Myelomeningocele is the most severe and complex form of spina bifida cystica, characterized by the herniation of both the spinal cord and meninges through a posterior vertebral defect. Almost universally co-occurring with Arnold–Chiari II malformation and hydrocephalus, its clinical management spans advanced fetal repair or early postnatal microsurgical closure paired with lifelong multidisciplinary care.
What Is Myelomeningocele?
Myelomeningocele is a severe form of neural tube defect in which both the spinal cord and meninges herniate through an opening in the vertebral column. The exposed neural tissue lies within a membranous sac or, in some cases, remains uncovered at birth. It represents the most complex and disabling variant of spina bifida cystica and is associated with lifelong neurological, orthopedic, and urological complications.
Under normal embryologic development, the neural tube completes its fusion by the end of the 4th gestational week. When this developmental sequence fails, the posterior vertebral arches do not form correctly, leaving the spinal canal unprotected and allowing neural tissue to protrude. Because functioning spinal nerve roots extend directly into the sac, myelomeningocele carries a far more significant neurological burden than an isolated meningocele. It is evaluated alongside related congenital central nervous system disorders such as Arnold–Chiari Malformation and pediatric hydrocephalus.
Embryology and Pathophysiology
Myelomeningocele arises from a primary failure of neurulation (neural tube closure) combined with a failure of mesodermal posterior arch vertebral bone formation. As a result, the meninges, cerebrospinal fluid (CSF), and neural placode elements herniate through an open posterior spinal defect.
Typical contents of the herniated sac include:
- Meningeal membranes (dura and arachnoid)
- Cerebrospinal fluid (CSF)
- Spinal cord parenchymal tissue
- Functional spinal nerve roots
The exposed neural tissue undergoes chemical damage from amniotic fluid exposure in utero and continues to deteriorate after birth if left unclosed. Nearly all cases are associated with Chiari II malformation, in which cerebellar tonsils and brainstem structures descend into the upper cervical spinal canal, obstructing CSF pathways to cause secondary hydrocephalus.
Common Locations
The anatomical level of the vertebral defect directly dictates the severity of motor and sensory impairment:
Lumbosacral Region (Most Common)
Affects lower lumbar and sacral spinal segments. Preserves upper extremity and trunk control, but impacts knee extension, foot dorsiflexion, bowel, and bladder control.
Thoracolumbar Spine
Involves lower thoracic and upper lumbar levels. Associated with more extensive lower extremity paralysis, abdominal core muscle weakness, and a higher risk of spinal deformities.
Cervical & High Thoracic Levels
Less common anatomical locations. Deficits are significantly more severe, impacting upper limb innervation, intercostal respiratory mechanics, and total body motor function.
Etiology and Risk Factors
Myelomeningocele results from a complex, multifactorial interplay of genetic susceptibility, maternal metabolic status, and environmental exposures during early embryogenesis:
- Inadequate Periconceptional Folic Acid Intake: Insufficient maternal folate prior to conception and during the first 28 days of pregnancy is the most well-established modifiable risk factor.
- Maternal Metabolic Conditions: Pre-gestational diabetes mellitus and maternal obesity significantly increase neural tube closure risks.
- Teratogenic Exposures: Exposure to antiepileptic medications (particularly valproic acid or carbamazepine) or maternal hyperthermia in early pregnancy.
- Genetic Predisposition: Family history of neural tube defects or specific chromosomal abnormalities.
Clinical Features and Presentation
The clinical presentation is readily apparent at birth and encompasses localized spinal findings along with systemic neurological complications:
Local Spinal & Neurological Deficits
- Open midline spinal defect or thin fluid-filled sac with delicate neural placode tissue
- Active cerebrospinal fluid (CSF) leakage if the membranous sac breaks
- Flaccid lower extremity weakness or complete paralysis below the lesion level
- Sensory dermatomal loss, absent deep tendon reflexes, and neurogenic bladder/bowel impairment
- Progressive orthopedic deformities (clubfoot, hip dislocation, scoliosis, or kyphosis)
Associated Central Nervous System Conditions
- Hydrocephalus: Present or developing in 80–90% of affected infants, requiring surgical CSF diversion (VP shunt or ETV).
