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1 Introduction

Appendicitis is the first surgical emergency in childhood. Obstruction of the lumen of the appendix is considered as the main causal mechanism of this pathology. It creates an increased intraluminal pressure leading to ischemia, congestion, and transmural infiltration by neutrophil granulocytes. Appendicitis may occur at any age and its diagnosis can be challenging especially among infants. Multiple factors may explain these difficulties.

Anatomically the appendix is usually located in the right iliac fossa. However, the variable position of the cecum, the direction of the appendix itself, and its length may dramatically modify the site and type of symptoms. Furthermore, the clinical expression of appendicitis may be very variable, including diarrhea, fever or even absence of fever.

The first challenge is to recognize early appendicitis before it gets complicated (perforation, abscess, peritonitis). The second challenge is to avoid negative appendectomies which will carry their own surgical risk for the future (e.g., adhesions causing small bowel obstruction).

A clinical examination by a senior pediatric surgeon is extremely efficient for the diagnosis, still the overall negative appendectomy rate ranges between 10 to 30% [1]. Laboratory tests (white blood cell count, C-reactive protein) are unable to reliably distinguish between patients with or without acute appendicitis, even if normal tests render this diagnosis very unlikely [2]. Thanks to imaging, the negative rate has been significantly reduced between 3 and 7% without an increase in perforation rate [1].

2 Imaging Strategies and Controversies

US, CT, and MR imaging are all excellent tools for the diagnosis of appendicitis. Numerous articles, meta-analysis [3, 4] have been published and controversies do persist [5,6,7] as to determine which imaging modality is the most accurate and the most adapted to the pediatric health care. Overall the choice between the different imaging techniques appears depending on the type of imaging unit available on site and on the experience of the local radiologists.

3 Ultrasound (US)

3.1 Classical Presentations

Linear high frequency probes 7 up to 15 MHz are needed to optimize the evaluation of a patient suspected of appendicitis. Convex probes with low frequency 3.5–6 MHz are also useful in order to provide a good overview of the inflammatory process, to look for its extension to the surrounding structures (such as associated abscesses) as well as to exclude differential diagnoses.

Appendicitis can be ruled out when a normal appendix is visualized in all its length (Fig. 10.1). For Wiersma and all it can be demonstrated in 82% of asymptomatic children [8].

Fig. 10.1
figure 1

Normal appendix on US in an 8-year-old-boy. The multilayered structure measures 5 mm in diameter and presents a blind ending (yellow arrow). It has to be demonstrated all along up to the cecum to establish its normality

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On US, signs to be considered as evidence (with different positive and negative predictive values) for appendicitis include:

  • The visualization of an enlarged and blind bowel-type structure connected to the cecum. The diameter considered as a significant abnormal enlargement is debated. Normal appendix diameter has been reported as below 6 mm [8,9,10] independently of the patient’s age. Above 6 mm, the diameter is usually considered abnormal but this by itself is not a highly specific sign [11]. A diameter up to 7 mm can be a normal finding [12, 13] or associated with asymptomatic lymphoid hyperplasia (thickening of the lamina propria) (Fig. 10.2) [14]. Still, a diameter equal or superior to 7 mm increases the specificity of this sign for diagnosing appendicitis [13].

  • In our experience, the thickness of the appendiceal wall is a more reliable sign than the diameter itself since it is less dependent on the lumen content which may increase the whole diameter of the appendix. There are controversies whether this wall thickness is age dependent [8, 9]. Above 3 mm, the wall thickening is definitely abnormal.

  • Peri-appendiceal fat inflammation appearing as increased hyperechogenicity is an important positive sign even if it can be found in other all inflammatory processes surrounding the appendix or even rarely in normal children. Its specificity increases when this echogenic fat surrounds circumferentially the appendix [11].

  • The presence of an appendicolith has a low predictive value. Indeed, it can be visualized in asymptomatic patients [15, 16].

  • A small amount of fluid within the lumen of the appendix [17].

  • An increased flow in the appendiceal wall as demonstrated by color Doppler US is a non-specific sign. However, its absence associated with the previous described positive signs would favor a perforation.

