Acute appendicitis

Last revised by Andrew Murphy on 18 Mar 2024

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

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 has a lifetime incidence of about 7%. It is rare in infants less than 2 years old when the appendix is funnel-shaped. Maximum incidence is around 20 years old which coincides with peak appendiceal lymphoid tissue. Older adults have a higher incidence of perforation and underlying appendiceal tumor1. Appendicitis is the most frequent non-O & G emergency in pregnancy and is associated with 10% foetal mortality and 0.5% maternal mortality. The gravid uterus may prevent omentum from walling-off the appendix.

The classical presentation consists of referred periumbilical pain (T10) which within a day or two localizes to McBurney’s point in the right iliac fossa and is associated with rebound tenderness, fever, nausea, vomiting, tachycardia, raised bilirubin and inflammatory markers and other signs of peritonitis 2. This progression is unhelpful in children who often present with vague and non-specific signs and symptoms. It also relies on the appendix being in the right iliac fossa, however the appendix can be located anywhere in the abdomen and even in the thorax and groin (see below).

General signs and symptoms include 1,2:

Lab testing often reveals leukocytosis and an elevated CRP, and an elevated bilirubin may also be present 40.

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 frequently caused by obstruction of the appendiceal lumen. The appendix continues to secrete mucus which raises intra-luminal pressure causing ischemia, initially antimesenteric, and subsequent gangrene and perforation. Stasis also causes bacterial overgrowth and gas-formation. The biofilm and glycocalyx are penetrated and the appendiceal wall is invaded. Omental fat migrates to the RIF surrounding the inflamed appendix and a phlegmon, an abscess or free purulent fluid may be observed. Age and presence of an appendicolith are important risk factors for perforation. Obstruction may be caused by 1,23:

One of the biggest challenges of imaging the appendix is finding it. Equally important is to recognize the wide range of normal and look for associated findings. Appendicitis should not be diagnosed by size alone. Normal appendices can measure 13 mm in width and 35 cms in length, so it is important to consider the ancillary findings of obstruction, ischemia, inflammation and perforation. Obstruction is highly associated with perforation and complications whereas cases of ‘simple’ non-obstructive appendicitis may have a benign course, resolving spontaneously. These cases may be caused by viruses.

Fecal loading of the cecum is common in acute appendicitis, but uncommon in other inflammatory diseases of the right 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. Possible locations are listed above.

Plain radiography is infrequently performed due to lack of sensitivity and specificity. Look for free gas, gas locules in an abscess or an appendicolith (7-15% of cases) 1. In the right clinical setting, finding an appendicolith raises the probability of acute appendicitis 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 gas-fluid levels is present in ~40% of perforations.

Ultrasound with its lack of ionizing radiation should be the investigation of choice in young patients and should be considered in women of child-bearing age. With a competent user, ultrasonography is reliable at identifying abnormal appendices, especially in thin patients. However, the identification of a normal appendix is less consistent, 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. Changes in position may also help to increase the visualization rate.

Findings supportive of the diagnosis of appendicitis include 5:

  • aperistaltic, non-compressible, fluid-filled blind-ending tube

  • >6 mm outer diameter (ultrasound measurements have been shown to be 1-2 mm less than CT measurements) 34

  • hyperechoic appendicolith with posterior acoustic shadowing

  • identification of wall layers

    • normal 5 layers implies non-necrotic (catarrhal appendicitis)

    • loss of wall stratification implies necrosis 18

    • gas locules in appendicitis indicate gangrene

  • periappendiceal hyperechoic indurated fat (>10 mm) surrounding a non-compressible appendix with a diameter >6 mm 11

  • periappendiceal complex fluid collection

  • periappendiceal reactive lymphadenopathy

  • wall thickening (3 mm or above)

    • mural or extramural hyperemia with color flow Doppler increases the specificity 17

    • vascular flow may be absent in a necrotic segment

  • 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 is also helpful.

A dynamic ultrasound technique using a sequential 3-step patient positioning protocol has been shown to increase the detection rate of the appendix 10. In the study, patients were initially examined in the conventional supine position, followed by the left posterior oblique 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 to the left, away from the appendix.

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 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). IV contrast medium can demonstrate the presence or absence of infarction.

CT findings include 1,2,4:

  • increased appendiceal diameter in acute appendicitis 32

    • ≥8-9 mm outer-to-outer diameter has been suggested as a cut-off value 30,33 but note that this overlaps with the upper limit of normal appendiceal diameter (<13 mm) 31,32

  • wall thickening (>3 mm), enhancement and stratification if no gangrene

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

  • periappendiceal inflammation

  • focal wall non-enhancement of necrotic segment (gangrenous appendicitis), a precursor to perforation

  • the presence of intraluminal, intramural or periappendiceal gas locules with an obstructed appendix strongly suggests necrosis (gangrenous appendicitis) 36-38; this is in contradistinction to the presence of intraluminal gas in a normal-appearing appendix

Less specific signs may be associated with appendicitis:

  • appendicolith  (high risk of perforation)

  • periappendiceal reactive lymphadenopathy

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 T2-weighted sequence, 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 (laparotomy) 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 that the patient is stable. In this situation, radiologists have a therapeutic role to play with percutaneous CT- or US-guided drainage of periappendiceal abscesses > 3cms. Smaller collections may be managed with antibiotics and interval appendectomy.

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 often has fatty lips, and then look for the appendix arising about 2 cms more inferiorly on the same side

  • inflammation may be initially limited to the distal end of the appendix (tip appendicitis). Further assessment with CT or MR is indicated if the tip is not seen on US

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

  • endometriosis affects the appendix in 4-22% of cases and is a challenging diagnosis on imaging. Nodular, inhomogeneous appendiceal thickening combined with non-specific, often cyclical symptoms and hypervascularity can hint at this condition 23

  • CT has drastically reduced the negative appendectomy (i.e. removal of a normal appendix) rate. This is important because appendectomy has been linked with mood/anxiety disorders (if performed in childhood) 39, inflammatory bowel disease, cardiovascular disease, type 2 diabetes, Parkinson disease and Alzheimer disease, and the risk of recurrent Clostridioides difficile colitis is 4 times greater

  • the presence of obstruction and appendicolith is associated with perforation

  • increased age is associated with a much higher risk of perforation, as well as tumor

  • the normal appendix is variable: maximum wall thickness occurs around 20 years of age, reflecting maximum lymphoid tissue

  • the normal appendix contains normal intraluminal gas, feces and mucus

  • when obstructed, continued mucus secretion fills and dilates the appendix causing ischemia, (fluid-filled, dilated appendix)

  • inflammation, swelling, increased fat and hyperemia can be seen in and around the appendix

  • subsequent appearance of gas locules indicates bacterial overgrowth and gangrene

  • non-obstructive appendicitis can be caused by viral infection (simple, self-limiting), Salmonella (necrotic), inflammatory bowel disease, etc.

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