Neural Tube Defects

• Neural tube formation and closure: The neural tube normally forms from the notochord-induced folding of the ectodermal neural plate by week 3 of development. Primary neurulation involves elevation and fusion of the neural folds in a rostral-to-caudal direction, typically completing by week 4 (day 28). The anterior neuropore closes around day 25, and the posterior neuropore by day 28. Secondary neurulation (formation of the caudal duct from the caudal eminence) continues through week 6 and contributes to the lumbosacral spine. Failure of primary neurulation results in open NTDs (spina bifida, anencephaly), while failures in secondary neurulation produce closed NTDs (tethered spinal cord, lipomeningocele). Genetic, epigenetic, and environmental factors disrupt the highly coordinated cell proliferation, migration, and apoptosis required for proper closure.

• Molecular mechanisms of defective closure: The planar cell polarity (PCP) pathway and canonical Wnt signaling are critical for coordinating neural fold elevation and convergent extension movements. Mutations in genes encoding components of these pathways (e.g., VANGL1, CELSR1, PRICKLE1) disrupt proper cell polarization and prevent adequate neural fold fusion. MTHFR (methylenetetrahydrofolate reductase) and other enzymes involved in one-carbon metabolism regulate DNA methylation and nucleotide synthesis; impaired folate metabolism compromises the methylation of histone and non-histone proteins essential for gene expression during neurulation. Inadequate methylation of key developmental regulatory genes (particularly those in the PAX3 and sonic hedgehog (SHH) pathways) prevents proper midline fusion. Elevated homocysteine levels (from impaired folate metabolism) may also be directly teratogenic through oxidative stress and altered methylation signaling. Additionally, mutations in genes like PAX3 (associated with Waardenburg syndrome) and VANGL2 directly impair neural fold morphogenesis.

• Environmental and metabolic disruption of neurulation: Folic acid deficiency reduces the availability of methylenetetrahydrofolate and impairs both methylation reactions and nucleotide synthesis, critically compromising the rapid cell divisions required during neurulation. Anticonvulsant drugs (particularly phenytoin, valproic acid, and phenobarbital) inhibit folate metabolism and may directly interfere with neural fold adhesion through alterations in homophilic cell adhesion molecules. Valproic acid is a known teratogen that causes NTDs through histone deacetylase inhibition, leading to aberrant gene expression and apoptosis in neuroepithelial cells. Maternal hyperglycemia in diabetes increases oxidative stress and impairs gene expression in the neuroepithelium. Retinoic acid excess (from isotretinoin or high-dose vitamin A) disrupts Hox gene expression patterns necessary for proper somite and neural tube development. Maternal hyperthermia (particularly in the first trimester) may denature critical developmental proteins and impair cell-cell signaling.

• Genetic factors: Multifactorial inheritance accounts for the majority of NTDs (approximately 90%), with multiple common genetic variants in folate metabolism genes (MTHFR C677T polymorphism), cell adhesion molecules, and developmental pathway genes conferring increased susceptibility. Mendelian inheritance patterns occur in approximately 5-10% of familial cases, including mutations in PAX3, VANGL1, CELSR1, PRICKLE1, MTHFR (homozygous), TYMS, and DHFR. Chromosomal abnormalities associated with NTDs include trisomy 18, trisomy 13, and deletion of chromosome 22q11. Syndromic NTDs occur in recognized genetic syndromes: Meckel-Gruber syndrome (ciliary dysfunction), Kallmann syndrome (midline defects including olfactory hypoplasia and hypogonadism), and Walker-Warburg syndrome (muscle-eye-brain disease with hydrocephalus and cobblestone cortex).

• Maternal metabolic and nutritional factors: Folic acid deficiency is the most modifiable risk factor; maternal serum folate <2.7 ng/mL significantly increases risk, while preconceptional supplementation with 400-800 mcg folic acid daily reduces risk by approximately 50%. Homozygous MTHFR C677T polymorphism reduces enzyme activity to 30% of normal, increasing both maternal homocysteine levels and fetal risk when combined with inadequate folate intake. Maternal obesity (BMI >30) is associated with increased NTD risk independent of folic acid intake, possibly through inflammatory mechanisms and impaired metabolism. Maternal diabetes (particularly pregestational type 1 or 2) increases NTD risk 2-10 fold through mechanisms of hyperglycemia-induced oxidative stress; HbA1c >8% in the first trimester correlates with sharply elevated risk.

• Medication-related causes: Antiepileptic drugs (particularly phenytoin, valproic acid, phenobarbital, and lamotrigine) increase NTD risk through folate antagonism and direct teratogenic effects; valproic acid carries a risk of approximately 1-2% (10-20 fold higher than baseline). Isotretinoin (a vitamin A derivative used for severe acne) is a potent teratogen causing NTDs in 25% of exposed pregnancies; women of childbearing age must be enrolled in iPLEDGE and use two forms of contraception. Methotrexate (used for cancer, autoimmune disease, ectopic pregnancy) is a folate antagonist causing NTDs when used in the first trimester. Trimethoprim (found in trimethoprim-sulfamethoxazole) inhibits dihydrofolate reductase and may increase NTD risk, particularly in the first trimester.

