CNS Pathology: Developmental Abnormalities Related to Neural Tube Defects and Other Malformations

Neural Tube Defects

  • Neural Plate and Tube Formation

    • The neural plate invaginates to form the neural tube.
    • Neural crests develop into the peripheral nervous system (PNS).
    • The wall of the tube develops into the central nervous system (CNS) while the lumen becomes the ventricles and spinal canal.

  • Closure of Neural Tube

    • Forms from the cranial to the caudal aspect (head to lower back).
    • Incomplete closure leads to neural tube defects.
    • Associated with low folate levels, particularly important before conception for prevention.

  • Detection

    • Neural tube defects can be detected during prenatal care via elevated alpha-fetoprotein (AFP) levels in amniotic fluid and maternal blood.

  • Types of Neural Tube Defects

    • Anencephaly

    • Results from failure of closure at the cranial end.

    • Characterized by absence of the skull and brain, leading to prominent eyes (frog-like appearance).

    • Associated with maternal polyhydramnios - Lack of CNS swallowing centers results in increased amniotic fluid.

    • Spina Bifida

    • Failure to close the posterior vertebral arch (caudal aspect).

    • May manifest as:

      • Spina Bifida Occulta: only a dimple or hair patch.
      • Meningocele: herniation of meninges.
      • Meningomyelocele: herniation of meninges and spinal cord.

Cerebral Aqueduct Stenosis

  • Definition

    • Congenital stenosis of the channel draining cerebrospinal fluid (CSF) from the third to fourth ventricles.
    • The choroid plexus produces CSF in the ventricles.

  • Consequences

    • Leads to accumulation of CSF, causing hydrocephalus (enlarged ventricles).
    • In infants, results in an enlarging head circumference.

Dandy Walker Malformation

  • Definition

    • Congenital failure to develop the cerebellar vermis, which separates the two sides of the cerebellum.

  • Presentation

    • Results in a massively dilated fourth ventricle with absent cerebellum, often with hydrocephalus.

Arnold Chiari Malformation

  • Definition

    • Congenital extension of the cerebellar tonsils through the foramen magnum.

  • Subtypes

    • Type 1: Generally asymptomatic.
    • Type 2: May obstruct CSF flow due to compression, leading to hydrocephalus.
    • High yield associations include meningomyeloceles and syringomyelia (a spinal cord lesion).

  • Important Associations

    • Hydrocephalus due to CSF obstruction.
    • Meningomyeloceles and syringomyelia in Type 2.