What causes the sawtooth sign on contrast enema?
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Answer:
The sawtooth sign is an irregular, serrated, spiculated contour of the distal colonic mucosa on contrast enema, produced by uncoordinated, non-propulsive contractions and muscular fasciculation of the aganglionic segment in Hirschsprung disease. Sensitivity is low, so a smooth aganglionic segment does not exclude the diagnosis, but specificity approaches 100%, making it a strong positive finding when present. It is assessed alongside the other contrast enema features: the transition zone, a funnel-shaped calibre change between the narrow distal aganglionic bowel and the dilated proximal colon; the rectosigmoid index, the ratio of maximum rectal to maximum sigmoid diameter, which is normally greater than 1 and reversed in Hirschsprung disease; and delayed retention of contrast beyond 24 hours on the delayed film. Contrast enema guides surgical planning and predicts the level of aganglionosis imperfectly, so suction rectal biopsy showing absent ganglion cells with hypertrophied nerve trunks remains the diagnostic gold standard.
Why is it called so?
The serrated, spiculated mucosal contour resembles the teeth of a saw.
Pathophysiology
Failure of craniocaudal migration of neural crest-derived ganglion cells leaves the distal bowel without a myenteric and submucosal plexus. The denervated segment cannot relax or generate coordinated peristalsis; instead it exhibits tonic contraction with disorganised, spastic, non-propagating contractions from denervation hypersensitivity and hypertrophied extrinsic nerve trunks. These fasciculating contractions indent the barium column irregularly along the aganglionic segment, and superimposed mucosal oedema and inflammation from stasis accentuate the spiculation, producing the saw-toothed outline.
Alternative names: Sawtooth irregularity, fasciculation irregularity, spiculated mucosal pattern
Other associated named signs: Transition zone, reversed rectosigmoid index, delayed contrast retention, and the lead pipe colon of chronic ulcerative colitis as a contrasting smooth-walled pattern
References
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