Spina Bifida

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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.

Ultrasonography
Sonographic diagnosis of spina bifida aperta requires a detailed understanding of not only spinal anomalies but also of fetal intracranial anatomy :
* 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 it.
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 spine.
* 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 this abnormality.

Differential :
* The false-positive rate for the banana sign has been reported as 0%
* Lateral thoracic meningocele
* Cephalocele
* 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.

Prognosis :
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.

Treatment :
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.

Reference :
* 1: Robinson AJ, Russell S, Rimmer S. The value of ultrasonic examination of the lumbar spine in infants with specific reference to cutaneous markers of occult spinal dysraphism. Clin Radiol. 2005 Jan;60(1):72-7.
* 2: Henriques JG, Pianetti Filho G, Costa PR, Henriques KS, Perpetuo FO. [Screening of occult spinal dysraphism by ultrasonography] Arq Neuropsiquiatr. 2004 Sep;62(3A):701-6. Epub 2004 Aug 24. Portuguese.
* 3: Cho FN, Kan YY, Yang TL, Huang HP, Hsu PH, Tai MH. Prenatal diagnosis of sharp-angled lumbosacral kyphosis with myelomeningocele and spina bifida in a fetus. J Ultrasound Med. 2004 Aug;23(8):1115-9.
* 4: Biggio JR Jr, Wenstrom KD, Owen J. Fetal open spina bifida: a natural history of disease progression in utero. Prenat Diagn. 2004 Apr;24(4):287-9.
* 5: Cherif A, Oueslati B, Marrakchi Z, Chaouachi S, Chaabouni M, Abassi M, Dammak M, Chaabouni H, Boukef-Largueche S. [Diastematomyelia: antenatal diagnosis with successful outcome, two cases] J Gynecol Obstet Biol Reprod (Paris). 2003 Sep;32(5):476-80.
* 6: Drugan A, Weissman A, Evans MI. Screening for neural tube defects. Clin Perinatol. 2001 Jun;28(2):279-87, vii. Review.
* 7. Pilu G: Ultrasound evaluation of the fetai neural axis. In 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, 1993.
* 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