The reasons for anal distension are as follows: 1, the anal opening is caused by hemorrhoids, hemorrhoids are perianal rectal mucosa flexure formation of a vein cluster, when usually eating spicy and stimulating food, or smoking and drinking, these things will play a certain role in stimulating the perianal mucosa, vein clusters pull the perianal nerves caused by the perianal distension. 2, the emergence of perianal diseases, such as perianal abscess, anal fissure, anal fistula, etc., these Diseases can cause pus cavity or fistula around the anus, when the stool passes through these positions, these unclean feces will stimulate the skin around the fistula to cause this feeling. 3, usually do not pay attention to dietary habits, eat too dry things lead to constipation. The anal opening is narrowed when the dry stool passes through the anus, part of the stool can not be discharged in time, causing perianal swelling.