Anorectal Malformations (Imperforate Anus)

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Last updated: April 6, 2026Bookmark

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
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
  - < 1cm → low lesion
  - ≥ 1 cm → high lesion
- 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

Reference Intervals ›

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

Calculator ›

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