British Journal of Radiology 75 (2002),775-781 © 2002 The British Institute of Radiology
Appendicitis: spectrum of appearances on helical CT
T C See, FRCS, FRCR1,
C S Ng, MRCP, FRCR1,
C J E Watson, MD, FRCS2 and
A K Dixon, MD, FRCP, FRCR1
University Departments of 1 Radiology and 2 Surgery, Addenbrooke's NHS Trust and the University of Cambridge, Hills Road, Cambridge CB2 2QQ, UK
Correspondence: C S Ng, Department of Radiology, Box 57, MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030-4095, USA
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Abstract
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Acute appendicitis has extremely varied clinical presentations. A delayed or missed diagnosis may result in severe adverse consequences. Helical CT is evolving as an important diagnostic aid, but the CT signs can be varied and can easily be overlooked by the unwary. This pictorial review illustrates the spectrum of radiological signs and appearances of appendicitis on helical CT.
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Introduction
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Acute appendicitis is one of the commonest causes of an "acute abdomen" and appendicectomy is the commonest emergency surgical operation performed worldwide [1].
The variable anatomical location of the appendix contributes to the multiplicity of clinical presentations of appendicitis (which are atypical in up to 30% [2]). In particular, appendicitis is able to masquerade as many other conditions, especially in the female patient. Because of the potentially severe consequences of delayed intervention, early surgery is usually indicated. To some extent this is at the expense of "negative" appendicectomies, which are reported to be as high as 45% in women of child-bearing age [3]. Nevertheless, perforation rates in the region of 20% are still reported [3].
Imaging techniques such as ultrasound and CT offer the potential to improve clinical outcome by increasing the accuracy of diagnosis. Ultrasound has the great advantage of being radiation free, however it has the relative disadvantages of being operator dependent and being limited in sensitivity [4].
In comparison, CT has greater sensitivity in the diagnosis of acute appendicitis, with reported accuracies of 93–98% [5, 6]. CT is also able to detect the presence and severity of complications, and in "negative" cases it may be able to detect alternative diagnoses. Furthermore, CT is a rapid technique and it has been shown to be cost effective [7].
However, CT diagnosis of acute appendicitis can be challenging, with signs ranging from subtle findings associated with early appendiceal inflammation to extensive abnormalities when complications supervene. This pictorial review illustrates the spectrum of radiological signs and appearances of appendicitis on helical CT. All cases were proven at surgery and histopathology.
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CT protocol
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The CT protocol for imaging suspected acute appendicitis is controversial and is undergoing change with the advent of multidetector CT systems. In particular there is debate regarding the value of intravenous (iv), oral and rectal contrast media, as well as the use of thin section collimation (e.g. 5 mm compared with 10 mm) [5, 6, 8–10]. However, whichever technique is used, consistently high accuracies (93–98%) are reported.
The advantages of adopting a CT protocol that does not utilize contrast medium include: ability to examine patients without introducing delays whilst waiting for contrast medium preparation (a potentially important consideration in ill patients); avoidance of possible masking of appendicoliths; elimination of the risks of adverse contrast-related reactions; and cost savings. However, interpretation of examinations without contrast media can be difficult, especially in patients with little intra-abdominal fat. In principle, good bowel opacification allows better identification of the relevant appendiceal and caecal apical changes; and iv contrast medium is able to highlight appendiceal wall enhancement and thickening. A limitation of such a strategy, however, is that the ill patient may not be able to tolerate oral or rectal contrast media. There are also compromises in consideration of section collimation: thin collimation provides better spatial resolution but increases radiation dose.
At our institution we typically utilize oral and iv contrast media with a section collimation of 5 mm (10 mm was used previously) and a helical pitch of 1.5.
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Normal anatomy
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The healthy appendix is mobile and variable in length (up to 20 cm [11]). It is usually retro-ileal; previous reports of the appendix usually being retrocaecal relate to cadaveric studies [12]. A small minority are paracaecal, pre-ileal or, rarely, subhepatic (in cases of arrested caecal descent), or they may even herniate through the inguinal canal. The appendix is not uncommonly closely related to the right ureter and psoas muscle.
On CT the appendix appears as a smooth, thin walled, tubular structure surrounded by mesenteric fat. It is generally considered that the transverse diameter should not exceed 6 mm, although some authors take an upper limit of 10 mm [10].
