Abstract
Ultrasound (US), computed tomography (CT), and magnetic resonance imaging (MRI) have high diagnostic accuracy in the diagnosis of acute appendicitis. Nowadays imaging is part of the diagnostic algorithm of appendicitis and the advantages are demonstrated by the reduced negative appendectomies rates. US is the preferred first line imaging in acute appendicitis. The US diagnosis of complicated and uncomplicated appendicitis is well characterized by different signs but the paradigm of US to take in mind is that the non-visualization of the appendix cannot exclude acute appendicitis. In US, when the appendix is not visualized, or US is inconclusive, second-line imaging examination should be performed. The aims of this pictorial are 1. to illustrate the US signs of complicated and uncomplicated AA; 2. to describe the conditions in which other imaging modalities should be performed to reach a definitive diagnosis.
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Introduction
The Acute Appendicitis (AA) diagnosis is assessed by combining clinical scores, laboratory data and imaging results [1, 2]. Clinical scores such as Adult Appendicitis Score, Appendicitis Inflammatory Response and RIPASA score identify patients with intermediate risk who need an imaging test to reach a diagnosis and are superior Modified Alvarado Score [3, 4]. Imaging significantly reduces the rate of negative appendicectomies in case of suspected AA. Ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) are highly accurate for the diagnosis of AA [1, 2], but according to the WSES Jerusalem guidelines (2020), US examination should be the first-line imaging [5]. The combination of US findings with clinical and laboratory parameters improve diagnostic accuracy, but when clinical suspicious persist after an US examination with negative or inconclusive results, a second-line imaging examination should be performed: a US re-evaluation, a CT or a MRI. CT and MRI should be preferred in children and pregnant women, whereas CT is usually warranted in adults and non-cooperative patients [1, 6]. The decision, regarding which diagnostic test is the best, depends on local expertise, imager availability, patient sex and age, and clinical conditions [1, 7]. Appendiceal lumen obstruction has always been considered the trigger of acute appendicitis, or lymphoid hyperplasia in young patients. The appendix obstructed keeps secerning mucus that determines appendiceal dilatation and increase of endoluminal pressure that can lead to vascular parietal damage and consequent ischemia and perforation with periappendiceal infection [8, 9]. Recently microbiota has been suggested to play an important role in the development of appendicitis because of the increased incidence of Crohn’s disease, ulcerative colitis, and Clostridium difficile infection after appendectomy [9, 10]. Appendix is now considered important for intestinal homeostasis and appendicitis may act as a trigger event in the development of bowel inflammatory diseases [11].
The treatment of appendicitis is strictly correlated to the transmural extension of the disease and it is defined as complicated or uncomplicated. Non-operative management is performed in selected patients with uncomplicated forms of AA for these reasons a distinction between complicated and uncomplicated forms of is fundamental. Imaging guides the treatment and a recently proposed classification [12] describes five imaging pattern: normal appendix (type 0), non-visualized appendix (type X), uncomplicated AA (type 1), complicated AA without perforation (type 2), and complicated AA with perforation (type 3). Instead, the CT staging of appendicitis define four grades: I normal, II mild appendicitis, III appendicitis with localized peritonitis, IV perforated appendicitis [13]. The classification between complicated and uncomplicated appendicitis varies among scientific society and imaging modalities but the presence of an appendicolith is considered as an independent prognostic risk factor for treatment failure in NOM of uncomplicated AA [1, 14, 15]. The role of imaging in the scenarios of operative or non-operative management requires accurate distinction between complicated appendicitis with high risk of perforation and uncomplicated appendicitis [12].
US signs of uncomplicated and complicated AA
Ultrasound is highly accurate in the diagnosis of appendicitis although its accuracy is less than CT, it may be influenced by different factors and the expertise of the operator, intrabdominal fat and appendix position are the principal factors [16,17,18,19]. The diagnostic criteria of acute appendicitis are the same among different imaging modalities (US, CT, MRI), they are divided in primary and secondary findings. The primary signs are related to appendix alterations, meanwhile secondary signs are related to periappendiceal inflammation [5, 6, 20, 21]. During US, appendix can be imaged with both high resolution, high frequency linear array 7.5–10 MHz and curvilinear array 3.5–7.0 MHz. The first is usually preferred also per the excellent detailed information, although the curvilinear probe is usually the first utilized to avoid any satisfaction errors (other diseases mimicking appendicitis) [22], to better localize the cecum and the terminal ileum, and in obese patients.
