Acute appendicitis

Last revised by Liz Silverstone on 28 Jul 2023

Acute appendicitis (plural: appendicitides) is an acute 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 may be simple and uncomplicated or complex, leading to gangrene and perforation 35. Chronic appendicitis is an uncommon entity 25. Acute appendicitis may be simple/uncomplicated or complicated by gangreneabscess, or perforation

If status post appendectomy, then stump appendicitis may still occur.

If the appendix failed to descend normally during development then a subhepatic appendicitis may be seen.

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 the 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:

Several clinical prediction and decision scores (rules) have been developed to improve diagnostic accuracy and reduce the rate of negative appendectomies, some of which are in routine clinical use:

Appendicitis is typically caused by obstruction of the appendiceal lumen, with the resultant build-up of fluid, suppurative inflammation, secondary infection, venous congestion, ischemia, and necrosis. Obstruction may be caused by 1,23:

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

Fecal loading of the cecum is associated with acute appendicitis, which is uncommon in other acute inflammatory diseases of the right side of the abdomen 24.

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%

Appendiceal diameter >6 mm has been commonly used as a cut-off for acute appendicitis but this has been shown as a sensitive but not specific sign of acute appendicits. The normal range of appendiceal diameter has been shown to be 2-13 mm (adults) 27 and 3-9 mm (pediatric) 28.

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

    • ultrasound measurements have been shown to be 1-2 mm less than CT measurements 34

  • hyperechoic appendicolith with posterior acoustic shadowing

  • distinct appendiceal wall layers

    • implies non-necrotic (catarrhal or phlegmon) 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

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 dynamic ultrasound technique using a sequential 3-step patient positioning protocol has been shown to increase the detection rate of appendix 10. In the study, patients were initially examined in the conventional supine position, followed by the left posterior oblique (LPO) position (45° 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 detection 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. Nonetheless, many radiologists advocate the use of oral contrast in patients with a low BMI (<25).

CT findings include 1,2,4:

  • appendiceal diameter is increased in acute appendicitis 32

    • ≥8-9 mm outer-to-outer diameter has been suggested as a cut-off value 30,33 but note this overlaps with the upper limit of normal appendiceal diameter (~9.5 mm) 31,32

  • wall thickening (>3 mm) and enhancement

  • thickening of the cecal apex: cecal bar sign, arrowhead sign

  • intraluminal fluid depth >2.6 mm in a dilated (>6 mm) appendix without periappendiceal inflammation 29

  • periappendiceal inflammation

  • focal wall non-enhancement representing necrosis (gangrenous appendicitis) and a precursor to perforation

Less specific signs may be associated with appendicitis:

MRI is recommended as the second-line modality for suspected acute appendicitis in pregnant 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 and perforated, initial nonoperative management is preferred provided the patient is stable. In this situation, radiologists have a therapeutic role to play with percutaneous CT- or US-guided drainages of periappendiceal abscess.

Recognized complications include 6:

When a complication occurs, it is said to be "complicated appendicitis".

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 more inferiorly on the same side

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

  • inflammation may be initially limited to the distal end of the appendix (tip appendicitis). It is crucial (particularly with US) to completely evaluate the appendix, and consider further assessment with cross-sectional imaging if it is only partially visualized, but the patient is clinically suspicious 

  • prior appendectomy does not completely rule out a recurrent stump appendicitis, the risk of which is significant if the appendiceal remnant is greater than 5 mm

  • appendiceal endometriosis is not uncommon, affecting 4-22% of patients with endometriosis, and is a challenging diagnosis on imaging. Nodular, inhomogeneous appendiceal thickening combined with non-specific, often cyclical symptoms can be hints of this condition 23

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Cases and figures

  • appendix position (diagram)
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  • macroscopic pathology
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  • macroscopic pathology
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  • Case 1: on ultrasound
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  • Case 2: appendicolith as seen on ultrasound
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  • Case 3: tip appendicitis
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  • Case 4: tip appendicitis
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  • Case 5 : acute appendicitis seen on transvaginal ultrasound
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  • Case 6: with perforation
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  • Case 7: on ultrasound
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  • Case 8: with appendicolith
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  • Case 9: with appendicolith and abscess formation
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  • Case 10: subhepatic appendicitis
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  • Case 11: appendicular abscess
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  • Case 12: acute appendicitis pre and post-op
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  • Case 13: retrocecal with appendicolith
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  • Case 14: long acute appendicitis anteromedial to uterus
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  • Case 15: acute appendicitis with long appendix
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  • Case 16: in pregnancy with rupture into perinephric space
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  • Case 17: appendicitis as seen on MRI in pregnancy
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  • Case 18: MRI in pregnancy
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  • Case 19: MRI in pregnancy
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  • Case 20
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