Keywords

FormalPara Case Example

An 11-year-old, 45 kg, female presents to a community emergency department with 8 hours of abdominal pain. The emergency physician requests surgical consultation in anticipation of possible transfer and reports that the patient has tenderness to palpation in the right lower quadrant, but is afebrile and has a normal white blood cell count. She would like to know if you would like imaging for this patient and, if so, what?

Introduction

Appendectomy is the most common urgent surgical procedure in children, with approximately 80,000 appendectomies performed annually in the United States [1]. Appendicitis was traditionally diagnosed clinically, based on history and physical exam, and surgeons accepted a 10–20% rate of finding a normal appendix at surgery (negative appendectomy) [2]. With improving technology, abdominal ultrasonography (U/S) gained popularity in the late 1980s [3]. As computed tomography (CT) became more ubiquitous, its use supplanted U/S [4]. Magnetic resonance imaging (MRI) use has increased slowly over the past few years as the harms of CT-associated radiation have been acknowledged and publicized. The increase in accuracy and availability of diagnostic imaging has resulted in a decrease in the negative appendectomy rate, from 20% to 2%, over the past three decades [2, 5]. Today, nearly 100% of pediatric patients undergo some type of imaging to establish a diagnosis of appendicitis; CT accounts for more than 50% of these studies [6]. The choice of imaging modality is highly dependent on clinician decision-making, as well as local availability and practices.

The ideal diagnostic imaging modality would expeditiously and accurately diagnose appendicitis without exposing the patient to additional harms and would be cost-effective. Each modality has limitations, risks, and benefits, which should be considered based on the clinical scenario. Adjuncts to imaging, such as radiology report templates, clinical scoring systems, and standardized algorithms for patient evaluation, have enhanced the value of diagnostic studies. Moreover, individual modalities should be thought of in conjunction with other study types as well as contextual and patient-related factors. The following review describes the advantages and limitations of each modality, followed by a discussion of ways to enhance the diagnostic utility of various imaging strategies.

Literature Review

Ultrasonography

Graded compression U/S was first described for the diagnosis of appendicitis in 1986 [3]. The traditional criterion for diagnosis of appendicitis by U/S is an appendiceal maximal outer diameter (MOD) >6 mm. Searle et al. looked at normal appendiceal diameter by age and found that the appendiceal diameter does not increase significantly above age 3 [7]. A retrospective review by Goldin et al. found that using diagnostic criteria of MOD ≥7 mm or wall thickness >1.7 mm had a sensitivity of 99% and specificity of 95% [8]. Among published meta-analyses of the diagnostic accuracy of U/S, sensitivity ranges from 0.88 to 0.91 and specificity from 0.90 to 0.97 (Table 5.1) [9,10,11,12]. Figure 5.1 shows the imaging findings of appendicitis by U/S.

Table 5.1 Ranges of sensitivity and specificity for different imaging modalities by source [9,10,11,12,13,14,15,16,17,18]
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Fig. 5.1
figure 1

Ultrasound positive for appendicitis. Transverse view (left) demonstrates periappendiceal free fluid (arrow). Longitudinal view (right) highlights appendiceal wall thickening, with a maximal outer diameter of 1.01 cm

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While MOD is a central component of U/S diagnosis of appendicitis, one criticism of U/S is the frequency of lack of visualization (full or partial) of the appendix, which occurs 18–75% of the time [19,20,21]. According to a multicenter prospective observational study by Mittal et al., U/S had a specificity of 97% and a sensitivity of 72.5% for the diagnosis of appendicitis in children ages 3–18 [22]. The primary reason for the lower sensitivity was the frequency with which the appendix was not visualized. In cases where the appendix was visualized, the sensitivity was 97.9%, and the specificity was 91.7%. However, lack of visualization has a high negative predictive value for appendicitis [23]. The combination of lack of visualization and absence of secondary findings reduces the likelihood of appendicitis to less than 2% [24].

