neural tube defect

Neural tube defects arise from incomplete closure of the neural tube, which normally occurs between days 21-28 of gestation (closure begins in the cervical region and proceeds bidirectionally). The neural tube gives rise to the brain and spinal cord; failure of the cranial end to close results in anencephaly (absence of major brain structures, incompatible with life), while failure of the caudal end causes spina bifida, which ranges from spina bifida occulta (mild, often asymptomatic vertebral defect covered by skin, sometimes with a tuft of hair) to more severe forms: meningocele (meninges protrude through vertebral defect) and myelomeningocele (meninges and spinal cord protrude, most severe and common form associated with neurological deficits). Encephalocele involves herniation of brain tissue and meninges through a skull defect.

Etiology: Folate deficiency is the most significant modifiable risk factor—folate is essential for DNA synthesis and methylation during neural tube formation. Other risk factors include maternal diabetes, obesity, hyperthermia (e.g., fever, hot tub use in early pregnancy), and certain antiepileptic drugs (valproic acid, carbamazepine) which inhibit folate metabolism. Genetic factors and family history also increase risk.

Screening/Diagnosis: Elevated maternal serum AFP (and amniotic fluid AFP) suggests an open NTD due to leakage of fetal proteins into amniotic fluid; this is followed by detailed ultrasound to confirm the defect and by amniocentesis for acetylcholinesterase if needed. Ultrasound findings include the "lemon sign" (scalloping of frontal bones) and "banana sign" (cerebellar abnormality) associated with Arnold-Chiari II malformation, which commonly accompanies myelomeningocele.

Prevention: All women of childbearing age are recommended to take folic acid (400 mcg/day) supplementation before conception and during early pregnancy; women with prior NTD-affected pregnancies or on antiepileptic drugs require higher doses (4 mg/day).

Clinical significance: NTDs are a leading cause of infant mortality and long-term disability, with manifestations including motor and sensory deficits below the level of the lesion, bowel/bladder dysfunction, hydrocephalus, and Chiari II malformation. Management is multidisciplinary, potentially involving prenatal surgical repair in select myelomeningocele cases, or postnatal repair to reduce risk of infection and further neural damage.