Surgery STAT: Diagnosis and surgical management of atresia ani in small animals

Atresia ani is a congenital embryological anomaly in which the hindgut fails to fully communicate with the perineum. The anus may be either stenotic or imperforate; atresia ani may appear alone or in combination with rectovaginal or rectovestibular fistula (RVF). In dogs, females and certain breeds including poodles and Boston Terriers are predisposed. There are few reports of atresia ani in cats, and most are females with concurrent RVF.

Photo 1: An 8-week-old female pug with type 1 atresia ani. A blunt probe is inserted into the stenotic canal. The ducts of the anal sacs are visible.

Atresia ani consists of four types of anomalies of worsening severity. Type I is congenital anal stenosis without imperforate anus (Photo 1). Type II and III anomalies constitute an imperforate anus a distance of either < 1.5 cm (type II) or > 1.5 cm (type III) away from a blind rectal pouch (Photo 2). Type IV anomalies are rare and involve a blind rectal pouch with normal terminal rectal development.

Photo 2: Illustration of type II atresia ani with RVF. The communication allows feces to exit the vulva.

Clinical signs and diagnosis

Puppies and kittens affected with atresia ani are often stunted and anorectic and have abdominal enlargement due to secondary megacolon. In patients with atresia ani alone, defecation is reduced (type I) or absent (types II to IV). Newborn puppies and kittens with atresia ani and confluent RVF may present with a history of chronic recurrent cystitis. Passage of feces from the vulva is the hallmark sign in females.

Photo 3: Contrast vaginogram from a 10-week-old Boston Terrier illustrates a wide communication between the dorsal wall of the vestibule and the rectum.

A contrast vaginogram is often helpful in determining the length and width of the communication in cases with concurrent RVF (Photo 3). Under deep sedation or general anesthesia, a curved hemostat can be inserted into the vulva to help identify the communication with the rectum (Photo 4A).

Photo 4A: An 8-week-old Jack Russell Terrier with an imperforate anus and RVF. A mosquito hemostat easily communicates with the rectal lumen, and the tip outwardly deflects the anal membrane.

Surgery and prognosis

Surgery to correct atresia ani is often delayed until 6 to 8 weeks of age. Fine instruments such as jewelers' forceps and tenotomy scissors are used to carefully dissect the skin of the anus inwardly and identify the rectal pouch. Rectal vaginal fistulas vary in depth and width of communication. Various authors have ligated, transected and oversewn or used hemostatic clips to attenuate the communication (Photos 4B, 4C).

Photo 4B: The rectovaginal communication is closed with hemostatic clips.

Complications after surgery include fecal incontinence, persistent megacolon, anal stricture and recurrent cystitis. In one study, three of six dogs that underwent anoplasty needed either revision or subtotal colectomy. The prognosis for patients with atresia ani with concurrent RVF appears better than those cases with atresia ani alone. The main concern in those cases is not persistent megacolon but rather persistent urinary tract infection. Preservation of the RVF for reconstruction of the anal canal and the anus in atresia ani with RVF was successfully performed in two dogs and might be a method of reducing postoperative incontinence.

Photo 4C: The appearance of the anoplasty 10 days postoperatively.

SUGGESTED READING

> Vianna ML, Tobias KM. Atresia ani in the dog: a retrospective study. J Am Anim Hosp Assoc 2005:41(5):317-322.

> Mahler S, Williams G. Preservation of the fistula for reconstruction of the anal canal and the anus in atresia ani and rectovestibular fistula in 2 dogs. Vet Surg 2005;34(2):148–152.

Dr. Gary Ellison is an ACVS Diplomate and professor and service chief of small animal surgery at The University of Florida College of Veterinary Medicine in Gainesville. When not working, he enjoys bicycling and automechanics.