Normal appendices can be identified in 52–75% of abdominopelvic CT studies, the higher value being obtained when imaging with thinner collimation and reconstruction intervals [13] (Figure 1
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Figure 1. Normal appendix. Non-distended, thin walled, retrocaecal appendix with normal surrounding fat (arrows). Note caecum (c), right common iliac vessels (v), ureter (u) and psoas muscle (p).
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Pathophysiology and natural history of appendicitis
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The underlying cause of appendicitis remains uncertain. However, occlusion by fecoliths or lymphoid hyperplasia appear to be a significant factor. Inflammation leads to wall thickening and distension, which gives rise to one of the key signs on CT.
If the inflammatory process is gradual, localization of sepsis by the small bowel and omentum forms an appendix mass, which may proceed to abscess formation.
Less commonly, if the inflammatory process advances rapidly it may lead to arterial thrombosis resulting in a gangrenous appendix and subsequent perforation and peritonitis. Delayed diagnosis and extremes of age are important predisposing factors for perforation. Early detection of appendicitis may reduce the perforation rate, as it is unusual within the first 12 h.
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CT appearances of early (uncomplicated) appendicitis
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The two diagnostic CT signs of acute appendicitis are unequivocal identification of an abnormal appendix, which is usually fluid-filled with a slightly thickened and circumferentially enhancing wall and dilated more than 6 mm in transverse diameter (Figure 2), or an appendicolith associated with pericaecal inflammatory stranding [4, 6, 9] (Figure 3).
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Figure 2. Typical appendicitis. 22-year-old man with intermittent abdominal pain and several admissions for possible appendicitis. CT with intravenous contrast medium shows a dilated appendix with an enhancing thickened wall (arrow).
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Figure 3. Appendicolith with inflammation. 53-year-old man with 3-day history of right iliac fossa pain, clinically not thought to be appendicitis. CT shows an appendicolith with peri-appendiceal fat stranding (arrow).
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The presence of an appendicolith alone without surrounding inflammation (Figure 4), or failure to visualize an abnormal appendix or appendicolith in the presence of pericaecal inflammatory changes, is suspicious but not diagnostic [4, 6, 9]. Similarly, findings of peri-appendiceal inflammation, including peri-appendiceal fluid collections, thickening of the appendiceal mesocolon, surrounding pararenal fascia or right lateroconal fascia (Figure 5) are suggestive signs, but unfortunately other inflammatory conditions in the lower abdomen or pelvis may give rise to similar appearances. Other CT findings that are suggestive of acute appendicitis include focal caecal apical thickening and the "arrowhead" sign (Figure 6) [6, 14].
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Figure 4. Appendicolith without inflammation. 12-year-old boy with three episodes of right iliac fossa pain in 4 months. Each episode settled with conservative treatment. CT shows an appendicolith without peri-appendiceal inflammation (arrow). Nevertheless, surgery and histology confirmed appendicitis.
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Figure 5. Lateral conal fascial thickening. 60-year-old man with 4-day history of peri-umbilical pain radiating to the right iliac fossa. CT with intravenous contrast medium shows a distended retrocaecal appendix with an enhancing wall (single arrow), together with thickening of the lateral conal fascia (paired arrows). c, caecum.
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Figure 6. "Arrowhead" sign. 64-year-old man with 3-day history of colicky lower abdominal pain and right iliac fossa tenderness. CT shows an "arrowhead" sign (arrow) with peri-appendiceal inflammation. The arrowhead sign results from the accumulation of gastrointestinal contrast medium between a symmetrically thickened caecal apex; the intraluminal collection of contrast "points" at the occluded orifice of the appendix.
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CT appearances of advanced appendicitis and its complications
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Gradual progression of appendicitis typically leads to adherence of the omentum and small bowel to the inflamed appendix, resulting in an inflammatory phlegmon-like mass (Figures 7 and 8). The appendix itself may be difficult to identify.
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Figure 7. Inflammatory mass. 37-year-old man with a 2-day history of right iliac fossa pain and two similar episodes 1 month and 1 year previously. CT following intravenous contrast medium shows a dilated appendix with a thickened enhancing wall (arrow) and an inflammatory mass. Note that the mass abuts the right psoas muscle (p).
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Figure 8. Inflammatory mass displacing adjacent structures. Same patient as Figure 6 . CT shows (a) fascial thickening in right paracolic gutter (arrows) and (b) inflammatory mass (arrows) surrounding an inflammed appendix, displacing bladder, labelled b, to the left.
