Disorders of Neural Tube Development
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:
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- 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:
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- 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:
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- Extracranial extension involving ventricular system.
- Cranial Meningocele:
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- CSF-filled lesion without neural tissue.
Characteristics of Occipital Encephaloceles:
- Commonly involve:
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- Occipital lobes
- Possibly cerebellar and brainstem tissue
- Low occipital encephaloceles extending into cervical spine:
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- 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:
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- Environmental factors:
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- Early gestational hyperthermia
- Radiation exposure
- Hypervitaminosis A
- Maternal diabetes
- May occur as part of recognized syndromes:
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- Meckel-Gruber syndrome
- Walker-Warburg syndrome