Diagnosing appendicitis by ultrasonography was first described by Puylaert in the 1980s.¹ However, many factors have prevented this modality from becoming standard of care in the diagnostic work-up of appendicitis. Its limitations were further highlighted in 1994 when Balthazar et al. reported on 100 cases of appendicitis, comparing ultrasound with computed tomography (CT) scanning; the study showed better accuracy, negative predictive value (NPV), and sensitivity with CT scanning.²

However, while CT scanning has become the standard of care in the diagnosis of appendicitis in both children and adults, its liberal use has come under fire recently because of the risk of malignancy due to its ionizing radiation.³ To prevent excessive radiation, multiple authors have proposed a protocol utilizing ultrasound as the first diagnostic modality for suspected appendicitis, followed by CT scan of the abdomen if the ultrasound is negative or equivocal.^4-6 This algorithm is feasible because of the inherent testing characteristics of ultrasonography in appendicitis. A meta-analysis by Doria et al. lists the overall sensitivity of ultrasound as 88% and 83% and its specificity as 94% and 93%, for children and adults, respectively.⁷ These characteristics make ultrasonography a good “rule in” test to confirm appendicitis as its specificity rivals that of CT scanning in some studies.

Unfortunately, its poor sensitivity in comparison to CT does not allow it to be utilized as a good “rule out” test, necessitating additional testing if the ultrasound result is not positive for appendicitis. Ultrasound is more accurate for detecting appendicitis in the pediatric population because the abdominal musculature of children generally has less fat content.^8-10

The opposite is true of CT scans, where intraperitoneal fat actually improves the diagnostic accuracy for appendicitis.¹¹ In this way, the protocol of ultrasound first followed by CT scan is ideally suited to help avoid ionizing radiation in pediatric patients. However, in today’s overcrowded ED, time and inability to perform the ultrasound examination are commonly cited as reasons for not obtaining an ultrasound examination in patients with suspected appendicitis. The literature shows that nonradiologists can perform this examination and obtain specificities as high as radiologists, even during the “off hours” between 5 p.m. and 8 a.m., when there is traditionally no radiology backup.¹²

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).

Appendiceal wall hyperemia as seen with color Doppler is another common finding in acute appendicitis (Fig. 5).20-22 Appendicoliths, which appear as hyperechoic foci that cast an anechoic shadow, can also sometimes be found within the lumen of an inflamed appendix.²² The presence of pericecal inflammatory changes such as hyperechoic fat or free fluid are often considered suggestive of but not specific for appendicitis.^21,22 Many argue that the appendix must be visualized in order to confirm the diagnosis. Unfortunately, identification of the appendix can often be difficult. The appendiceal visualization rate varies by institution, with one study citing a range of 22%-98%.²³ Interestingly, a recent study by Pacharn et al. reported a 95% NPV for the exclusion of acute appendicitis even if the appendix was not visualized.²⁴ However, such results require a great deal of experience and as a result most still consider an ultrasound without visualization of the appendix to be nondiagnostic. Further imaging is warranted in this situation. Only an ultrasound that demonstrates a compressible, blind-ending appendix in the absence of surrounding inflammatory changes is considered truly normal.^22,25

Limitations to Visualization

Several factors can make diagnosing acute appendicitis with ultrasound challenging. Technical aspects affecting the ability of the sonographer to achieve adequate compression of the RLQ such as obesity, severe pain or abdominal guarding, excessive bowel gas, and an uncooperative patient can all affect the accuracy of the ultrasound.^26,27 Operator experience can also affect the study result.

