## Most Common Site of Neural Tube Defects **Key Point:** The lumbosacral region is the most common site for neural tube defects (NTDs), accounting for approximately 80% of all spina bifida cases globally and in the Indian population. ### Anatomical Basis The neural tube closes in a bidirectional manner: 1. Primary closure begins at the junction of the future cervical and thoracic regions (around day 21–22) 2. Closure proceeds both cranially (toward the anterior neuropore) and caudally (toward the posterior neuropore) 3. The posterior neuropore (lumbosacral region) closes last, around day 27–28 of gestation Because the lumbosacral region closes last, it has the longest window of vulnerability to teratogenic insults (folic acid deficiency, maternal diabetes, antiepileptic drugs, hyperthermia). ### Frequency Distribution of NTDs | Site | Frequency | Clinical Presentation | |------|-----------|----------------------| | Lumbosacral | 80% | Spina bifida, meningomyelocele, lower limb paralysis | | Thoracic | 10% | Paraplegia, bowel/bladder dysfunction | | Cervical | 5% | Upper limb involvement, respiratory compromise | | Occipital | <5% | Encephalocele, rare | **High-Yield:** In Indian populations, the incidence of NTDs is higher than in Western countries (1 in 250–500 births), and lumbosacral involvement remains the predominant form. **Clinical Pearl:** Lumbosacral NTDs often present with a visible tuft of hair, dimple, or subcutaneous mass over the lower back in newborns—signs that should prompt immediate imaging and neurosurgical evaluation. **Mnemonic:** **LAST** = Lumbosacral is the Anatomically Slowest To close (and thus most vulnerable). [cite:Langman's Embryology 14e Ch 17]
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