During a routine second-trimester ultrasound at 18 weeks, a fetal neural tube defect is suspected. Which is the most common site of neural tube defect in the Indian population?

Explanation

## Neural Tube Defects: Site Distribution ### Most Common Site of NTD **Key Point:** Lumbosacral spina bifida is the most common site of neural tube defect globally and in the Indian population, accounting for approximately 75–80% of all spinal dysraphisms. ### Epidemiology and Site Distribution | Site | Frequency | Clinical Features | Prognosis | |------|-----------|-------------------|----------| | **Lumbosacral** | 75–80% | Lower limb paralysis, bowel/bladder dysfunction, clubfoot | Variable; depends on level | | **Thoracic** | 10–15% | Paraplegia, scoliosis, respiratory compromise | Generally poor | | **Cervical** | 5–10% | Upper limb weakness, tetraplegia | Severe; often incompatible with life | | **Occipital** | <5% | Encephalocele, hydrocephalus | Often lethal | ### High-Yield Facts **High-Yield:** The lumbosacral region is the last to close embryologically (around week 6), making it the most vulnerable to failure of neural tube closure. This explains why it is the most common site of NTD. **Clinical Pearl:** In the Indian population, the prevalence of neural tube defects is higher than in Western populations, with lumbosacral spina bifida being the predominant form. This is partly due to lower folate intake and higher rates of consanguinity in certain regions. ### Ultrasound Findings at Different Sites ```mermaid flowchart TD A[Neural Tube Defect Suspected]:::outcome --> B{Location on USG?}:::decision B -->|Lumbosacral| C[Spina bifida aperta<br/>Loss of skin coverage<br/>Meningocele/Myelomeningocele]:::action B -->|Thoracic| D[Severe kyphosis<br/>Meningomyelocele<br/>Hydrocephalus common]:::action B -->|Cervical| E[Severe neurological deficit<br/>Often associated with<br/>Klippel-Feil syndrome]:::action B -->|Occipital| F[Encephalocele<br/>Severe hydrocephalus<br/>Poor prognosis]:::urgent C --> G[Measure alpha-fetoprotein<br/>Confirm with MRI]:::action D --> G E --> G F --> G ``` ### Mnemonic for NTD Sites **"LUMBOSACRAL LAST"** — The lumbosacral region closes last embryologically (week 6), making it the most common site of neural tube defect (75–80%). ### Associated Findings on Prenatal USG - **Lumbosacral NTD:** Splaying of posterior elements, loss of skin coverage, meningocele or myelomeningocele sac, clubfoot, lower limb contractures, polyhydramnios (if severe). - **Hindbrain herniation (Chiari II malformation):** Banana sign (curved cerebellum), lemon sign (frontal bone scalloping), ventriculomegaly. - **Elevated maternal serum alpha-fetoprotein (MSAFP):** Levels 2–3 times normal in open NTD; normal in closed NTD. **Warning:** Closed spina bifida (with intact skin) may not be detected on ultrasound alone; MRI is required for confirmation and surgical planning. [cite:Callen's Ultrasonography in Obstetrics and Gynecology 6e Ch 8]