Voluntary Muscle Structures

In the normal continent patient, the voluntary muscle structures are represented by the levators, the muscle complex, and the parasagittal fibers. Normally, they are used only for brief periods of time when the fecal mass, pushed by the involuntary peristaltic contraction of the rectosigmoid colon, reaches the anorectal area. This voluntary contraction keeps the stool in the rectum by closing the anus around the time of defecation. These muscles are used only occasionally during the rest of the day and night.

Patients with ARMs have abnormal voluntary striated muscles with different degrees of hypodevelopment. Some patients are close to normal whereas some patients do not have any voluntary sphincter muscles. Also, patients with Hirschsprung disease may have suffered damage to this sphincter mechanism at the time of pull-through, and patients with spinal abnormalities may have deficient innervation of these muscles.

Anal Canal Sensation

Voluntary muscles are used only when the patient has the sensation that determines that it is necessary to use them. To appreciate that sensation, the patient needs feedback that is derived from an intact anal sensory mechanism.

Exquisite sensation in normal individuals resides in the anal canal. Except for patients with rectal atresia, most patients with ARMs are born without an anal canal. Therefore, sensation does not exist or is rudimentary. Patients with spinal conditions may lack this anal canal sensation as well.7,8 Those with Hirschsprung disease are born with a normal anal canal, but this can be injured if not meticulously preserved at the time of their pull-through (Fig. 36-1).^5,6 Also, perineal trauma may result in an injured or destroyed anal canal.⁹

It seems that most individuals can perceive distention of the rectum. This point is important for patients undergoing pull-through procedures for imperforate anus as the distal rectum and anus must be placed precisely within the sphincter mechanism. This sensation seems to be a consequence of stretching of the voluntary muscles (proprioception). The most important clinical implication of this point is that patients might not feel liquid or soft fecal material because such stool consistency does not distend the rectum. Thus, to achieve some degree of sensation and bowel control, the patient must have the capacity to form solid stool. This is especially important in a patient without a good anal canal to give sensory cues. This point is relevant in children with ulcerative colitis who have undergone an ileoanal pull-through procedure. They can suffer from varying degrees of incontinence due to their incapacity to form solid stool, but their normal sphincter muscles and anal canal allow them to overcome this problem. Most need treatments that bulk up the stool.

Bowel Motility

In a normal individual, the rectosigmoid colon may remain quiet for up to several days, depending on defecation habits or dietary consumption. During that time, the previously mentioned sensation and voluntary muscle structures are not active because the stool, if it is solid, remains inside the colon. The patient can feel the peristaltic contraction of the rectosigmoid that occurs before defecation. Voluntarily, the normal individual can then relax the striated rectal muscles, which allows the rectal contents to migrate down into the highly sensitive area of the anal canal. There, the anal canal provides feedback concerning stool consistency. The voluntary muscles are used to push the rectal contents back up into the rectosigmoid and to hold them until the appropriate time for evacuation. At the time of defecation, the voluntary muscle structures relax, and the stool exits the anus helped by gravity, rectal contraction, and abdominal wall muscles.

Most patients with an ARM suffer from some disturbance of this sophisticated bowel motility mechanism. Patients who have undergone a posterior sagittal anorectoplasty (PSARP) or any perineal approach in which the most distal part of the bowel was preserved can develop an overefficient bowel reservoir (megarectosigmoid) (Fig. 36-2). The main clinical manifestation is constipation, which seems to be more severe in patients with lower defects.³ Constipation that is not aggressively treated, combined with an ectatic distended colon, eventually leads to more severe constipation. A vicious cycle ensues, with worsening constipation leading to more rectosigmoid dilation, leading to more severe constipation, and then to overflow incontinence.

Patients with an ARM, who are managed with operations in which the most distal part of the bowel was resected (Fig. 36-3), behave clinically as individuals without a rectal reservoir. Depending on the amount of colon removed, the patient may have loose stools. In these cases, medical management consists of enemas, a constipating diet, and medications to slow down the colonic motility. Patients with Hirschsprung disease undergo operative resection of the distal aganglionic colon and rectum. However, their normal anal canal and sphincter mechanism (if properly preserved) allows the vast majority of them to be continent despite the lack of a rectal reservoir. Some patients with Hirschsprung disease have bowel hypermotility and need medications to slow the colon. Amazingly, some patients with an injured anal canal can be continent if their motility is normal because the regular contraction of the rectosigmoid can translate into a successful voluntary bowel movement.

Which Children Have True Fecal Incontinence?

In children with ARMs, 75% who have undergone a correct and successful operation have voluntary bowel movements after the age of 3 years.¹ About half of these patients occasionally soil their underwear. These episodes of soiling are usually related to constipation. When the constipation is treated properly, the soiling frequently disappears. Thus, approximately 40% overall have voluntary bowel movements and no soiling, and behave normally. Children with good bowel control still may suffer from temporary episodes of fecal incontinence, especially when they experience diarrhea.

Some 25% of all patients with ARMs suffer from true fecal incontinence, and are the patients who need bowel management to be kept clean. As previously noted, certain patients with Hirschsprung disease and those with spinal problems can suffer from true fecal incontinence as well. For these patients, similar principles of bowel management learned from treatment of patients with ARMs can be applied.¹⁰

For children with ARMs, the surgeon should be able to predict in advance which patient will likely have a good functional prognosis and which child will have a poor prognosis. Table 36-1 shows the most common indicators for a good or poor prognosis. After primary repair and colostomy closure, it is possible to establish a functional prognosis (Table 36-2). Parents should be given the information regarding their child’s realistic chances for bowel control to avoid needless frustration at the age of toilet training.

Once the diagnosis of the specific anorectal defect is established, the functional prognosis can be predicted. If the child’s defect is associated with a good prognosis such as a vestibular fistula, perineal fistula, rectal atresia, rectourethral bulbar fistula, or imperforate anus with no fistula, one should expect that the child will have voluntary bowel movements by the age of 3 years provided the sacrum and spine are normal. These children will need careful attention to avoid fecal impaction, constipation, and soiling.³

If the child’s defect is associated with a poor prognosis, such as a very high cloaca with a common channel longer than 3 cm, a rectobladder neck fistula, an associated myelomeningocele, or a very hypodeveloped sacrum, the child will most likely need a bowel management program with enemas to remain clean. This should be implemented at 3 to 4 years of age and before starting school. Children with a rectoprostatic fistula have a 50% chance of having voluntary bowel movements and continence. In these children, an attempt should be made to achieve toilet training by the age of 3 years. If this proves unsuccessful, bowel management should be implemented. The ability to toilet train can be reassessed each summer after school has ended.

In patients who have undergone repair of an imperforate anus and who have fecal incontinence, reoperation to relocate a misplaced rectum or repair a rectal prolapse should be considered if the child was born with a good sacrum, a good sphincter mechanism, and a malformation with good functional prognosis. A repeat PSARP can be performed and the rectum relocated within the sphincter mechanism.¹¹