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.
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Terminology
Acute appendicitis (plural: appendicitides) may be simple and uncomplicated or complex, leading to gangrene, abscess, or perforation 35. Chronic appendicitis is an uncommon entity 25.
If status post appendicectomy, then stump appendicitis may still occur.
If the appendix fails to descend normally during development then subhepatic appendicitis may be seen.
Epidemiology
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 tumour 1. Appendicitis is the most frequent non-obstetric emergency in pregnancy and is associated with 10% foetal mortality and 0.5% maternal mortality. The gravid uterus may prevent the omentum from walling off the appendix.
Clinical presentation
The classical presentation consists of referred periumbilical pain (T10) which within a day or two localises 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:
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localised pain and tenderness
the most common locations are retrocaecal (65%) or pelvic (30%)
rebound tenderness over the appendix (e.g. RIF: McBurney sign)
pelvic pain, diarrhoea, and tenesmus (pelvic appendix)
flank pain (retrocaecal appendix)
groin pain - appendix within an inguinal hernia (Amyand hernia) or a femoral hernia (De Garengeot hernia)
right upper quadrant pain (subhepatic appendicitis or mobile caecum) 22
pain in the left abdomen (non-rotation or situs inversus)
chest pain (diaphragmatic hernia)
nausea and vomiting
loss of appetite
constipation
Lab testing often reveals leucocytosis 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 appendicectomies, some of which are in routine clinical use:
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in children, clinicians sometimes use other scores for the same purpose:
Pathology
Appendicitis is frequently caused by obstruction of the appendiceal lumen. The appendix continues to secrete mucus which raises intra-luminal pressure causing ischaemia, initially antimesenteric, and subsequent gangrene and perforation. Stasis also causes bacterial overgrowth and gas formation. The biofilm and glycocalyx are penetrated allowing bacterial invasion. Omental fat migrates to the RIF surrounding the inflamed appendix and a phlegmon, an abscess or free purulent fluid may be observed. Age and the presence of an appendicolith are important risk factors for perforation. Obstruction may be caused by 1,23:
lymphoid hyperplasia, predominantly in young patients (~60%)
appendicolith (~33%)
faecolith
foreign bodies (~4%)
Crohn disease or other rare causes, e.g. stricture, tumour, parasite
appendiceal tumour (usually in patients over 50 years old)
Radiographic features
One of the biggest challenges of imaging the appendix is finding it. Equally important is to recognise 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 cm in length, so it is important to consider the ancillary findings of obstruction, ischaemia, 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 viral infection.
Faecal loading of the caecum 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 ileocaecal valve and the apex of the caecum. This relationship is maintained even when the caecum is mobile. Possible locations are listed above.
Plain radiograph
Plain radiography is infrequently performed due to a 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 caecal 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
Ultrasound with its lack of ionising 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 gradually increasing pressure exerted to displace normal overlying bowel gas. Changes in position may also help to increase the visualisation rate.
Findings supportive of the diagnosis of appendicitis include 5:
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target (Bull's eye) sign on transverse view indicates bowel wall thickening in the context of acute inflammation 41
fluid-filled hypoechoic centre
echogenic submucosa
hypoechoic muscularis propria
aperistaltic, non-compressible, fluid-filled blind-ending tube
>6 mm outer diameter (ultrasound measurements are 1-2 mm less than CT measurements) 34
hyperechoic appendicolith with posterior acoustic shadowing
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identification of wall layers
normal 5 layers imply 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
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wall thickening (3 mm or above)
mural or extramural hyperaemia with colour flow Doppler increases the specificity 17
vascular flow may be absent in a necrotic segment
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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 visualised structure is the appendix is clearly essential and requires demonstration of it being blind-ending and arising from the base of the caecum. 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 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
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:
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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 caecal apex: caecal bar sign, arrowhead sign
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periappendiceal inflammation
fat stranding
thickening of the lateroconal fascia or mesoappendix
extraluminal fluid, especially if complex
phlegmon (inflammatory mass)
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
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 and prognosis
Treatment is appendicectomy, 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 generalised 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 >3 cm. Smaller collections may be managed with antibiotics and interval appendicectomy.
Complications
Recognised complications include 6:
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perforation: in 10-20% of cases and suggested by peritonitis and imaging findings: 21
most specifically, appendiceal abscess or extraluminal gas; also periappendiceal phlegmon and fluid 20
generalised peritonitis due to free perforation
pylephlebitis: infective thrombophlebitis of the portal circulation
necrotising fasciitis
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:
inflammatory bowel disease, especially Crohn disease, which may affect the appendix
other causes of terminal ileitis
right-sided diverticulitis
acute epiploic appendagitis
obstruction by appendiceal or caecal tumour
isolated appendiceal submucosal lipomatosis 26
Valentino syndrome (from perforated peptic ulcer)
enlarged normal appendix as almost 50% of asymptomatic patients can have an appendix diameter greater than 6 mm on CT 13
Practical points
on CT, identify first the ileocaecal valve, which often has fatty lips, and then look for the appendix arising about 2 cm 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 appendicectomy 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 appendicectomy (i.e. removal of a normal appendix) rate. This is important because appendicectomy 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 tumour
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, faeces and mucus
when obstructed, continued mucus secretion fills and dilates the appendix causing ischaemia, (fluid-filled, dilated appendix)
inflammation, swelling, increased fat and hyperaemia 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.