Anorectal Malformations

​Did you know that 1 in 5000 babies are born without an anus? This is called an anorectal malformation also known as imperforate anus. This is a congenital condition affecting approximately 1 in 5,000 infants. We will explore the embryological basis of the condition, its clinical presentation, diagnostic techniques, and the surgical management strategies. The article is based on a learning event at a paediatric surgery clinic, where we observed patients undergoing anal dilation after corrective surgery and had the opportunity to learn from the medical team. We will also discuss the gold standard surgical procedure, Posterior Sagittal Anorectoplasty (PSARP), and the crucial role of post-operative anal dilation in preventing complications.

​Understanding Anorectal Malformations:

​Anorectal malformations (ARMs) are birth defects that occur when the anus and rectum don’t develop correctly. This can result in various anatomical abnormalities. These range from a rectum that ends in a blind pouch, to one that connects to other structures like the urethra, bladder, or vagina via a fistula. ARMs are thought to arise from abnormal development during the 4th to 7th week of gestation. Specifically, the uro-rectal septum, which divides the cloaca (a common chamber for urinary, genital, and intestinal tracts), fails to properly descend, or the anal membrane fails to rupture.

​ARMs are generally classified into two main types. This classification is based on the level of the rectal pouch relative to the levator ani muscle complex:

​● High Type: The rectal pouch ends above the puborectalis sling. This type is more severe. It often presents with no visible anal opening and a fistula to the urinary tract in males (recto-urethral) or the vagina in females (recto-vaginal).

​● Low Type: The rectal pouch ends below the puborectalis sling. This type is less severe. It is often linked to a visible but ectopically placed anus, a stenotic anus, or an anus covered by a thin membrane. A perineal fistula may also be present.

​The specific type of ARM determines the surgical approach and prognosis. They can occur as isolated defects. Alternatively, they can be part of a syndrome, like VACTERL association (Vertebral defects, Anal atresia, Cardiac defects, Tracheo-esophageal fistula, Renal anomalies, and Limb defects).

​How do we find out?

​The diagnosis of an ARM is usually made shortly after birth. 

Key clinical signs include:

• An absent anal opening upon examination of the newborn.
• Failure to pass meconium (the first stool) within the first 24–48 hours of life.
• Passage of meconium from an abnormal location, such as the urethra or vagina, indicating a fistula.
• Abdominal distension and vomiting, which can be signs of bowel obstruction.

​Radiological imaging is crucial for diagnosis and classification. A cross-table lateral X-ray (also known as an invertogram), taken 12–24 hours after birth, helps decide the level of the rectal pouch. A radiopaque marker, such as a coin, is taped to the perineum. 

If the distance between the air in the rectal pouch and the marker is greater than 1 cm, it suggests a high type ARM, while a distance of less than 1 cm indicates a low type.

​Other diagnostic tests, like ultrasound of the kidneys and echocardiogram, are performed to rule out associated congenital anomalies.

How do we fix it?

​The management of ARMs primarily involves surgery, tailored to the specific type of malformation.

For low-type ARMs, a primary Posterior Sagittal Anorectoplasty (PSARP), a definitive repair, is often performed shortly after birth. High-type ARMs, typically need a staged approach. This begins with a diverting colostomy in the newborn period to allow for bowel decompression and growth. The definitive repair, PSARP, is then performed at around 6–12 months of age. The colostomy is closed a few months after the PSARP, once the neo-anus has healed.

​The Posterior Sagittal Anorectoplasty (PSARP) is the gold standard surgical technique. It involves a midline incision from the sacrum to the perineum. The surgeon mobilizes the rectum. They ligate any fistulae. They then pull the rectum through the centre of the sphincter muscle complex to create a new anus. This procedure preserves the sphincter muscles, which are crucial for future bowel control.

​A vital part of the post-operative care is the anal dilation regimen.  This begins about two weeks after the PSARP and involves the use of Hegar dilators to gradually stretch the neo-anus. This is a crucial step to prevent anal stenosis, a common late complication. The regimen involves increasing the size of the dilators over several months.  While Hegar dilators are standard, surgeons use more affordable alternatives, like specially made candle dilators, in resource-limited settings. The goal is to achieve an age-appropriate anal calibre, which helps guarantee proper bowel function and prevents constipation. Patients are also prone to constipation and may need daily flushes to clear retained stools.

​ Long-term Outcome:

​The long-term prognosis for patients with ARMs depends on the type of malformation and the presence of associated anomalies.

• ​Low-type ARMs generally have a better prognosis and a higher likelihood of achieving good bowel control.
• ​High-type ARMs have a more guarded prognosis, and faecal incontinence and chronic constipation are common late complications. A poorly developed sacrum can be a marker for a poorer continence prognosis.

Many individuals with ARMs can achieve good functional outcomes and lead healthy lives with appropriate surgical management. And more importantly, consistent post-operative care, including the anal dilation regimen, is a must for a good functional outcome.