Residency · Residency · Neurosurgery

Myelomeningocele and Neural Tube Defects

Overview

Neural tube defects are congenital malformations resulting from failure of neural tube closure during the third to fourth week of embryogenesis. The spectrum ranges from anencephaly, which is incompatible with life, to spina bifida occulta, which is often entirely asymptomatic. Myelomeningocele is the most common open neural tube defect compatible with survival, involving herniation of the spinal cord and meninges through a vertebral bony defect. The incidence is approximately 0.5 to 1 per 1000 births, varying by geography and the degree of folic acid supplementation in the population. Periconceptional folic acid supplementation at 0.4 milligrams daily, or 4 milligrams daily for women with a prior affected pregnancy, has dramatically reduced the incidence.

Embryology

Normal Neural Tube Closure

The neural plate forms by day 18 of gestation. Primary neurulation involves the neural folds elevating and fusing in the midline between days 21 and 28. Closure begins at multiple initiation sites and progresses bidirectionally. The anterior neuropore closes by day 25 and the posterior neuropore by day 28. Secondary neurulation, which forms the conus medullaris and filum terminale from the caudal cell mass, occurs after day 28.

Failure of Closure

Failure of anterior neuropore closure produces anencephaly. Failure of posterior neuropore closure produces myelomeningocele, with the lumbosacral region being most common. Failure of secondary neurulation results in occult spinal dysraphism including tethered cord and lipomyelomeningocele.

Classification of Neural Tube Defects

Open Neural Tube Defects

Open NTDs involve exposed neural tissue. Anencephaly features an absent cranial vault and cerebral hemispheres and is incompatible with life. Encephalocele involves brain and meningeal herniation through a skull defect, most commonly occipital in Western populations and frontoethmoidal in Southeast Asia. Myelomeningocele features protrusion of the spinal cord and meninges through an open vertebral defect with the neural tissue, termed the neural placode, directly exposed. Myelocele is a variant where the neural placode lies flush with the skin surface and is less protuberant.

Closed Neural Tube Defects

Closed NTDs are skin-covered. Meningocele involves meningeal herniation through a bony defect with the spinal cord in its normal position. Lipomyelomeningocele features a fatty mass attached to the spinal cord or placode through a bony defect. Spina bifida occulta is a defect in the vertebral arch without herniation, usually discovered incidentally on radiography and present in 10 to 20 percent of the population.

Myelomeningocele: Detailed Discussion

Prenatal Diagnosis

Maternal serum alpha-fetoprotein is elevated in open NTDs and serves as a screening test at 15 to 20 weeks gestation. Ultrasound detects more than 90 percent of open NTDs by 18 to 20 weeks, revealing the "lemon sign" of frontal bone scalloping and the "banana sign" of cerebellar abnormality from the associated Chiari II malformation. Amniocentesis showing elevated AFP and acetylcholinesterase confirms an open NTD. Fetal MRI provides detailed anatomic characterization.

Associated Anomalies

Chiari II malformation is present in virtually 100 percent of myelomeningocele patients. It involves caudal displacement of the cerebellar vermis, fourth ventricle, and brainstem through the foramen magnum, caused by chronic CSF leak in utero that leads to underdevelopment of the posterior fossa. This can produce brainstem dysfunction including stridor, apnea, and dysphagia, particularly in neonates.

Hydrocephalus develops in 80 to 90 percent of patients from the Chiari II malformation and aqueductal obstruction, requiring shunting in approximately 80 percent. Tethered spinal cord is universal at birth, with symptomatic re-tethering after repair occurring in 10 to 30 percent. Orthopedic problems include clubfoot, hip dysplasia or dislocation, scoliosis, and kyphosis. Neurogenic bladder affects the majority of patients, often requiring clean intermittent catheterization. Latex allergy occurs in up to 60 percent, necessitating latex-free precautions from birth.

Neurological Level and Function

The neurological level determines motor function and ambulation potential. Thoracic-level lesions produce no lower extremity function and the patient is wheelchair-dependent. High lumbar lesions at L1-L2 allow hip flexion but only non-functional ambulation. Mid lumbar lesions at L3-L4 permit knee extension and hip adduction, enabling community ambulation with bracing. Low lumbar lesions at L5 allow ankle dorsiflexion and hip abduction with community ambulation, possibly requiring ankle-foot orthoses. Sacral lesions at S1-S3 preserve ankle plantarflexion with variable bowel and bladder function and permit full community ambulation.

Postnatal Closure

Closure should be performed within 24 to 72 hours of birth to minimize infection risk from meningitis and ventriculitis. The infant is positioned prone with gentle handling of the placode. The surgeon identifies the neural placode and releases it from surrounding skin and arachnoid attachments. The placode is reconstructed into a tube, a process called placode neurulation, and placed dorsal side down into the spinal canal. The dura is closed, potentially requiring dural substitutes or fascial grafts. Myofascial closure using paraspinal fascia and muscle flaps covers the repair, followed by skin closure that may require rotation flaps for large defects. Tension on the closure must be avoided as it promotes wound breakdown and CSF leak. Postoperatively, the infant is positioned prone or laterally, and head circumference is monitored for developing hydrocephalus.

The MOMS Trial

The Management of Myelomeningocele Study, published in 2011, was a landmark randomized controlled trial comparing prenatal repair before 26 weeks gestation with standard postnatal repair. Prenatal repair significantly reduced the need for shunting by 12 months from 82 percent to 40 percent, improved the Chiari II malformation with reversal of hindbrain herniation, improved motor function at 30 months including the ability to walk independently, and improved mental development and Bayley scores.