• Infectious and environmental factors: Maternal fever in the first trimester, particularly from maternal infection (influenza, rubella, varicella), increases NTD risk; the critical period is weeks 3-4 when neural tube closure occurs. Maternal alcohol use disorder increases risk through acetaldehyde-induced impaired metabolism and folate depletion; fetal alcohol spectrum disorder may include midline defects. Maternal smoking increases NTD risk modestly, possibly through effects on folate metabolism and increased oxidative stress. Geographic and seasonal variation (lower folic acid intake in winter months, dietary patterns in different regions) contributes to epidemiologic variation. Socioeconomic factors correlate with NTD risk primarily through determinants of nutrition and healthcare access.

Spina Bifida (Most Common Open NTD - 70% of NTDs)

• Cardinal symptom—weakness and sensory loss: Varies dramatically with spinal level of the lesion; lumbar and lumbosacral lesions (most common, 60% of cases) cause paraplegia, lower extremity weakness (gastrocnemius and tibialis anterior preserved with L4-L5 lesions; absent with S1-S2 lesions), and loss of sensation below the lesion. Thoracic lesions cause complete paraplegia. Cervical lesions (rare, 5% of cases) may cause quadriplegia and respiratory impairment if the lesion extends above C3-C4 (phrenic nerve level). Weakness is typically flaccid at birth and may become spastic as pyramidal tract damage evolves over infancy and childhood.

• Orthopedic manifestations: Talipes equinovarus (clubfoot) is present at birth in approximately 40% of spina bifida cases due to paralysis of dorsiflexors and evertors; hip dislocation or dysplasia occurs in 60-70% due to imbalance of hip flexor/extensor and abductor/adductor muscles; scoliosis develops in 60% of cases with thoracic lesions due to unequal paraspinal muscle innervation. Vertebral anomalies (hemivertebrae, block vertebrae) may be present.

• Urological and bowel dysfunction: Neurogenic bladder occurs in 75-90% of spina bifida patients due to loss of sacral parasympathetic innervation (S2-S4); presents with incontinence, urinary retention, or overflow incontinence depending on the pattern of sphincter denervation. Neurogenic bowel with fecal incontinence occurs in 40-50% due to loss of external anal sphincter control (S2-S4) and reduced rectal sensation; constipation is often more problematic than incontinence. Early catheterization programs and bowel management are critical to prevent upper urinary tract deterioration and social complications.

• Hydrocephalus and Chiari malformation: Hydrocephalus develops in 80% of patients with open spina bifida due to associated Chiari II malformation (herniation of cerebellar vermis, fourth ventricle, and lower medulla through the foramen magnum into the cervical spinal canal); presents with progressive developmental delay, vomiting, headache, and neck stiffness. Chiari II malformation may cause stridor (from brainstem compression of vagal nuclei), apnea, and feeding difficulties. The presence of Chiari malformation drives the decision for prenatal surgical repair.

• Neurogenic pain and tethering symptoms: Tethered spinal cord (caused by thickened filum terminale or scarring) develops in 20-40% of spina bifida patients, presenting with progressive lower back pain, worsening leg pain, progressive weakness, scoliosis progression, and changes in bladder/bowel function. Syrinx formation (post-traumatic syringohydromyelia) may develop years later, causing progressive pain, weakness, and sensory loss above the original lesion level.

Anencephaly (Second Most Common Open NTD - 20% of NTDs)

• Gross anatomic defect: Absence of major portions of the brain, meninges, and skull vault; the brain tissue directly exposed to amniotic fluid undergoes necrosis. Polyglobulia (elevated hemoglobin) is characteristic and may be detected prenatally. Most cases are stillborn or die within hours to days of delivery; surviving infants (rare) lack cerebral cortex consciousness and awareness.

• Associated malformations: Facial defects (ocular hypertelorism, cleft palate), cardiac anomalies, and limb malformations frequently coexist. The diagnosis is essentially incompatible with life, though brainstem reflexes may persist temporarily, making anencephaly a leading indication for prenatal diagnosis and pregnancy counseling.

Closed Spinal Dysraphism (5-10% of NTDs)

• Occult presentation: Tethered spinal cord, lipomeningocele, myelomeningocele, and diastematomyelia (split spinal cord) may present without an obvious skin opening; often discovered through investigation of cutaneous stigmata: dimple at midline, hairy patch (faun's tail), capillary hemangioma, lipoma, or skin sinus tract overlying the lumbosacral region.

