We present the 1st case report of an additional enteric smooth muscle layer in a patient with Mowat-Wilson syndrome and Hirschsprung disease. After resection of the aganglionic colon at the age of 5 months, our patient initially suffered from intermittent constipation, and subsequently by the age of 5 years, he developed ongoing diarrhea requiring medical treatment for more than a decade. Although the exact mechanism of abnormal gut motility in this case is unknown, we postulate that the supernumerary muscle and its associated neural plexus may be responsible for the patient's unusual late complication in treated Hirschsprung disease.
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