  • Lack of compressibility of the appendix has a poor positive predictive value. It has been reported to be the most commonly false-positive finding (80%) [11].

  • Local sonographic tenderness, presence of gas within the lumen of the appendix and lymph nodes within the root of the mesentery are not considered as accurate positive diagnosis signs.

Fig. 10.2
figure 2

Lymphoid hyperplasia of the appendix in US in an asymptomatic 6-year-old patient: the appendix is enlarged only on the mucosal layer with preservation of the multilayer differentiation

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In summary, the association of several different US signs increases the accuracy of US and ascertains the diagnosis (Fig. 10.3). Once confirmed, it becomes mandatory to search for potential complications.

Fig. 10.3
figure 3

Non-perforated acute appendicitis on US-Color Doppler in a 12-year-old-girl. (a) US sagittal scan: The appendix is enlarged surrounded by an increased echogenic fat. (b) Color Doppler: Hypervascularization is demonstrated within the appendix wall

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Of course, if an alternative diagnosis is identified during the US exploration, it rules out the diagnosis of appendicitis.

3.2 Complications

Perforation of the appendix is a major pejorative stage in the evolution of appendicitis; its frequency increases as the age at presentation decreases since the classic clinical signs and symptoms tend to be absent. Therefore, the diagnosis might be delayed in youngest patients [18].

The prevalence of perforation has been reported to range from 23 to 88% [19]. Perforation opens the way to the two most major life-threatening complications, abscesses and peritonitis. Noteworthy, the therapeutic options have recently evolved and there is a wide discussion regarding the optimal treatment (medical vs surgical treatment) of a simple acute appendicitis [20]. Even perforation with abscesses would now be treated with antibiotics first [21]. In order to decrease surgical complications, an appendectomy will be performed in a second stage after disappearance of infection and inflammation. A clinical, biological, and ultrasound follow-up will confirm the regression of the abscesses. In case of absence of response to the antibiotic treatment, a percutaneous drainage can be performed before to finally consider a surgical option.

US features suggesting perforation (isolated or complicated) include [19, 22,23,24] (Fig. 10.4):

  • A focal loss of differentiation of the appendix wall.

  • An appendicolith outside the lumen of the appendix.

  • The absence of appendiceal hyperhemia on color Doppler.

  • A hypoechoic and hyperhemic ill-defined fatty infiltration surrounding the abnormal appendix (=phlegmon)

  • A well-limited heterogeneous collection containing or not air bubbles (with comet-type hyperechogenicities) and surrounded by a hyperemic capsule of variable thickness (=abscess)

  • Due to the peritonitis, US may demonstrate several associated features

    • Loculated echogenic fluid collection(s) within the peritoneum

    • A hyperechogenicity of the periportal space

    • A diffuse increase of thickness and echogenicity of the peritoneal fat

    • A diffuse thickening of the outer layers (serosa, muscularis propria) of the small bowel

      • A small bowel ileus

      • Free air bubble(s) in the peritoneal cavity (=pneumoperitoneum). These bubbles appear as hyperechoic isolated spots and must be searched within the surrounding inflammatory fat, close to the anterior liver capsule and under the anterior peritoneal surface.

Fig. 10.4
figure 4

US of a perforated appendix in an 11-year-old-boy: a heterogeneous well-limited fluid collection surrounds an inflamed appendix; part of the perforated appendix wall is not visible anymore (yellow arrow). The adjacent mesenteric fat is inflamed and hyperechoic

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The complexity of appendicitis and its complications highlights once again the need for the exploration of the entire abdomen when performing US.

4 Appendicitis and CT

The lack of peritoneal fat in children younger than 10 years is an important limitation to visualize the appendix on CT [25]. A normal appendix is demonstrated on CT in 55.6% of patients older than 10-year but only in 38.7% in younger patients [25]. However, the absence of the visualization of the appendix has been reported to have a very high negative predictive value up to 98.7% as for the diagnosis of appendicitis [26].

CT has numerous advocates and has demonstrated in a meta-analysis high sensitivity and specificity, respectively 94 and 95%—for the diagnosis of appendicitis in children [3].