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Should diagnosis be delayed, inflammatory masses may progress to collections (Figure 9), or inflammation may spread to adjacent structures such as the psoas muscle (Figure 10), pelvic organs and dependent intraperitoneal pouches (Figure 11). Collections and abscesses may also seed to unusual and remote locations, which adds to the varied CT appearances of advanced appendicitis (Figure 12a).
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Figure 9. Progression of inflammatory mass to an abscess collection. 17-year-old man with 2 days of vomiting and mild lower abdominal pain. (a) CT with intravenous (iv) contrast medium on admission shows a 4 cm phlegmonous soft tissue mass with surrounding fat stranding (arrow). Initial management plan was for antibiotic therapy to be followed by interval appendicectomy. (b) CT with iv contrast medium 9 days later, obtained because of clinical deterioration, shows a liquefied right iliac fossa abscess (arrow) with a locule of gas.
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Figure 10. Involvement of the psoas muscle. 6-year-old boy with a 6-day history of vomiting and intermittent pyrexia initially considered to be a viral infection. Pain subsequently localized to the right iliac fossa. Ultrasound examination was indeterminate. CT with intravenous contrast medium shows a distended appendix with a thick enhancing wall (single arrow) together with inflammatory involvement (paired arrows) of the right psoas muscle (p) (compare with left psoas muscle). A retrocaecal appendicitis and a psoas abscess were confirmed at surgery.
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Figure 11. Intra-abdominal abscesses. 32-year-old man with a 1-week history of diffuse abdominal pain, distension and vomiting. CT with intravenous contrast medium shows a large abscess collection with an enhancing wall in the rectovesical pouch, containing small locules of gas (arrow) (r, rectum; b, bladder). There were further collections posterior to the ascending colon and caecum, and anterior to the sigmoid colon (not shown). The walls of the caecum and the proximal ascending colon were also thickened, but the appendix was not identified at CT. Surgery revealed a perforated appendix and confirmed the large pelvic collection.
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Figure 12. Perforation and collections in unusual locations. Same patient as Figure 3. CT with intravenous contrast medium shows (a) collection in the left subhepatic space (arrow), anterior to the stomach (s) and (b) free intraperitoneal gas anterior to and posterior to the liver, and in the falciform fissure (arrows).
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In cases of a perforated acute appendicitis (Figure 12b), identification of the correct cause can be extremely challenging on CT, since in such cases it may not be possible to identify the appendix or there may only be non-specific CT signs (as with the case in Figure 11).
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CT or clinical entities that may mimic acute appendicitis
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A distended appendix on CT, with or without peri-appendiceal inflammation, may be seen in other appendiceal diseases, for example mucocele, carcinoid, carcinoma or mucus-secreting tumours of the appendix (Figure 13).
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Figure 13. Mucocele of the appendix mimicking appendicitis. 75-year-old man. CT shows a distended appendix without peri-appendiceal inflammation (arrows). Histopathology showed a distended appendix containing a mucus-secreting adenoma with no evidence of acute inflammation.
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Conversely, clinical entities that may mimic acute appendicitis include Crohn's disease, ureteric calculi, pyelonephritis, Meckel's diverticulitis, ovarian disease, ectopic pregnancy, caecal diverticulitis, epiploic appendagitis and segmental omental infarction [10, 15].
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False negative CT
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Reported false negative rates for acute appendicitis utilizing CT are up to 7% [15]. False negatives may arise when the appendix is not identified (a particular problem in thin individuals) or when it does not appear distended, or when there are only non-specific radiological signs or coincidental findings are (erroneously) attributed to the cause of pain (such as ovarian cysts). Clinical correlation is essential in these situations.
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Conclusions
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CT is highly beneficial in the investigation of suspected acute appendicitis. The pathognomonic CT signs of early appendicitis are a dilated appendix (transverse diameter >6 mm), and an appendicolith in the presence of abnormal peri-appendiceal fat. There are, however, a variety of other ancillary signs and careful evaluation is essential.
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Acknowledgments
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We would like to acknowledge the assistance of the CT radiographers, as well as the radiology and surgical registrars in assisting with this project.
Received for publication July 16, 2001.
Revision received November 30, 2001.
Accepted for publication December 6, 2001.
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Abstract
Introduction
