Congenital CNS malformations

Neural Tube Defects - The Unclosed Chapter

Failure of neural fold fusion in the 3rd-4th week gestation, linked to maternal folate (B9) deficiency.

• Anencephaly: No forebrain, "frog-like" appearance, polyhydramnios.
• Spina Bifida:
  • Occulta: Vertebral arch defect. Tuft of hair or skin dimple.
  • Meningocele: Meninges herniate through defect.
  • Myelomeningocele: Meninges and spinal cord herniate.

⭐ ↑ Acetylcholinesterase (AChE) in amniotic fluid is a highly specific confirmatory test, whereas ↑ Alpha-fetoprotein (AFP) is a sensitive screening marker found in both maternal serum and amniotic fluid.

Forebrain Anomalies - Frontal Lobe Flubs

• Holoprosencephaly (HPE): Failure of prosencephalon (forebrain) cleavage.
  • Spectrum: Alobar (no separation) → Semilobar → Lobar (least severe).
  • Associated with midline facial defects: cyclopia, proboscis, cleft lip/palate.
  • Genetic links: Mutations in Sonic Hedgehog (SHH) pathway genes.
  • Chromosomal ties: Strongly associated with Trisomy 13 (Patau) & Trisomy 18.

⭐ Maternal diabetes and alcohol use are significant risk factors for developing holoprosencephaly.

📌 Mnemonic: Prosencephalon problems in Patau syndrome.

Posterior Fossa Defects - Cerebellar Conundrums

• Chiari I Malformation
  • Cerebellar tonsillar ectopia (>5 mm) below foramen magnum.
  • Adult presentation: headaches, ataxia, cranial nerve palsies.
  • Strong association with syringomyelia.
• Chiari II (Arnold-Chiari)
  • Herniation of cerebellar vermis and medulla through foramen magnum.
  • Features: tectal beaking, small posterior fossa, hydrocephalus.
• Dandy-Walker Malformation
  • Agenesis of cerebellar vermis with cystic dilation of 4th ventricle.
  • Enlarged posterior fossa.
  • Presents with hydrocephalus and developmental delay.
• Joubert Syndrome
  • Agenesis of the vermis presenting with the "Molar Tooth Sign" on axial MRI.

⭐ Chiari II malformation is almost invariably associated with a lumbosacral myelomeningocele.

Syringomyelia - Central Canal Chaos

• Fluid-filled cavity (syrinx) within the cervical spinal cord, often associated with Chiari I malformation.
• Disrupts the anterior white commissure, leading to a bilateral, "cape-like" loss of pain and temperature sensation across the upper back and arms.
• Fine touch, vibration, and proprioception are typically preserved initially.

⭐ Look for a patient presenting with burns on their hands they didn't feel, a classic sign of lost pain/temp sensation with preserved dorsal column function.

High‑Yield Points - ⚡ Biggest Takeaways

• Neural tube defects are linked to maternal folate deficiency and detected by elevated α-fetoprotein (AFP) in maternal serum/amniotic fluid.
• Chiari II malformation involves cerebellar tonsillar herniation and is strongly associated with lumbosacral myelomeningocele.
• Dandy-Walker malformation shows an absent cerebellar vermis and cystic dilation of the 4th ventricle.
• Holoprosencephaly, a failure of forebrain cleavage, is associated with Trisomy 13 and SHH gene mutations.
• Syringomyelia presents with a "cape-like" bilateral loss of pain and temperature in the upper extremities.