Ultrasound scans are frequently performed in children with varying degrees of benefit in assisting the diagnosis of appendicitis. The sonographic signs are infrequently reported but have been shown to aid the diagnosis [
9‐
23]. Our study aimed to investigate the diagnostic utility of USS by finding sonographic signs associated with appendicitis, and which differentiate SA and CA in children.
The negative appendicectomy rate was similar between the group who had USS and the group who did not (
p = 0.18); we believe this is related to an excellent accuracy of clinician gestalt in diagnosing appendicitis in children presenting to our emergency department [
2]. The time interval between USS and surgery can be an important factor in occurrence of perforation; different patient-dependent factors (e.g., need for resuscitation) and patient-independent factors (e.g., availability of the operating theatre and operating team) can prolong this interval, potentially affecting the occurrence of perforation in patients that are delayed to theatre. However, we found that the time interval between USS and surgery was slightly longer in the SA group compared to the CA group, although the difference was not statistically significant. Due to the retrospective nature of the study, we do not have an exact explanation for this finding but we speculate that children diagnosed with CA on USS were deemed to require surgery more urgently than children diagnosed with SA. In this respect, the USS might add to the clinical decision making in prioritising children with CA requiring urgent surgery.
Appendix visualisation and ultrasound accuracy
Visibility of sonographic signs is markedly reduced if the appendix itself is not visualised. Our study had an overall 62% conclusive scan rate, closely linked to our overall 61% appendix visualisation rate. Studies of diagnostic accuracy of USS for paediatric appendicitis report a wide range of visualisation, with two Australian studies reporting 41% and 92% [
5,
7]. Cundy et al. attribute their superior visualisation rate to diligent paediatric sonographers who employ multiple techniques and use tightly curved transducers more suited to smaller patients [
6]. Reddan et al. were able to improve visualisation rates to 69% in a subsequent study, following sonographer training and implementation of a worksheet [
13]. Aside from implementation of a worksheet, other institutional variables such as sonographer experience, type of hospital (regarding paediatric volume), and time of day also contribute [
22]. Visualisation is also dependent on patient factors, such as duration of symptoms and their clinical presentation pre-USS. Patients who have a high suspicion of appendicitis could have developed more sonographic signs than clinically equivocal patients [
7,
12,
22,
24].
Our study demonstrated appendiceal visualisation rates of 92% in SA, with only 48% NoA viewed. One explanation is that non-inflamed appendices are more difficult to visualise due to their smaller size and absent secondary features. However, our study included a higher appendicitis prevalence (36%) than Cundy et al. (28%) implying a smaller patient volume in our study but also suggesting consistent visualisation is still possible. Our CA visualisation rate (72%) was lower than SA, which we attribute to the difficulty delineating an appendix amongst associated inflammatory changes.
With non-visualisation of the appendix, secondary signs were present in 16% of USS, well within the range of 5–23% reported in the literature [
10,
24‐
26]. In our cohort, a quarter of this group had appendicitis (26%), and 69% of these were CA. This relatively low rate of USS diagnosed appendicitis compared to Partain et al. (42%), is likely due to their stricter definition of a secondary sign including a ‘significant amount of fluid’ rather than our ‘peri-appendiceal fluid’, which is often physiological [
12]. However, Held et al. report a 17% appendicitis rate, likely explained by the majority of their USS being in the non-visualised category (76%) [
10]. When neither the appendix nor secondary signs were viewed, our appendicitis rate was within the range of previously reported values 2–9% [
10,
12,
24]. It has been suggested that these children can be observed or discharged with increased confidence, although it remains case dependent.
Overall, our reported accuracy was inferior to previous meta-analyses reporting pooled sensitivity (88–89%) and specificity (94–97%) [
27,
28]. However, this included heterogenous data from 12,926 children from different institutions. For example, some centres excluded non-diagnostic USS from analysis, which overestimate the true accuracy. For diagnosing CA over SA, our sensitivity was higher than previously reported values of 23–44%, with specificity similar at 93–100% [
17‐
19,
29].
Our sonographic features were analysed using an established institutional worksheet. This provides sonographers with a conventional framework to ensure a systematic and thorough examination and ensures a standardised outcome source which increases the quality of our retrospective study. The application of a standardised worksheet is lacking in existing published literature [
9,
10,
12,
19‐
21]. Therefore, their retrospective data are limited to sonographic signs that radiologists report, which means the relevant absence of signs may not be detailed.
Primary sonographic signs suggesting appendicitis
The maximal appendiceal diameter is the most commonly studied sonographic sign. Our service uses 6 mm as a threshold, (sensitivity 97%, specificity 69%). However, our analysis suggests 7 mm is more accurate (sensitivity 86%, specificity 86%), particularly in the NegA group. This is similar to other studies, with some suggesting three categories [
30‐
33]. We suggest balancing the statistical superiority of a more specific 7 mm threshold with the clinical risk of false negatives.
