Appendicitis is inflammation of the vermiform appendix. It is a very common condition in general radiology practice and is a major cause of abdominal surgery in young patients. CT is the most sensitive modality to detect appendicitis.


Acute appendicitis is typically a disease of children and young adults with a peak incidence in the 2nd to 3rd decades of life 1.

Clinical presentation

The classically presentation consists of periumbilical pain (referred) which within a day or later localizes to McBurney's point and is associated with fever, nausea and vomiting 2. This progression is only seen in a minority of cases and is unhelpful in children who often present with vague and nonspecific signs and symptoms. It also relies on the appendix being in a 'normal' position, which is not the case in a significant number of cases (see below).

General signs and symptoms include 1-2:

  • fever
  • localised pain and tenderness
    • RLQ pain over appendix (i.e. McBurney sign)
    • pelvic pain, diarrhoea and tenesmus (pelvic appendix)
    • flank pain (retrocaecal appendix)
    • groin pain (appendix within an inguinal hernia - Amyand's hernia) or femoral hernia - De Garengeot's hernia)
  • leucocytosis
  • nausea and vomiting
  • atypical location: within pelvis (30%), extra-peritoneal (5%)


Appendicitis is typically caused by obstruction of the appendiceal lumen, with resultant build up of fluid, secondary infection, venous congestion, ischaemia and necrosis. Obstruction may be caused by 1:

Radiographic features

One of the biggest challenges of imaging the appendix is finding it. Once confidently identified whether or not it is normal, is relatively straightforward in most instances.

The location of the base of the appendix is relatively constant, located roughly between the ileocecal valve and the apex of the caecum. This relationship is maintained even when the caecum is mobile.

The location of the tip of the appendix is much more variable, especially as the length of the appendix has a large range (2-20 cm) 8. The distribution of positions has been described as 7-8:

  • behind the caecum (ascending retrocaecal): 65%
  • inferior to the caecum (subcaecal): 31%
  • behind the caecum (transverse retrocaecal): 2%
  • anterior to the ileum (ascending paracaecal preileal): 1%
  • posterior to the ileum (ascending paracaecal retroileal): 0.5%

Plain radiography is infrequently able to give the diagnosis, however, is useful for identifying free gas, and may show an appendicolith in 7-15% of cases 1. In the right clinical setting, finding an appendicolith makes the probability of acute appendicitis up to 90%.

If an inflammatory phlegmon is present, displacement of caecal gas with mural thickening may be evident.

Small bowel obstruction pattern with small bowel dilatation and air-fluid levels is seen in approximately 40% of perforations.


Ultrasound with its lack of ionizing radiation should be the investigation of choice in young patients, and is effective in competent hands in identifying abnormal appendixes, especially in thin patients. However, the identification of a normal appendix is more problematic, and in many instances, appendicitis cannot be ruled out.

The technique used is known as graded compression, and uses the linear probe over the site of maximal thickness, with gradual increasing pressure exerted to displace normal overlying bowel gas.

Findings supportive of the diagnosis of appendicitis include 4:

  • aperistaltic, noncompressible, dilated appendix ( >6 mm outer diameter)
  • appendicolith
  • distinct appendiceal wall layers
  • echogenic prominent pericaecal fat
  • periappendiceal fluid collection
  • target appearance (axial section)

Confirming that the structure visualised is the appendix is clearly essential and requires demonstration of it being blind ending and arising from the base of the caecum.

A recently described dynamic ultrasound technique using a sequential 3 step patient positioning protocol has been shown to increase the visualisation rate of the appendix 9. In the study, patients were initially examined in the conventional supine position, followed by the left posterior oblique position (45o LPO) and then a “second-look” supine position. Reported detection rates increased from 30% in the initial supine position to 44% in the LPO position and a further increase to 53% with the “second-look” supine position. Slightly larger absolute and relative visualisation rates were seen in children. The authors suggested that the effect of the LPO positioning step improved the acoustic window by shifting bowel contents. 


CT is highly sensitive (94-98%) and specific (up to 97%) for the diagnosis of acute appendicitis and allows for alternative causes of abdominal pain to also be diagnosed.
The need for contrast (IV, oral or both) is debatable and varies from institution to institution. Findings include 1-3:

  • dilated appendix with distended lumen ( >6 mm diameter) 3
  • thickened and enhancing wall
  • thickening of the caecal apex (up to 80%): caecal bar sign, arrowhead sign
  • periappendiceal inflammation, including stranding of the adjacent fat and thickening of the lateroconal fascia or mesoappendix.
  • extraluminal fluid
  • inflammatory phlegmon
  • abscess formation
  • appendicolith may also be identified

Treatment and prognosis

Treatment is appendicectomy, which can be performed either open or laparoscopically 5. Mortality from simple appendicitis is approximately 0.1%, but is as high as 5% in perforation with generalised peritonitis 5.

In approximately 30% of cases where the appendix has become gangrenous, recovery is complicated by abdominal/pelvic abscess formation. It is in this situation that radiologists have a therapeutic role to play with percutaneous CT or US guided drainages.


Recognised complications include 5:

Differential diagnosis

Clinically, the most common differential is that of mesenteric adenitis, which can be differentiated by the identification of a normal appendix and enlarged mesenteric lymph nodes.

The imaging differential includes:

Practical points

  • on CT, identify first the ileocaecal valve, which usually has lipomatous walls, and then look for the appendix

  • >6 mm outer diameter is a reliable measurement to characterise appendicitis in all imaging modalities

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