71 Neural Tube Defect

Abdulrahman J. Sabbagh, Abdulrahman Yaqub Alturki, José Luis Montes, Jean-Pierre Farmer, and Jeffrey Atkinson

Clinical Presentation

A 32-year-old woman is referred to you from an obstetrician. She is 22 weeks pregnant, gravida 2 para 1 (G2P1) but otherwise healthy with a normal first daughter.

A uterine ultrasound was done followed by a fetal magnetic resonance imaging (MRI) scan (Fig. 71.1).
She has a cesarean section and a lesion is found in the midline on her infant’s back.
An MRI of the infant is done in the first few hours of its life (Fig. 71.2).

Fig. 71.1 Prenatal magnetic resonance imaging scans with (A) T2-weighted axial and (B) sagittal sections of the brain in utero. (C) T2-weighted sagittal and (D) coronal sections of the spine are also shown.

Questions

What is the diagnosis? Describe the MRI shown in Fig. 71.1.
Give five features on this MRI of the brain and spinal cord that are characteristic of this diagnosis. Why is there pneumocephalus and air in the spinal canal?
Describe methods of evaluation for this condition in the prenatal period.
What are the most common sites for neural tube defects (NTDs)?
How would you assess the level of the defect clinically?
What do you tell the parents regarding survival (with or without treatment), intelligence, ambulation, and urinary continence? What are the causes of early and late mortality?
Highlight a management plan.
When should you operate on this patient? What are the goals and principles of the surgery?
Describe surgical options for repair of such large defects in the skin.
What are the risk factors associated with NTDs?
How can NTDs be prevented?

Fig. 71.2 (A) Preoperative and intraoperative photograph of the skin defect depicting neural placode (P) and nerve roots. (B) Magnetic resonance imaging scans obtained shortly after birth with sagittal T2-weighted image of the spine and T1-weighted image of the brain.

Answers

What is the diagnosis? Describe the MRI shown in Fig. 71.1.
- The diagnosis is of Chiari II malformation with an open myelomeningocele (MMC).
- The prenatal MRI shows a T2-weighted axial section of the brain in utero depicting asymmetrical ventriculomegaly. The axial section at the lumbar level shows an open MMC (arrow). Sagittal and coronal sections show evidence of the MMC and Chiari II.
Give five features on this MRI of the brain and spinal cord that are characteristic of this diagnosis. Why is there pneumocephalus and air in the spinal canal?
- Characteristic findings include the following¹:
  - Medullary kinking
  - Tectal beaking
  - Enlarged Massa intermedia
  - Elongation and/or cervicalization of the medulla
  - Low attachment of tentorium
  - Hydrocephalus
  - Syringomyelia in the area of the cervicomedullary junction
  - Dysgenesis of the corpus callosum
- Pneumocephalus and air in the spinal canal indicate open MMC.
Describe methods of evaluation for this condition in the prenatal period.
- Methods of evaluation include
  - Mother serum α-fetoprotein (AFP)
  - Prenatal ultrasound (high NTDs)
  - If both are positive then an amniocentesis may be performed for amniotic AFP and acetylcholinesterase levels (the diagnostic accuracy for NTD rises to more than 97%).²
  - Fetal MRI is an option when available, as it gives accurate information regarding associated anomalies and may help in prognostication.
What are the most common sites for neural tube defects (NTDs)?
- Most common sites of occurrence include the lumbosacral area in 50% of cases, followed by the thoracolumbar area in 35% of cases³
How would you assess the level of the defect clinically?
- The defect level is determined by assessing the lowest level of neurologic function (Table 71.1).
What do you tell the parents regarding survival (with or without treatment), intelligence, ambulation, and urinary continence? What are the causes of early and late mortality?
- Prognosis is described below.^5–7
  - Survival
    - Without treatment, only 14–30% survive infancy.
    - With treatment, 85% survive.
  - Intelligence
    - 70–80% will have normal intelligence quotients.
    - Mental delays may be related to shunt malfunctions, primary microgyri, or absence of corpus callosum.
  - Ambulation
    - 50% are ambulatory.
    - Up to 80% can ambulate with bracing. However, 80% of these will decrease over time because of weight gain by the patients.
  - Urinary incontinence
    - 90–95% are incontinent but are able to stay dry with intermittent catheterizations.
  - Mortality
    - Early mortality is related to Chiari malformation complications such as aspiration and respiratory arrest
    - Late mortality may be related to shunt complications, urosepsis, as well as progressive respiratory compromise due to kyphoscoliosis.
Highlight a management plan.
- A management plan is described below.
  - Items related to Chiari II malformation¹
    - Measure head circumference to follow the rate of growth due to risk of hydrocephalus.
    - Obtain a head ultrasound within 24 hours of birth.
    - Check for inspiratory stridor and apneic episodes.
  - Items related to the defect⁸
    - Measure the size of the defect.
    - If the lesion is ruptured, start antibiotics.
    - Cover the lesion with a piece of sterile Telfa (Tyco Healthcare, Mansfield, MA)
    - Keep the patient in Trendelenburg position (to keep pressure off lesion).
    - Plan early surgical closure.
  - General assessment and management
    - Neonatologist assessment for other abnormalities
    - Urologic consultation and regular urinary catheterization
    - Orthopedic consultation for spine, hip, and knee deformities
When should you operate on this patient? What are the goals and principles of the surgery?
- Timing of surgery should be within 48–72 hours.^6,9
- The goals of surgical repair include reconstruction of the neural tube and its coverings, avoidance of meningitis, and protection of the remaining functional tissue in the neural placode.
- The principles of the surgery include
  - Reconstruction of the neural tube (Fig. 71.3A).
    - The placode is dissected from the surrounding tissue by incising the junctional zone.
    - All dermal remnants are resected, and the neural tube is reconstituted by closing the pia with a 7–0 monofilament suture.
- Reconstruction of the thecal sac (Fig. 71.3B)
  - The dura is dissected free from its junctions with the fascia and skin.
  - The goal is a watertight closure without causing constriction of the closed neural placode.
- Often the defect is large enough to require a dural patch (Fig. 71.3C)
  - Tissue sealant is preferably used at the end of dural closure.
- Midline fascia and skin closure in layers (Fig. 71.3D)
Describe surgical options for repair of such large defects in the skin.
- Several options are available and usually best done by a plastic surgeon. They include
  - Circumferential skin release dissection (used in small- to medium-sized defects)
  - Rameris procedure: medial advancement of bilateral bipedicled musculocutaneous flap based on the latissimus dorsi and maximus gluteus without any relaxing incisions or skin grafting¹⁰
  - Flaps used include latissimus dorsi myocutaneous flap for thoracolumbar defects and gluteus maximus myocutaneous flap for lower defects.¹¹
  - The junctional zone or the cyst wall membrane can be used as a graft.
  - Alternatively, artificial dermis can be used.¹²
What are the risk factors associated with NTDs?
- Risk factors include the following (the associated percentage risk of having a child with NTD is in brackets)²:
How can NTDs be prevented?
- Prevention can be achieved by supplementing expecting mothers with folic acid.
  
  Expecting mothers with no known risk factors should take folic acid at the dose of 0.4 mg/day.
  
  Those with a higher risk factor should take folic acid at 4 mg/day (for the duration of one month before pregnancy through the first trimester).¹³