Secondary findings that may support the diagnosis of appendicitis include appendiceal wall thickness ≥3 mm, non-compressibility of the appendix, wall hyperemia on color Doppler, periappendiceal mesenteric fat stranding, presence of an appendicolith, free fluid, a periappendiceal hypoechoic halo indicative of appendiceal wall edema, lymphadenopathy, and abdominal tenderness during U/S examination. Published literature provides contradictory data on the diagnostic value of secondary signs for the diagnosis of appendicitis with U/S. Goldin et al. reported that incorporating secondary findings into diagnostic criteria did not increase the sensitivity or specificity of U/S [8]. Trout et al. found that periappendiceal fat stranding was the only secondary finding that was statistically significant in predicting the presence of appendicitis [25]. In one retrospective review of imaging in negative appendectomies, the most common U/S findings that misled radiologists were non-compressibility (56%) and sonographic tenderness (56%), followed by RLQ lymphadenopathy (50%) [26]. Nevertheless, the presence of secondary findings in the setting of a non-visualized appendix or the absence of secondary findings in the setting of a borderline MOD may increase diagnostic accuracy [20].

The primary limitation of U/S is its operator dependence. Techniques for improved appendix visualization have been reported, including standard supine scanning, followed by left posterior oblique scanning if the appendix is not visualized, and then “second-look” supine scanning [27]. Increased use of U/S, i.e., practice, is also associated with decreased non-visualization of the appendix and improved diagnostic accuracy [19]. Sensitivity is higher at centers with higher U/S utilization, and nondiagnostic U/S studies are more prevalent at community hospitals, where CT is more often employed as the first imaging test [22, 28].

Additionally, U/S is commonly thought to perform worse than other imaging modalities in overweight or obese patients [29,30,31]. Increased abdominal wall thickness and retrocecal location are associated with decreased rates of visualization in adult studies [32]. However, U/S has been shown to maintain its diagnostic utility in children regardless of patient body habitus [33]. Techniques to improve visualization in patients with a large body habitus include posterior manual compression and use of lower-frequency transducers [34].

Utilizing a standardized radiology report template that incorporates secondary signs has been shown to improve U/S diagnostic clarity. Nielsen et al. formulated a template that defined an abnormal appendix as one with a MOD ≥7 mm and a maximal wall thickness ≥1.7 mm [35]. Secondary signs included hyperechogenic periappendiceal fat, fluid collection consistent with abscess, and local dilation and hypoperistalsis of bowel. For their final impression, radiologists were required to choose between four categories: (1) normal appendix, (2) appendix not visualized or partially visualized without secondary signs of appendicitis, (3) appendix not visualized or partially visualized with secondary signs of appendicitis, and (4) acute appendicitis. Categories 3 and 4 were considered positive for appendicitis. This template nearly eliminated nondiagnostic exams and improved diagnostic accuracy. Sensitivity improved from 67% to 92%. In other studies, the use of U/S templates has reduced use of CT, improved diagnostic accuracy, and reduced costs [36,37,38].

Rapidity of the test and repeatability are additional benefits of U/S. Increased sensitivity with increased duration of abdominal pain and repeated scans has been reported [39, 40]. U/S alone and in conjunction with algorithms that include U/S first have demonstrated lower costs and decreased use of CT [41,42,43,44,45]. The trend toward increased U/S use has not produced a concomitant increase in negative appendectomies, time to surgery, perforations, or missed appendicitis [4, 46,47,48].

Computed Tomography

CT findings of appendicitis are similar to those seen on U/S: appendiceal MOD >6 mm, increased wall thickness (target sign), wall hyperemia, periappendiceal mesenteric fat stranding, presence of an appendicolith, free fluid, and abscess [49]. Figure 5.2 demonstrates imaging by CT of confirmed appendicitis. Non-visualization of the appendix is possible on CT and argues against appendicitis with a very high (99%) negative predictive value [50]. The advantages of CT are its high sensitivity and specificity, operator independence, relatively quick acquisition time, widespread availability, and ability to identify alternate diagnoses.

