Acute inflammation of the vermiform appendix. It is one of the most common abdominal emergencies, affecting between 7 and 12 % of the population. On gross pathology the following stages of acute appendicitis can be distinguished:
- simple appendicitis where the inflamed appendix is still viable;
- gangrenous appendicitis characterized by focal and diffuse necrosis; and
- perforated appendicitis characterized by necrosis and destruction of the appendix.
The diagnosis of acute appendicitis is usually based on clinical symptoms and laboratory tests without the need for radiological imaging. However, about one third of patients with acute appendicitis show atypical clinical symptoms and physical findings. In this group of patients radiological imaging can play an important clinical role.
Until the advent of ultrasound (US) and CT plain radiography and barium studies were used for the diagnosis of acute appendicitis. Plain radiographic films are abnormal in 50% of cases but the findings are nonspecific. On barium studies complete opacification of the appendix is believed to exclude acute appendicitis. Nonfilling or incomplete opacification of the appendix in the presence of an extrinsic mass effect on the caecum generates a high index of suspicion for the diagnosis. Diagnostic accuracy of barium studies has been reported to be as high as 91%. However, nonfilling of the normal appendix may occur occasionally in normal patients and it may be difficult to distinguish a partially filled from a completely filled appendix.
Over the last decade the use of ultrasound as the primary diagnostic modality for acute appendicitis became widespread. When ultrasound transducer is used as a compressor to displace bowel loops and to compress the caecum, the normal appendix can be visualized in a high percentage of cases, and a high level accuracy has been reported in the diagnosis of acute appendicitis. A sensitivity of 75 - 90%, a specificity of 86 - 100 % and overall accuracy of 87 - 96% have been reached with ultrasound. Sonographic criteria for acute appendicitis (Fig.1) include:
- detection of a fluid-filled distended appendix with a diameter of more than 5 mm;
- a wall thickness of 3 mm or greater;
- absence of peristalsis and noncompressibility of the appendix; and
- pericaecal inflammatory changes.
Colour Doppler sonography and power Doppler sonography may show increased flow but perforated appendicitis or abscess may show an absence of flow. Sonographic pitfalls are related to retrocaecal location of the appendix, a perforated appendix, a gas-filled appendix, tumoural enlargement of the appendix or other conditions that may cause acute lower right quadrant pain such as infectious ileocaecitis, acute epiploic appendagitis, diverticulitis of the right colon, right-sided segmental infarction of the omentum and mesenteric adenitis.
The CT diagnosis of acute appendicitis is based on the visualization of an abnormal appendix (Fig.2) or of an appendicolith (Fig.3) with pericaecal inflammatory changes. The inflamed asymmetrically thickened wall of the appendix shows clear contrast enhancement following intravenous contrast medium injection. The inflammatory changes in the surrounding tissues are characterized by hazy linear strands in the mesenterial fat, fluid collections in the case of abscess or inhomogeneous soft tissue density in the case of phlegmon. The wall of the surrounding bowel loops, particularly of the ileocaecal segment may also show changes of secondary inflammation such as thickening or abnormal pattern of contrast enhancement following intravenous contrast medium administration. In some studies a higher sensitivity and accuracy was obtained for CT than for ultrasound in the same group of patients studied prospectively. In general CT is particularly useful for visualizing periappendiceal inflammatory masses. CT is the most reliable technique for differentiating between abscess and phlegmon subsequent to acute appendicitis. This distinction is clinically relevant as an abscess can be drained percutaneously while a periappendiceal phlegmon may be treated conservatively. Because of its excellent sensitivity and specificity ultrasound should be used as the primary diagnostic modality in infants, children and women of childbearing age suspected of acute appendicitis whereas CT should be used primarily if perforation or abscess formation is suspected because it will better depict the exact characteristics of the inflammatory mass (solid or liquefied), and its extent and location.
Gastrointestinal Imaging from GE Healthcare: Appendicitis, acute
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Appendicitis, acute, Fig.1
Appendicitis, acute, Fig.2
Appendicitis, acute, Fig.3
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