Primary signs
During US, the appendix appears as a blind-ended peristaltic tubular structure. The diameter of normal appendix, when visualized, is less than 6 mm, the length is extremely variable [23, 24]. An appendiceal diameter > 6 mm on axial axis (serosa to serosa) under probe compression is considered a positive sign but it should not be considered alone but always in association with other signs because the appendix could be dilated by endoluminal material and not obstructed; the appendix is non-compressible although in case of perforation it may become compressible [25,26,27,28,29,30]. One clinical signs that should be considered is the positive McBurney Sign that is the pain evocated during the compression with the probe of the non-compressible appendix [28]. A parietal thickness > 3 mm (mucosa to serosa) is considered a pathological sign. Another sign related to wall appearance is the parietal stratification and its loss, with non-visualization of the mucosal surface and increased thickness of an hyperechogenic submucosa. With the progress of the inflammation, the hyperechogenic submucosa tends to present focal interruption due to the transmural progression of the disease indicting the presence of complicated appendicitis. In advanced complicate stages, the parietal wall can appear thinned and interrupted. The loss of normally echogenic submucosal layer is a uniquely helpful discriminator of complicated versus uncomplicated appendicitis [31]. Increased Doppler flow can be detected within the inflamed appendiceal wall, this sign is specific but has low sensitivity [9, 32]. The appendix might contain colonic material including gas or fluid. Endoluminal gas was considered an exclusion criterion of AA indicating appendix patency, although it is characteristic of gangrenous appendicitis [33]. Appendicolith has been considered as a characteristic sign of appendicitis, but its significance has been strongly remodulated because it is present in 4% of asymptomatic patients and in less of 50% of patients with AA. In symptomatic patients with appendicolith and other inflammatory changes of the appendix should be considered for an early appendicectomy even in absence of other complicated signs of appendicitis [34,35,36] (Figs. 1, 2).
Secondary signs
Secondary signs are related to periappendiceal inflammation. Hyperechogenicity of periappendiceal fat, reactive lymph nodes, free fluid, abscess and thickening of peritoneum can be detected. Inflamed fat appears as echogenic thick zone around the appendix and sometimes it can present mass effect on adjacent structures [9, 12, 21, 28].
Free fluid can be assessed in the right lower quadrant, around the appendix but also between bowel loops. The terminal ileum can appear hypoperistaltic (sentinel loop) [37].
Uncomplicated and complicated appendicitis.
Differentiation between a complicated and uncomplicated AA is crucial for the correct management. The presence of an appendicolith is an independent prognostic risk factor for treatment failure in NOM of uncomplicated AA, in these cases, an early surgical approach is recommended [1, 35]. Focal interruption of submucosa layer indicates transmural progression of the inflammation progress and is consider a sign of complicated appendicitis such as interruption of the parietal wall (complicated without perforation) [31]. The presence of abscess or periappendiceal phlegmon indicated a complicate appendicitis with perforation. The role of the US in staging appendicitis is not accurately stated, although US can represent a definitive diagnostic modality in selected case. First, when appendix is completely visualized during US, intraluminal appendicolith can be detected and patients can be stratified. Moreover, the US can accurately identify wall discontinuity and focal defects that imply transmural extension of inflammatory process and perforation in complicated AA. Peri-appendicular abscesses and fluid collections are easily detected during US too, although in severe complicated forms, with larger fluid collection in the right lower quadrant, the cecal appendix may not be clearly recognizable anymore and a second line imaging technique is required. In spite of the high accuracy of CT and US in selected patients, no safe differentiation between uncomplicated and complicated forms can be made upon imaging alone. Hence, a combination of clinical scores, laboratory value (white blood cell count and C-reactive Protein), US, and CT findings are usually preferred to reach a correct diagnosis [13, 38] (Figs. 3, 4).
Acute appendicitis: when ultrasound is not enough
Despite the high accuracy of US, there are still some scenarios in which other imaging methods (CT or MRI) should be performed [1, 16]: (1) when the appendix is not clearly or completely visualized during US, (2) when the inflammatory changes involve the appendix and the other structures in the right lower quadrant, (3) when ultrasound detects “chaos in right lower quadrant”.
