Reflex anal dilation (
RAD) is 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. RAD was 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. The finding of RAD alone is not considered indicative of
sexual abuse, and a normative sample of children not suspected of having been sexually abused found that 49 per cent of children showed anal dilation either continuously or intermittently, though the dilation exceeded 20 mm in only 1.2 per cent of cases.