3.3\ Appendicitis

Appendicitis affects all age groups, peaking in the early 20s and then gradually declining with increasing age. Appendicitis is 1.4 times more frequent in men compared to women. The most common causes of appendicitis are obstruction of the lumen by fecalith, lymphoid follicle hyperplasia, foreign bodies, and tumors. Variations in the appendiceal location make the clinical assessment of appendicitis difficult. The position of the appendix is retroperitoneal in about 30% of cases. In the remaining 70% of intraperitoneal appendices, the location can vary from retro-cecal to retro-ileal, deep pelvic, and rarely right upper quadrant location. Suspected appendicitis is the commonest cause of emergency abdominal surgery; however, clinical diagnosis can be difficult and approximately 20% of appendicectomy cases are false-positive diagnoses (Paulson et al. 2003). In women of reproductive age, the error rate can be as high as 40%, because acute gynecological processes can mimic the clinical findings of acute appendicitis (Andersson et al. 1992).

Perforation and abscess formation can complicate appendicitis in 38–55%, with the highest rates occurring in children and in elderly patients.

3.3.1\ Imaging Findings

On CT the normal appendix appears as a tubular structure with a diameter of less than 6 mm that often contains air or contrast media. CT findings of acute appendicitis include enlargement of the appendix (>6 mm in outer diameter), enhancement of the thickened appendiceal wall, and fat stranding of the peri-appendiceal region (Fig. 17) (Rao et al. 1997). Signs indicative of perforation include extra-luminal air, extraluminal appendicolith, a defect in the enhancing appendiceal wall, and an abscess or phlegmon (Horrow et al. 2003). A phlegmon is characterized by diffuse inflammation of the peri-appen- diceal fat with no or small, ill-defined fluid collections. An abscess is a well-delineated fluid collection with rim enhancement (Horrow et al. 2003). Focal thickening of the cecum can be seen secondary to the inflammatory process

a

b

Fig. 17  CT findings in acute appendicitis. Axial Ct through the right lower quadrant (a, b). The tubular enhancing structure with a diameter of 9 mm is the dilated appendix (arrow). It is surrounded by marked fat stranding of the peri-cecal fat and adjacent facial thickening. At the base of the appendix (arrow), thickening of the cecum can be seen, which presents the arrowhead sign (b). A small fluid collection is seen along the surface of the psoas muscle (b)

and has been described as the arrowhead sign (Rao et al. 1997). The appearance on MR is similar to that described on CT, including thickening of the appendiceal wall, a dilated fluid-filled lumen, and increased intensity of peri-appendi- ceal tissue on T2-weighted imaging or contrastenhanced images (Fig. 18) (Nitta et al. 2005). Extra-intestinal fluid-filled hyperintense lesions with walls that are hypointense on T2-weighted images and thick on the contrast-enhanced images are indicative of abscesses. The presence of air on MRI or CT allows the definitive diagnosis of an abscess (Oto et al. 2005).