Overview
Definition
A neural tube defect (NTD; spinal dysraphism) is a congenital defect in the covering of the central nervous system, resulting from a failure of the neural tube to close spontaneously during the 3rd or 4th week of embryonic development.
Classification
- Open NTD (80% of cases): a midline defect of the vertebral bodies with different degrees of protrusion of the meninges and central nervous system (CNS)
- Defect along the spinal cord:
- Meningocele: Only the meninges protrudes.
- Meningomyelocele: Both meninges and spinal cord protrude (most common NTD).
- Defect of the cranium:
- Cranial meningocele: Meninges protrude.
- Cranial encephalocele: Both the meninges and brain stem/cerebellum/cerebral cortex protrude.
- Anencephaly: complete failure of the cephalic neural tube to close resulting in fully exposed fetal brain (not compatible with life)
- Defect along the spinal cord:
- Closed NTD: a midline defect of the vertebral bodies without protrusion of the meninges or neural tissue
- Without subcutaneous mass: spina bifida occulta
- With subcutaneous mass:
- Lipomyelomeningocele
- Lipomeningocele
Epidemiology
- 1 in 1,500 births in the United States
- 2nd-most common major congenital anomalies (most common are cardiac anomalies)
- Incidence:
- Has decreased with prenatal folic acid supplementation and screening
- Varies by ethnicity and geographic region
- Female neonates are affected more commonly.
- Maternal risk factors during pregnancy:
- Folate deficiency
- History of NTD in previous children (5%–10% recurrence with 1–2 affected siblings)
- Folic acid-depleting medications (valproic acid, triamterene, trimethoprim, sulfasalazine)
- Poorly controlled maternal diabetes
- Maternal obesity
Embryology
- Conception to 3rd week of gestation (gastrulation):
- Single cell → single-layered blastula → 3-layer gastrula: ectoderm, endoderm, and mesoderm
- 3rd–4th week:
- Ectoderm → differentiates into neuroectoderm (creating neural plate) → cell replication in neural plate → neural crests + neural fold
- Mesoderm → differentiates into notochord → signals neural fold to enlarge/fuse → neural tube (neurulation)
- Neural tube “zips up” from the middle outward → cranial (head) and caudal (tail) openings close approximately on days 24 and 28
Neurulation: the differentiation and growth of the neural plate into the neural tube during the first trimester of gestation.
PNS: Peripheral nervous system
Clinical Presentation
Most NTDs are discovered during prenatal screening. Open NTDs are evident at birth, but closed NTDs may have more subtle presentations.
| Name | Description | Clinical features |
|---|---|---|
| Myelomeningocele | Spinal cord and meningeal herniation | The higher the lesion in the spine, the more severe the symptoms:
|
| Meningocele | Meningeal herniation without spinal cord involvement |
|
| Encephalocele | Meninges ± brain tissue protruding from cranial defect |
|
| Anencephaly | Major component of the brain and skull is absent. |
|
| Spina bifida occulta | Midline vertebral body fusion defect without protruding dura or neural tissue |
|
Types of NTD and their respective defect
Image by Lecturio.
A and B: Posterior and lateral photographs demonstrating a lumbosacral myelomeningocele
Image: “Lumbosacral Myelomeningocele” by US National Library of Medicine. License: CC BY 2.0
Tuft of hair over the lower back in a patient with spina bifida
Image: “Hypertrichiosis” by Department of Pediatric Surgery, Maulana Azad Medical College, New Delhi. License: CC BY 3.0
Diagnosis
Prenatal screening
- Serum/amniotic α-fetoprotein (maternal serum α-fetoprotein; MSAFP): high levels at 15–20 weeks of gestation suggestive of NTD, but not diagnostic
- Ultrasound: Visualization of defect is 98% specific and can be used to confirm diagnosis (closed NTD may not be detected).
Postnatal imaging
- Open NTD: ultrasound commonly used; magnetic resonance imaging (MRI) can be used if further detail required
- Closed NTD: ultrasound indicated with following cutaneous signs:
- Palpable subcutaneous mass
- Hairy patch overlying spine
- Dermal sinus
- Dimples > 5 mm deep or > 25 mm from anal verge
- Skin tags or tail-like appendages
- Hyperpigmented patches or deviation of the gluteal cleft
Management
Prenatal
- Prevention with folic acid (vitamin B9) supplementation:
- Decreases the incidence of NTD by around 70%
- Preconception dose: 0.4 mg/day of folic acid for at least 1 month prior to conception and throughout pregnancy
- A higher dose (usually 4 mg/day) is indicated for the following:
- A previous pregnancy affected by an NTD
- A positive family history for NTD
- Use of folic acid-depleting medications
- Pre-gestational diabetes
- Delivery: cesarean section recommended
Postnatal
- Closed NTD:
- May require no intervention
- Ongoing follow-up for lower neurological symptoms (incontinence, constipation, difficulty ambulating)
- Open NTD:
- Keep warm and in prone position.
- Cover NTD with sterile wet dressing.
- Surgical management:
- Neurosurgical intervention for all open NTDs
- Closed NTD typically does not require surgery
References
- Boon, R. L. (2010). Textbooks of Paediatrics.
- Winn, H. R. (2011). Youmans Neurological Surgery E-Book. Elsevier Health Sciences.
- Goetzl, L.M. Folic acid supplementation in pregnancy. UpToDate. Retrieved Nov 2, 2020, from https://www.uptodate.com/contents/folic-acid-supplementation-in-pregnancy?search=neural%20tube%20defects&topicRef=460&source=see_link#H17
- Dukhovny, S., Wilkins-Haug, L. Open neural tube defects: Risk factors, prenatal screening and diagnosis, and pregnancy management. UpToDate. Retrieved Nov 2, 2020, from https://www.uptodate.com/contents/open-neural-tube-defects-risk-factors-prenatal-screening-and-diagnosis-and-pregnancy-management?search=neural%20tube%20defects&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1#H27
