= sac covered by leptomeninges containing CSF + variable amount of neural tissue; herniated through a defect in the posterior / anterior elements of spine

Prevalence: 1÷1,000–2,000 births (in Great Britain 1÷200 births); twice as common in infants of mothers >35 years of age; Caucasians >Blacks >Orientals; most common congenital anomaly of CNS

Etiology: localized defect of closure of caudal neuropore (usually closed by 28 days); persistence of neural placode causes derangement in the development of mesenchymal + ectodermal structures

• positive family history in 10%
• neural placode = reddish neural tissue in the middle of back made up of open spinal cord
• normal skin / cutaneous abnormality: pigmented nevus, abnormal distribution of hair, skin dimple, angioma, lipoma
• MS-AFP (≥2.5 S.D. over mean) permits detection in 80% (2–5% PPV) if defect not covered by full skin thickness

Recurrence rate: 3–7% chance of NTD with previously affected sibling / in fetus of affected parent

Associated with:

1. Hydrocephalus (70–90%): requiring ventriculoperitoneal shunt in 90%
   • 25% of patients with hydrocephalus have spina bifida!
2. Chiari II malformation (99%)
3. Congenital / acquired kyphoscoliosis (90%)
4. Vertebral anomalies: vertebral body fusion, hemivertebrae, cleft vertebrae, butterfly vertebrae
5. Diastematomyelia (20–46%): spinal cord split above (31%), below (25%), at the same level (22%) as the myelomeningocele
6. Duplication of central canal (5%) cephalic to + at level of placode
7. Hemimyelocele (10%) = two hemicords in separate dural tubes separated by fibrous / bony spur: one hemicord with myelomeningocele on one side of midline, one hemicord normal / with smaller myelomeningocele at a lower level
   • impaired neurological function on side of hemimyelocele
8. Hydromyelia (29–77%) cranial to placode ← disturbed CSF circulation
9. Chromosomal anomalies (10–17%): trisomy 18, trisomy 13, triploidy, unbalanced translocation
   • In 20% no detectable associated anomalies!
10. Tethering of spinal cord (70–90%)
11. Arachnoid cyst (2%) ← developmental deficiency during formation of arachnoid / dura mater with a subdural location

Distribution: thoracic (2%), thoracolumbar (32%), lumbar (22%), lumbosacral (44%)

Location:

1. cranial meningocele = encephalocele
2. dorsal / posterior meningocele
3. anterior sacral meningocele
4. lateral thoracic / lumbar meningocele

OB-US:

• detection rate of 85–90%; sensitivity dependent on GA (fetal spine may be adequately visualized after 16–20 weeks GA); false-negative rate of 24%
  • spinal level estimated by counting up from last sacral ossification center = S4 in 2^nd trimester + S5 in 3^rd trimester (79% accuracy for ± spinal level)
  • may have clubfoot / rocker-bottom foot
  • polyhydramnios
  • Spine:
    • loss of dorsal epidermal integrity
    • soft-tissue mass protruding posteriorly + visualization of sac
    • widening of lumbar spine with fusiform enlargement of spinal canal:
      • splaying (= divergent position) of ossification centers of laminae with cup- / wedge-shaped pattern (in transverse plane = most important section for Dx)
      • absence of posterior line = posterior vertebral elements (in sagittal plane)
      • gross irregularity in parallelism of lines representing laminae of vertebrae (in coronal plane)
    • anomalies of segmentation / hemivertebrae (33%) with short-radius kyphoscoliosis
    • tethered cord + lumbar / lumbosacral myelomeningocele
  • Head:
    • “lemon” sign = concave / linear frontal contour abnormality located at coronal suture strongly associated with spina bifida
    • “banana” sign
      Prevalence: in 96% of fetuses ≤24 weeks; in 91% of fetuses >24 weeks
    • “nonvisualization” of cerebellum
    • effaced cisterna magna (100% sensitivity)
      • A normal cisterna magna is 3–10 mm deep and usually visualized in 97% at 15–25 weeks GA
    • BPD <5^th percentile during 2^nd trimester (70% sensitive)
    • HC <5^th percentile (35% sensitivity)
    • ventriculomegaly (40–90%) with choroid plexus incompletely filling the ventricles (54–63% sensitivity) = “dangling” choroid on dependent side
      Prevalence: in 44% of myelomeningoceles <24 weeks GA; in 94% of myelomeningoceles during 3^rd trimester

Plain films:

• bony defect in neural arch
• deformity + failure of fusion of lamina
• absent spinous process
• widened interpedicular distance
• widened spinal canal

Rx:

1. Possibly elective cesarean section at 36–38 weeks GA (may decrease risk of contaminating / rupturing the meningomyelocele sac)
2. Repair within 48 hr

Postoperative complications:

1. Postoperative tethering of spinal cord by placode / scar
2. Constricting dural ring
3. Cord compression by lipoma / dermoid / epidermoid cyst
4. Ischemia from vascular compromise
5. Syringohydromyelia

Prognosis:

1. Mortality 15% by age 10 years
2. Intelligence: IQ <80 (27%); IQ >100 (27%); learning disability (50%)
3. Urinary incontinence: 85% achieve social continence (scheduled intermittent catheterization)
4. Motor function: some deficit (100%); improvement after repair (37%)
5. Hindbrain dysfunction associated with Chiari II malformation (32%)
6. ventriculitis: 7% in initial repair within 48 hours, more common in delayed repair >48 hours