An appendicolith was a significant predictor of appendicitis in our cohort with specificity 94% (adjusted OR 1.15,
p = 0.003), similar to Partain et al. (adjusted OR 7.9 [95% CI 1.7–37.2]) and Telesmanich et al. (OR 15.8 (
p = 0.03)), with Trout et al. finding no association [
12,
14,
15]. An appendicolith has been generally associated with failure of non-operative management; however, none of the “appendicolith-positive” patients in our series (6%) developed appendicitis after 12 months follow-up; a longer follow-up will be needed to confirm this finding.
Hyperaemia on doppler was most frequently seen in SA (86%), and significantly associated with appendicitis. This is consistent with most previous studies [
12,
15,
34,
35].
Non-compressibility on probe pressure significantly predicted appendicitis, and had increased prevalence in SA. Potentially CA may have been decompressed with a perforation at the time of USS, or the appendiceal structure could be distorted amongst heterogenous inflammatory echoes. This sign is uncommonly reported in the literature [
15].
Absence of luminal gas was significantly associated with appendicitis and most frequent in SA. There is a paucity of reported data studying this sign, with our data suggesting it has utility to rule in appendicitis (specificity 92%), with its absence having little value (sensitivity 43%).
Secondary sonographic signs suggesting appendicitis
Focal pain with transducer pressure is uncommonly studied due to its similarity to palpation [
11]. However, we found significant association with appendicitis, present in 97%, and probe tenderness carried the highest sensitivity (97%) and lowest specificity (44%) of all the sonographic signs. Its relative frequency in children who had a NegA (92%), could suggest clinicians value clinical right iliac fossa tenderness for diagnostic evaluation.
Peri-appendiceal fluid was not significantly associated with SA, consistent with the literature [
9,
11,
13‐
15,
35]. However, other studies reported significance of right lower quadrant fluid, with some additionally finding free fluid [
10,
12,
35]. This emphasizes the importance of location, volume and character descriptors, as free fluid can be physiological in small amounts, whereas heterogenous fluid can suggest pathology.
The presence of peri-appendiceal echogenic fat was most associated with appendicitis in our cohort, which is consistent with the published literature [
11,
14,
15,
35].
Primary sonographic signs predicting complicated over simple appendicitis
The statistically optimal threshold maximal appendix diameter was 10.1 mm for CA (sensitivity 54%, specificity 75%), which is larger than previously studied (9 mm) [
21]. An appendicolith was a significant predictor of CA, which has been reported previously [
16,
17,
20,
21,
23]. Visualisation of a blind-ending structure is included as a confirmatory variable on our worksheet to ensure viewing of the correct anatomy. It was negatively associated with CA which could be due to difficulty identifying the appendix tip amongst inflammatory tissue. Hyperaemia, non-compressibility, and absent luminal gas were not associated with CA in our study although some previous studies have found them to be helpful [
17,
22].
Secondary sonographic signs predicting complicated over simple appendicitis
Probe tenderness was negatively associated with CA, which was unexpected with unclear clinical significance. It has not been previously studied in the literature and reflects a subjective, non-specific yet sensitive sign of CA. Peri-appendiceal fluid was detected most frequently and significantly associated with CA, although it was also present in both SA and children without appendicitis. Heterogenous fluid has been reported as the most predictive sign for CA [
17,
20,
22,
23]. We hypothesize this finding reflects perforation with leaked intra-luminal content or inflammatory exudate, and should be further characterised in a sonographic worksheet or report. Echogenic fat was not associated with CA as it was also common in SA, consistent with most previous studies [
16,
17,
20‐
23].
Strengths and limitations
Strengths of our study include a large study population over a relative short period and the use of a standardised worksheet, reducing the variability in reporting, and ensuring accurate data collection. Limitations of our study stem from its retrospective design. Our literature review revealed other sonographic signs reported to be associated with appendicitis, such as appendiceal mural thickness and bowel signs which were not assessed in our study. Peri-appendiceal fluid is one of the signs found to be good differentiator between SA and CA; however, our study did not specifically address if it was simple or heterogeneous; we plan to add this information to our worksheet in the future. Investigating inter-observer reliability between sonographers or radiologists was another potential limitation. This would have allowed the operator-dependence of USS to be quantified and whether experience of sonographers improved their performance. Additional patient data such as body mass index, clinical presentation details, time from onset of symptoms to USS, and complication rates could enable a thorough investigation into factors affecting USS accuracy. It would also be useful to explore the clinician’s views about USS and appendicectomy, allowing us to gauge their perceived utility and how certain findings influence their management.