Current Issue - 2007, Volume 2 Number 3

REVIEW ARTICLE

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IMAGING AS AN AID TO THE DIAGNOSIS OF ACUTE APPENDICITIS

Ultrasonography
Ultrasound (US) has been in regular use for nearly four decades as a diagnostic tool for the investigation of right lower quadrant pain in gynaecology but had no place of prominence in the investigation of similar pain in general surgical practice. Its use to visualise the inflamed appendix during early attempts to do so was limited by the interference of overlying gas-filled loops of intestines. More recently, however, the technique of graded compression has been used to overcome this handicap by displacing such loops of bowel away from the appendix without undue discomfort to the patient; so the virtues of ultrasound scanning to establish the diagnosis of acute appendicitis are clearer today.27,28 Ultrasound appearances of the inflamed appendix are rather characteristic in that the inflamed organ is usually non-compressible and there is dilatation of the lumen. Less often, thickening of the neighbouring caecum as well as the presence of peri-appendiceal fluid might be noticed.

A marked reduction in the negative appendicectomy rate was shown after the introduction, in recent years, of ultrasound imaging for the diagnosis of acute appendicitis.27 Ultrasound is relatively inexpensive, can be arranged with speed, does not need injection of contrast, and is safe even when investigating a pregnant patient. During the examination, the appendix is identified as a blind-ending, nonperistaltic loop of bowel attached to the caecum. The investigation is considered positive for acute appendicitis if the appendix measures 6 mm or more in diameter and even more likely with thickening of the appendiceal wall itself and the demonstration of periappendiceal fluid. The demonstration of an appendicolith is also highly suggestive of the diagnosis.21

Diagnosis of acute appendicitis during pregnancy is notoriously difficult. Some of the early symptoms such as nausea and vomiting might unfortunately be mistaken for features of pregnancy itself as would an elevated neutrophil count. The displacement upwards and sideways of the appendix by the enlarging uterus adds to the diagnostic difficulty in that the area of maximum tenderness on the abdominal wall may be well away from McBurney’s point. Furthermore, the laxity of the abdominal wall associated with pregnancy often prevents the development of the expected rebound tenderness and guarding. Ultrasound of course would be a useful investigation to resolve such diagnostic doubt under these circumstances, enabling early surgery since foetal mortality rises sharply if the appendix perforates during pregnancy.4

In the early days, the likelihood of acute appendicitis was excluded if the appendix was not visualised on ultrasound but today such a study would be considered inconclusive and other imaging modalities recommended. If acute appendicitis is excluded sonograpically, a survey of the remainder of the abdomen and pelvis would usually be done for evidence of other pathology to account for the patient’s clinical features. The sonographic diagnosis of acute appendicitis has a reported sensitivity of 86 to 96 per cent and a specificity of 85 to 98%.29,30,33

Computed tomography
Computed tomography, mostly in the form of helical scanning is now more widely used to diagnose acute appendicitis, particularly in North America. In a CT scan, as indeed it does on US, the inflamed appendix appears swollen with a thickened wall and with peri-appendiceal fat stranding. Other evidence of acute inflammation such as thickened mesoappendix and periappendiceal fluid is also looked for. Faecoliths, where present, are rather easily visualised. As with US, CT scanning may also help identify other intra-abdominal inflammatory processes that mimic acute appendicitis.

There is documented improvement in diagnostic accuracy with the use of CT scanning in patients suspected to have appendicitis. It has yielded even higher figures for accuracy compared to US.31 The use of CT resulted in a lowering of the negative appendicectomy rate as observed in several studies32 whereas in women an even more remarkable reduction from 24% to 5% was achieved.33 There are however some obvious disadvantages in that CT scanning is still relatively expensive, exposes the patient to significant radiation, and of course is contraindicated in pregnancy.

CONCLUSION
Acute appendicitis still remains largely a clinical diagnosis supported at times by basic laboratory investigation. Where there is doubt, it is justifiable, indeed advisable, to resort to the use of imaging techniques to clear such doubt rather than subject the patient to an unnecessary appendicectomy. In the same vein, undue delays in performing early surgery on deserving patients would of course be avoided.

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