Susan M.Cera and Steven D.Wexner
Fecal continence relies on normal sensory, motor, and reflex activity of the colon, rectum, and anus. Stool consistency, colonic transit, rectal sensation, neural integrity, and sphincter function all have a role in individual control of stool and gas. The principal component in facilitating continence is the anal sphincter, whereas the hemorrhoidal cushions, sensory epithelium of the anal mucosa, intrinsic anorectal reflexes, and pelvic floor muscles contribute to its function. The anal sphincter complex is composed of the internal anal sphincter (IAS), the external anal sphincter (EAS), and the puborectalis muscle. The pudendal nerve innervates the EAS and puborectalis whereas the innervation of the rectum, pelvic floor muscles, and IAS is a complex system of sympathetic and parasympathetic neurons supplied by the pelvic and sacral nerves. Damage to any muscular or neural component of the sphincter mechanism may result in fecal incontinence and possible need for surgical intervention. The two most common causes of fecal incontinence are sphincter dysfunction and neuropathy.
Physical disruption of the sphincter may occur secondary to surgical, obstetric, or other trauma. Surgical trauma most often occurs after anorectal surgical procedures used in the treatment of anal fistulas, hemorrhoids, tumors, or anal fissures. Each of these surgical injuries is similar, in that the IAS, the EAS, or both may be injured. Anal fistulo-tomy inherently causes some form of damage to the sphincter mechanism, and the risk of incontinence may be proportional to the amount of muscle divided. Alternatives to fistulotomy involve controlled transection of the muscle with cutting setons, instillation of fibrin glue, or endorec-tal advancement flaps.
Chronic fissures are frequently treated by lateral internal sphincterotomy in which a portion or all of the internal sphincter is transected. The internal sphincter, which consists of smooth muscle under autonomic involuntary control, is responsible for the resting pressure in the anal canal. When divided, the resting pressure is lowered, allowing the anal fissure to heal. The vast majority of patients have no untoward effects from this technique, but a small minority may develop transient incontinence to gas. A few individuals will develop chronic incontinence, which may be more severe and associated with involuntary loss of liquid or solid stool. Alternatives to this procedure have been developed and include medications, such as nitrate paste and calcium channel blocking cream, to relax the internal sphincter. These therapies are now often the first line treatment in the management of anal fissures.
Sphincter injury resulting from hemorrhoid surgery is fortunately rare and usually avoided by careful attention to the anatomy.
Trauma can injure the anal sphincter mechanism; impalement injuries may result in sphincter disruptions, rectal perforations, and soft tissue damage to the perineum. Their treatment is often staged with initial debridement and fecal diversion followed by delayed definitive repair when the sepsis is resolved and the injury has healed.
Obstetric injury is the most common cause of sphincter damage and occurs as the result of a tear in the perineum or an episiotomy. Parous women or women who undergo assisted evacuations are at the highest risk for sphincter laceration. The sphincter damage may be occult for many years; it is common for this dormant injury to clinically manifest as fecal incontinence years to decades postpartum.
The surgical treatment of fecal incontinence is categorized into two groups: reparative procedures that focus on repair or augmentation of the sphincter muscle, versus replacement procedures in which the goal is reconstruction, replacement, or external modulation of the sphincter apparatus. The choice of procedure is dependent on the underlying etiology of the incontinence as well as on complete evaluation of the anorectal and pelvic floor anatomy. The type and severity of injury found on evaluation and physiologic testing directs therapy to the individual's problem. If possible, repair of the native sphincter mechanism is undertaken before resorting to major reconstructive surgery, artificial sphincters, neurostimulation, or stoma. In particular, patients who are candidates to undergo sphincter repair often have injuries from physical disruption of the EAS complex.
Types of anal sphincter repairs include direct apposition sphincteroplasty, overlapping sphincteroplasty, anterior plication (reefing), Parks' postanal repair, and total pelvic floor repair. Sphincteroplasty involving direct apposition is traditionally reserved for the acute setting of traumatic sphincter laceration. However, recent data have challenged this dogma. The overlapping sphincteroplasty is the most widely accepted procedure for secondary repairs in patients with isolated anterior defects. Anterior plication, postanal repair, and total pelvic floor repair do not involve direct repair of the muscle but theoretically attempt to augment the function of the sphincter mechanism by restoring the lax muscular architecture to its original anatomic configuration. These procedures are seldom performed today but remain important surgical options in patients who have failed conservative measures but do not desire more complex forms of surgical therapy.
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