- Aug 7th, 2014
- Emberlynn Liu
During four weeks at Children’s emergency department, six of fourteen patients who presented with right lower quadrant pain were diagnosed with appendicitis. Many of those presenting with right lower quadrant pain also complained of other classic findings such as periumbilical pain radiating to right lower quadrant, fever, anorexia, nausea or vomiting. Pelvic ultrasound in addition to ultrasound of appendix was performed in female patients complaining of right lower quadrant pain with good visualization of the appendix in five patients and one obese patient requiring a computed tomography (CT) scan of the abdomen. While ultrasound of appendix is performed commonly in the pediatric population, CT abdomen seems to be more favorable in the adult population. I was curious what the actual literature behind ultrasounds of the appendix and how the study compares to CT scans in the diagnosis of appendicitis.
On ultrasound, the normal appendix should appear as a blind ended aperistaltic tubular structure that originates from the base of the cecum with a wall thickness of 2mm or less. Sonographic diagnosis of appendicitis requires an incompressible, blind ended tubular structure with wall thickness of 3 mm or more and outer anterior-posterior diameter of 6 mm or more. Other findings suggestive of appendicitis include presence of fecalith, hyperechoic periappendicular fat, peritoneal fluid or a collection. The study is largely dependent on the operator and sensitivity can range from 76-90% and specificity between 83-100%. Diagnosis with ultrasound is complicated by equivocal studies in the setting where the appendix is not well visualized. In multiple meta-analyses comparing ultrasound to CT scans, the sensitivity and specificity of ultrasounds are slightly lower than that of CT scans and reports generally have more heterogeneous estimates for the two compared to CT scans.
With a negative or equivocal ultrasound, CT scan is the next best choice. The normal appendix on CT appears as a tubular structure adjacent to the cecum that is collapsed or filled with fluid, contrast or air. The outer diameter should not exceed 6 mm and the wall thickness should be less than 3 mm. Periappendiceal fat should be homogenous and signs of inflammation such as linear fat stranding and local fascial thickening should not be seen. CT scans have sensitivity and specificity of 90-100%, but exposes patients to ionizing radiation which is most concerning in the pediatric and pregnant populations. In general, CT scans are preferred in those patients who are poor sonography candidates including individuals who are obese, have possible appendiceal perforation with abscess formation, or in evaluating other possible pathology on the differential including pyelonephritis, ureteral stones or genitourinary pathology.
Other options for evaluating the appendix include diagnostic laparoscopy and magnetic resonance imaging (MRI). Diagnostic laparoscopy allows for direct visualization of the appendix and definitive treatment, but it is also associated with increased morbidity and expense. It has a positive predictive value is 93% and negative predictive value is 71%. MRIs have been shown to be accurate in diagnosing appendicitis with one studying showing 100% sensitivity and 98% specificity. However, its low availability, high cost, and long study duration has made it unavailable routinely for clinical use.
While the literature seems to suggest the use of ultrasound is also applicable to the general adult population, CT scans are more commonly used because adults have a higher number of differential diagnosis for right lower quadrant pain that may also be visualized using the CT. Furthermore, ultrasonography depends much more on the operator of the device than CT scans.
Reference: Wild JRL, Abdul N, Ritchie JE, Rud B, Freels S, Nelson RL. Ultrasonography for diagnosis of acute appendicitis (Protocol). Cochrane Database of Systematic Reviews 2013, Issue 2. Art. No.: CD010402. DOI: 10.1002/14651858.CD010402.