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Articles » Central nervous system » Spine » Neural tube defect: Spine » Spina bifida
2000-08-20-19 Spina bifida  © Gonçalves www.thefetus.net/
Spina bifida

Luis F. Gonçalves, MD - Maria Verônica Munoz Rojas, MD, Gloria Valero MD*

Florianopolis, Brazil
*Magdalena Sonora, Mexico

Definition

Also known as myelomeningocele, meningocele and rachisquisis, spina bifida can be defined as a midline vertebral defect, resulting in exposure of neural contents to the amniotic fluid. In the immense majority of cases, the defect is located on the posterior vertebral arches. In rare cases, the defect is caused by division of the vertebral body[1].

Incidence

It is the most common CNS malformation. Incidence varies according to geographic location, ethnic background and year seasons. It is more prevalent in the British Isles and less common in Eastern countries. Likewise, spina bifida is more frequent in Caucasians when compared to Orientals or Africans. These differences seem to persist even after eventual migrations, suggesting that genetic influence is more pronounced than the environmental one.

Etiology

The pattern of inheritance is multifactorial. It can also occur as part of Mendelian syndromes, chromosomal disorders or secondary to exposure to teratogenic agents like valproic acid, aminopterin and thalidomide. Diabetes mellitus is a predisposing maternal factor.

Embryology

The neural tube derives from the ectoderm. The Table below describes the chronology of neural tube development[2].

Gestational age
Menstrual age
CRL
Events

16 days

30 days
(4 wks + 2 days)

1 mm

neural crest formation

21 days

35 days
(5 wks)

1,5 - 2 mm

neural crest starts to close from midline, advancing to rostral and caudal ends

24 days

38 days
(5 wks + 3 days)

2,5 mm

closure of the anterior neuropore

24 days

42 days
(6 wks)

4 mm

closure of the posterior neuropore

Diagnosis

The spine should be systematically examined in all fetuses beginning at 12 weeks. Ultrasonographic examination should be conducted in standardized planes: coronal plane, transverse plane or sagittal plane. We usually begin examining the spine in the coronal plane, making sure that an equal amount of fetal tissue is observed in both parts of the fetus. Next, each vertebra is examined in the transverse plane. In case a defect is detected during the examination in one or both of the planes described below, sagittal sections are obtained in order to determine the extent of the defect1.

Coronal sections

In coronal sections, spina bifida may be recognized by splaying of the vertebral lateral pedicles.

Diagram illustrating separation of the lateral pedicles in a coronal section through the fetal spine.

Normal spine demonstrated in a coronal section.

Coronal section through the spine of a fetus with spina bifida. Observe the separation of the lateral pedicles and compare with the image above.

Axial sections

In axial or transverse sections, separation of the posterior ossification centers is observed, along with a skin defect and exposure of neural contents to the amniotic fluid. In most cases, a myelomeningocele sac can be seen.

Diagram illustrating a transverse section through a fetus with spina bifida

Transverse section through the vertebra of a fetus with spina bifida.

Longitudinal sections

As mentioned above, longitudinal sections are used to determine the extension of the anomaly once it is diagnosed.

Diagram illustrating the sagittal section through the spine of a fetus affected by spina bifida.

Sagittal section through the lumbosacral vertebrae of a normal fetus.


Sagittal section of the lumbosacral spine of a fetus with spina bifida.

Indirect signs

There are two indirect signs that may lead to the suspicion of spina bifida during an ultrasound examination, generally before the actual spinal lesion can be observed. The fist is called the "lemon sign", consisting of a more or less pronounced depression at the level of the metopic suture, giving to the calvarium the shape of a lemon. The second sign is known as the banana sign and is 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 aspect of a banana. Both signs are the consequence of the Arnold-Chiari Malformation type II, commonly associated with spina bifida[3].

Lemon sign.

Banana sign.