Scenario 1
During US, the appendix is not visualized or not completely visualized
One paradigm of US diagnosis of AA is that the lack of visualization of the appendix does not exclude AA. In case of inconclusive US, a second line imaging is required. In many of these cases, the appendix may be retrocecal or pelvic, being difficult to be displayed in US [28, 29]. Normal appendix in adults asymptomatic patients can be visualized during US in 50–71% of population and it is negatively influenced by intrabdominal fat [39,40,41], oppositively during CT and MRI, normal appendix is visualized in almost 80% of the population and are positively influenced by the presence of intrabdominal fat [41, 42]. It is important to underline this concept, because in case of clinical and laboratory suspicions of appendicitis, not only the non-visualization of the appendix cannot exclude appendicitis, moreover the appendix, also if normal, should be assessed completely in its length because inflammation can be distant from the orifice and can be limited to the tip (tip appendicitis) [9, 43] (Figs. 5, 6, 7).
Scenario 2
Too many inflammatory changes in the RLQ
Many non-surgical conditions may determine inflammatory changes in the right lower quadrant that do not originate in the appendix, but instead may be in the cecum or in the ileum or secondary to pelvic inflammatory disease such as cecal diverticulitis, epiploic appendicitis, omental infarction, infectious colitis, inflammatory bowel diseases, malignancies etc. These processes lead to secondary thickening and inflammation of the appendix (reactive appendicitis) and clinically present acute RLQ pain. A careful and accurate differential diagnosis should be made between surgical and non-surgical conditions and the appendix should be confidently individuated as the epicenter of the inflammation. US can be inaccurate and it requires high level of confidence and expertise. Therefore, when there are too many inflammatory changes in the RLQ that involve all the structures, secondary imaging is necessary to make a definitive diagnosis, and prevent erroneous management. Appendiceal involvement in Crohn’s disease may be observed and is commonly seen in combination with involvement of the terminal ileum and cecum [44]; thickening of the appendix and terminal ileum should alert radiologists to rule out the presence of Crohn’s disease [5], on the other hand, Crohn’s may manifest with firstly granulomatous appendiceal involvement [45, 46]. A misdiagnosis and surgical treatment of secondarily appendix inflammation in these patients may evolve in long-term post-surgical complications. Generally during US, the inflammatory changes are diffuse and the epicenter is not clearly identified in the appendix, radiologists should perform second line imaging to achieve a confident diagnosis to determine the cause and to stratify patients for surgical or - non-surgical management [47, 48] (Figs. 8, 9, 10).
Scenario 3
The “chaos” in right lower quadrant
Severe inflammatory changes in the right lower quadrant may determine difficulties in the recognition during US of anatomical structures. Fluid collections and involvement of adjacent organs behind the appendix cannot be localized accurately during US. In these cases, second line imaging is required to determine the cause, the extension and structure involvement in the inflammatory process. Although ultrasound may be diagnostic in assessing the presence of an inflammatory process in the RLQ, it cannot be sufficient in determining the cause and the extension. Second line imaging such as CT or MRI, in stable and selected patients, are necessary to define the cause and involvement of RLQ and pelvic structures (Fig. 11).
Conclusion
A multidisciplinary approach involving clinical assessment, scoring systems, and appropriate imaging is essential for accurate diagnosis and optimal patient care in suspected acute appendicitis. Each component of this approach contributes to overall diagnostic accuracy and helps healthcare professionals for the correct management. Nowadays, appendicitis management can be operative or non-operative, this approach is also sustained by the emerging immune and homeostatic role of the appendix. Discriminating between complicated and non-complicated appendicitis is necessary, but imaging alone cannot be accurate and imaging findings need to be always correlated to laboratory test and clinical picture. The US should be the first imaging modality to approach patients with suspected appendicitis, but the non-visualization or non-complete visualization of the appendix do not exclude appendicitis. Furthermore, there are different scenarios that require a second line imaging: when the appendix is non visualized, when there are too many inflammatory changes or parietal thickening of cecum and ileum, and when the inflammatory process is extremely severe determining the chaos in the RLQ.
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Comune, R., Tamburrini, S., Durante, A. et al. Ultrasonography (US) examination of acute appendicitis (AA): diagnosis of complicated and uncomplicated forms and when US is not enough. J Med Imaging Intervent Radiol 11, 14 (2024). https://doi.org/10.1007/s44326-024-00002-5
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DOI: https://doi.org/10.1007/s44326-024-00002-5