How can topical nitroglycerin help in treating chronical anal fissure?

Relaxation: The theory w/ anal fissures is that it is due to spasms of the anal sphincter so that when you have a bm, it tears the canal (like pooping shards of glass). The nitro is meant to relax the sphincters.

Relaxes the muscle: Relaxing the internal anal sphincter muscle may allow the fissure to heal. Nitroglycerin is a smooth muscle relaxant.