous fat, or those in whom a retrocecal appendix is suspected. Be advised, the further the appendix is from your probe the more challenging your imaging will be.

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05 Technique Tips: Allow the patient to direct the ultrasound probe to the point of maximal tenderness.  Begin your scan there.  Look for a non-compressible round structure about 1cm in diameter.  Always image in at least two planes: once you find a cross-sectional view of what appears to be an inflamed appendix, adjust your probe to look for a long-axis view.  Confirm that the structure has a blind-tip at one end to avoid confusion with vascular or other structures. In some cases, you may also see a hyperechoic appendicolith with posterior shadowing.

06 Diagnostic Criteria: Diagnostic criteria for acute appendicitis are as follows: a non-compressible, aperistaltic blind-ended tubular structure which is greater than 7mm in diameter and connects to the cecum. Checking for non-compressibility involves pushing down with the ultrasound probe to see if the structure you are viewing is flattened at all. Intestines usually will demonstrate peristalsis which can be seen in real time if you look long enough.

07 Other Signs: On a transverse image of the appendix, look for the “target sign” of inflamed muscularis propria surrounded by edema and inflammatory changes.  You might see other sonographic clues, including periappendiceal fluid, prominent pericecal fat with stranding, a hyperechoic appendicolith within the tubular appendix, or presence of an abscess or phlegmon.

08 Negative Studies: You or your friendly sonographer must visualize a normal, compressible appendix on ultrasound to definitively rule-out appendicitis.  Unlike with CT scan, this is rarely the case.  If the appendix is not seen on ultrasound, you have a non-diagnostic study, so consider an 8-hour return visit for ongoing pain, a surgical consultation, or a CT scan.

09 Know Thy Surgeon:  Not all surgeons are comfortable going to the OR based on an ultrasound that is positive for appendicitis. If the pretest probability is not already very high, your surgical colleagues may still request a CT scan.  Don’t let that prevent you from doing the right thing for your patient. If you do more ultrasounds you and/or your ultrasound techs will become more proficient and your surgeons will eventually get more comfortable when repeated ultrasounds are confirmed by either CT or operative findings.  If you impress your surgeons, they may even want to start borrowing your machine to do their own ultrasounds.

10 Disadvantages: Ultrasound is far less likely to accurately identify other causes of right lower quadrant pain such as a kidney stone, Crohn’s disease, right sided diverticulitis and mesenteric adenitis. Keep this and your differential diagnosis and relative levels of suspicion in mind when choosing your initial imaging study.

11 Pediatrics: You should be aware of the most recent recommendation of the American College of Radiology from the “Choosing Wisely” campaign, which states, “Don’t do computed tomography (CT) for the evaluation of suspected appendicitis in children until after ultrasound has been considered as an option.” Although CT is accurate in the evaluation of suspected appendicitis in the pediatric population, ultrasound is nearly as good in experienced hands. Since ultrasound will reduce radiation exposure, ultrasound is the preferred initial consideration for imaging examination in children. If the results of the ultrasound exam are equivocal, it may be followed by CT. This approach is cost-effective, reduces potential radiation risks and has excellent accuracy, with reported sensitivity and specificity of 94 percent.

Brady Pregerson (@TheSafetyDoc) manages a free online EM Ultrasound Image Library and is the editor of the Emergency Medicine Pocketbook series. For more info visit EMresource.org.

Teresa S. Wu (@TeresaWuMD) is the Associate Residency Director, and Director of Ultrasound and Simulation Programs and Fellowships, for the Maricopa EM Program in Phoenix.

 

 

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