Anal stenosis (also referred to as anal stricture) is an abnormal narrowing of the anus that often occurs after colon and rectal surgery. The diagnosis of anal stenosis is suggested by a history of constipation and difficulty in passing stool. Patients often have a history of anal surgery, inflammatory bowel disease, radiation treatment, or anorectal infection. The diagnosis is confirmed by physical examination.
The anal canal may be defined as the part of the alimentary tract from the anal verge to the anorectal ring (at the level of the levator muscle). Scarring and contracture may occur in a narrow bandlike fashion at the anal verge or occasionally may involve the entire anal canal with a thick, unyielding contracture. Severe stenosis may be defined by the inability to pass an 11-mm scope or the index finger into the anal canal. Stenosis of this degree is usually symptomatic and often requires operative treatment.
Sometimes the stenosis is due to an abnormally high tone in the internal sphincter. This condition is sometimes referred to as anismus (internal sphincter spasm). Anal manometry may be of value in documenting the spasm; a “saw tooth” pressure pattern is common. If the stenosis is due to scarring or a tumor, manometry is not very helpful. However, measurement of rectal compliance, anorectal sensation, and the integrity of the rectoanal inhibitory reflex will complete the picture of the patient’s adaptation to his or her condition.
Classification of Anal Stenosis
Anal stenosis may be broadly classified by its cause ( Box 10-1 ).
Sphincter spasm (anal fissure)
Inflammatory bowel disease
Congenital malformation (imperforate anus, stenosis, or membrane)
Neoplasm (benign or malignant anal, perianal, or rectal lesions)
Trauma (lacerations, crush, thermal injury, chemical injury)
Infection (tuberculosis, lymphogranuloma venereum, schistosomiasis, syphilis, actinomycosis)
Anal spasm (anismus) causes a tight anal canal that is painful and impossible to examine in the office. One of the common causes of internal sphincter spasm is an anal fissure. However, the spasm of a fissure is not a true stenosis. Painful anal spasm is an indication for examination with use of an anesthetic. Narrowing associated with spasm will disappear as the anesthetic relaxes the sphincters, whereas a stricture due to scarring will persist. Stenosis associated with internal sphincter spasm and a painful anal fissure is generally treated with a lateral internal sphincterotomy, which is discussed in Chapter 3 .
Postoperative scarring is the most common cause of an anal stenosis. The anal canal is a small oval tube, and incisions made in the mucosa heal with contracture, producing a circular scar that inevitably tightens. This outcome is characteristic of all circular wounds, such as anastomoses. Ileal pouch–anal anastomoses can be hand sewn or stapled. Stapled anastomoses are prone to weblike strictures because of the diverting ileostomy, but these strictures are easily dilated at the time of stoma closure. Hand-sewn anastomoses can form denser strictures that may be symptomatic, often requiring repeated dilations. Anal stenosis is also common after a hand-sewn coloanal anastomosis for rectal cancer, especially when neoadjuvant radiation has been administered. The radiation makes stenosis relatively resistant to dilation, and thus frequent stretching with use of an anesthetic may be needed.
Anal mucosal stripping after stapled ileal pouch–anal anastomosis for retained anal transition zone is a potent cause of a dense anal stenosis. To avoid this consequence, consider stripping half the circumference at one operation and the other half later, after the first half has healed.
Postoperative strictures at the anal verge or in the external anal canal are usually due to excision of excessive amounts of anoderm during anorectal operations—especially hemorrhoidectomy—or overambitious electrocoagulation/excision of anal warts.
Stenosis Due to Chronic Diarrhea
The anal canal relies on passage of a formed stool for maintenance of its suppleness and dilatability. In persons with chronic diarrhea, the anus never naturally stretches, and over time it tends to lose the ability to dilate. This tendency is worse in patients with anal inflammation, such as that found in association with inflammatory bowel disease (Crohn-related colitis and ulcerative colitis). The presence of active inflammatory bowel disease, especially with rectal involvement or suppuration, may also prevent surgical correction of a stricture. Occasionally, fecal diversion will be required in this group of patients. Gentle dilation after induction of anesthesia with a lubricated dilator has been reported to have a good effect in some patients. This success is thought to be related to the absence of new wounds in these patients with inflammatory bowel disease and carries the caveat of using minimal dilation that is just adequate to allow the passage of semiliquid stool without disrupting the sphincters.
Anal stenosis may develop in elderly and senile patients or patients with Alzheimer disease who chronically abuse laxatives. Care must be taken to avoid surgery in these patients whose continence may, in part, depend upon the presence of anal stenosis. Both the anal stenosis and the atrophy of the sphincters are due to passage of only liquid stools and the lack of natural dilatation by formed movements. Use of mineral oil has been implicated in these patients. Caution is advised in management because correction of the anal stenosis may result in severe incontinence. Education with regard to bowel habits and diet may alleviate symptoms.
Other causes of stenosis are listed in Box 10-1 .
Other Classification Schemes for Anal Stenosis
Other classification schemes for anal stenosis have been described. On the basis of severity, Milsom and Mazier distinguished mild, moderate, and severe anal stenosis. Persons with mild anal stenosis have a tight anal canal that can be traversed by a well-lubricated index finger or a medium Hill-Ferguson retractor. In persons with moderate anal stenosis, forceful dilatation is required to insert either the index finger or a medium Hill-Ferguson retractor. In persons with severe anal stenosis, neither the little finger nor a small Hill-Ferguson retractor can be inserted unless a forceful dilatation is employed. Furthermore, stenoses may be diaphragmatic (characterized by a thin strip of constrictor tissue in the presence of inflammatory bowel disease), ringlike or anular (after development of surgical or traumatic lesions of a length <2 cm), or tubular (with a length >2 cm). Based on the level of anal canal affected, stenosis also may be distinguished as low, middle, high, or diffuse. Low stenosis, which occurs in 65% of patients, is located at the distal anal canal at least 0.5 cm below the dentate line. Middle stenosis, which occurs in 18.5% of patients, is 0.5 cm proximal to 0.5 cm distal to the dentate line. High stenosis, which is found in 8.5% of patients, is proximal to 0.5 cm above the dentate line. Diffuse stenosis, found throughout the anal canal, affects 6.5% of patients.