The following is a 2.6 MB video of a case that demonstrates most of the findings of spina bifida. Note the lemon and banana signs, the abrupt opening of the spine at the high lumbar level with a large meningomyelocele and the associated clubfeet. Select images of the clip are reproduced below:

 

Lemon-shaped head, the banana shaped cerebellum and thinned cisterna magna

  

 

Coronal views showing abnormal spacing of the posterior ossification centers

  

  

 

Transverse axial views of the lumbar spine showing splaying of the posterior elements and a myelomeningocele.

  

Clubfoot deformity secondary to the myelomeningocele

Associated anomalies

Almost all cases of spina bifida are associated with the Arnold-Chiari type II malformation, characterized by herniation of the cerebellar vermis through the foramen Magnum. In these cases, the fourth ventricle is dislocated towards the neural canal, the posterior fossa is shallow and the tentorium is also displaced downwards. This finding is almost invariably associated with obstructive hydrocephalus.

Congenital hip dislocation and clubfeet are frequently associated with spina bifida and are secondary to variable inervation deficits of the hip and thigh muscles.

Classification

The most common location for spina bifida is the lumbosacral region. 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 (CSF) and some times neural content covered by meninges. When the herniation sac contains only CSF, the anomaly is called meningocele; if neural tissue is present, the anomaly is called meningomyelocele.

Prognosis

Spina bifida is a serious congenital anomaly. The neonatal morbidity and mortality rate is estimated as 25%. The majority of the children without treatment die in the first few months of life. Survival rate of those treated in the immediate neonatal period approaches 40% at seven years. Twenty five percent of these children are almost totally paralyzed, 25% require intense rehabilitation and 25% do not have a significant lower extremity dysfunction. Seventeen percent will have normal continence in long term follow up. The presence of severe hydrocephalus is considered a poor prognostic sign[4].

Genetic counseling and recurrence risk

Similar to anencephaly, the majority of the cases of spina bifida are compatible with a multifactorial model. Genetic factors seem important because of familial incidence, whereas geographic variation suggests an environmental cause. Concordance and discordance have occurred in monozygous twins. An increased incidence of neural tube defects occurs in women who have diabetes during pregnancy. Also, women who take valproic acid for a seizure disorder are at increased risk for anencephaly if their medication has been consumed prior to conception or during the first trimester of pregnancy.

Causative factors act on the developing embryo between the 16th and 26th day after conception.

The recurrence risk is 1:20 (5%) and if there is a second affected child the risk rises to 13%.

Prevention

All women of reproductive age should consume at least 0.4 mg (400 mcg) of folic acid daily to prevent neural tube defects. For women who have previously had a fetus affected with anencephaly, the Centers for Disease Control and Prevention (CDC), recommends increasing the intake of folic acid to 4 mg (4000 mcg) per day beginning at least one month prior to conception (Committee on Genetics, 1999)[5].

Treatment

The three options available once a fetus with spina bifida is diagnosed are:

termination of the pregnancy

intrauterine surgery (see links below)

expectant management with closure of the vertebral defect and correction of associated anomalies in the neonatal period

Links

Children´s Hospital of Philadelphia
Jama - Fetal Surgery for Spina Bifida at Philadelphia
Fetal Surgery for Spina Bifida at Vanderbilt University
Jama - Fetal Surgery for Spina Bifida at Vanderbilt

References

[1] Romero R, Pilu G, Jeanty P, Ghidini A, Hobbins JC. Prenatal Diagnosis of Congenital Anomalies. Norwalk: Appleton&Lange, 1988.

[2] Moore KL, Persaud TV: The Developing Human: Clinically Oriented Embryology, 5th edition, Philadelphia: WB Saunders Company, 1993.

[3] Campbell J, Gilbert WM, Nicolaides KH, Campbell S. Ultrasound screenign for spina bifida: carnial and cerebellar signs in a high risk population. Obstet Gynecol 1987;70:247.

[4] Bianchi DW, Cromblehome TM, D"Alton ME. Fetology: : Diagnosis & Management of the Fetal Patient. McGrawHill, 2000.

[5] Folic acid for the prevention of neural tube defects. American Academy of Pediatrics. Committee on Genetics. Pediatrics 1999;104: 325-7.

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