REVIEW ARTICLE
IMAGING AS AN AID TO THE DIAGNOSIS OF ACUTE APPENDICITIS
As would be expected, abdominal pain is the most common presenting symptom. The classic sequence of central abdominal pain shifting to the right lower quadrant of the abdomen usually within 12 to 24 hours of onset, sometimes even earlier, followed at times by vomiting, as first described by Murphy13, however occurs in less than half of the patients.14 It is also well-known that the variable anatomical position of the appendix contributes to the actual site of maximal pain and tenderness thus confusing the clinical picture even further,15 e.g. pain and tenderness in the right loin when the inflamed organ is retrocaecal or retrocolic in position. Other often mentioned clinical features such as constipation or diarrhoea are somewhat less constant. The presence of a low-grade fever (up to 38°C) with tachycardia is relatively common but a significant elevation of temperature is present in less than a fifth of patients.14
Examination of the abdomen in most patients usually reveals localised tenderness and guarding in the right iliac fossa. Rebound tenderness is often present but looking for percussion tenderness would be a more humane way of gathering the same information. The patient often lies rather still too since any movement exacerbates the pain.16 The site of maximum tenderness is usually at McBurney's point as first observed by McBurney himself.17 Findings on per rectal or per vaginal examination may be normal, although tenderness towards the right may be felt particularly if the inflamed appendix happens to lie in the pelvic position. A positive Rovsing's sign (where deep palpation of the left iliac fossa causes pain in the right lower quadrant), a positive psoas sign (where the patient is seen to lie in bed with the right hip flexed for some relief of pain since the inflamed appendix lies on the right psoas muscle) and a positive obturator sign (pain felt in the hypogastrium on flexion and internal rotation of the right hip) may help in the diagnosis of less obvious cases.
If the diagnosis of appendicitis is clear from the history and clinical examination at the time the patient is first seen, then no further investigation is necessary and prompt surgical treatment is indicated18 but since that is so in only about half of the patients,8 acute appendicitis can be one the most difficult diagnoses to make with certainty resulting in delays in treatment, unnecessary hospital admissions for observation and at times unnecessary surgery.
BLOOD TESTS
Laboratory tests, particularly the white blood cell count (WBC) are often done in the initial evaluation of patients with right lower quadrant pain in order to confirm or exclude the suspected diagnosis. Eighty per cent of patients with acute appendicitis are found to have a leucocytosis and 80% of these harbour a neutrophilia greater than 75%. This, being a sensitive indicator of underlying inflammation, was almost universally used to aid the diagnosis of acute appendicitis early on but on its own has a low specificity and thus low predictive value for appendicitis, being evident in such a large number of other acute inflammatory conditions causing similar pain in the right lower quadrant.19 Added to this is the interesting but unfortunate fact that the proportion of gangrenous and perforated appendixes in patients with a normal white cell count may be the same as in those with a raised count.20 C-reactive protein (CRP) can also be used. A normal pre-operative CRP in a patient clinically suspected to have acute appendicitis is most often associated with a normal appendix but if raised along with at least one other inflammatory marker the diagnosis of acute appendicitis becomes more likely16.
The overall diagnostic accuracy achieved by the history, physical examination and laboratory tests even at the present time is only about 80 per cent, at best.21
IMAGING
Diagnostic imaging is not among the recommended routine where the clinical assessment points to a clear diagnosis of acute appendicitis but where the latter is equivocal, making use of imaging to clarify the diagnosis may be considered. A few newer radiologic modalities have been shown to improve patient outcomes.22
Plain abdominal radiography
Plain abdominal radiography, being the most readily available among them was naturally the first investigation thus used. Localised ileus in the right lower quadrant of the abdomen detected as a dilated sentinel loop of bowel, air-fluid levels in loops of bowel in the right lower quadrant on an erect abdominal film, an increase in the soft tissue density in the same area, obliteration of the psoas shadow or lumbar scoliosis may be detectable in up to half of the patients with early acute appendicitis.23 An opaque faecolith may be apparent in the right lower quadrant in fewer than 5% of patients subject to abdominal radiography. Plain radiography, however, has a low sensitivity and specificity for the diagnosis of acute appendicitis and can even be misleading.24 So, generally speaking, plain abdominal radiographs are not really recommended unless other conditions (e.g. perforation of a viscus, intestinal obstruction, ureteric calculus) are considered in the differential diagnosis.7
Likewise, as other more convenient imaging techniques are available, barium enema is now used infrequently25 although there was a tendency to use barium studies to exclude the condition in the past.
