Fetal abnormalities » Spine
Open spina bifida
- 1 in 1,000 fetuses at 12 weeks’ gestation.
- Diagnosis of spina bifida requires the systematic examination of each neural arch, from the cervical to the sacral region, both transversely and longitudinally. In the transverse scan, the normal neural arch appears as a closed circle with an intact skin covering, whereas in spina bifida the arch is ‘U’-shaped and there is an associated bulging meningocele (thin-walled cyst) or myelomeningocele. The extent of the defect and any associated kyphoscoliosis are best assessed in the sagittal scan.
- Site of open spina bifida: lumbosacral (65%), sacral (24%), thoracolumbar (10%), cervical (1%).
- Open spina bifida is associated with the Arnold-Chiari II malformation with caudal displacement of the brain stem and obliteration of the cisterna magna.
- At 11-13 weeks’ gestation, in the mid-sagittal view of the head the lower part of the brain between the sphenoid bone anteriorly and the occipital bone posteriorly can be divided into the brain stem in the front and a combination of the fourth ventricle and cisterna magna in the back. In most cases of open spina bifida the diameter of the brain stem is increased, the distance between the brain stem and the occipital bone (BSOB) is decreased and the ratio of the brain stem to BSOB is >1.0.
- In the second-trimester, more than 95% of fetuses have frontal bone scalloping (lemon sign), and obliteration of the cisterna magna with either an ‘absent’ cerebellum or abnormal anterior curvature of the cerebellar hemispheres (banana sign). A variable degree of ventricular enlargement is present in virtually all cases of open spina bifida at birth, but in only about 70% of cases in the mid-trimester.
- Chromosomal abnormalities (mainly trisomy 18), single mutant genes, and maternal diabetes mellitus or ingestion of antiepileptic drugs, are implicated in about 10% of the cases.
- Risk of non-chromosomal syndromes is low.
- Detailed ultrasound examination.
- Fetal karyotyping in the presence of associated anomalies.
- Follow-up scans every 4 weeks to monitor evolution of ventriculomegaly, possible kyphoscoliosis and development of talipes.
- In utero closure of spina bifida (open surgery requiring hysterotomy or by fetoscopy), reduces the need for postnatal shunt placement and may improve motor and urologic function.
- Standard obsetric care and delivery. There is no evidence that delivery by cesarean section reduces handicap.
- The 5-year mortality rate is about 35%, with 20% dying during the first 12 months of life. About 25% of fetuses with spina bifida are stillborn
- The surviving infants usually have paralysis in the lower limbs and double incontinence; despite the associated hydrocephalus requiring surgery, intelligence is often normal.
- Affected parent or one sibling: 5%.
- Two affected siblings: 10%.
- Supplementation of the maternal diet with folate (5 mg/day) for 3 months before and 2 months after conception reduces the risk of recurrence by about 75%.