Fig. 5.2
figure 2

CT scan demonstrates dilated appendix (solid arrow) with periappendiceal mesenteric fat stranding (hollow arrow)

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CT has strong test characteristics in all populations, somewhat better than U/S, with a sensitivity of 0.90–0.95 and specificity of 0.92–0.95 (Table 5.1) [9, 13, 14]. Sensitivity and specificity improve slightly with intravenous contrast, but enteric contrast is generally unnecessary [51]. In children, however, several systematic reviews have shown that the sensitivity and specificity of CT are similar to U/S [9, 15]. CT’s lower performance in children is due to relative lack of body fat, which makes distinguishing the appendix from surrounding structures more difficult [10, 52, 53]. In contrast to U/S, CT’s high sensitivity and specificity remain consistent across institutions. In their meta-analysis of 9, 356 pediatric patients, Doria et al. reported little variation between hospitals of institution-specific sensitivity and specificity [10].

CT is now ubiquitous in the United States, with more than 34 scanners per million population as of 2007 [54]. Fourteen percent of children presenting to the emergency department with non-traumatic abdominal pain undergo CT (Fig. 5.3) [55]. Being faster than MRI, CT is useful for obtaining high-quality imaging in younger children, who may not be able to remain still a prolonged time period. However, CT completion may take significantly longer than U/S from time of order to performance [56]. CT also identifies extra-appendiceal findings suggestive of other diagnoses more often than U/S [57, 58]. Because of these properties, CT may reduce the rate of negative appendectomies in children younger than 5 and girls older than 10 years of age [59].

Fig. 5.3
figure 3

Imaging trends in the United States for pediatric abdominal pain patients presenting to the emergency department [55]

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Despite strong test characteristics, ease, and efficiency of performance, the ionizing radiation produced by CT raises concern. Focused exams and low-dose techniques, which have demonstrated equivalence to traditional techniques, have reduced radiation exposure but cannot eliminate it [60]. Children are particularly vulnerable to the effects of radiation, as their developing tissues are more sensitive and they have a longer remaining life span during which oncogenic effects may manifest [61]. Age at time of exposure impacts the risk of malignancy, with age inversely related to risk [62]. With growing recognition of the harms of healthcare-associated pediatric radiation exposure, limiting the utilization of CT is strongly recommended by national bodies such as the American College of Radiologists, the National Cancer Institute, the American Academy of Pediatrics, the American Pediatric Surgical Association, the Image Gently Alliance, and the Joint Commission [61, 63,64,65,66].

In the United States, individuals are exposed to approximately 3 mSv of background radiation per year. For reference, 1 mSv is equivalent to 1 mGy if the radiation type is gamma rays. An abdominal CT scan delivering 10 mSv is expected to cause cancer in the lifetime of 1 in 1000 male patients who are 10 years old at the time of imaging [67]. An estimated 4–9 million CT scans are performed annually on US children, 11% of which are obtained to evaluate for appendicitis [68, 69]. One year’s worth of pediatric CTs is projected to cause 4870 cancers, but the true risk is unknown [70].

CT use in pediatric patients is associated with community, non-children’s hospitals (NCH), older children, females, and patients with higher body mass index [71, 72]. Multiple studies have demonstrated that NCHs are more likely than children’s hospitals (CH) to utilize CT to diagnose pediatric appendicitis [71,72,73]. The majority of pediatric patients with suspected appendicitis undergo CT, likely because 66–82% of these patients initially present to community and NCHs [74, 75]. Anderson et al. have also showed that the size-specific dose estimate and effective dose of radiation are significantly higher and have greater variance at NCHs that are not involved in a dose reduction program (Fig. 5.4) [76, 77].

