Congenital Anorectal Malformations and Hirschsprung Disease in the Neonate

Pathophysiology of Anorectal Malformations

ARMs are a congenital group of disorders that occur when the hindgut fails to develop in the appropriate anatomic position and with appropriate caliber. One out of every 4000 to 5000 newborns is born with a congenital ARM. The incidence is slightly higher in males. Prenatal diagnosis is generally uncommon. A minority of cases present after the neonatal age, with the majority of patients developing symptoms in the first weeks of life. Prompt, early diagnosis allows for appropriate counseling, operative planning, and overall management in the neonatal period. The goal of surgery is to restore normal anatomy and in so doing allow for normal function and motility with minimal long-term morbidity.

Normal Continence

The three primary components that maintain bowel continence are voluntary muscle constriction, anal canal sensation, and bowel motility. Voluntary muscle structures include the levator muscles, the striated muscle complex, and the external sphincter. Neonates with ARMs will have varying degrees of hypodevelopment of these voluntary muscles. For these voluntary muscles to contract appropriately, sensation of the anal canal must be intact. In general, patients with ARMs are born without an anal canal or with a malformed anal canal, thus severely impairing sensation and inhibiting voluntary muscle contracture. Lastly, peristaltic contraction of the distal large bowel, the rectosigmoid, is usually felt prior to defecation. This leads to relaxation of the voluntary rectal muscles, which allows the contents within the rectum to be interrogated by the sensate anal canal. These voluntary muscles can either then push the rectal contents back into the sigmoid until the appropriate time for evacuation or, together with a Valsalva maneuver, evacuate the contents. Rectosigmoid motility is disrupted in ARMs, contributing to an inability to achieve appropriate continence.

Clinical Features of Anorectal Malformations

As mentioned, ARMs compose a spectrum of disorders. Some disorders such as rectoperineal fistulas occur in both males and females, whereas other disorders are unique to each sex. Risk factors for the development of congenital ARMs include prematurity, being small for gestational age, and possibly maternal obesity and smoking.

In general, ARMs can be classified according to whether the fistula or hindgut opening is “high” or “low.” Patients with “low” fistulas have anterior fistulas that open onto the perineal surface and are best treated with early surgical corrections. Repairs of low fistulas usually do not require diversion and can be repaired in a single-stage procedure.

In some patients, the fistula cannot be clearly visualized and the anatomy is not clearly delineated. This is because the fistula location is “high,” occurring at the bladder or urethra (rectobladder neck or rectourethral) in males or at the posterior fourchette or within the introitus (rectovestibular fistula) in females. The most severe form, seen in females, is a cloaca in which the neonate has a single perineal orifice, with the rectum and genitourinary tract sharing a common channel. In other cases still, there may be normally developed anus and sphincter muscles without a fistula, yet the rectum is strictured or atretic. In general, high fistulas are repaired in multiple stages to minimize risk of injury to neighboring structures and to allow for the delineation of the variant anatomy prior to definitive repair.