Yet, there is a raising concern in relation with the potential radiation risk associated with the use of CT. It has been calculated that 10,000 CT scans would increase the risk of cancer in 13 children; but conversely, not performing these 10,000 CT would lead to 280 missed diagnoses of appendicitis [3].

Whatever the arguments leading to perform CT, an optimized dose reduction strategy needs to be applied in order to minimize the patient’s exposure while maintaining a diagnostic level quality of images. The aim is being to obtain the best diagnosis accuracy.

There is no well-established CT protocol in the literature to explore a child with suspected appendicitis.

  • Adaptation of the tube current (mA) to the child body weight has been proposed by Donnelly et al. [27]. For instance, in a patient between 27 and 36 kg, 100 mA is sufficient while between 45.1 and 70 kg the mA can range between 140 and 150 mA.

  • Recommendations from the SFIPP society propose for a 10-year-old child, 32 kg weight, a CTDI of 7 ± 3 mGy for exploring a body of 35 cm (www.sfip-radiopediatrie.org).

  • Some authors prefer to limit the exploration from the lower pole of the right kidney [28] down to the pubic symphysis while others start the acquisition at the level of the diaphragm in order to consider all possible differential diagnoses [6].

  • Agreement does exist to perform the acquisition after IV contrast injection contrast media during the portal phase (1.5–2 cm3/kg with a maximum of 140 cm3).

  • No oral or rectal opacification is usually recommended [29].

  • Slice thickness varies in the literature from 1.5 to 5 mm [26].

  • Coronal and sagittal reconstructions are recommended mostly to increase the level of confidence for the diagnosis.

4.1 Classical Presentation

Diagnosis criteria’s include the following [30]

  • An appendiceal caliber >6 mm: This measurement has been questioned in the literature since normal caliber of the appendix may raise above 7 mm and increases from birth till 6–7 years [31].

  • An increased appendiceal wall enhancement.

  • A peri-appendicular fat stranding.

  • An appendicolith. However, Lowe et al. [32] report the presence of such finding in 3% of completely asymptomatic children.

  • The presence of fluid of more than 2.6 mm of thickness within the lumen of the appendix [33].

  • A focal symmetric thickening of the ceco-appendiceal junction (arrowhead sign) or a mass effect on the cecum [34].

None of these signs are highly specific, but the accuracy of diagnosis increases when a combination of signs is observed (Fig. 10.5).

Fig. 10.5
figure 5

Retro-cecal sub-hepatic appendicitis (yellow arrow) on coronal reformatted CE-CT in a 16-year-old-boy

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Noteworthy, the presence of gas within the lumen of the appendix does not exclude acute inflammation.

As mentioned, the visualization of a normal appendix on its whole length excludes the diagnosis of appendicitis.

4.2 Complications

Peritonitis is more frequent in children due to more rapid perforation than in adults.

Arguments for perforation include:

  • Focal loss of enhancement of the wall of the appendix; this sign has the best accuracy

  • The demonstration of a phlegmon: ill-defined inflammatory mass of heterogeneous density with an increased volume of inflamed fat associated with a various amount of fluid.

  • The presence of an abscess: well-limited fluid collection of various densities surrounded by an enhancing wall of variable thickness (Fig. 10.6).

  • Extra-luminal air

  • Appendicolith outside the appendix

Fig. 10.6
figure 6

CE-CT on a 15-year-old-boy. Perforated appendicitis surrounded by an abscess (large air fluid level collection on the midline)

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Using these criteria the overall accuracy of CT is reported as higher than 96% [35].

5 MR Imaging and Appendicitis

In order to avoid irradiation hazards, MR imaging has gained acceptance for the evaluation of suspected appendicitis as a first line imaging exploration or when the contribution US is limited especially in obese children [36,37,38,39].

Using either a 1.5 or a 3 T unit magnet, a fast MR imaging protocol is performed without the need for IV injection. Three Tesla allows shorter examination time and higher resolution images. The MR imaging examination time ranges from 8 to 35 min.