Fig. 5.4
figure 4

Size-specific dose estimate by patient age at children’s hospitals compared to non-children’s hospitals [76]

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CHs have led the effort to reduce CT utilization in children. As MRI has become increasingly available, it has begun to replace CT as a secondary modality after inconclusive U/S and is less commonly used as the primary study [72]. In addition to the radiation risk, CT use is not associated with better patient outcomes [78]. Other disadvantages of CT compared to U/S include cost and potential for allergic reaction or kidney injury from iodinated contrast agents. With radiation dose being cumulative, repeat CTs are not recommended.

Magnetic Resonance Imaging

The diagnostic features of appendicitis on MRI are similar to those previously mentioned. A study by Leeuwenburgh et al. described nine MRI features predictive of appendicitis: appendix diameter >7 mm, appendicolith, periappendiceal fat infiltration, periappendiceal fluid, absence of gas in the appendix, appendiceal wall destruction, restricted diffusion of the appendiceal wall, and lumen or focal fluid collections [79]. Systematic reviews and meta-analyses have reported sensitivity from 0.96 to 0.99 and specificity ranging from 0.96 to 0.97 (Table 5.1) [9, 16,17,18]. In addition to strong test characteristics, MRI is also operator independent, provides alternative diagnoses as frequently as CT, and does not expose patients to ionizing radiation [80]. Figure 5.5 shows an MRI demonstrating appendicitis.

Fig. 5.5
figure 5

MRI demonstrates dilated appendix with periappendiceal enhancement consistent with inflammation

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MRI does have several limitations, especially in the pediatric population. MRI may take longer to perform than either U/S or CT. The youngest children, those who cannot cooperate, or those with claustrophobia may require sedation to complete the exam. MRI is not as widely available as the other modalities and is more expensive as a stand-alone test than its comparators.

Traditional MRIs, with or without contrast, take longer to perform than U/S or CT. However, at centers where it is available, “fast” MRI has mitigated this problem. 3-T MRI produces a magnetic field twice as powerful as the more common 1.5T MRI, which typically decreases scanning time by half while retaining strong test characteristics [81]. In combination with 3-T MRI machines, parallel processing and newer body coils aid in reducing scan time, making it possible to scan children with free breathing, no IV contrast, and no sedation [82]. The benefit of gadolinium enhanced images has not been conclusively established [17, 83, 84].

MRI is not as widely available as CT or U/S [54]. Hospital characteristics such as higher total expenditures and network affiliation are associated with its availability [85]. Even where available, MRI use has increased very slowly, comprising only 1–2% of all imaging for suspected appendicitis [47]. Otero et al., in a large retrospective study of trends and costs over time, found that while imaging costs of all studies increased slightly, it was at a much lower rate than overall hospital costs [47]. However, MRI only accounted for approximately 1% of imaging in that review. Heverhagen et al. reported that MRI is cost-effective as an isolated imaging modality because it decreases the rate of negative laparotomy, but their study did not compare MRI to U/S or CT [86]. Anderson et al. demonstrated a small, yet significant, increase in radiology costs with a large increase in MRI use (from 1% to 25%) in a single institution [46].

The utility of MRI as the first imaging test for appendicitis has not been established; nevertheless, it is a reliable alternative to CT when U/S is inconclusive. The slow transition from CT to MRI as the secondary imaging modality has not shown a change in outcomes. Several studies have reported no difference in time to antibiotic administration, time to surgery, negative appendectomy, perforation rate, or length of stay [46, 82, 87]. Future studies are necessary to evaluate the repercussions of this change in imaging strategy.

Discussion

Imaging for suspicion of pediatric appendicitis has become a common practice in the United States. As a result, tolerance for negative appendectomies and missed diagnoses has significantly decreased. Based on the available literature and current guidelines, U/S should be the first-line imaging modality in children with suspected appendicitis [88, 89]. MRI, where available, should be considered as a second-line exam in lieu of CT. CT may be beneficial when MRI cannot be performed, in older or obese children and in exigent circumstances. Imaging for suspected appendicitis is cost-effective for the reduction of negative appendectomies and decreased length of stay [90].