- Chiari II Malformation: Hindbrain herniation causing potential stridor, swallowing difficulties, or central apneas.
- Tethered Cord Syndrome: Secondary anchoring of the spinal cord at the surgical repair site during childhood growth.
Prenatal Diagnosis
Advances in fetal medicine allow accurate prenatal screening and early surgical planning:
Targeted Prenatal Ultrasound
Identifies the posterior vertebral arch opening, protruding sac, and cranial signs resulting from Chiari II malformation (e.g., scalloping of frontal bones ["lemon sign"] and curved cerebellum ["banana sign"]).
Fetal MRI & Maternal Serum Testing
Fetal MRI provides high-resolution anatomical mapping of neural placode tissue, ventriculomegaly, and hindbrain herniation. Maternal Serum Alpha-Fetoprotein (MSAFP) screening reveals marked elevation in open neural tube defects.
Treatment and Management
Management mandates a specialized neurosurgical approach, offering both prenatal (fetal) and early postnatal repair options:
1. Postnatal Surgical Repair
The standard treatment protocol involves early microsurgical closure of the defect within the first 24 to 72 hours of life. The goals are to dissect and reconstruct the neural placode, achieve watertight dural closure, prevent central nervous system infection (meningitis), and reconstruct muscle/skin coverage over the spine.
2. Fetal (In Utero) Surgery
Selected pregnancies meeting MOMS trial criteria may undergo prenatal myelomeningocele repair between 19 and 26 weeks of gestation. Fetal repair halts amniotic fluid neurotoxicity, reverses hindbrain herniation (Chiari II), reduces the need for postnatal hydrocephalus shunting, and improves long-term motor outcomes.
3. Lifelong Multidisciplinary Care
Optimal long-term functional outcomes require ongoing coordination across pediatric neurosurgery, urology (clean intermittent catheterization and neurogenic bladder management), orthopedics, physical rehabilitation, and developmental pediatrics.
Prognosis
Overall prognosis depends on the anatomical level of the lesion, promptness of surgical closure, and management of associated hydrocephalus. While lower limb paralysis and neurogenic bowel/bladder dysfunction require long-term management, modern surgical techniques, shunt systems, and targeted physical rehabilitation allow many individuals with myelomeningocele to achieve functional mobility, independence, and normal cognitive development.
Frequently Asked Questions
What is myelomeningocele?
Myelomeningocele is the most severe form of spina bifida, where an opening in the spine allows both the spinal cord and protective meningeal membranes to push out into a visible sac on the back.
How does myelomeningocele differ from meningocele?
In a meningocele, the sac contains only fluid and meningeal membranes, leaving the spinal cord intact inside the spine. In a myelomeningocele, the spinal cord and nerve roots extend into the sac, causing permanent nerve damage.
What is Chiari II malformation in spina bifida?
Chiari II malformation is a brain anomaly present in nearly all infants with myelomeningocele, where parts of the cerebellum and brainstem slip downward into the top of the spinal canal, blocking fluid flow and causing hydrocephalus.
Why is fetal surgery performed for myelomeningocele?
Fetal surgery repairs the spinal defect in utero before birth, protecting the spinal cord from toxic exposure to amniotic fluid, reversing Chiari II malformation, and reducing the child's need for a VP shunt.
Can a person with myelomeningocele walk?
Ability to walk depends on the anatomical level of the spinal lesion. Children with lower lumbar or sacral defects frequently walk with braces or crutches, while those with higher defects may rely on wheelchairs for mobility.
How can myelomeningocele be prevented?
Taking a daily folic acid supplement (400 mcg to 4 mg daily) prior to conception and during early pregnancy reduces the risk of neural tube defects, including myelomeningocele, by up to 70%.