The use of multi-channel phased array coils with large coverage and parallel processing is mandatory.

Three-plans single-shot turbo spin-echo sequences associated with axial T2-weighted fat saturation acquisitions are obtained with free breathing. Alternative protocols replace the sagittal single-shot TSE by a coronal T2-weighted fat saturation sequence [38, 39].

A respiratory triggering may be used. Slice thickness can range from to 3 to 5 mm.

In more complex cases, the use of DWI sequences and Gadolinium IV injection T1 Fat-Saturation sequences may be required.

The normal appendix in asymptomatic patient can be demonstrated in 48% [40] to 86% [41] of patients while using an upper limit diameter of 7 mm [40].

5.1 Classical Presentation

MR imaging signs for appendicitis include: (Fig. 10.7)

  • Markedly hyperintense T2-weighted thickened wall of the appendix

  • Markedly hyperintense T2-weighted peri-appendicular tissues

  • Dilated appendix >6 mm

  • Presence of an appendicolith appearing as a focal signal void in T2-weighted sequences in a dependent position.

  • Free fluid in the pelvis

Fig. 10.7
figure 7

MR imaging Axial T2-weighted fat saturation sequence: Enlarged inflamed appendix (yellow arrow) with a small appendicolith in a 12-year-old obese patient

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5.2 Complications

Perforation must be considered in case of [37] (Fig. 10.8):

  • Localized fluid collection(s): they appear as hypersignal on T2-weighted sequences with or without heterogeneous content, with or without air fluid level or entrapped air within a thick fluid. The collections being surrounded by a hyposignal rim of various thicknesses.

  • Free peritoneal air appearing as a signal void outside the bowel lumen in a non-dependent position.

  • Direct visualization of the perforation of the appendicular wall

Fig. 10.8
figure 8

Perforated appendix in a 5-year-old girl: MR imaging T2-weighted sequences. (a) Deep perforated appendicitis (yellow arrow). (b) Associated pelvic abscess (yellow arrow)

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6 Unusual Presentations

6.1 Appendicitis in Infant and Neonates

Less than 3% of appendicitis occur in children younger than 3 years. Symptoms are non-specific, including abdominal pain, diarrhea, vomiting, abdominal distension, and fever. Due to a delayed diagnosis, the perforation rate is higher and the perforation occurs more rapidly in this age group.

6.2 Ectopic Locations

Due to wide variation in its length and position and due to the variable positions of the cecum itself, the symptoms related to appendicitis may be very atypical.

  • Retrocecal appendix: it is the second most frequent appendix location [42]. Due to this anatomical position, symptoms may mimic cholecystitis or pyelonephritis. Perforation may lead to abscesses in the pararenal space and/or in the perihepatic spaces.

  • Left side appendix: it can be associated with malrotation, situs inversus, or mobile cecum.

  • Amyand hernia: It is defined by the presence of the appendix within the inguinal canal. The appendix can be located in the inguinal canal in 1% of children but less than 0.1% will be complicated by an inflammation [43].

  • De Garangeot hernia: It is referred to an appendicitis within the femoral canal.

  • Intrathoracic appendix is associated with a diaphragmatic hernia

6.3 Pylephlebitis

Pylephlebitis refers to a septic thrombophlebitis of the portal vein system. The diagnosis of this rare complication of appendicitis in children is often delayed. Prolonged fever and abdominal pain of unknown origin are the main clinical signs. Even if they have not yet been reported in children, the potential risks are bowel ischemia, liver abscesses, and late portal hypertension. CE-CT is the method of choice recommended for such diagnosis [44]. It will reveal the extent of the hypodense thrombus within the lumen of the portal system and the signs of appendicitis. US-Doppler may potentially achieve this diagnosis but the technique will rather be used to follow up the resolution of the thrombosis. Anticoagulation associated with the adapted antibiotic treatment is advised by most authors to prevent bowel ischemia after appendectomy [45].

6.4 Uretero-Hydronephrosis

Uretero-hydronephrosis may occur secondarily to adjacent inflammatory reaction associated with the appendicitis and therefore, it could be mistaken for a urinary tract infection.

