Intussusception is one of the most common causes of intestinal obstruction in early childhood. Despite this, it is often difficult to diagnose correctly on clinical grounds at first presentation to the medical practitioner. Historically, when intussusception has been suspected, a plain X ray of the abdomen has been the initial investigation, even though it is usually normal.
In recent years, the high sensitivity (over 98%) and specificity (almost 100%) of ultrasonography in experienced hands have made it the preferred initial imaging modality. Yet plain abdominal X rays are still performed routinely in many institutions. Both Nelson and Rudolph's paediatric textbooks recommend ultrasonography for suspected intussusception.[5, 6] A recent survey of European Paediatric Radiologists found that plain films were used in 72.5% of cases and ultrasonography at 93%. However, only 21% relied on ultrasonography alone, and 45.5% performed both plain X rays and ultrasonography. This raises the question as to what is the role of plain radiological imaging in the initial investigation of a child with suspected intussusception.
Use of a plain abdominal X ray as a diagnostic tool
Purported roles of a plain X ray of the abdomen in suspected intussusception:
- ■ Confirmation of the diagnosis: Sometimes, the leading edge of the intussusceptum can be seen.
- ■ Demonstration of small bowel obstruction
- ■ Pneumoperitoneum, which is taken as a contraindication to attempted enema reduction
However, the majority of plain X rays of the abdomen in intussusception are normal or have non-specific features. Potential signs of intussusception on plain film include: a mass, evidence of small bowel obstruction or paucity of bowel gas distally. The mass can be indistinct and poorly defined, or reveal a ‘target sign’ (a round soft tissue mass with concentric lucencies, representing the intussusception and mesenteric fat within) or the ‘crescent sign’ (a crescent of gas seen within the colon, representing the apex of the intussusceptum).[2, 8-12]
It has been well documented that the sensitivity and specificity of the plain X ray for intussusception are poor compared with ultrasonography. A recent Best Evidence Topic report found that the sensitivity of plain radiology ranges from 36 to 90%, and a specificity of 45–90%. Two studies have shown that 23–24% of patients with proven intussusception have a completely normal X ray.[9, 14] One study described a significant increase in the sensitivity (from 60.3 to 74.1%) and specificity (from 25.6 to 58.1%) by taking a decubitus view along with a plain frontal supine film view (KUB view) of the abdomen when compared with KUB view alone.
Although the sensitivity and specificity of plain radiology can be improved by careful choice of view, it still does not reach anywhere near the same levels of accuracy as an ultrasound examination (sensitivity 98%, specificity 100%). It also has the disadvantage of unnecessary radiation exposure in a young child. Public Health England estimates this to be 0.7 mSv for each abdominal film; the equivalent of 4 months background radiation exposure and for a paediatric population is estimated to be associated with a 1 in 15 000 additional lifetime risk of fatal cancer.
Use of a plain abdominal X ray to identify a small bowel obstruction
Demonstration of a small bowel obstruction is consistent with (but not diagnostic of) intussusception. Often, the same information can be obtained on the demonstration of a distended, mildly tender and tympanic abdomen with obstructive bowel sounds. Ultrasonography is a much more accurate method of demonstrating intussusception with a high specificity.
The problem is that demonstration of small bowel obstruction alone does not influence management. Many years ago, small bowel obstruction was considered a relative contraindication to attempting an enema reduction. However, there is now good evidence that even with barium enema reduction (which has a lower success rate than gas enemas), the presence of small bowel obstruction on plain X ray of the abdomen is simply an indicator of a lower likelihood of successful enema reduction but is not per se a contraindication to performing the enema. Bowel obstruction in itself should not influence the decision to attempt non-operative reduction. There is a correlation between radiological evidence of small bowel obstruction on X ray and longer duration of symptoms (as might be expected), and both can be predictive of the success rate of enema reduction; but neither feature on its own is an indication for surgery without a prior attempt at non-operative reduction of the intussusception. Most intussusceptions, in the presence of air fluid levels on abdominal radiology, are still reducible non-operatively.
Use of a plain abdominal X ray to detect pneumoperitoneum
There are no reports of pneumoperitoneum evident on plain X ray of the abdomen at presentation, with or without signs of peritonitis. Pneumoperitoneum is much more likely to occur during attempted reduction rather than on presentation. There are three cases reported of children in India who presented late with the operative finding of perforation of the bowel (erosion of the intussuscipiens with prolapse of the intussusception through the defect), but these children had no evidence of pneumoperitoneum on pre-operative imaging, nor did they have signs of peritonitis. A reasonable conclusion is that if pneumoperitoneum does occur in intussusception, it must be extraordinarily rare and is not a valid reason for performing plain radiological imaging as a routine, and certainly is not a requirement prior to ultrasonography or attempted reduction of intussusception.