Adjuvant techniques exist that can assist in the visualization of the appendix when one is unable to identify it utilizing graded compression alone. Posterior manual compression can be performed by placing one hand on the patient’s back, posterior to the RLQ, and applying forced compression in the anteromedial direction while still applying graded compression with the transducer on the anterior abdominal wall (Fig. 6). This causes displacement of the RLQ bowel structures and can reduce the distance from the transducer to the retrocecal or retrocolic spaces, potentially increasing the resolution of the appendix. Because the most common location of the appendix is retroileal and subcecal, this technique is effective in a majority of patients. However, it is limited when the appendix lies inferior to the iliac crest because the bony pelvis will not allow anterior displacement in this position.^21,28

A retrocecal appendix is often difficult to identify as high-frequency transducers may fail to appropriately visualize deep structures.^21,26,27,29 One can aid in the visualization of the retrocecal appendix by either scanning via a lateral flank approach or by turning the patient to the left lateral decubitus position in order to obtain views posterior to the cecum (Fig. 7).^22,29,30 The left lateral decubitus technique may also be helpful in pregnant patients with suspected appendicitis, with this position shifting the uterus away from the RLQ and allowing easier visualization of the appendix.³¹ Additionally, one author promotes using a 500- to 1,000-cc saline enema for nonvisualization of the appendix in children and cites improved visualization rates with this method in the pelvic, retrocecal, and retroileal position.³¹

A perforated appendix can result in an inconclusive or false-negative study.^21,26,27,29 This is likely due to difficulty with adequate compression of the abdomen due to guarding or dilatation of bowel loops as a result of peritonitis and/or the failure to recognize the decompressed appendix after pus evacuation.²⁶

Pitfalls

The accuracy of ultrasonography is operator dependent and requires both skill and experience.^22-27

The appendiceal diameter should be measured from outer wall to outer wall to obtain an accurate measurement. Additionally, it is important to visualize the entire length of the appendix, including the distal tip, to confirm that it is blind-ending, as the most common cause of misdiagnosed appendicitis by ultrasonography is mistaking the terminal ileum for an inflamed appendix.^21,22,27 Visualization of the appendiceal tip is also important so that early appendiceal inflammation, which can often be confined to the distal tip, is not missed.^21-29 Finally, other disease entities such as colonic diverticulitis, Crohn’s disease, pelvic inflmmatory disease, epiploic appendagitis, and terminal ileitis can result in false-positive scans.^21-29

Conclusion

Bedside ultrasound is helpful in patients with suspected appendicitis to confirm the diagnosis. Further imaging may be warranted if the ultrasound is equivocal.

