An imperforate anus is a congenital condition where the normal anal opening is absent. It is often associated with abnormal pelvic muscles, sphincters, and innervation. It may include fistulas opening into the vaginal vault or perineum, colourethral fistulas, or a blind rectum. Rectourethal fistula is more common in boys, while a rectovestibular fistula is more common in girls.
Anorectal malformations are slightly more common in boys.
Spectrum of anorectal malformations
| Term | Description |
|---|---|
| Simple | Membranous covering or fistulas opening into the perineum |
| Complex | Cloaca or fistulas opening into the genitourinary tract. |
Wingspread classification (Anatomical level)
| Type | Anatomy | Key Features | Prognosis |
|---|---|---|---|
| Low | Rectum below the levator ani | Perineal fistula | Good continence |
| Intermediate | At the level of the levator ani | A fistula is often present | Variable |
| High | Above the levator ani | Complex fistulas | Poorer continence |
Peña Classification (Fistula-Based)
| Demographic | Fistulas |
|---|---|
| Males | Perineal fistula (low), rectobulbar urethral fistula, rectoprostatic urethral fistula, rectovesical fistula (high), no fistula (rare, severe) |
| Females | Perineal fistula, rectovestibular fistula (most common), rectovaginal fistula, cloaca (most complex) |
- Risk factors
- Genetic associations
- Currarino syndrome
- Townes-Brocks syndrome
- Pallister-Hall syndrome
- Trisomy 13
- Trisomy 18
- Trisomy 21
- Family history of imperforate anus
- Environmental risk factors
- Maternal diabetes
- Obesity
- Paternal smoking
- Genetic associations
- Associated anomalies
- VACTERL association
- MURCS
- OEIS complex
- Pathophysiology
- The hindgut ends in a common cavity known as the cloaca. The cloaca is divided by the urorectal septum into anterior (urogenital sinus) and posterior (anorectal canals)
- At weeks 7 – 8, the cloacal membrane breaks down to form the urogenital opening and anal opening.
- Mesenchymal migration forms the levator ani muscles, which establish the external anal sphincter complex
- Developmental errors in anorectal malformations include:
- Failure of cloacal septation → fistulas or cloaca (severe form)
- Abnormal cloacal membrane breakdown → imperforate anus
- Arrested caudal migration of the rectum → high lesion (above the levator ani) or low lesion (below the levator ani)
- Abnormal mesodermal development → incontinence and sacral anomalies
- Abnormal innervation → tethered cord and spinal dysraphism → poor voluntary bowel control and neurogenic bowel dysfunction
- Signs and symptoms
- Absent anal opening
- Failure to pass meconium
- Abdominal distension
- Meconium in the perineum (perineal fistula), urethra (rectourethral fistula), or vagina (rectovaginal/vestibular fistula)
- Anal pit or misplaced opening
- Flat perineum or absent gluteal folds
- In girls:
- 1 opening → cloaca
- 3 openings → vestibular fistula
- Urinary tract infection
- Especially in boys due to a colourethral fistula
- Differentials
- Anal stenosis
- Anteriorly placed anus – this is a normal variant
- Cloacal anomalies
- Other fistulous conditions
- Investigations
- Prone cross-table lateral X-ray
- Perineal ultrasound
- Distal colostogram to define fistula anatomy
- Abdominopelvic ultrasound
- MRI of the pelvis to assess the pelvic musculature and spinal cord (tethered cord)
- Echocardiography to rule out cardiac defects
- Spine imaging
- Treatment
- Nil per oral
- Nasogastric tube for decompression
- Intravenous fluids and parenteral nutrition
- Evaluate within 24 – 36 hours
- Surgical repair
- Primary anoplasty (perineal approach) for low lesions
- High/complex lesions have a 3-stage approach: diverting colostomy → definitive repair (Posterior sagittal anorectplasty – PSARP) → colostomy closure
- Immediate surgical repair for the supralevator type
- May delay surgical repair if there is an external opening
- Urinary catheter
- Anal dilatations 2 weeks after surgery to prevent strictures
- Monitor feeding and wound healing
- Education about stoma and skin care
- Complications
- Sepsis
- Wound infection
- Anastomotic leak
- Anal stenosis
- Recurrent fistula
- Rectal prolapse
- Constipation
- Incontinence