Dysraphism of the Posterior (Spinal) Neural Tube
Dysraphism of the Posterior (Spinal) Neural Tube
- Located posterior to the point of initial neural tube closure (below the foramen magnum).
Terminology and Definitions
- Spinal Dysraphism: A broad spectrum of anomalies involving variable degrees of non-fusion of neural, vertebral, and mesenchymal tissues.
- Spina Bifida: Specifically refers to incomplete bony vertebral closure around the spinal cord.
- Dysraphic lesions categorized based on skin coverage:
- Open Dysraphism: Not skin-covered (e.g., myelomeningocele, myeloschisis).
- Closed Dysraphism: Skin-covered lesions (e.g., meningocele, lipomeningocele).
Open Spinal Dysraphism (OSD)
Pathophysiology
- Occurs due to regional failure of neural tube closure (~3rd week of gestation).
- Fundamental defect: Non-disjunction of neural and cutaneous ectoderm.
- Prevents mesenchyme from separating neural tube from skin, thus impeding vertebral development.
- Results in exposure of ependymal-lined central canal (placode) to external environment.
Types of Open Dysraphism
- Myeloschisis (Myelocele):
- Placode flush with skin surface.
- Myelomeningocele:
- Neural tissue displaced dorsally by expanded anterior subarachnoid space.
- Placode protrudes externally beyond skin level.
Pathogenesis: Two-Hit Hypothesis
- Neurological deficits result from:
- Primary neural developmental defect.
- Secondary damage from amniotic fluid exposure (chemical, inflammatory, physical insults).
Epidemiology
- Incidence: ~0.5–1.0 per 1000 live births (regional variability).
- Predominantly sporadic, multifactorial aetiology.
Aetiological Factors
- Folate Metabolism Disturbances:
- Antenatal folate significantly reduces OSD incidence.
- MTHFR gene mutations implicated in ~20% of cases.
- Genetic Syndromes (rare):
- Trisomy 18, Meckel–Gruber syndrome, Lehman syndrome.
- Teratogens & Maternal Factors:
- Antiepileptics: Valproate, Carbamazepine.
- Vitamin A toxicity.
- Maternal diabetes, obesity, hyperthermia.
Clinical Features and Associations
Location & Associated Malformations
- 80% occur at thoracolumbar or lumbosacral levels.
- Commonly associated with Chiari II Malformation:
- Hindbrain herniation due to CSF leak.
- Small posterior fossa, cerebellar crowding.
- Displaced cerebellar tonsils/vermis.
- Brain anomalies: Callosal agenesis, tectal beaking, periventricular heterotopias, enlarged massa intermedia, migrational disorders.
- Split cord malformation (diastematomyelia) in ~40%.
Prenatal Diagnosis
Diagnostic Tools
- Maternal serum alpha-fetoprotein (AFP):
- Elevated in open neural tube defects.
- Normal in closed neural tube defects.
- Amniotic Fluid Tests:
- AFP, acetylcholinesterase (confirmatory).
- Imaging:
- Fetal ultrasound/MRI identify spinal defect.
- Ultrasound signs: Lemon sign (calvarium shape), Banana sign (cerebellar shape).
- Associated findings: microcephaly (may resolve), macrocephaly (hydrocephalus), talipes equinovarus, hip dislocation.
- Karyotyping if other anomalies detected.
Differential Diagnoses
- Sacrococcygeal teratoma
- Closed neural tube defects (meningocele, lipomyelomeningocele, myelocystocele)
Prognosis
Factors Influencing Prognosis
- Level of spinal lesion
- Degree of posterior fossa crowding (Chiari II severity)
- Presence of hydrocephalus (85% of cases)
- Associated cerebral anomalies (impact cognition, seizures)
- Brainstem dysfunction (feeding, apnea, stridor; 35% mortality if present)
Survival and Outcomes
- 75% survive into adulthood.
- 14% mortality before age 5 despite intensive management.
- Cognitive Outcomes:
- IQ ~102 without hydrocephalus.
- IQ ~95 with uncomplicated shunt.
- IQ ~73 with shunt infection.
- ~70% IQ >80; ~50% independent as adults.
- Motor Function:
- Lesions below S1: ambulation unaided.
- L4–L5 lesions: 50% ambulatory with aids.
- Above L2 lesions: wheelchair dependency, scoliosis common.
- Bladder/Bowel Dysfunction: Nearly universal.
Management
Prevention
- Primary focus:
- Folate supplementation.
- Optimal dose: 4 mg/day starting ≥3 months pre-conception.
- Associated with 83% reduction in OSD.
Delivery Method
- Controversial best delivery mode.
- Pre-labour Caesarean section associated with improved lower-extremity outcomes.
- Less rigorous studies show no difference between vaginal and Caesarean deliveries.
Postnatal Surgical Management
- Surgical closure: 24–72 hours after birth.
- Early closure reduces infection risk.
- De-tether spinal cord; close dura and skin.
- Hydrocephalus management:
- Shunt placement (often deferred briefly to allow healing).
- Nearly half develop shunt complications within first year.
