Functional Immaturity of the Colon (Small Left Colon and Meconium Plug Syndrome)
Historically 'meconium plug syndrome' and small left colon syndrome have been considered as separate, if possibly overlapping, entities. The main finding in both entities is that of a narrow recto-
sigmoid and descending colon with a transition to normal size or dilated colon typically at the level of the splenic flexure. The term 'meconium plug syndrome' was used initially, reflecting the belief that the obstruction was probably due to 'an alteration in the character of the most distal portion of the meconium mass' (CLAtwoRtHy et al. 1956). Subsequent authors observed that a small left colon with proximal obstruction could be seen in the absence of the meconium plug, and termed the syndrome 'small left colon syndrome', in the belief that the abnormality was due to the smallness of the colon rather than the abnormality of the meconium. Finally the term 'functional immaturity of the colon' has come into widespread use and reflects the overlapping or synonymous entities of 'small left colon syndrome' and 'meconium plug syndrome' (Berdon et al. 1977).
The underlying aetiology remains unknown but is likely due to impaired intestinal motility, leading to local inspissation of meconium and secondary low colonic obstruction. Presentation is in new born infants, normally within the first 24 h of life. Approximately 50% of infants have diabetic mothers. The signs are those of abdominal distention, vomiting and failure to pass meconium. The AXR will show evidence of a low obstruction with multiple dilated loops of bowel. Occasionally there is a bubbly appearance in the colon which raises concern about the differential diagnosis of submucosal air including necrotising enterocolitis. Investigation is by water soluble contrast enema. A catheter is passed per rectum and contrast introduced under fluoroscopic guidance. If a meconium plug is present there may be a ribbon-like filling defect in the small left colon with a larger filling defect in an otherwise normal appearing transverse colon (Fig. 6.10). The hyper-osmolality of the contrast medium often causes the meconium plug to shell away from the bowel wall and meco-nium is usually evacuated during or soon after the enema examination, resulting in relief of obstruction. If a meconium plug is not present, a small left colon will be seen usually with an abrupt change of calibre at the splenic flexure. The images confirm the diagnosis and usually no further treatment is necessary with the infant normally passing meconium from the ascending and transverse colon soon after the study. Though infants with functional immaturity of the colon usually do well, a benign course is not invariable and perforation may occur.
A differential diagnosis in infants with these findings includes that of Hirschsprung's disease. However, it is unusual for a case of Hirschsprung's
disease to have a transition zone at the splenic flexure, the transition zone in functional immaturity is often quite abrupt whereas the transition zone in Hirschsprung's disease may be cone shaped and gradual, and finally in Hirschsprung's disease aganglionic colon is usually of near normal calibre whereas in functional immaturity the left colon is usually small. In newborns with functional immaturity the rectum is usually quite distensible unlike in Hirschsprung's disease when the aganglionic colon usually remains of uniform calibre to the anus. If any doubt about the diagnosis remains after treatment by contrast enema, a rectal biopsy should be obtained.
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