6.5 The Concepts of Spontaneously Resolving, Recurrent, and Chronic Appendicitis

Interestingly, 7–30% of patients that are operated for appendicitis have sustained one or more episodes of right iliac fossa pain [46,47,48].

  • Resolving appendicitis is defined by the association of clinical, biological (possible elevation of the WBC count but no elevation of the erythrocyte sedimentation rate) and imaging signs of appendicitis that disappear spontaneously within less than 48 h. The frequency of such entity has been reported to be at least 8% [48]. In this group of patients, lymphoid hyperplasia may play a major role. US shows an isolated thickening of the lamina propria (but not of the others layers) which can create a transient obstruction of the appendix lumen that will become symptomatic.

  • Recurrent appendicitis is defined as multiple subtle episodes of right iliac fossa pain, confirmed by histology to be related to appendicitis. Recurrences has been shown to occur between 19 and 38% of cases, most of them within a year [17, 48]. An initial thickness of the appendix above 8 mm [48], a residual intraluminal fluid within the appendix [17] or an appendicolith are features predicting recurrence [49].

  • Chronicity is defined as continuous symptoms affecting the right iliac fossa lasting for more than several weeks and associated with chronic inflammatory or fibrotic changes of the appendix at histology [50].

The imaging features of the entities described above are identical to those observed in the classical acute appendicitis [46, 50].

6.6 Stump Appendicitis

Recurrent episode of acute appendicitis may occur after appendectomy, when the residual stump is too long, usually above 0.5 cm. It can occur either after laparotomy or coelioscopy [51]. The challenge is to consider this diagnosis knowing the past surgical history. Only a few cases have been described in pediatric patients [52]. They occur after a few days or even years post appendicectomy. Imaging findings are similar to classical acute appendicitis.

6.7 Infection on an Appendicolith (Fig. 10.9)

When left after appendectomy, an appendicolith may become infected and clinically symptomatic including high fever and abdominal pain. It can be localized anywhere in the peritoneal cavity but usually in the most dependent part of the abdomen and pelvis (Morison’s pouch, Douglas cul-de-sac). It has also been reported to be located within the pleural cavity [53]. CT is the modality of choice to search for this lost and infected calcified left-over which can be associated with an abscess. Percutaneous retrieval under CT guidance has been published but the usual treatment remains surgical [54].

Fig. 10.9
figure 9

Infection on a left-behind appendicolith. (a) US: hyperechoic foci (yellow arrow) with posterior shadowing surrounded by a hypoechoic tissue reaction. (b) CT: small calcification (yellow arrow) surrounded by hyperdense tissue and inflamed fat

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6.8 Fistula

Fistulas developing during an episode of acute appendicitis have been exceptionally reported [55,56,57,58]. The fistulous tract can form between the appendix and the skin, the umbilicus, the rectum, the vagina, or the bladder. The latter has never been reported in pediatric patients [58]. Imaging will search for the inflammation and contact between the appendix and any of these sites. Air bubbles may be seen within the fistulous tract.

7 Differential Diagnoses of Acute Bacterial Appendicitis

One has to keep in mind that in the daily practice of an emergency department, when appendicitis is suspected on the basis of clinical evidence, the final diagnosis will be different in one out of 2 patients.

7.1 Differential Diagnoses Which Directly Involve the Appendix [59]

  • Ascites such as in the setting of nephrotic syndrome may increase the diameter of the appendix (Fig. 10.10).

  • Transient inflammatory bowel disease:

    • Lymphoid hyperplasia of the appendix [14]: The presence of lymphoid tissue within the mucosa and the submucosa of the appendix is a normal finding. It may be responsible for an increased diameter of the appendix during a transient viral inflammatory process (gastroenteritis, mesenteric adenitis). Other findings, especially, peri-appendiceal fat inflammation, hyperhemia of the appendiceal wall must be present to consider a diagnosis of appendicitis.

    • Any kind of terminal ileitis may mimic appendicitis and can involve the appendix. This includes infectious ileitis and neutropenic colitis. The most frequent pathogens agents being E. coli, Salmonella, Shigella, Campylobacter, and Yersiniosis.

