## Neural Tube Defects: Classification by Timing and Location **Key Point:** Anencephaly results from failure of closure of the **anterior neuropore** (cranial end) around week 4 of gestation, leading to complete absence of the cerebral hemispheres and vault of the skull. ### Pathogenesis of Anencephaly The anterior neuropore normally closes around **day 25** of embryonic development. Failure at this stage results in: - Absence of the cerebral hemispheres - Absence of the skull vault - Exposure of neural tissue to amniotic fluid - Incompatibility with life (stillbirth or neonatal death) ### Comparison of Neural Tube Defects | Defect | Location | Neuropore Involved | Timing of Closure Failure | Prognosis | | --- | --- | --- | --- | --- | | **Anencephaly** | Cranial | Anterior | ~Day 25 | Lethal | | **Spina bifida occulta** | Lumbar/sacral | Posterior | ~Day 27 | Benign, asymptomatic | | **Meningomyelocele** | Lumbar/sacral | Posterior | ~Day 27 | Variable; often requires surgery | | **Encephalocele** | Occipital/frontal | Anterior (partial) | ~Day 25 (incomplete) | Variable; depends on contents | **High-Yield:** Anencephaly is the most severe anterior neural tube defect and is **always fatal**. It is the second most common neural tube defect after spina bifida. ### Clinical Pearl **Maternal serum alpha-fetoprotein (AFP)** is markedly elevated in anencephaly (often >10 MoM), making it detectable on prenatal screening. Ultrasound shows the characteristic "frog-eye" appearance (exophthalmos) due to absence of the skull vault. **Mnemonic: ANTE** — **A**nencephaly = **N**o brain, **T**erminal, **E**arly closure failure (anterior neuropore) 
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