A 28-year-old primigravida from rural Maharashtra presents at 18 weeks of gestation for antenatal screening. Ultrasound reveals a large defect in the lumbosacral region with herniation of meninges, cerebrospinal fluid, and neural tissue through the vertebral defect. The fetus shows associated lower limb paralysis and loss of sphincter control. Maternal serum alpha-fetoprotein (AFP) is markedly elevated. What is the most likely diagnosis?

## Diagnosis: Myelomeningocele **Key Point:** Myelomeningocele is the most common and severe form of open neural tube defect (NTD), characterized by herniation of both meninges AND neural tissue (spinal cord) through a vertebral defect. ### Clinical Features of Myelomeningocele | Feature | Myelomeningocele | Meningocele | Anencephaly | Encephalocele | |---------|------------------|-------------|-------------|---------------| | **Contents** | Meninges + spinal cord | Meninges only | Absent brain vault | Brain tissue + meninges | | **Neurological deficit** | Yes (paralysis, incontinence) | No or minimal | Incompatible with life | Variable | | **AFP elevation** | Marked | Mild–moderate | Severe | Moderate | | **Location** | Lumbosacral > thoracic | Lumbosacral | Occipital region | Occipital > frontal | | **Prognosis** | Guarded; lifelong disability | Good if closed early | Lethal | Fair–good | **Clinical Pearl:** The presence of **lower limb paralysis and sphincter dysfunction** is pathognomonic for myelomeningocele — it indicates spinal cord involvement. Meningocele does not cause neurological deficits because the cord is not herniated. ### Embryological Basis 1. **Neural tube closure defect** occurs between weeks 3–4 of gestation (primary neurulation). 2. **Failure of fusion** of neural folds in the midline → persistent communication between neural canal and amniotic cavity. 3. **Herniation of spinal cord** through the vertebral defect → exposure to amniotic fluid → further damage and scarring. 4. **Marked AFP leakage** into amniotic fluid and maternal circulation due to open defect. ### Why Markedly Elevated AFP? **High-Yield:** Open neural tube defects (anencephaly, myelomeningocele) cause **massive AFP leakage** from exposed fetal tissue into amniotic fluid and then into maternal serum. This makes **maternal serum AFP > 2.5 MoM (multiples of median)** a sensitive screening marker for open NTDs at 15–20 weeks gestation. ### Management Overview ```mermaid flowchart TD A[Prenatal diagnosis of myelomeningocele]:::outcome --> B{Gestational age?}:::decision B -->|< 24 weeks| C[Counseling + termination option]:::action B -->|≥ 24 weeks| D[Fetal MRI for assessment]:::action D --> E[Delivery planning]:::action E --> F[Postnatal neurosurgical closure within 48 hrs]:::action F --> G[Long-term: orthopedic, urological, rehabilitation care]:::action ``` **Mnemonic for NTD types: MACE** - **M**yeloMeningocele (cord + meninges) - **A**nencephaly (absent brain vault) - **C**erebral/spinal defects - **E**ncephalocele (brain herniation) **Key Point:** Folic acid supplementation (5 mg daily) **periconceptionally and throughout first trimester** reduces NTD risk by ~70%. This is why all women of reproductive age in India are advised folic acid supplementation.