Definition

• The neural tube usually closes between 15 and 28 days post-conception.  Failure of normal closure results in a neural tube defect (NTD).  90% of NTDs are spina bifida or anencephaly.
  • Anencephaly is caused by a defect in the closure of the anterior portion of the neural groove. This results in an unfused partially developed forebrain and incompletely developed calvarium. This condition is always lethal.
  • Spina bifida is caused by failure of closure of the posterior portion of the neural tube.
  • A meningomyelocoele (MM), the most common type of spina bifida, is a sac like structure which protrudes through a defect in the vertebral arches, containing meninges, CSF and spinal cord and /or nerve roots.
  • Meningocoeles are rare. The structure contains meninges and CSF only.
  • Spina bifida occulta is a common anomaly consisting of a midline defect of the vertebral bodies without protrusion of spinal cord or meninges.  The incidence is reported to be between 5% and 40%.  Most often it is asymptomatic and of no consequence, but it can be associated with developmental abnormalities or tethering of the spinal cord.
• Both meningomyelocoeles and meningocoeles may occur anywhere along the spinal cord but are most common in the lumbar and sacral area.

• The degree of neurological impairment depends on the position and extent of the lesion. There will usually be loss of sensation in corresponding dermatome, complete or partial paralysis of skeletal muscles and often bladder and/or anal sphincter paralysis.
• The Chiari II malformation (herniation of the cerebellar vermis through the foramen Magnum, with dislocation of the fourth ventricle towards the neural canal, and downward displacement of the tentorium) is present in 95% of children with myelomeningocoele. This is associated with obstructive hydrocephalus.
  • Other abnormalities of CNS development (e.g. polymicrogyria, heterotopias, aqueduct stenosis) may co-exist.
• Orthopaedic abnormalities related to disrupted innervation of muscles (i.e. dislocated hips, club feet) are common.
• There may be an associated spinal deformity (scoliosis or kyphosis).
• Other major abnormalities of other organ systems are unusual.

Aetiology

• The aetiology is felt to be multifactorial.  However, risk factors include:
  • previous pregnancies have been affected with NTDs
  • some anticonvulsants such as carbamazepine and sodium valproate
  • pregnancies complicated by diabetes at conception
• 10% of infants may have an associated chromosomal defect (e.g. trisomy 18, triploidy, or single gene disorders). A referral to the genetic service should be arranged, and karyotype considered as indicated.

Incidence

• The true incidence of NTDs appears to be decreasing in most parts of the world.
• Estimates of rate approximate 0.2-0.4 per 1000 livebirths.
• In 1999 in New Zealand, the prevalence of neural tube defects was 0.5 per 1000 live births. Many pregnancies complicated by NTDs result in termination – therefore the incidence increases to 0.91 per 1,000 total births and terminations.

Antenatal Issues

• Periconceptional folate supplementation has been shown to reduce the incidence of NTDs, with data suggesting that a daily intake of 0.4mg of folic acid reduces the risk of 1st neural tube defects by up to 70%.
• Current New Zealand recomendations are for women to take 800 micrograms of folic acid 4 weeks prior to and 12 weeks after conception. Women at high risk require 5mg daily.
• Problems exist in that a real proportion of NZ pregnancies are not planned and preconceptual folate is not taken.

Screening and Detection

Alpha feto protein (AFP) from amniotic fluid is very reliable except when the defect is closed. Maternal serum AFP is less reliable, detecting between 50-90% of open anomalies, but is falsely positive in 5%.

USS: Diagnosis often made at 16-18 week USS. Often seen as lateral displacement of the spinal pedicles, but the sac may also be visualised. An axial cross section through the fetal skull may demonstrate a “lemon head” shape, with a depression of the metopic suture in the frontal region. Visualisation of the posterior fossa may demonstrate a “banana sign”, observed at the level of the cerebellum and cistern magna. It is caused by herniation of the cerebellar vermis through the foramen magnum, giving the cerebellum the appearance of a banana. USS should evaluate for other structural abnormalities of limbs, brain, kidneys, and other organs.

Coronal view of spine   Sagittal view of spine   "Lemon" shaped skull

Counselling

• Parents will usually have seen the neurosurgeon prior to delivery to discuss treatment options and prognosis.
• Genetic counselling – 2-4% recurrence risk.

Place and mode of delivery

• Tertiary institution with access to neurosurgical services.
  • Delivery by elective lower segment caesarean section (LSCS) prior to the onset of labour may result in better motor function than vaginal delivery or LSCS after a trial of labour.

Postnatal Management

• The goal is for neurosurgical closure within 24 hours.
• Some parents may opt, after discussions with the neurosurgeon, for palliative care or conservative management.

Postoperative Care

1. Will usually be undertaken on the Neurosurgical and Neurology ward (26A) at Starship Hospital.
2. A urinary catheter usually remains in situ postoperatively (preferably until renal imaging has been undertaken).

Prognosis

• Children require multidisciplinary follow-up including the neurosurgical team, a developmental or general paediatrician, urologists, orthopaedic surgeons, and physiotherapists.  There should be a supervising “overviewing” paediatrician.
  • Children with spina bifida from West and North are seen by Dr Bobby Tsang, Paediatrician at Wilson Centre.  Those from Central and South are seen by Dr Raewyn Gavin, Paediatrician at Greenlane Clinical Centre.
• Hydrocephalus is a common occurrence after surgery but may not develop for some weeks. However most become evident in the first 10 days post closure.
  • The baby should be watched with serial head circumference measures and ultrasound imaging.
  • A shunt should be inserted if there is clear evidence of progressive ventriculomegaly or increasing head circumference.
  • Shunt infections and revisions are common throughout childhood.
• In the absence of treatment, the mortality of myelomeningocoele is approximately 50%.
• Normal intellect should be assumed in the absence of significant hydrocephalus or dysmorphism.
• Children with MM have a lifetime increased risk of latex sensitivity and therefore need to be cared for in a latex-free environment.
• Urological and orthopaedic referral will be organised by the neurology/neurosurgery services.

References