However, prenatal repair carries significant maternal risks including premature delivery at an average of 34 weeks versus 37 weeks, uterine dehiscence at the repair site, oligohydramnios, placental abruption, and the requirement for cesarean delivery in all future pregnancies. Fetal repair is now offered at specialized centers for selected patients meeting MOMS criteria: gestational age 19 to 25 weeks, MMC from T1 to S1, hindbrain herniation on MRI, and no kyphosis greater than 30 degrees. Fetoscopic repair is emerging as a less invasive alternative to open hysterotomy and is currently under investigation.

Hydrocephalus Management in MMC

Serial head circumference measurements after closure guide management. Progressive ventriculomegaly or clinical signs such as a bulging fontanelle or sunsetting prompt intervention. VP shunt insertion is the most common approach. ETV may be attempted in select cases but has lower success rates in MMC-associated hydrocephalus. Intervention is typically required within one to two weeks of closure if progressive.

Chiari II Malformation Management

Most patients remain asymptomatic from the Chiari II malformation itself. When symptomatic in neonates, presenting with stridor, apnea, feeding difficulties, or opisthotonos, the first step is ensuring the VP shunt is functioning, as shunt malfunction can exacerbate brainstem symptoms. If the shunt is functional and symptoms persist, posterior fossa decompression with suboccipital craniectomy, C1-C2 laminectomy, and duraplasty is performed. In older children, symptoms include headache, syringomyelia, and progressive scoliosis; tethered cord should be evaluated and released if present, as untethering may improve syringomyelia.

Tethered Cord in MMC

All MMC patients have a tethered cord at birth inherent to the malformation. Symptomatic re-tethering after repair occurs in 10 to 30 percent, manifesting as progressive scoliosis, worsening leg function, changes in bladder function, or back pain. MRI demonstrates a low-lying conus and adherent cord at the repair site, though interpretation after prior repair is difficult. Urodynamics provide important baseline data and a monitoring tool. Treatment is surgical untethering with re-exploration and release of adhesions from the placode. Prophylactic untethering is not recommended; surgery is reserved for symptomatic deterioration.

Occult Spinal Dysraphism

Lipomyelomeningocele

Lipomyelomeningocele presents as a subcutaneous lipoma attached to the spinal cord through a bony defect. It is skin-covered and often presents as a lumbar subcutaneous mass. The lipoma-cord attachment produces tethering. Surgical untethering involves debulking the lipoma, releasing the cord, and reconstructing the dura. Whether to operate on asymptomatic patients remains controversial, balancing surgical risk against natural history.

Dermal Sinus Tract

A dermal sinus tract is an epithelium-lined connection from the skin to the spinal cord or dura that carries a risk of ascending infection causing meningitis or abscess. Cutaneous stigmata include a dimple, hair tuft, or hemangioma located above the gluteal crease. Treatment requires complete surgical excision of the tract and any associated dermoid or epidermoid.

Split Cord Malformation

Split cord malformation, previously termed diastematomyelia, involves the spinal cord being divided by a bony septum (Type I) or fibrous septum (Type II). It is associated with tethering and may present with progressive neurological deficit. Treatment involves excision of the septum and untethering.

<image>Photograph of a newborn with a lumbosacral myelomeningocele showing the neural placode (exposed neural tissue) surrounded by the zona epitheliosa and transitional skin, with the open defect visible as a reddish-pink mass of neural tissue protruding through the midline lumbar back, covered by a moist transparent membrane</image>

<image>Sagittal T2-weighted MRI of the brain in a child with Chiari II malformation associated with myelomeningocele, demonstrating caudal displacement of the cerebellar vermis and brainstem through the foramen magnum, beaking of the tectum, a small posterior fossa, and associated hydrocephalus with dilated lateral ventricles</image>

<image>Intraoperative photograph during postnatal myelomeningocele closure showing the neural placode being reconstructed (neurulated) with the dorsal surface rolled inward, the dural edges being approximated over the placode, and the paraspinal myofascial flaps being mobilized for layered closure</image>

Clinical Pearls

Folic acid supplementation at 0.4 milligrams daily before conception reduces NTD risk by 50 to 70 percent; women with a prior NTD-affected pregnancy should take 4 milligrams daily. The MOMS trial changed clinical practice by demonstrating that prenatal MMC repair reduces shunt dependency and improves motor outcomes; eligible patients should be referred to fetal surgery centers. Latex allergy should always be assumed in MMC patients, with latex-free precautions implemented from birth and maintained throughout life. Shunt malfunction should always be excluded first in any MMC patient presenting with worsening neurological function, before attributing symptoms to tethered cord or Chiari II decompensation. Baseline urodynamic studies should be obtained early because changes in urodynamics may be the earliest sign of re-tethering, often preceding motor deficits. A sacral dimple below the gluteal crease is almost always benign, but dimples above the gluteal crease or those associated with other cutaneous stigmata such as a hairy patch, hemangioma, or skin tag warrant imaging to rule out occult dysraphism. Multidisciplinary care involving neurosurgery, urology, orthopedics, rehabilitation, and social work is essential for optimal long-term outcomes in MMC patients.

References

1. Adzick NS et al. A randomized trial of prenatal versus postnatal repair of myelomeningocele. N Engl J Med. 2011;364(11):993-1004.
2. Copp AJ et al. Neural tube defects: recent advances, unsolved questions, and controversies. Lancet Neurol. 2013;12(8):799-810.
3. McLone DG, Knepper PA. The cause of Chiari II malformation: a unified theory. Pediatr Neurosci. 1989;15(1):1-12.
4. Bowman RM et al. Spina bifida outcome: a 25-year prospective. Pediatr Neurosurg. 2001;34(3):114-120.
5. MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the MRC Vitamin Study. Lancet. 1991;338(8760):131-137.

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