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Diagnostic Criteria

Sonographic criteria for acute appendicitis include a non-compressible appendix with an outer AP diameter of at least 6 mm (Kessler et al. 2004), mural thickness of 3 mm or greater, or the presence of an appendicolith in an appendix of any size (Figs. 6.1, 6.2). The two most useful ultrasound signs are those of an appendiceal diameter of >6 mm (sensitivity 98%, specificity 98%, positive predictive value (PPV) 98%, negative predictive values (NPV) 98%) and non-compressibility (sensitivity 96%, specificity 96%, PPV 96%, NPV 96% (Kessler et al. 2004)). If the appendix measures less than 6 mm in diameter, particularly if compressible, this should be considered normal. The use of colour Doppler may be helpful in showing hyperaemia associated with appendiceal wall inflammation.

Presence of a hypo-echoic fluid collection containing an appendicolith or a fluid collection adjacent to a gangrenous appendix is diagnostic for a peri-appendiceal abscess (Fig. 6.3).

Appendicitis may occur at the tip of the appendix and therefore the appendix must be visualized throughout its length to exclude a distal 'tip appendicitis'. The normal appendix may be difficult to visualize due to its small luminal diameter and easy compressibility. However, with operator experience it should be identified in the majority of cases. Features of a normal appendix include: (1) the typical normal sonographic appearance of bowel, with an

Fig. 6.1a-c. Acute appendicitis in a 12-year-old boy. a Longitudinal ultrasound image showing a swollen appendix (markers) which measured 7 mm. Note the increased echogenicity in the wall. b Transverse image (TS) showing the concentric layers of the inflamed appendix. c TS image showing two sections of the curved appendix and it's relationship to the iliac vessels

Fig. 6.1a-c. Acute appendicitis in a 12-year-old boy. a Longitudinal ultrasound image showing a swollen appendix (markers) which measured 7 mm. Note the increased echogenicity in the wall. b Transverse image (TS) showing the concentric layers of the inflamed appendix. c TS image showing two sections of the curved appendix and it's relationship to the iliac vessels

Fig. 6.2. Acute appendicitis in a 14-year-old boy showing the presence of an appendicolith with posterior acoustic shadowing on ultrasound
Fig. 6.3. Ultrasound of a heterogenous appendix mass in a 12-year-old girl

echogenic submucosal ring, (2) terminating with a blind tip (3), origin from the base of the caecum (4), and lack of peristalsis as it is part of the colon.

If the clinical signs and symptoms strongly suggest appendicitis, but it has not been possible to demonstrate the appendix, a retrocaecal appendix may be present. Various operator techniques can be employed to improve delineation of the retrocaecal appendix. These include manoeuvres to scan the patient in a left posterior oblique position. Ancillary techniques for demonstrating a 'hidden' appendix include the additional use of the posterior manual compression technique, upward graded compression technique (Lee et al. 2005), using a low frequency contrast transducer, or left/lateral decubitus change of body position.

It is reported that graded compression sonogra-phy with additional operator dependent techniques can yield a sensitivity of up to 99%, specificity of 99% and an accuracy of 99% for acute appendicitis (Lee et al. 2005).

The lack of visualization of the appendix with US has a negative predictive value of 90% in experienced hands (Kessler et al. 2004). The identification of an appendix measuring less than 6 mm in diameter is a very accurate indication to exclude appendicitis with a negative predictive value of between 98%-100% (Kessler et al. 2004; Rioux 1992; Rettenbaoher et al. 2001).

Secondary signs may help the sonographer establish the diagnosis of appendicitis. These include prominent echogenic fat in the right lower quadrant suggestive of an inflammatory process, the presence of appendiceal mass, the presence of free fluid or of a collection, or the presence of free gas seen over the liver on ultrasound. However, the secondary signs are not specific for acute appendicitis because these may also be seen with other intra-abdominal pathology.

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