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Myelomeningocele (Meningomyelocele) - A Neonatologist's Deep Dive

Terminology note: The terms myelomeningocele (MMC) and meningomyelocele are used interchangeably. "Myelomeningocele" is the preferred modern term. It is a subset of open spina bifida and the most severe common form of neural tube defect (NTD) compatible with life.

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1. The Anatomy - What Exactly Is It?

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2. Embryology - Why Does It Happen?

1. First hit: Primary failure of neural tube closure at ~week 4
2. Second hit: Progressive damage from chronic exposure of the open neural placode to neurotoxic amniotic fluid throughout gestation - amniotic fluid contains substances directly toxic to neural tissue, and there is also mechanical trauma

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3. Epidemiology

• Incidence: approximately 1 in 3,000 live births (post-folate supplementation era)
• Folate supplementation has reduced MMC prevalence by ~50%, but a plateau has been reached
• 25-40% of MMC pregnancies are terminated following prenatal diagnosis
• Most common in lumbar and sacral regions (>75% of cases)
• Slight female predominance

• Maternal folate/B12 deficiency (most important modifiable factor)
• Valproic acid during early pregnancy (1-2% risk of NTD - one of the highest teratogen risks known)
• Carbamazepine
• Family history of NTDs
• Prior affected pregnancy
• Maternal obesity, diabetes, hyperthermia in early pregnancy

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4. Prenatal Diagnosis

• Maternal serum AFP (MSAFP) at 14-16 weeks - elevated in ~85% of open NTDs. Open defects leak AFP into amniotic fluid then into maternal blood.
• Amniotic fluid AFP + acetylcholinesterase (more specific than AFP alone)
• Second-trimester ultrasound (18-20 weeks): Detects >90% of NTDs

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5. Associated Anomalies - What Else to Look For

Chiari II Malformation (THE most important associated anomaly)

• Cerebellar vermis + caudal brainstem (medulla, sometimes pons) herniate through enlarged foramen magnum
• Small posterior fossa
• Kinked medulla
• Polymicrogyria can be seen

• Stridor (inspiratory) - from vocal cord palsy due to cranial nerve X dysfunction
• Apnea - central (brainstem compression) or obstructive
• Swallowing dysfunction - aspiration risk
• Central hypoventilation - life-threatening
• Opisthotonus in severe cases
• These symptoms = brainstem dysfunction = 35% mortality by 5 years even with repair

Hydrocephalus

• Present in 64-85% of MMC cases requiring VP shunt (lumbosacral lesions)
• Mechanism: obstruction of CSF flow at the fourth ventricle due to Chiari II malformation
• Higher lesion level = higher risk of shunt-dependent hydrocephalus
• Mean IQ with VP shunt: ~80 (low normal); lower than those who don't require a shunt
• Monitor for shunt malfunction throughout childhood

Other associations:

• Tethered cord syndrome - progressive neurological deterioration post-repair as cord gets re-tethered
• Latex allergy - critically important. Children with MMC have a dramatically higher prevalence (10-40%) of latex allergy from repeated early latex exposure during multiple procedures. Strict latex-free environment from day 1.
• Craniolacunia (Lückenschädel) - depressed non-ossified areas on inner calvaria
• Syringomyelia
• Orthopedic deformities (hip dislocation, clubfoot, scoliosis, kyphosis)
• Neurogenic bladder and bowel

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6. The Lesion Level - Functional Anatomy for Neonatologists

• Urinary retention with overflow incontinence if untreated
• Risk of upper urinary tract damage (hydronephrosis, reflux, renal failure)
• Clean intermittent catheterization (CIC) must be started early - often within days of birth
• Baseline renal ultrasound and urodynamics in first weeks of life

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7. Neonatal Presentation and Initial Assessment

At Delivery - What You See

• Intact sac: Translucent membrane covered sac containing CSF, meninges, and neural tissue - relatively protected
• Ruptured/exposed placode: Raw, red, weeping area - urgent surgical priority (meningitis risk)
• Look for motor posturing of lower limbs (flaccid = bad)

Neurological Examination in the Delivery Room

Immediate Neonatal Orders (First Hour)

