Dysraphism of the Entire Neural Tube

Craniorachischisis Totalis

  • Most severe form of neural tube defect.
  • Results from complete failure of neurulation.
  • Entire neural plate remains uncovered by mesoderm and ectoderm (skin).
  • Lesions are incompatible with life.
  • Typically leads to spontaneous abortion early in gestation.

Dysraphism of the Anterior Neural Tube

Anencephaly

  • Results from failure of neural tube closure anterior to the foramen magnum.
  • Severe forms extend forward to anterior neuropore at lamina terminalis.
  • Leaves entire dorsal surface of cerebrum and brainstem uncovered.
  • No identifiable neural structures above the brainstem on fetal imaging.
  • Almost universally fatal shortly after birth.
Alternative Pathogenesis Explanation:
  • May arise from primary developmental failure of:
    • Overlying mesoderm (skull, meninges).
    • Non-neural ectoderm (skin, scalp).
  • Secondary degeneration of exposed prosencephalic structures.

Encephaloceles

  • Localized defects of neural tube closure anterior to the foramen magnum.
  • Extracranial protrusion of a cystic mass containing:
    • Meninges
    • Neural tissue
    • Cerebrospinal fluid (CSF)

Types and Locations:

  • Occipital encephaloceles (most common)
  • Frontal encephaloceles (often extend into nasal cavity; called frontoethmoidal encephaloceles)
  • Temporal and Parietal encephaloceles (less common)

Classification:

  • Meningoencephalocystocele:
    • Extracranial extension involving ventricular system.
  • Cranial Meningocele:
    • CSF-filled lesion without neural tissue.

Characteristics of Occipital Encephaloceles:

  • Commonly involve:
    • Occipital lobes
    • Possibly cerebellar and brainstem tissue
  • Low occipital encephaloceles extending into cervical spine:
    • Associated with Chiari III malformation (downward herniation of cerebellar tissue).

Associated Intracranial Complications:

  • Hydrocephalus (up to 50% of patients)
  • Microcephaly
  • Subependymal heterotopias
  • Agenesis of corpus callosum

Diagnostic Considerations:

  • Often covered by skin; normal levels of maternal and amniotic fluid alpha-fetoprotein (AFP).
  • Pathogenesis likely multifactorial involving:
    • Environmental factors:
      • Early gestational hyperthermia
      • Radiation exposure
      • Hypervitaminosis A
      • Maternal diabetes
  • May occur as part of recognized syndromes:
    • Meckel-Gruber syndrome
    • Walker-Warburg syndrome