Neural tube defects

Etiology

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• Maternal risk factors
  • Folate deficiency during pregnancy due to:
    • Insufficient folate supplementation
    • Drugs that interfere with folate metabolism
      • Methotrexate
      • Anticonvulsants: valproate, carbamazepine
      • Sulfonamides, trimethoprim
• Pregestational diabetes mellitus

Closed spinal dysraphism

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Spina bifida occulta

• Description
  • Most common closed NTD
  • Cause: failure of vertebral arch fusion
  • Vertebral bone defect without herniation
  • The spinal cord, meninges, and overlying skin remain intact.
• Clinical features
  • Most commonly affects the lower lumbar or sacral region
  • Often asymptomatic (may be an incidental finding in imaging)
  • Possible symptoms at the level of the vertebral defect:
    • Lumbar skin dimple
    • Collection of fat
    • Patch of hair
    • Vascular malformations (hemangioma, port-wine stain).
• Diagnostics
  • Normal AFP

Diagnostics

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Prenatal period

• Screening test (16–18 weeks’ gestation): ↑ AFP in maternal serum (MSAFP)
  • MSAFP is only elevated in open NTDs.
    • Because the fetal tissues (which are producing AFP) are directly exposed to the amniotic fluid, there is a significantly increased leakage of AFP from the fetus into the amniotic fluid.
  • MSAFP is not elevated in spina bifida occulta.
• Ultrasonography (18–20 weeks’ gestation)
  • Characteristic findings depend on the specific defect.
    • Findings in anencephaly
      • Cranial vault and brain tissue are absent.
      • Residual, disorganized cerebellar and/or brainstem tissue may be present.
      • Bulging eyes and underdeveloped forehead
      • Associated with polyhydramnios
• Amniocentesis: ↑ AFP and ↑ AChE in amniotic fluid (increase in open NTDs only)
  • During fetal development, AChE is present in the developing nervous system tissues, including the neural tube.
  • Used as confirmation test when MSAFP is elevated but ultrasound findings are inconclusive
  • When both AFP and AChE are elevated, an open NTD is very likely.

Differential diagnostics

Tethered cord syndrome

• Definition & Pathophysiology:
  • Spinal cord stretch/traction injury due to mechanical anchoring of the conus medullaris below L1–L2 (thickened filum terminale, lipoma, spina bifida occulta).
  • Strongly associated with spina bifida occulta and other forms of closed spinal dysraphism.
  • Exacerbated during growth spurts (age 5–15) or spinal flexion.
• Clinical Presentation:
  • Cutaneous stigmata (lumbosacral midline): “Faun tail” (tuft of hair), sacral dimple (>0.5 cm wide, >2.5 cm above anal verge), subcutaneous lipoma, or hemangioma.
  • Neuromusculoskeletal: Progressive LE weakness, gait instability, mixed UMN/LMN signs, progressive scoliosis, and foot deformities (pes cavus, hammer toes). c
  • Urologic / GI: Neurogenic bladder (new-onset incontinence, daytime wetting, recurrent UTIs), chronic constipation, encopresis.
• Diagnosis:
  • Initial (< 4–6 months old): Spinal Ultrasound (viable prior to posterior arch ossification).
  • Confirmatory (> 6 months old & Gold Standard): Lumbosacral MRI (demonstrates low-lying conus < L2 and thickened filum > 2 mm).
  • Ancillary: Urodynamic studies (assesses detrusor hyperreflexia/sphincter dyssynergia).
• Key Differentials:
  • Cauda equina syndrome: Acute onset, massive disc herniation, saddle anesthesia, absent congenital skin stigmata.
  • Charcot-Marie-Tooth (CMT): Distal hereditary polyneuropathy (“inverted champagne bottle” legs), normal spine imaging.
• Management:
  • Definitive: Early surgical detethering (laminectomy w/ filum terminale transection) to stop progressive irreversible neuro/urologic loss.
  • Supportive: Clean intermittent catheterization (CIC), anticholinergics (oxybutynin), PT/OT, corrective orthotics/surgery.
• Complications:
  • Irreversible neurogenic bladder → hydronephrosis → chronic kidney disease (CKD).
  • Permanent paraparesis and postoperative re-tethering from scar tissue.