Fetal Surgery
MOMS Trial Findings
- Fetal myelomeningocele repair (19–26 weeks gestation, T1–S1 lesions):
- Decreased shunt-dependent hydrocephalus incidence.
- Reduction in Chiari II malformation severity.
- Concerns:
- Scar dehiscence, preterm birth risks.
- Requires strict candidate selection, protocol adherence.
Clinical Takeaways:
- OSD is a significant neural tube defect associated with extensive morbidity and high healthcare costs.
- Early detection and intervention significantly impact long-term outcomes.
- Multidisciplinary care essential (neurology, neurosurgery, rehabilitation, urology, orthopaedics).
Closed Spinal Dysraphism (CSD)
- Definition: Congenital spinal anomalies covered by skin.
- Examples include:
- Meningoceles
- Lipomeningoceles
- Lipomyelomeningoceles
- Myelocystoceles
- Spinal lipomas
- Usually present as skin-covered mass lesions.
- Chiari II malformation typically rare, but possible.
Key Clinical Features
- Skin-covered lesions, often raising the skin surface.
- Typically no significant reduction with antenatal folate supplementation.
- May be challenging to distinguish from open spinal dysraphism prenatally.
Specific Types of CSD
1. Meningoceles
- Description:
- CSF-filled meningeal sacs, covered by skin.
- Typically no neural elements within the sac.
- Underlying spinal cord usually intact, occasionally malformed into a placode.
- Anatomical distribution:
- Commonly thoracic; lumbar location less common (arising from secondary neurulation defects).
- Neurological outcome:
- Typically normal or near-normal initially.
- Potential late complications: cord tethering causing urinary, bowel, and motor disturbances.
- Rare complicated form:
- Contains neural elements or placode.
- Associated with Chiari II malformations and poorer prognosis.
2. Myelocystoceles
- Description:
- Hydromyelic distension of central canal of spinal cord herniating through vertebral defect, covered by skin.
- Types:
- Terminal Myelocystocele (caudal spinal cord).
- Cervical Myelocystocele (less common).
- Associated features (Terminal myelocystoceles):
- Linked with caudal regression syndrome.
- Commonly involves severe bladder, bowel, and lower limb motor deficits.
- Potential late-developing Chiari II malformations.
- Differential diagnosis:
- Sacrococcygeal teratoma.
- OEIS (omphalocele-exstrophy-imperforate anus-spinal defect) complex.
- Myelomeningocele.
3. Lipomyelomeningoceles
- Description:
- Skin-covered placode with adjacent lipoma.
- Lipoma continuous with subcutaneous fat.
- Typically prevents neural tissue protrusion; associated with cord tethering.
- Pathogenesis:
- Premature disjunction between neural and cutaneous ectoderm.
- Mesenchyme (future adipose tissue) interposed prematurely, impairing neural tube closure.
- Clinical outcomes:
- Generally favorable compared to open myelomeningoceles.
- Brain typically normal; Chiari II malformation rare.
- Urinary continence outcomes better.
4. Spinal Lipomas
- Associated mechanism:
- Early disjunction of neural and cutaneous ectoderm.
- Interposition of mesenchyme between neural folds developing into fatty tissue.
- Clinical significance:
- May lead to tethered cord syndrome, neurological dysfunction.
- Brain typically unaffected, no Chiari malformation.
Caudal Regression Syndrome
- Definition:
- Spectrum of lower spine anomalies due to maldevelopment or agenesis of caudal cell mass.
- Clinical associations:
- Dramatically increased risk (200-fold) in maternal diabetes.
- Outcomes:
- Severe urological, orthopedic, and motor complications depending on extent of malformation.
Sacrococcygeal Teratomas (SCT)
- Description:
- Rare fetal tumors arising at sacrococcygeal region.
- No neural tube defect or Chiari malformation.
- Features:
- More common in females; more malignant in males.
- Appearance: cystic, solid, or mixed; protrusion from sacral region.
- Clinical complications:
- Can cause high-output cardiac failure (vascular lesions).
- May result in polyhydramnios, fetal hydrops.
Diagnostic Considerations in CSD
- Prenatal imaging (Ultrasound/MRI) essential for diagnosis.
- Differential diagnosis includes open spinal dysraphism, sacrococcygeal teratomas, OEIS complex.
Prognosis and Management
- Prognosis varies widely:
- Good neurological outcome in uncomplicated meningoceles.
- Poorer outcome if associated with tethered cord syndrome, Chiari malformation, neurological elements involved.
- Lipomyelomeningoceles typically have better prognosis than open lesions.
- Myelocystoceles associated with significant morbidity due to underlying caudal regression.
- Management involves:
- Surgical repair of defect.
- De-tethering of spinal cord.
- Multidisciplinary approach (urology, orthopedics, rehabilitation).
- Monitoring for delayed complications (e.g., tethered cord, urinary dysfunction, motor deterioration).
Clinical Takeaways
- Closed spinal dysraphisms have distinct clinical, diagnostic, and prognostic implications compared to open lesions.
- Often associated with subtle but potentially significant long-term neurological outcomes, especially with tethered cord syndrome.
- Require careful prenatal diagnosis and individualized multidisciplinary management plans.