Appendicitis is inflammation of the vermiform appendix. It is a very common condition in general radiology practice and is one of the main reasons for 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.

The classical presentation consists of periumbilical pain (referred) which within a day or later localizes to McBurney point with associated 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 non-specific 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
  • localized pain and tenderness
    • right lower quadrant pain over appendix (i.e. McBurney sign)
    • pelvic pain, diarrhea and tenesmus (pelvic appendix)
    • flank pain (retrocecal appendix)
    • groin pain - appendix within an inguinal hernia (Amyand hernia) or a femoral hernia (De Garengeot hernia)
  • leukocytosis
  • nausea and vomiting
  • atypical location:
    • within the pelvis (30%)
    • extraperitoneal (5%)
    • left iliac fossa (rare), found in patients with a long appendix, intestinal malrotation, situs inversus and those with a mobile cecum

The Alvarado score is a clinical score that can be useful risk stratify patients. In children clinicians sometimes use other scores such as a PAS or pARC score 3 for the same purpose.

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

One of the biggest challenges of imaging the appendix is finding it. Once confidently identified, assessing its normality is relatively straightforward.

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

The location of the tip of the appendix is much more variable, especially as the length of the appendix has an extensive range (2-20 cm) 9. The distribution of positions is described as 8,9:

  • behind the cecum (ascending retrocecal): 65%
  • inferior to the cecum (subcecal): 31%
  • behind the cecum (transverse retrocecal): 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 cecal gas with mural thickening may be evident.

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

Ultrasound with its lack of ionizing radiation should be the investigation of choice in young patients. With a competent user, ultrasonography is reliable at identifying abnormal appendices, 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, using the linear probe over the site of maximal tenderness, with gradual increasing pressure exerted to displace normal overlying bowel gas.

Findings supportive of the diagnosis of appendicitis include 5:

  • aperistaltic, non-compressible, dilated appendix (>6 mm outer diameter)
    • appears round when compression is applied
  • hyperechoic appendicolith with posterior acoustic shadowing
  • distinct appendiceal wall layers
    • implies non-necrotic (catarrhal or phlegmone) stage
    • loss of wall stratification with necrotic (gangrenous) stages 18
  • echogenic prominent pericaecal and periappendiceal fat
  • periappendiceal hyperechoic structure: amorphous hyperechoic structure (usually >10 mm) seen surrounding a non-compressible appendix with a diameter >6 mm 11
  • periappendiceal fluid collection
  • target appearance (axial section)
  • periappendiceal reactive nodal prominence/enlargement
  • wall thickening (3 mm or above)
    • mural hyperemia with color flow Doppler increases the specificity 17
    • vascular flow may be lost with necrotic stages
  • alteration of the mural spectral Doppler envelope 16
    • may support diagnosis in equivocal cases
    • a peak systolic velocity > 10 cm/s suggested as a cutoff
    • a resistive index (RI) measured at > 0.65 may be more specific

Confirming that the structure visualized is the appendix is clearly essential and requires demonstration of it being blind-ending and arising from the base of the cecum. Identifying the terminal ileum confidently is also helpful.

A recently described dynamic ultrasound technique using a sequential 3-step patient positioning protocol has been shown to increase the visualization rate of the appendix 10. 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 visualization 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 also to be diagnosed. The need for contrast (IV, oral or both) is debatable and varies from institution to institution. Oral contrast has not been shown to increase the sensitivity of CT 12.

CT findings include 1,2,4:

  • dilated appendix with distended lumen ( >6 mm diameter) 4
  • thickened and enhancing wall
  • thickening of the cecal apex (up to 80%): cecal 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
  • periappendiceal reactive nodal prominence/enlargement
  • non-enhancement of the mucosa representing necrosis and a precursor to perforation

MRI is recommended as the second line modality for suspected acute appendicitis in pregnancy patients, where available 14,15. Protocols vary widely, but most include imaging in three planes with a rapidly acquired sequence with T2 weighting, and some include T2 fat-suppressed imaging. MRI findings mirror those of other modalities, with luminal distension and widening, wall thickening and periappendiceal free fluid.

Treatment is appendectomy, which can be performed either open or laparoscopically 6. Mortality from simple appendicitis is approximately 0.1% but is as high as 5% in perforation with generalized peritonitis 6.

In ~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.

Recognized complications include 6:

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:

  • on CT, identify first the ileocecal valve, which usually has fatty lips, and then look for the appendix

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

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Article information

rID: 922
Synonyms or Alternate Spellings:
  • Acute uncomplicated appendicitis
  • Acute appendicitis

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Cases and figures

  • Figure 1: macroscopic pathology
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  • Figure 2: appendix position (diagram)
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  • Case 1: with thickened appendix
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  • Case 2: appendicolith
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  • Appendicolith:

    Case 3: with appendicolith
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  • Case 4
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  • Case 5
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  • Case 6
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  • Case 7
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  • Case 8
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  • Right lower quadr...
    Case 9
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  • Case 10
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  • Case 11: retrocecal
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  • Case 12: with ovarian vein thrombophlebitis
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  • Case 13: with subsequent abscess
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  • Pelvic CT

    Case 14: pelvic abscess with appendicolith and gas
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  • Perforated append...
    Case 15: perforated with appendicolith
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  • Case 16
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  • Case 17: complicated by an appendiceal abscess
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  • Case 18: terminal appendicitis
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  • Right adnexal ten...
    Case 19: on transvaginal ultrasound
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  • Case 20: terminal appendicitis on ultrasound
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  • Case 21
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  • Case 22
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  • Case 23: with perforation
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  • Case 24: pediatric acute appendicitis
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  • Case 25: with appendicolith
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  • Case 26: with localized perforation and abscess
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  • Case 27
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  • Case 28
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  • Case 29: MRI in pregnancy
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