U01.12.004 Neural tube defects

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Neural tube defects (NTDs) are congenital malformations resulting from failure of the neural tube to close completely by the 4th week of embryonic development.

They are strongly associated with:

Maternal folate deficiency
Teratogenic drugs such as valproate and carbamazepine (antiepileptics that interfere with folate metabolism)

Pathophysiology

During the 4th week, the neural plate folds to form the neural tube, which later differentiates into the brain and spinal cord.
Incomplete closure leads to:
- Open NTDs (exposed tissue, ↑ AFP & AChE)
- Closed NTDs (no exposure, skin intact)

Diagnostic Markers

Test	Finding	Comment
Maternal Serum Alpha-Fetoprotein (AFP)	↑ in open NTDs	AFP leaks from exposed neural tissue
Amniotic Fluid Acetylcholinesterase (AChE)	↑ in open NTDs	Confirms open defect
Ultrasound	Visualization of spinal/cranial defects	Routine prenatal screening

Classification of NTDs

Spinal Dysraphism (Spinal NTDs)

Type	Type of Defect	Features	Key Clinical Points
Spina bifida occulta	Closed	Caudal neural tube fails to close, no herniation, dura intact	A tuft of hair or a skin dimple over the defect
Meningocele	Open	Herniation of meninges only	Fluid-filled sac, no neural tissue
Myelomeningocele	Open	Herniation of meninges + neural tissue (spinal cord/cauda equina)	Neurologic deficits below the lesion
Myeloschisis	Open	Completely exposed neural tissue, no skin or meningeal covering	The most severe form

Cranial Dysraphism (Cranial NTDs)

Type	Description	Clinical Findings
Anencephaly	Failure of the rostral neuropore to close → absence of forebrain and open skull	Polyhydramnios (↓ fetal swallowing), incompatible with life

Key Points to Remember

NTDs develop by week 4 → prevention depends on maternal folate before conception.
↑ AFP and AChE in amniotic fluid = open NTD.
Valproate and carbamazepine are major teratogens.
Anencephaly = absent brain and skull; spina bifida occulta = only bony defect.

Learning Objective

By the end of this topic, students should be able to:
Identify the major types, causes, and diagnostic findings of neural tube defects, and relate them to embryologic development and clinical presentations.