Furthermore, this diagnostic study can also be performed by an emergency physician in less than 5 minutes with specificities of 90% and higher depending on the age of the patient and the machine used for the examination.¹³

Indications

Any patient who presents to the emergency department with abdominal pain who is suspected to have appendicitis could benefit from sonographic imaging of their appendix. There are certain populations in which ultrasonography has higher reported specificities or can help delineate other abdominal or pelvic pathology. These populations include pediatric patients (as previously stated), females, and slender patients. Ultrasonography should be the first radiologic study obtained in the diagnostic work-up of such patients. Ultrasound may help delineate pathology in the abdomen or pelvis of female patients who present with RLQ pain such as tubo-ovarian abscesses, ovarian cysts, ectopic pregnancy, or other pathology of the female reproductive system.

Although there are no known studies specifically addressing ultrasound for appendicitis in nonpregnant female patients, subgroup analyses show specificities of 85%-100% in selected studies.^14,15

Unfortunately, these characteristics are not reflected in pregnant patients when ultrasound is used for imaging appendicitis. In this population, poor testing characteristics stemming from difficulty in visualizing the appendix and studies with small sample sizes limit the use of ultrasound during the second and third trimesters, compared with other imaging modalities such as MRI.^16,17

Additionally, there have been various studies demonstrating that patients with lower body mass indexes have higher rates of detection of appendicitis using ultrasound.^8-10 Although these were primarily pediatric studies, the same general concepts could be extrapolated to adult patients (male or female) even though no current studies exist in adults looking at BMI.

Performing the Study

The patient should be placed in the supine position for the ultrasound examination, and a high-frequency linear array transducer should be applied to the anterior abdominal wall over the area of maximal tenderness (Fig. 1). All studies should be performed in both the transverse and longitudinal planes with a technique referred to as “graded compression,” where the examiner exerts gentle pressure in the area of interest using the ultrasound probe and either one or two hands to palpate the RLQ in the same way as when performing an abdominal examination. Utilizing varying pressure, this method is used to decrease the distance between the ultrasound probe and the pathology and eliminate overlying bowel gas, which can cause overlying bowel gas artifact.¹

The patient should be given adequate analgesia (a short-acting narcotic is recommended), as the inability to tolerate compression may obscure the image, decreasing the accuracy of the scan due to bowel gas. The ascending colon should be identified first as it appears as a nonperistaltic structure containing gas and fluid (Fig. 2). The probe should then be moved inferiorly to identify the peristaltic and compressible terminal ileum; the terminal ileum lacks haustra, which helps differentiate it from the cecum. These structures are best visualized in the longitudinal view. The appendix should arise from the cecal tip approximately 1 cm below the terminal ileum. The psoas muscle and iliac vessels can be used as landmarks, as the appendix will generally appear anterior to these structures.¹⁸ Once the appendix has been visualized, its diameter should be measured from outer wall to outer wall.

Diagnosing Acute Appendicitis

The accepted criteria for diagnosing acute appendicitis by ultrasonography are the identification of a noncompressible, blind-ending tubular structure in the longitudinal axis that measures greater than 6 mm in diameter and lacks peristalsis (Fig. 3).¹³, ^19-21 In the transverse view, the distended appendix has a target-like appearance (Fig. 4).