anal dilation reflex anal dilation https://en.wikipedia.org/wiki/Reflex_anal_dilation https://www.wikidata.org/wiki/Q7307346 RAD The reflexive dilation of the human anus to a diameter greater than two centimeters in response to the parting of the buttocks or anal stimulation, such as brushing with a medical instrument. It has been theorized to be a clinical marker associated with anal sexual assault in children, and has been associated with other signs of sexual assault but also appears in children with severe chronic constipation and those subject to invasive medical treatments of the anus.