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 localises 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:
- localised pain and tenderness
- nausea and vomiting
- atypical location: within the pelvis (30%), extraperitoneal (5%)
The Alvarado score is a clinical score that can be useful to help risk stratify.
Appendicitis is typically caused by obstruction of the appendiceal lumen, with the resultant build-up of fluid, secondary infection, venous congestion, ischaemia and necrosis. Obstruction may be caused by 1:
- lymphoid hyperplasia (~60%)
- appendicolith (~33%)
- foreign bodies (~4%)
- Crohn disease or other rare causes, e.g. stricture, tumour, parasite
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 ileocaecal 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 an extensive range (2-20 cm) 8. The distribution of positions is 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 present in ~40% of perforations.
Ultrasound with its lack of ionising 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 thickness, with gradual increasing pressure exerted to displace normal overlying bowel gas.
Findings supportive of the diagnosis of appendicitis include 4:
- aperistaltic, non-compressible, dilated appendix (>6 mm outer diameter)
- distinct appendiceal wall layers
- 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 10
- periappendiceal fluid collection
- target appearance (axial section)
- periappendiceal reactive nodal prominence/enlargement
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. 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 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 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 11.
CT 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
- peri-appendiceal 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
MRI is recommended as the second line modality for suspected acute appendicitis in pregnancy patients, where available 13,14. 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 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 ~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:
- perforation: in up to 13-30% of cases
- abscess formation: appendiceal abscess
- generalised peritonitis
- pylephlebitis: infective thrombophlebitis of the portal circulation
- complicating hepatic abscess
The imaging differential includes:
- inflammatory bowel disease, especially Crohn disease, which may affect the appendix
- other causes of terminal ileitis
- appendiceal mucocoele
- lymphoid hyperplasia
- pelvic inflammatory disease (PID)
- right-sided diverticulitis
- Meckel diverticulitis
- acute epiploic appendagitis
- omental infarction
- appendiceal malignancy
- 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 12
on CT, identify first the ileocaecal valve, which usually has fatty lips, 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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The appendix can be affected by numerous inflammatory, infectious and neoplastic conditions:
- appendiceal mucocele
- appendiceal intussusception
neoplasms of the appendix
- appendiceal epithelial neoplasms
- Goblet cell carcinoid of the appendix
- appendiceal neuroendocrine tumours
- appendiceal lymphoma