    • Chronic inflammatory bowel diseases: The appendix can be inflamed in patients with Crohn disease. The correct diagnosis of Crohn can be achieved only when associated terminal ileum and the cecum are even more thickened and inflamed than the appendix itself. Furthermore, the fistulous tracts that are seen within the adjacent inflamed fat will confirm the diagnosis of Crohn. The diagnosis is much more difficult when the Crohn’s disease is limited to the appendix or when appendix is involved in case of ulcerative colitis [60].

    • Mucocele: finding a large appendix over 6 mm of diameter is frequent in cystic fibrosis. Interestingly, acute appendicitis is less frequent in this population [61] (Fig. 10.11). Mucocele without underlying cystic fibrosis has been reported in adolescents [59].

    • Tumor of the appendix:

      • Carcinoid of the appendix: This rare tumor is considered more aggressive in children than in adults. Due to its small size, this lesion is usually discovered at histology or may appear on imaging as a focal thickening of the appendix layers.

      • Lymphoma: Burkitt lymphoma is the most frequent subtype of lymphoma that can affect the appendix. The appendix is grossly enlarged and there is loss of wall differentiation. Associated tumoral bowel infiltration and large lymph nodes are usually present.

      • Inflammatory pseudo-tumor has exceptionally been described to involve the appendix in pediatric patients [62].

  • Foreign bodies may be visualized on imaging within the lumen of an inflamed appendix (see also Chap. 26).

  • Worms: Obstruction of the appendiceal lumen by ascaris lumbricoides is a common tropical problem that may mimic appendicitis. The diagnosis is achieved by ultrasound. It shows the worm’s alimentary canal as a simple or double hyperechoic bands with a hypoechoic center associated with curling movements, alone or within a complex echogenic mass [63].

  • Torsed appendicitis: Torsion of the appendix has been rarely reported in pediatric patients [64]. On US, a target like appearance at the base of the appendix is the clue for diagnosis. The other findings are similar to those present in classical appendicitis.

Fig. 10.10
figure 10

Increased thickness of a normal appendix on US in a 5-year-old boy. The luminal content and the associated ascites are responsible for this increased diameter

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Fig. 10.11
figure 11

Mucocele in 4-year-old-child with cystic fibrosis

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7.2 Differential Diagnoses Which Do Not Directly Involve the Appendix

These will be only mentioned. Readers will find their imaging description in the related chapters.

  • Ileitis

  • Meckel diverticulitis

  • Epiploic appendagitis

  • Omental infarction

  • Cholecystitis

  • Acute pyelonephritis, renal or ureteral stones

  • Ovarian disease (ovarian torsion, hemorrhagic cyst, abscess) or fallopian inflammation (salpingitis, pyosalpinx, tubo-ovarian abscess)

  • Right inferior pulmonary infection

8 Post-Operative Complications

Apart from the left behind appendicolith (Fig. 10.12) and stump appendicitis already mentioned, the most frequent post-operative complication is abscess formation (without these underlying causes).

Fig. 10.12
figure 12

Large post-appendicectomy abscess containing a tiny appendicolith treated with antibiotics. (a) US shows a hyperechoic structure with posterior shadowing (yellow arrow) surrounded by a well-limited heterogeneous fluid collection. (b) CE-CT demonstrated a well-limited enhancing thickened wall and hypodense contents with a tiny calcification (yellow arrow)

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Recurrent abscesses after initial or delayed appendicectomies performed for perforated appendicitis with abscesses have been reported to range from 1 to 24% of cases [65]. Noninvasive treatment, with or without antibiotics, or invasive (surgical or drainage under radiological guidance) treatment remains debated.

Conclusion

Appendicitis due to its numerous presentations and mimickers remains a clinical challenge. Ultrasound, MR, or CT are all highly valuable techniques to look for this diagnosis and search for its differential diagnoses as well as assess complications. The choice of the type of imaging is under the responsibility of the radiologist. When CT is used, an optimized strategy, including IV injection and dose reduction need to be used.