Updated: 9/22/2018

Neural Tube Defects

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Snapshot
  • A 38-year-old woman at 12 weeks gestation presents to her obstetrician for prenatal care. She denies any vaginal discharge, bleeding, and at times notices fetal movement. She has a medical history of seizures. Laboratory testing is significant for an elevated α-fetoprotein level. On ultrasound, polyhydramnios is appreciated with an absence of the brain and calvaria. It was explained to her that her fetus is diagnosed with anencephaly.
Introduction
  • Embryology
    • neurulation
      • at the third week of gestation
      • the process of neural tube formation which gives rise to the
        • spinal cord and cerebrum
      • impairment in neurulation results in
        • neural tube defects
      • process
        • the notocord induces the formation of the neural plate
        • the neural plate (neuroectoderm) will give rise to the central nervous system
          • elevation of the lateral edges of the neural plate develop
            • the neural folds which eventually form the
              • neural tube
        • the neural tube fuses in the
          • cervical region and then fuses
            • in both the cephalad and caudal direction, eventually forming the
              • cranial neuropore
                • closes on day 25
                • forms three primary vesicles
                  • forebrain (prosencephalon) which forms into
                    • telencephalon
                    • diencephalon
                  • midbrain (mesencephalon) which forms into
                    • mesencephalon
                  • hindbrain (rhombencephalon) which forms into
                    • metencephalon
                    • myelencephalon
              • caudal neuropore 
                • closes on day 27
                • failure to close leads to spina bifida
        • the neural crest cells give rise to the
          • peripheral nervous system and non-neural structures
  • Etiologies
    • folic acid deficiency
    • syndromes such as
      • Chiari II malformation
    • medications such as
      • valproic acid and other
        • folic acid antagonists such as
          • carbamazepine
          • phenobarbital
          • phenytoin
          • sulfasalazine
          • triamterene
          • trimethoprim
Neural Tube Defects
 
Neural Tube Defects
Neural Tube Defect
Comments
Anencephaly
  • Cranial neural tube is exposed due to
    • failure of the rostral neuropore to close at day 25
  • Findings
    • ↑ AFP
    • ultrasonography shows
      • absence of the brain and calvaria
      • polyhydramnios
  • This is not compatible with survival
Spina bifida occulta 
  • Secondary to
    • failure of the caudal neuropore to close
      • without herniation
  • Findings
    • normal AFP
    • tuft of hair or skin dimple at site of bony defect
Spina bifida with meningocele
  • Secondary to
    • failure of the caudal neuropore to close
      • with protusion of the meninges through the bony defect
  • Findings
    • ↑ AFP 
    • associated with spina bifida cystica
Spina bifida with meningomyelcele
  • Secondary to
    • failure of the caudal neuropore to close
      • with protusion of the meninges and spinal cord through the bony defect
  • Findings
    • ↑ AFP
    • associated with Chiari II malformation
Spina bifida with myeloschisis
  • Secondary to
    • failure of the caudal neuropore to close
      • with the spinal cord able to be seen externally
  • Findings
    • ↑ AFP
    • ↑ AChE
 
 

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