Symptomatic hemorrhoids come from the inflammation of the mucosa and interstitial tissue surrounding the normal veins of the anorectum. These veins provide the normal pathway for return of blood from the anus and rectum back through the circulation to the heart. Important is the fact that the veins in the rectum drain into the portal circulation and thus to the liver prior to returning to the heart, while the veins of the anus drain directly into the central circulation.
A fistula is an abnormal communication between the inside of a hollow organ and the skin. In the case of an anal fistula the communication is from inside the rectum to the skin outside the anus or on the buttock. The inciting cause is trauma of defecation which causes a break in the mucosa of the rectum. Bacteria track into the tissue under the mucosa and form an abscess. The abscess eventually finds its way to the skin where it ruptures and drains. The outer opening may heal but the inner opening remains and permits the cycle to repeat itself. Patients often present with years of repeated abscesses in this area which drain, heal and return again.
Surgical treatment involves opening the entire fistula from its outer opening to its inner opening and allowing the wound to heal from the inside out thus obliterating the fistula. If the fistula track runs deep to the anal sphincter muscle, the muscle will need to be divided; however, doing so carries a risk of causing fecal incontinence.
A fissure is simply a crack or tears in the anal skin in the anal canal. The cause is also straining with bowel movement. They are often exquisitely painful. So much so, that when I see a patient in excruciating pain in the anus, the patient will not allow me to do a digital exam and I cannot see a thrombosed hemorrhoid, I will usually just treat for a fissure as the presumptive diagnosis. Surgical treatment involves doing a lateral sphincterotomy. The skin in the outer part of the anus is opened slightly to expose the sphincter muscle and the muscle is divided superficially. This does not carry the same risk of incontinence as when dividing the muscle for a fistula, because the location and degree of muscle division can be completely controlled by the surgeon. With a fistula, the area of muscle involvement is dictated by the disease.
Surgery for fissures can often be avoided with the use of topical Nitroglycerine applied directly to the anus. Nitroglycerine relaxes the muscles of the anal sphincter the same way it relaxes the smooth muscle of blood vessels when treating Coronary Artery Disease. It may cause a low blood pressure in normal patients even when applied to the anus in this manner and therefore, may not be suitable for everyone. It is; however, a good first line treatment option that often resolves the problem and avoids the need for surgery.