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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
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 nutritionEvaluate 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
Biochemistry
ACTH P: <80 ng/L ALT P: 5–35 U/L Albumin P: 35–50 g/L Aldosterone P: 100–500 pmol/L Alk. phosphatase P: 30–130 U/L α-Amylase P: 0–180 IU/dL α-Fetoprotein S: <10 kU/L Angiotensin II P: 5–35 pmol/L ADH P: 0.9–4.6 pmol/L AST P: 5–35 U/L Bicarbonate P: 24–30 mmol/L Bilirubin P: 3–17 μmol/L BNP P: <50 ng/L CRP P: <10 mg/L Calcitonin P: <0.1 mcg/L Calcium (ionized) P: 1.0–1.25 mmol/L Calcium (total) P: 2.12–2.60 mmol/L Chloride P: 95–105 mmol/L Cholesterol P: <5.0 mmol/L VLDL P: 0.128–0.645 mmol/L LDL P: <2.0 mmol/L HDL P: 0.9–1.93 mmol/L Cortisol AM P: 450–700 nmol/L Cortisol Midnight P: 80–280 nmol/L CK ♂ P: 25–195 U/L CK ♀ P: 25–170 U/L Creatinine P: 70–100 μmol/L Ferritin P: 12–200 mcg/L Folate S: 2.1 mcg/L FSH P: 2–8 U/L ♂; >25 menopause GGT ♂ P: 11–51 U/L GGT ♀ P: 7–33 U/L Glucose (fasting) P: 3.5–5.5 mmol/L Growth hormone P: <20 mu/L HbA1C (DCCT) B: 4–6% HbA1C (IFCC) B: 20–42 mmol/mol Iron ♂ S: 14–31 μmol/L Iron ♀ S: 11–30 μmol/L Lactate (venous) P: 0.6–2.4 mmol/L Lactate (arterial) P: 0.6–1.8 mmol/L LDH P: 70–250 U/L LH P: 3–16 U/L Magnesium P: 0.75–1.05 mmol/L Osmolality P: 278–305 mosmol/kg PTH P: 0.8–8.5 pmol/L Potassium P: 3.5–5.3 mmol/L Prolactin ♂ P: <450 U/L Prolactin ♀ P: <600 U/L PSA P: 0–4 mcg/mL Protein (total) P: 60–80 g/L Red cell folate B: 0.36–1.44 μmol/L Renin (erect) P: 2.8–4.5 pmol/mL/h Renin (recumbent) P: 1.1–2.7 pmol/mL/h Sodium P: 135–145 mmol/L TBG P: 7–17 mg/L TSH P: 0.5–4.2 mU/L T4 P: 70–140 nmol/L Free T4 P: 9–22 pmol/L TIBC S: 54–75 μmol/L Triglycerides P: 0.50–2.3 mmol/L T3 P: 1.2–3.0 nmol/L Troponin T P: <0.1 mcg/L Urate ♂ P: 210–480 μmol/L Urate ♀ P: 150–390 μmol/L Urea P: 2.5–6.7 mmol/L Vitamin B12 S: 0.13–0.68 nmol/L Vitamin D S: 50 nmol/L
Arterial Blood Gases
pH 7.35–7.45 PaCO₂ 4.7–6.0 kPa PaO₂ >10.6 kPa Base excess ±2 mmol/L
Urine
Cortisol (free) <280 nmol/24h Hydroxyindole acetic acid 16–73 μmol/24h Hydroxymethylmandelic acid 16–48 μmol/24h Metanephrines 0.03–0.69 μmol/mmol cr. Osmolality 350–1000 mosmol/kg 17-Oxogenic steroids ♂ 28–30 μmol/24h 17-Oxogenic steroids ♀ 21–66 μmol/24h 17-Oxosteroids ♂ 17–76 μmol/24h 17-Oxosteroids ♀ 14–59 μmol/24h Phosphate (inorganic) 15–50 mmol/24h Potassium 14–120 mmol/24h Protein <150 mg/24h Protein/creatinine ratio <3 mg/mmol Sodium 100–250 mmol/24h
Haematology
WCC 4.0–11.0 ×10⁹/L RBC ♂ 4.5–6.5 ×10¹²/L RBC ♀ 3.9–5.6 ×10¹²/L Hb ♂ 130–180 g/L Hb ♀ 115–160 g/L PCV ♂ 0.4–0.54 L/L PCV ♀ 0.37–0.47 L/L MCV 76–96 fL MCH 27–32 pg MCHC 300–360 g/L RDW 11.6–14.6% Neutrophils 2.0–7.5 ×10⁹/L (40–75%) Lymphocytes 1.0–4.5 ×10⁹/L (20–45%) Eosinophils 0.04–0.44 ×10⁹/L (1–6%) Basophils 0–0.10 ×10⁹/L (0–1%) Monocytes 0.2–0.8 ×10⁹/L (2–10%) Platelets 150–400 ×10⁹/L Reticulocytes 0.8–2.0% / 25–100 ×10⁹/L Prothrombin time 10–14 s APTT 35–45 s
Paediatric
Pulse Rate (bpm) Neonate 140–160 Infant <1yr 120–140 1–5 years 110–130 5–12 years 80–120 >12 years 70–100 Respiratory Rate (tachypnoea) 0–2 months ≥60/min 2–12 months ≥50/min 1–5 years ≥40/min >5 years ≥30/min Blood Pressure (mmHg) Term 65/45 1 year 75/50 4 years 85/60 8 years 95/65 10 years 100/70 Weight Formulas 3–12 months (a + 9)/2 kg 1–6 years 2a + 8 kg >6 years (7a − 5)/2 kg Haemoglobin (g/dL) Term newborn 13–20 1 month 11–18 2 months 10–15 1–2 years 10–13 >2 years 11–14 MUAC (6 months–5 years) Obese >17.5 cm Normal 13.5–17.4 cm At risk 12.5–13.4 cm Moderate malnutrition 11.5–12.4 cm Severe malnutrition <11.5 cm Developmental Milestones Social smile 1.5 months Head control 4 months Sits unsupported 7 months Crawls 10 months Stands unsupported 10–12 months Walks 12–13 months Talks 18 months CSF WBC (/mm³) Term newborn 0–25 >2 weeks 0–5