Nitroglycerin is more rapidly absorbed if blood flow in the anoderm is high. Sphincterotomy Overview A fissure is a tear in the skin of the anus, usually caused when a large, hard stool overstretches the anal opening and damages the fragile skin. Common causes of rectal pain can result from conditions that also may be accompanied by blood in the stool include: Anal fissures are a common problem. Muscle paralysis occurs within hours and the effect remains for 3—4 months, until there has been axonal regeneration with the formation of new nerve terminals. Anal fissures are very common in infants, but they may occur at any age.