quote from "Pediatric Colorectal Conundrums: Case Studies: From Fundamental to Advanced (Pediatric Colorectal Surgery)" by Marc Levitt, Thomas Xu, Hussein Wissanji -

"Anorectal manometry is sometimes used to diagnose HD; however, its diagnostic accuracy is limited in children under the age of 12 months. It is a helpful adjunct in diagnosing older children who struggle with constipation due to a delayed diagnosis of HD. One of the characteristics of HD is that patients cannot relax their internal anal sphincter in response to rectal distension, otherwise known as the rectoanal inhibitory reflex (RAIR). Anorectal manometry can screen for the absence of this reflex, which is typical of HD. If the RAIR is absent, a rectal biopsy to assess for HD should be done."