Imaging is rarely performed in isolation of history and physical and laboratory tests. The incorporation of commonly performed blood tests, such as a complete blood count, has demonstrated improved predictive value when used in conjunction with U/S [91]. The Alvarado Score, the Pediatric Appendicitis Score (PAS), the Appendicitis Inflammatory Response (AIR) Score, and similar tools have been used with great success to triage patients for imaging, to choose the imaging modality, or to support imaging results [43, 91,92,93,94,96]. Saucier et al. used the PAS to selectively image pediatric patients with U/S first, while Bachur et al. integrated the PAS with U/S results to determine next steps [43, 97]. Blitman et al. found a negative predictive value of 99.6% in patients with a low Alvarado Score and inconclusive U/S [92].

There is strong evidence that clinical guidelines or pathways for the diagnosis of appendicitis are efficient and cost-effective; they also reduce radiation exposure even if CT is included in the algorithm [40, 43, 46, 82, 98, 99, 100]. Published pathways are varied but generally include an U/S first protocol [101]. The LeBonheur pathway, as described by Saucier et al., had a diagnostic accuracy of 94% and was shown to be the 2nd most cost-effective strategy compared to U/S of all patients, clinical judgment alone, CT of all patients, overnight observation with surgical evaluation, and no imaging [102]. U/S of all patients was the most cost-effective; however, stratification by PAS had improved diagnostic accuracy with only a moderate increase in cost. U/S as the initial imaging modality with CT or MRI reserved for inconclusive U/S results increases overall diagnostic accuracy and decreases cost without sacrificing time to antibiotics or surgery [103, 104].

Continuing Controversies and Areas for Study

The evidence supports the use of U/S as the first diagnostic imaging test in children with suspected appendicitis. The utility of MRI first or MRI second has not been conclusively established, and more studies are needed to evaluate MRI alone or as part of an algorithm. Moreover, the role of CT, in light of radiation concerns, has not been fully determined. Several authors have suggested that MRI should replace CT as the secondary imaging modality after inconclusive U/S, but this practice has not been widely implemented, and the repercussions of such a shift have not been fully investigated. The Cochrane Collaboration is in the process of conducting a systematic review and meta-analysis of U/S and MRI for the diagnosis of acute appendicitis [105, 106]. They are also conducting a systematic review for evidence of the benefit of CT for acute appendicitis in adults.

Case Example Discussion

The case mentioned in the beginning of this chapter highlights the conundrums of imaging for suspected appendicitis in children. In this female patient with right lower quadrant pain, multiple diagnoses must be considered, including appendicitis, gynecologic pathologies such as ovarian torsion, or mesenteric adenitis. An ultrasound should be ordered for this patient. If the ultrasound is inconclusive, the emergency physician ought to consider the characteristics of their facility, transfer logistics, and patient/family preferences prior to ordering additional or repeat imaging.

In this case you recommended an U/S, which was obtained first. It demonstrated free pelvic fluid but was nondiagnostic. After you discussed the U/S results, the emergency physician ordered an MRI of the abdomen and pelvis on your recommendation, which showed a dilated appendix with periappendiceal inflammation and a normal right ovary. The patient was started on antibiotics, and an appendectomy was successfully performed several hours later.

Conclusion

Imaging for suspected pediatric appendicitis is an invaluable diagnostic tool, but modality selection can be controversial. First-line U/S is the evidence-based recommendation of the authors and the American College of Radiology [88]. Additional imaging is at the discretion of the provider, and multiple factors should be considered including availability of secondary modalities, exam and laboratory findings, surgeon assessment, patient characteristics, and patient/family preference.

Clinical Pearls

  • Imaging is a valuable tool that has significantly improved our ability to correctly diagnose appendicitis and lower our negative appendectomy rate.

  • U/S is a useful and safe first-line mode of imaging, but is operator dependent.

  • MRI may provide a nonionizing radiation imaging option with a similar accuracy to CT scan.