Purpose: To investigate the incidence and treatment of internal anal sphincter achalasia (IASA) in Japan based on an analysis of data from a nationwide retrospective cohort study of the allied disorders of Hirschsprung’s disease. Methods: Five cases of definitive IASA were collected from a nationwide retrospective cohort study conducted from 2001 to 2010 and a search of the Japanese literature. Results: Symptoms developed during the neonatal period in two patients, during early childhood in two, and at school age in one. Symptoms included abdominal distension with severe constipation (n = 4) and enterocolitis (n = 1). Rectocolonography showed megarectum and no narrow segment in most of the patients. All patients were negative for rectosphincteric reflex. The presence of ganglion cells was demonstrated by H&E or AChE staining from rectal mucosal biopsies or resected full-thickness segments. Two patients were treated conservatively, and three were treated surgically by internal anal sphincter myotomy (n = 2) or Lynn procedure (n = 1), with satisfactory outcomes. Conclusion: IASA is a rare but distinct entity in Japan. Although the clinical features of IASA resemble those of short- and ultrashort-segment HD, characteristic pathological findings include the presence of ganglion cells. The outcomes of both conservative and surgical treatment are good.
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