• Progressive neurologic decline: Presents in infancy and childhood with progressive lower extremity weakness, progressive scoliosis, progressive changes in bladder/bowel function, and lower back pain. Diagnosis is made by MRI, and surgical untethering may halt progression.

Prenatal Presentation

• Elevated maternal serum alpha-fetoprotein (MSAFP): Detected in routine second-trimester screening; opens neural tissue leaks AFP into amniotic fluid and maternal serum. MSAFP > 2.5 multiples of median (MoM) warrants further evaluation with detailed ultrasound and possible amniocentesis for amniotic fluid alpha-fetoprotein (AFAFP) and acetylcholinesterase.

• Ultrasound findings: Splaying of posterior elements, abrupt termination of spinal canal, lemon sign (concave frontal bones due to increased intracranial pressure from Chiari malformation), banana sign (abnormal curvature of cerebellum), ventriculomegaly.

• Prenatal diagnosis—biochemical screening (second trimester, weeks 15-20):
• Maternal serum alpha-fetoprotein (MSAFP): Measured at 16-18 weeks gestation; elevated levels (>2.5 MoM) indicate increased risk of open NTDs (sensitivity 80-85% for spina bifida, 90% for anencephaly when compared to gestational age-matched controls). False positives occur with incorrect dating, multiple gestations, and certain maternal conditions; accurate ultrasound dating is mandatory before interpreting MSAFP.
• First trimester screening (weeks 11-14): Pregnancy-associated plasma protein-A (PAPP-A) is decreased in NTDs; combined with nuchal translucency (NT) measurement and maternal age, provides early risk stratification. This screening is increasingly used to allow earlier diagnosis and decision-making.

• Prenatal imaging—ultrasound****:
• Detailed anatomy ultrasound (18-22 weeks): Gold standard for visualization; demonstrates spinal defect with splayed posterior vertebral elements, exposed neural tissue, sac protruding from spinal canal. Cranial signs of Chiari malformation: lemon sign (flattening of frontal bones), banana sign (exaggerated curvature of cerebellar hemisphere), ventriculomegaly (lateral ventricle width >10 mm suggests hydrocephalus). Abdominal wall defects and cardiac anomalies should be assessed.
• Fetal MRI (increasingly used after 20 weeks): Provides superior soft tissue definition and precise determination of lesion level, presence of Chiari malformation, spinal cord involvement, and associated anomalies; useful for counseling on prognosis and planning surgical intervention.

• Prenatal diagnosis—amniotic fluid analysis:
• Amniotic fluid alpha-fetoprotein (AFAFP): Measured on amniotic fluid obtained by amniocentesis (usually performed at 16-18 weeks if MSAFP elevated); AFAFP >2.0 MoM is highly specific for open NTDs (sensitivity >99% for open spina bifida, essentially 100% for anencephaly). Acetylcholinesterase presence in amniotic fluid is confirmatory for open NTD.
• Genetic testing: Cell-free fetal DNA testing may identify chromosomal abnormalities (trisomy 13, 18, 21) that may be associated with NTDs. Karyotyping of amniotic fluid cells excludes chromosomal disorders.

• Postnatal diagnosis—clinical examination and imaging:
• Physical examination: Visible spinal defect (meningocele with or without nerve tissue), cutaneous stigmata (dimple, hair, hemangioma), motor and sensory level determination (testing reflexes, strength, and sensation), assessment of hydrocephalus (head circumference, bulging fontanelle, suture separation). Occult lesions require suspicion based on cutaneous stigmata.
• Neuroimaging—spinal ultrasound and MRI:

Prevention (the highest-yield "treatment")

• Folic acid supplementation: The USPSTF recommends 400–800 mcg of folic acid daily for all persons capable of pregnancy, begun at least one month before conception and continued through the first trimester; ACOG and CDC recommend a high-dose 4 mg daily regimen for women with a prior NTD-affected pregnancy. Mandatory FDA fortification of enriched grain products supplements, but does not replace, individual supplementation.
• Modify teratogen exposure: Per AAN/ACOG guidance on epilepsy in pregnancy, valproic acid and topiramate should be avoided in women of reproductive potential when an alternative exists; isotretinoin requires iPLEDGE enrollment; preconception glycemic optimization is advised in pregestational diabetes.

Perinatal and immediate neonatal management (open myelomeningocele)

• Delivery planning: Delivery at a center with pediatric neurosurgery; ACOG notes cesarean delivery is commonly planned for open defects to avoid trauma to exposed neural placode.
• Sac protection: Position prone, cover the defect with sterile, saline-moistened non-adherent gauze, avoid pressure, and maintain a strictly latex-free environment from the first hospital day — this is the key intervention preventing later latex anaphylaxis.
• Antimicrobials: Broad-spectrum IV antibiotics (e.g., an antistaphylococcal agent plus a third-generation cephalosporin) until closure, to prevent ventriculitis/meningitis.

