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:
    1. Primary neural developmental defect.
    2. 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.