An anal fissure (see the images below) is a painful linear tear or crack in the distal anal canal, which, in the short term, usually involves only the epithelium and, in the long term, involves the full thickness of the anal mucosa. Anal fissures develop with equal frequency in both sexes; they tend to occur in younger and middle-aged persons.
Failure of medical therapy is an indication for surgical therapy. Controversy mostly involves continued efforts to find a medical therapy for anal fissure that is as successful as the surgical therapy for the condition.
A thorough knowledge of the anatomy of the anal canal is vital for effective surgical treatment of an anal fissure.
The anal canal may be defined in two ways, as follows:
Functional (or surgical) anal canal
Anatomic anal canal
These two terms are often used interchangeably, even though they do not mean the same thing. The surgical anal canal is approximately 4 cm long and extends from the anal verge or intersphincteric groove distally to the anorectal ring, proximally. The anatomic anal canal is only approximately 2 cm long and extends from the anal verge distally to the dentate line proximally.
The dentate line is the junction of the ectoderm and endoderm in the anal canal. The anal verge is an anocutaneous line approximately 2 cm distal to the dentate line. The anal verge marks the beginning of the anal canal.
The internal anal sphincter is a smooth muscle that is the most distal extension of the inner circular smooth muscle of the colon and the rectum. It is 2.5-4 cm long and is normally 2-3 mm thick. The internal sphincter is not under voluntary control and is continuously contracted to prevent unplanned loss of stool.
The external anal sphincter is striated muscle that forms a circular tube around the anal canal. Proximally, it merges with the puborectalis and the levator ani to form a single complex. Control of the external anal sphincter is voluntary.
Pathophysiology and Etiology
The exact etiology of anal fissures is unknown, but the initiating factor is thought to be trauma from the passage of a particularly hard or painful bowel movement. Low-fiber diets (eg, those lacking in raw fruits and vegetables) are associated with the development of anal fissures. No occupations are associated with a higher risk for the development of anal fissures. Prior anal surgery is a predisposing factor because scarring from the surgery may cause either stenosis or tethering of the anal canal, which makes it more susceptible to trauma from hard stool. 
Initial minor tears in the anal mucosa due to a hard bowel movement probably occur often. In most people, these heal rapidly without long-term sequelae. In patients with underlying abnormalities of the internal sphincter, however, these injuries progress to acute and chronic anal fissures. Studies of the internal anal sphincter and of anal canal physiology have been performed with varied results, but at least one abnormality is likely present in the internal anal sphincter of many anal fissure patients.
The most commonly observed abnormalities are hypertonicity  and hypertrophy of the internal anal sphincter, leading to elevated anal canal and sphincter resting pressures. The internal sphincter maintains the resting pressure of the anal canal; anal-rectal manometry can be used to measure this pressure. Most patients with anal fissures have an elevated resting pressure, which returns to normal levels after surgical sphincterotomy.
The posterior anal commissure is the most poorly perfused part of the anal canal. In patients with hypertrophied internal anal sphincters, this delicate blood supply is further compromised, thus rendering the posterior midline of the anal canal relatively ischemic.
This is thought to account for why many fissures do not heal spontaneously and may last for several months. Pain accompanies each bowel movement as this raw area is stretched and the injured mucosa is abraded by the stool. The internal sphincter also begins to spasm when a bowel movement is passed. This spasm has two effects: First, it is painful in itself, and second, it further reduces the blood flow to the posterior midline and the anal fissure, contributing to the poor healing rate.
Approximately 1-6% of patients have a recurrence of their anal fissure after sphincterotomy. The recurrence rate is higher after a sphincter stretch. If a patient develops a recurrence after a sphincterotomy, it could be from recurrent disease or from an improperly or incompletely performed initial sphincterotomy.
In the event of a recurrence, medical management should be attempted again, but if no relief is obtained, the surgeon must evaluate whether the original sphincterotomy was adequate. Evaluation can be performed by means of palpation during examination under anesthesia or by means of endoanal ultrasonography. If the sphincterotomy was incomplete, it can be completed on the initial side or redone on the opposite side. If the first sphincterotomy was complete, a second sphincterotomy can be completed on the opposite side.
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