Definitive and escalation therapy

• Surgical closure: Postnatal repair within roughly the first 24–72 hours of life is standard.
• Prenatal (in utero) repair: The MOMS trial established that fetal repair before ~26 weeks reduces the need for shunting and improves motor outcome versus postnatal repair, at the cost of prematurity and uterine dehiscence; candidacy follows ACOG/SMFM criteria and mandates cesarean for all subsequent deliveries.
• Hydrocephalus: Ventriculoperitoneal shunt or endoscopic third ventriculostomy with choroid plexus cauterization.
• Neurogenic bladder: Clean intermittent catheterization plus an antimuscarinic (oxybutynin); escalate to intradetrusor onabotulinumtoxinA or augmentation cystoplasty. Bowel programs use osmotic laxatives and antegrade continence enemas.
• Anencephaly: No surgical option — comfort-focused palliative care and perinatal counseling.

Neurosurgical — several are emergencies

• Shunt malfunction or infection (emergency): Obstruction raises ICP; in infants look for **bulging fontanelle, rapidly rising head circumference, vomiting, lethargy, and *sunsetting eyes*** (impaired upgaze from tectal compression); in older children, headache and papilledema. Fever plus shunt dysfunction suggests ventriculitis (commonly coagulase-negative staphylococci). Immediate imaging and neurosurgical consultation.
• Symptomatic Chiari II malformation (emergency): Brainstem compression injures vagal and hypoglossal nuclei — inspiratory stridor, central apnea, dysphagia, and opisthotonos. This is the leading cause of death in infants with myelomeningocele; first step is to confirm the shunt is working, since decompensation often reflects untreated hydrocephalus.
• Tethered cord and syringomyelia: Traction and scar at the repair site cause loss of previously attained milestones — new leg weakness, back/leg pain, rapidly progressive scoliosis, or new incontinence. Any deterioration from a stable baseline is the signal, and warrants MRI.
• Meningitis/ventriculitis (emergency) from an unclosed or leaking sac.

Systemic

• Latex anaphylaxis (emergency): Repeated mucosal latex exposure sensitizes these patients at very high rates; presents intraoperatively as hypotension, bronchospasm, urticaria. Treat with intramuscular epinephrine 0.3 mg (adult dose; 0.01 mg/kg in children). Prevention is lifelong latex avoidance.
• Upper tract urologic deterioration: Detrusor–sphincter dyssynergia raises storage pressures → vesicoureteral reflux, hydronephrosis, recurrent pyelonephritis, and CKD — historically a major cause of late mortality. Signalled by rising creatinine or hydronephrosis on surveillance renal ultrasound.
• Orthopedic: Progressive scoliosis, hip dislocation, contractures, and insensate-skin pressure ulcers and unnoticed fractures.
• Neurocognitive: Nonverbal learning disability with fluent but superficial speech (cocktail party personality), executive dysfunction, and shunt-related precocious puberty.

• Elevated MSAFP → the single best next step is ultrasound, not amniocentesis: Confirm gestational dating and exclude multiple gestation and fetal demise first, since incorrect dating is the most common cause of a "positive" screen.
• The cranial signs beat the spine: Lemon sign (scalloped frontal bones) and banana sign (wrapped, effaced cerebellum) reflect Chiari II and are often easier to see than the vertebral defect itself.
• Only open defects raise AFP: Skin-covered lesions — spina bifida occulta, lipomeningocele, and closed meningocele — have a normal MSAFP and normal amniotic fluid acetylcholinesterase. This is the classic distractor. Amniotic fluid acetylcholinesterase is the confirmatory test that distinguishes fetal open NTD from maternal/placental blood contamination.
• AFP direction is the trisomy discriminator: high AFP = open NTD; low AFP = trisomy 21 and trisomy 18.
• Folic acid dosing is a two-tier answer: 400–800 mcg daily for all who could become pregnant (USPSTF), but 4 mg daily for a woman with a prior NTD-affected pregnancy (ACOG/CDC), started before conception — supplementation after a missed period is too late, because the neuropore closes by day 28.
• The drug examiners test is valproic acid (also carbamazepine); the mechanism to state is folate antagonism plus histone deacetylase inhibition.
• Sacral dimple, hair tuft, or lumbosacral hemangioma with any neurologic or urologic sign: image the cord — ultrasound before ~6 months while the posterior elements are unossified, otherwise MRI. Isolated spina bifida occulta on a plain film is an incidental, asymptomatic finding requiring nothing.
• Two reflexes on any spina bifida vignette: a child with new weakness, scoliosis, or incontinence after years of stability has a tethered cord until proven otherwise; and every patient needs lifelong latex precautions, with intraoperative hypotension and bronchospasm signaling latex anaphylaxis.