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17 General introduction and outline of the thesis Contrast enema Contrast enemas are used to identify anatomic abnormalities of the anorectum. After infusion of contrast fluid into the rectum an abdominal X-ray is obtained, visualizing the distribution of contrast fluid in the distal gastrointestinal tract. Contrast enemas can be useful to detect mechanical causes of constipation such as anatomical abnormalities or complications after colorectal surgery. 51 Ultrasonography Transabdominal ultrasonography has been used to measure the transverse rectal diameter. 52,53 An increased rectal diameter (>30 mm) is often considered to be suggestive for fecal impaction. 54,55 Although transabdominal ultrasonography is a promising technique, there is currently insufficient evidence that the transverse diameter can be used as a reliable predictor of constipation and fecal impaction in children. 33,56 Manometry Manometry assesses intraluminal pressure and contact force in the gastrointestinal tract, thereby providing insights into contractile motor patterns of the gut. Anorectal manometry Anorectal manometry provides information about anorectal neuromuscular function. This test allows assessment of the rectoanal inhibitory reflex, anal sphincter pressure, rectal sensation, and defecation dynamics. The rectoanal inhibitory reflex is a physiological reflex that induces the internal anal sphincter to relax in response to rectal distension, thereby enabling defecation to take place. To assess the rectoanal inhibitory reflex during anorectal manometry, rectal distension is generated by inflating a balloon attached to the end of the manometry catheter. Since anorectal manometry can be used to assess the rectoanal inhibitory reflex , it is used as a tool to rule out Hirschsprung’s disease. 33 Hirschsprung’s disease is the most common congenital gut motility disorder. It is characterized by the absence of enteric ganglions (aganglionosis) in the distal gut. Due to this aganglionosis the physiologic rectoanal inhibitory reflex is absent. The performance and analysis of anorectal manometry data requires considerable experience and therefore the procedure is primarily performed in specialized centers. Colonic manometry Colonic manometry is used as a tool to identify abnormalities in colonic contractile activity. Although protocols differ between centers, colonic manometry studies generally include recording periods in a fasted state, after ingestion of a meal and, particularly in children, after administration of a stimulant laxative. 57