Spina bifida is a congenital defect of the fetus's spine resulting
from the failure of closure of the neural tube. The most common
location for spina bifida is the lumbo-sacral region. The two
main types of spina bifida are spina bifida aperta and spina bifida
occulta. In 85% of the cases the defect is open and in 15% of
the cases spina bifida is classified as occulta. In spinal bifida
occulta the defect is covered by skin and is not usually diagnosed
prenatally. In open spina bifida, the most common presentation
is a cystic tumor in the lumbosacral region, containing cerebrospinal
fluid and some times neural content covered by meninges. In cases
where the meninges protrude through this defect, it is called
a meningocele. If neural tissue is involved, it is a myelomeningocele.
These lesions may occur anywhere along the spine but are more
common in the lumbar and sacral regions.
The prevalence of spina bifida is estimated to be around 1 in
1000 births, but Incidence varies according to geographic location.
The etiology of spina bifida is multifactorial. It has been associated
with chromosomal abnormalities (trisomy 13 and 18, triploidy),
exposure to teratogenic agents like valproic acid and carbamazepine,
maternal hyperthermia and maternal diabetes.
Sonographic diagnosis of spina bifida aperta requires a detailed
understanding of not only spinal anomalies but also of fetal intracranial
* spinal anomalies : The ultrasound diagnosis
of spina bifida is based on the evaluation of the fetal spine,
The spine should be systematically examined in all fetuses beginning
at 12 weeks but it usually clearly visualized by 16 weeks of gestation.
Each vertebra has three ossification centers, one anterior and
two posterior. The normal posterior ossification centers are seen
as two closely spaced parallel lines of echoes. When spina bifida
is present, there is spraying of the posterior ossification centers
on the transverse and longitudinal scans.
At the level of the lesion, the ossification centers are more
widely spaced than those in the vertebral segments above and below
In the transverse cut the ossification centers assume an O shape
because the two posterior elements tilt toward the midline. If
the pedicle ossification centers assume a splayed V-, C-or U-shaped
appearance when viewed in the transverse section, the diagnosis
of spina bifida should be considered.
If a meningocele or myelomeningocele is present and intact, the
sac bulges in the amniotic fluid, and the anomaly is recognized
as a cystic formation arising from the posterior aspect of the
* Fetal intracranial anomalies : Spina bifida is
frequently associated with Arnold-Chiari malformation type II
(inferior displacement of the cerebellum, which assumes a flattened
and centrally curved shape, "banana sign"), and with notching
of the frontal bone (also seen in normal fetuses, "lemon sign").
- Arnold-Chiari : Between 80 and 90% seen as an abnormal banana-shaped
cerebellum or absent cerebellum.
- Lemon-shaped head : Seen in 90-95% of fetuses with spina bifida.
- Ventriculomegaly : 75% of cases of spina bifida demonstrate
* The false-positive rate for the banana sign has been reported
* Lateral thoracic meningocele
* Bifid sacrum artifact : A bifid sacrum artifact is a skewed
representation of normal anatomy and should not be interpreted
as a true anomaly. It is produced by a steeply angled parasagittal
scanning plane that intersects normal structures. Sonograms of
the distal fetal spine can be misleading and deceptive. The normal
spine is constructed of multifaceted anatomic structures that
change in appearance and relative position throughout gestation.
These complex structures can be seriously misinterpreted if scanning
planes are skewed or rotated off axis.
the quoted mortality rate is 25%. Of the survivors, 25% are completely
paralyzed, 25% are almost completely paralyzed, 25% need intensive
rehabilitation, and 25% have no significant lower limb dysfunction.
Neonatal neurology sequel depend on the severity and location
of the spina bifida defect. Multiple anomalies, involvement of
the thoracic cavity, and craniorachischisis all carry worse prognoses
than an isolated lumbosacral defect.
The three options available once a fetus with spina bifida is
diagnosed are: * termination of the pregnancy and ultrasound follow-up
should be performed every 2-3 weeks or *intrauterine surgery or
* expectant management with closure of the vertebral defect and
correction of associated anomalies in the neonatal period.
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examination of the lumbar spine in infants with specific reference
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diagnosis of sharp-angled lumbosacral kyphosis with myelomeningocele
and spina bifida in a fetus. J Ultrasound Med. 2004 Aug;23(8):1115-9.
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defects. Clin Perinatol. 2001 Jun;28(2):279-87, vii. Review.
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Callen PW, editor: Ultrasonography in obstetrics and gynecology,
ed 4, Philadelphia, 2000, WB Saunders.
* 8. Bail RH, Filly RA, Goldstein RB et al: The lemon sign: not
a spécifie indicator of meningomyelocele, / Ultrasound Med 12:131-134,
* 9. Van den Hof MC, Nicolaides KH, Campbell J et al: Evaluation
of thé lemon and banana signs ip one hundred thirty fetuses with
open spina bifida, Am J Obstet Gynecol 162:322-327, 1990.
* 10. Campbell J, Gilbert WM, Nicolaides KH et al: Ultrasound
screening for spina bifida: cranial and cerebellar signs in a
high-risk population, Obstet Gynecol 70:247-250, 1987.
* 11. Chervenak FA, Duncan G, Ment LR et al: Périnatal management
of meningomyelocele, Obstet Gynecol 63:376-380, 1984.
Image : Ben Jelloun
text and reference : GOUBAA