1. Positioning: Prone or lateral decubitus - never supine over the sac. Place on a "doughnut" (ring pad) to protect the sac.
2. Sterile moist saline gauze over the defect (non-adherent) - prevents desiccation and infection
3. Latex-free environment immediately - use only latex-free gloves, equipment
4. Thermoregulation: High insensible losses from the exposed defect - prone to hypothermia and heat loss. Incubator.
5. Fluid management: Increase maintenance fluids to account for insensible losses from the sac
6. IV access - anticipate early surgery
7. Cranial ultrasound - assess ventricular size, rule out acute hydrocephalus
8. Renal ultrasound - baseline upper tract
9. Neurosurgical consultation - within hours for surgical planning
10. Urology consultation - for bladder management plan

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8. Timing and Type of Repair

Postnatal Repair

• Standard of care for decades
• Goal: Close within 24-72 hours of birth (reduces meningitis risk, though exact timing evidence varies)
• Rationale for urgency: open defect = direct route for ascending infection; CSF leak
• Surgical steps: separation of placode from surrounding skin, dural closure, fascial closure, skin closure
• Anesthesia considerations: prone positioning for intubation (doughnut under the sac), risk of brainstem herniation on induction, anticipate significant insensible fluid loss, vocal cord palsy risk

In Utero (Fetal) Repair - The MOMS Trial Revolution

• Reduced need for VP shunt placement before 1 year (40% vs 82%)
• Reversal of hindbrain herniation (Chiari II improvement)
• Better lower extremity motor function: 44.8% vs 23.9% walked independently
• Function ≥2 levels better than anatomic level: 26.4% vs 11.4%
• Better Bayley Mental Development Index scores
• Better Peabody Motor Scale scores

• Higher risk of preterm birth, preterm PROM
• Uterine dehiscence at delivery (11%)
• Maternal hysterotomy scar (affects future pregnancies - cesarean delivery required)
• Two perinatal deaths in each group

• Singleton pregnancy
• MMC between T1 and S1 with hindbrain herniation
• Gestational age 19-25+6 weeks at evaluation
• Normal chromosomes
• No other major fetal anomalies
• No maternal contraindications

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9. Postoperative Neonatal Care

• Positioning: Still prone for 5-7 days to allow wound healing; avoid pressure on repair
• Monitor for hydrocephalus: Head circumference daily; serial cranial ultrasounds; fontanelle examination. May develop or worsen post-repair as CSF dynamics change.
• VP shunt decision: Most will need a shunt (64-85% of lumbosacral MMC). Usually placed days to weeks after closure.
• Wound care: Monitor for CSF leak at repair site, infection
• Ventilatory monitoring: Apnea, central hypoventilation from Chiari II - may require prolonged monitoring, some require home apnea monitor
• Feeding: Assess swallow function before oral feeds - risk of aspiration with Chiari II-related bulbar dysfunction. NG feeds initially.
• Bladder management: Initiate CIC as soon as urology clears (typically within days)

• Ophthalmology: strabismus, papilledema
• Audiology: sensorineural hearing loss can be associated
• Neurodevelopmental team engagement
• Social work / family counseling

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10. Long-Term Complications and Follow-up (The Lifespan Perspective)

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11. Prevention - What You Tell Parents and Primary Care

• ALL women of childbearing age: 0.4 mg (400 mcg) daily (CDC recommendation)
• Women with a prior NTD pregnancy or on antiepileptic drugs: 4 mg/day (10x dose), starting 1 month before conception and continuing for first 3 months
• Mechanism: folate is essential for DNA synthesis, methylation reactions, and neural tube closure. Deficiency impairs closure at exactly the critical window (days 18-28)
• Fortification of cereal/grain products in many countries has reduced NTD rates by ~30-50%

• Valproic acid (1-2% NTD risk)
• Carbamazepine
• Trimethoprim (folate antagonist)
• Methotrexate

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12. Quick Summary for the Bedside

1. Posterior midline sac (lumbosacral most common)
2. Flaccid lower limbs or absent voluntary movement
3. Absent anal wink
4. Macrocephaly / bulging fontanelle (hydrocephalus)
5. Possible stridor (Chiari II vocal cord palsy)

1. Latex-free protocol - from this moment
2. Prone positioning with sac protected (sterile moist gauze + doughnut)
3. Thermoregulation in incubator (high insensible losses)
4. IV access + extra fluids
5. Cranial ultrasound + renal ultrasound
6. Urgent Neurosurgery + Urology consults

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