Neural Tube Defects

Neural tube defects (NTDs) are the 2nd-most common type of congenital birth defects. Neural tube defects can range from asymptomatic (closed NTD) to very severe malformations of the spine or brain (open NTD). Neural tube defects are caused by the failure of the neural tube to close properly during the 3rd and 4th week of embryological development. The most common type of open NTD is meningomyelocele, which involves both the meninges and neural tissue. The etiologies of NTD are multifactorial, ranging from maternal nutrition to genetic determinants. Prenatal diagnosis by ultrasound and maternal α-fetoprotein level is common. Management of open NTDs is mainly surgical.

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Overview

Definition

A neural tube defect (NTD; spinal dysraphism) is a congenital defect in the covering of the central nervous system, resulting from a failure of the neural tube to close spontaneously during the 3rd or 4th week of embryonic development.

Classification

  • Open NTD (80% of cases): a midline defect of the vertebral bodies with different degrees of protrusion of the meninges and central nervous system (CNS)
    • Defect along the spinal cord:
      • Meningocele: Only the meninges protrudes.
      • Meningomyelocele: Both meninges and spinal cord protrude (most common NTD).
    • Defect of the cranium:
      • Cranial meningocele: Meninges protrude.
      • Cranial encephalocele: Both the meninges and brain stem/cerebellum/cerebral cortex protrude. 
      • Anencephaly: complete failure of the cephalic neural tube to close resulting in fully exposed fetal brain (not compatible with life)
  • Closed NTD: a midline defect of the vertebral bodies without protrusion of the meninges or neural tissue
    • Without subcutaneous mass: spina bifida occulta
    • With subcutaneous mass: 
      • Lipomyelomeningocele
      • Lipomeningocele

Epidemiology

  • 1 in 1,500 births in the United States 
  • 2nd-most common major congenital anomalies (most common are cardiac anomalies)
  • Incidence:
    • Has decreased with prenatal folic acid supplementation and screening
    • Varies by ethnicity and geographic region
  • Female neonates are affected more commonly.
  • Maternal risk factors during pregnancy:
    • Folate deficiency 
    • History of NTD in previous children (5%10% recurrence with 12 affected siblings)
    • Folic acid-depleting medications (valproic acid, triamterene, trimethoprim, sulfasalazine)
    • Poorly controlled maternal diabetes
    • Maternal obesity

Embryology

  • Conception to 3rd week of gestation (gastrulation): 
    • Single cell → single-layered blastula  → 3-layer gastrula: ectoderm, endoderm, and mesoderm
  • 3rd4th week:  
    • Ectoderm → differentiates into neuroectoderm (creating neural plate) → cell replication in neural plate →  neural crests + neural fold
    • Mesoderm → differentiates into notochord  signals neural fold to enlarge/fuse →  neural tube (neurulation)
    • Neural tube “zips up” from the middle outward → cranial (head) and caudal (tail) openings close approximately on days 24 and 28
Timeline Neurulation

Neurulation: the differentiation and growth of the neural plate into the neural tube during the first trimester of gestation.
PNS: Peripheral nervous system

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Clinical Presentation

Most NTDs are discovered during prenatal screening. Open NTDs are evident at birth, but closed NTDs may have more subtle presentations.

Table: Clinical features of major neural tube defects
NameDescriptionClinical features
MyelomeningoceleSpinal cord and meningeal herniationThe higher the lesion in the spine, the more severe the symptoms:
  • Brainstem: type 2 Chiari malformation (cerebellar tonsillar herniation) with hydrocephalus
  • Neck: quadriplegia
  • Lumbosacral defect:
    • Paraplegia/flaccid paralysis
    • Loss of touch and pain sensations in lower limbs
    • Anesthesia in perianal area
    • Bowel + bladder incontinence
MeningoceleMeningeal herniation without spinal cord involvement
  • Fluctuant mass
  • Transilluminates
  • Covered by skin
EncephaloceleMeninges ± brain tissue protruding from cranial defect
  • Hydrocephalus
  • Spasticity
  • Paralysis
  • Seizures
  • Microcephaly
  • Developmental delay
  • Vision problems
  • Developmental and growth retardation
AnencephalyMajor component of the brain and skull is absent.
  • Stillbirth/early death
  • Spastic with deafness/blindness
Spina bifida occultaMidline vertebral body fusion defect without protruding dura or neural tissue
  • Usually asymptomatic
  • If spinal cord is involved (tethered cord syndrome), may develop incontinence, constipation, or ataxia
  • May be accompanied by abnormal overlying skin with a hemangioma, discoloration, pit, lump, dermal sinus, or hairy patch
Types of NTD and their respective defect

Types of NTD and their respective defect

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Diagnosis

Prenatal screening

  • Serum/amniotic α-fetoprotein (maternal serum α-fetoprotein; MSAFP): high levels at 1520 weeks of gestation suggestive of NTD, but not diagnostic
  • Ultrasound: Visualization of defect is 98% specific and can be used to confirm diagnosis (closed NTD may not be detected).

Postnatal imaging

  • Open NTD: ultrasound commonly used; magnetic resonance imaging (MRI) can be used if further detail required
  • Closed NTD: ultrasound indicated with following cutaneous signs:
    • Palpable subcutaneous mass
    • Hairy patch overlying spine
    • Dermal sinus
    • Dimples > 5 mm deep or > 25 mm from anal verge
    • Skin tags or tail-like appendages
    • Hyperpigmented patches or deviation of the gluteal cleft

Management

Prenatal

  • Prevention with folic acid (vitamin B9) supplementation:
    • Decreases the incidence of NTD by around 70%
    • Preconception dose: 0.4 mg/day of folic acid for at least 1 month prior to conception and throughout pregnancy 
    • A higher dose (usually 4 mg/day) is indicated for the following:
      • A previous pregnancy affected by an NTD
      • A positive family history for NTD
      • Use of folic acid-depleting medications
      • Pre-gestational diabetes
  • Delivery: cesarean section recommended

Postnatal

  • Closed NTD: 
    • May require no intervention
    • Ongoing follow-up for lower neurological symptoms (incontinence, constipation, difficulty ambulating) 
  • Open NTD:
    • Keep warm and in prone position.
    • Cover NTD with sterile wet dressing. 
  • Surgical management:
    • Neurosurgical intervention for all open NTDs
    • Closed NTD typically does not require surgery

References

  1. Boon, R. L. (2010). Textbooks of Paediatrics.
  2. Winn, H. R. (2011). Youmans Neurological Surgery E-Book. Elsevier Health Sciences.
  3. Goetzl, L.M. Folic acid supplementation in pregnancy. UpToDate. Retrieved Nov 2, 2020, from https://www.uptodate.com/contents/folic-acid-supplementation-in-pregnancy?search=neural%20tube%20defects&topicRef=460&source=see_link#H17
  4. Dukhovny, S., Wilkins-Haug, L. Open neural tube defects: Risk factors, prenatal screening and diagnosis, and pregnancy management. UpToDate. Retrieved Nov 2, 2020, from https://www.uptodate.com/contents/open-neural-tube-defects-risk-factors-prenatal-screening-and-diagnosis-and-pregnancy-management?search=neural%20tube%20defects&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1#H27

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