References

1. Puylaert JB. Acute appendicitis: US evaluation using graded compression. Radiology 1986;158(2):355-60.
2. Balthazar EJ, Birnbaum BA, Yee J, et al. Acute appendicitis: CT and US correlation in 100 patients. Radiology 1994;190(1):31-5.
3. Brenner DJ, Hall EJ. Computed tomography – an increasing source of radiation exposure. N. Engl. J. Med. 2007;357(22):2277-84.
4. Garcia Peña BM, Mandl KD, Kraus SJ, et al. Ultrasonography and limited computed tomography in the diagnosis and management of appendicitis in children. JAMA 199915;282:1041-6.
5. Ramarajan N, Krishnamoorthi R, Barth R, et al. An interdisciplinary initiative to reduce radiation exposure: Evaluation of appendicitis in a pediatric emergency department with clinical assessment supported by a staged ultrasound and computed tomography pathway. Acad. Emerg. Med. 2009;16(11):1258-65.
6. Poortman P, Oostvogel HJ, Bosma E, et al. Improving diagnosis of acute appendicitis: Results of a diagnostic pathway with standard use of ultrasonography followed by selective use of CT. J. Am. Coll. Surg. 2009;208(3):434-41.
7. Doria AS, Moineddin R, Kellenberger CJ, et al. US or CT for diagnosis of appendicitis in children and adults? A meta-analysis. Radiology 2006;241(1):83-94.
8. Abo A, Shannon M, Taylor G, Bachur R. The influence of body mass index on the accuracy of ultrasound and computed tomography in diagnosing appendicitis in children. Pediatr. Emerg. Care 2011;27(8):731-6.
9. Josephson T, Styrud J, Eriksson S. Ultrasonography in acute appendicitis: Body mass index as selection factor for US examination. Acta Radiol. 2000;41(5):486-8.
10. Hörmann M, Scharitzer M, Stadler A, et al. Ultrasound of the appendix in children: Is the child too obese? Eur. Radiol. 2003;13(6):1428-31.
11. Grayson DE, Wettlaufer JR, Dalrymple NC, Keesling CA. Appendiceal CT in pediatric patients: Relationship of visualization to amount of peritoneal fat. Am. J. Roentgenol. 2001;176(2):497-500.
12. Pohl D, Golub R, Schwartz GE, et al. Appendiceal ultrasonography performed by nonradiologists: Does it help in the diagnostic process? J. Ultrasound Med. 1998;17(4):217-21.
13. Fox JC, Solley M, Anderson CL, et al. Prospective evaluation of emergency physician performed bedside ultrasound to detect acute appendicitis. Eur. J. Emerg. Med. 2008;15(2):80-5.
14. Pohl D, Golub R, Schwartz GE, et al. Appendiceal ultrasonography performed by nonradiologists: Does it help in the diagnostic process? J. Ultrasound Med. 1998;17(4):217-21.
15. Gaitini D, Beck-Razi N, Mor-Yosef D, et al. Diagnosing acute appendicitis in adults: Accuracy of color Doppler sonography and MDCT compared with surgery and clinical follow-up. Am. J. Roentgenol. 2008;190(5):1300-6.
16. Lehnert BE, Gross JA, Linnau KF, et al. Utility of ultrasound for evaluating the appendix during the second and third trimester of pregnancy. Emerg. Radiol. 2012 Feb. 28 (Epub ahead of print).
17. Israel GM, Malguria N, McCarthy S, et al. MRI vs. ultrasound for suspected appendicitis during pregnancy. J. Magn. Reson. Imaging 2008;28(2):428-33.
18. Sivit CJ, Applegate KE, Stallion A, et al. Imaging evaluation of suspected appendicitis in a pediatric population: Effectiveness of sonography versus CT. Am. J. Roentgenol. 2000;175(4):977-80.
19. Schwerk WB, Wichtrup B, Rothmund M, Rüschoff J. Ultrasonography in the diagnosis of acute appendicitis: A prospective study. Gastroenterology 1989;97(3):630-9.
20. Stone MB, Chao J. Emergency ultrasound diagnosis of acute appendicitis. Acad. Emerg. Med. 2010;17(1):E5.
21. Lee JH. Sonography of acute appendicitis. Semin. Ultrasound CT MR. 2003;24(2):83-90.
22. Brown MA. Imaging acute appendicitis. Semin. Ultrasound CT MR. 2008;29:293-307.
23. Taylor GA. Suspected appendicitis in children: In search of the single best diagnostic test. Radiology 2004;231:293-5.
24. Pacharn P, Ying J, Linam LE, et al. Sonography in the evaluation of acute appendicitis: Are negative sonographic findings good enough? J. Ultrasound Med. 2010;29:1749-55.
25. Cole MA, Maldonado N. Evidence-based management of suspected appendicitis in the emergency department. Emerg. Med. Pract. 2011;13(10):1-29.
26. Ang A, Chong NK, Daneman A. Pediatric appendicitis in “real-time”: The value of sonography in diagnosis and treatment. Pediatr. Emerg. Care 2001;17(5):334-40.
27. Ma OJ, Mateer J, Blavais M. Emergency Ultrasound. 2nd ed. 2007. McGraw Hill.
28. Lee JH, Jeong YK, Hwang JC, et al. Graded compression sonography with adjuvant use of a posterior manual compression technique in the sonographic diagnosis of acute appendicitis. Am. J. Roentgenol. 2002;178(4):863-8.
29. Jeffrey RB, Jain KA, Nghiem HV. Sonographic diagnosis of acute appendicitis: interpretive pitfalls. Am. J. Radiol. 1994;162:55-9.
30. Rioux M. Sonographic detection of the normal and abnormal appendix. Am. J. Roentgenol. 1992;158:773-8.
31. Han TI. Sonographic visualization of the appendix with a saline enema. J. Ultrasound Med. 2002;21:511-6.