The transition zone between aganglionic and normal colon is determined by biopsy and frozen section. The level is usually in the sigmoid colon. In this case it was higher, just proximal to the splenic flexure.
A "leveling colostomy" is made just proximal to the transition zone after mobilizing the colon through the left lower quadrant incision. Postoperative function of the colostomy is confirmation of the integrity of the proximal colon.
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