Diagnostic ultrasound ( PDFDrive )

CHAPTER 35 The Fetal Spine 1225
TABLE 35.3 Deinition of Terms for Spinal Abnormalities
Term Deinition Comment
Spinal dysraphism
(neural tube defect)
Failure of part of neural tube to close.
This disrupts both differentiation of
central nervous system and induction
of vertebral arches.
Spina biida Defect in posterior midline neural arch. Arches fail to fuse along dorsal midline
and fail to enclose vertebral canal.
Spina biida occulta Vertebral arches of a single vertebra fail to fuse. Underlying neural tube differentiates
normally; does not protrude from
vertebral canal.
Meningocele
Myelomeningocele
Rachischisis (e.g.,
myeloschisis)
Cranioschisis (e.g.,
exencephaly,
anencephaly)
Inionschisis
Dura and arachnoid protrude from vertebral canal
through spina biida defect in posterior midline neural
arches.
Dura, arachnoid, and neural tissue protrude from
vertebral canal through spina biida defect in posterior
midline neural arches.
Neural folds corresponding to future spinal cord fail to
fuse and fail to differentiate (myeloschisis), invaginate,
and separate from surface ectoderm.
Neural folds corresponding to future brain fail to fuse
and fail to differentiate, invaginate (exencephaly,
anencephaly), and separate from surface ectoderm.
Failure of neural tube to differentiate properly and close
in occipital and upper spinal region.
The deformed underdeveloped spinal
cord is exposed dorsally.
This is the most severe form of spinal
neural tube defect.
The brain is represented by an exposed
dorsal mass of undifferentiated
neural tissue.
U.S. Food and Drug Administration ordered that fortiication
with folate of all enriched grain products be started no later than
January 1, 1998 (0.14 mg per 100 g of grain). Honein et al. 51
demonstrated a 19% reduction in NTDs in the United States as
an efect of folic acid fortiication of grains. his study did not
take into account the large percentage of NTDs that are prenatally
diagnosed and electively terminated. A study in Nova Scotia
demonstrated a decrease of annual incidence of NTDs by 54%
ater implementation of folic acid fortiication, from 2.58 in 1000
births from 1991 to 1997 to 1.17 in 1000 births from 1998 to
2000. 52 his study included terminated pregnancies, which is
important because more than 50% of all pregnancies afected
with NTDs in Nova Scotia resulted in elective termination. A
failure to include these terminated pregnancies may underestimate
the beneit of folic acid–fortiied grains. In Canada, folic acid
fortiication of grain products was legislated to begin in November
1998, at levels similar to U.S. levels. Since then in Canada, the
prevalence of NTDs nationally has decreased from 1.58 in 1000
births before fortiication to 0.86 in 1000 births during the fullfortiication
period, a 46% reduction. 7 Geographic diferences
almost disappeared ater fortiication began. he observed reduction
rate was greater for spina biida (53%) than for anencephaly
and encephalocele (38% and 31%, respectively).
Lipomyelomeningocele (LMMC) is a type of NTD similar
to myelomeningocele, with a prevalence of about 0.5 in 10,000
births. Studies in Hawaii and Canada have shown that LMMC
rates are not afected by folic acid fortiication, unlike the signiicant
rate reduction in myelomeningoceles. LMMC seems to
be pathogenetically distinct from myelomengocele. 53,54
he risk of NTD rises to 20 to 30 per 1000 live births for
women with a previous infant with an NTD. his constitutes
about a 10-fold increase in risk over the general population. 55
A meta-analysis of randomized trials of folic acid for the prevention
of recurrent NTDs demonstrated an 87% reduction in NTDs
in women who took supplements before the start of pregnancy. 3
Other factors that increase the risk of NTD include anticonvulsant
therapy with valproic acid or carbamazepine (10-20 : 1000),
warfarin and vitamin A use, pregestational diabetes, obesity,
parent with spina biida (11 : 1000), and sibling of fetus with
multiple vertebral defects and scoliosis (15-30 : 1000). 56 Low
maternal vitamin B12 status may also be a risk factor for NTDs.
In Ontario, Ray et al. 57 demonstrated a tripling of the risk for
NTD in the presence of low maternal B12 status, as measured
by serum holotranscobalamin at 15 to 20 weeks’ gestation.
Risk Factors for Neural Tube Defect (NTD)
Folic acid deiciency
Previous sibling with NTD
Maternal anticonvulsants
Valproic acid
Carbamazepine
Maternal warfarin
Maternal vitamin A
Pregestational diabetes
Obesity
Parent with spina biida
Low maternal vitamin B12