Failure of closure of the neural tube during neurulation results in neural tube defects. Neurulation, both primary and secondary, are completed by approximately 32 post-ovulatory days.
All cases of anencephaly, approximately 80% of spina bifida, and 18% of cephaloceles are open lesions. In cases of closed neural tube defects, the maternal serum alpha-fetoprotein (MSAFP) is normal and the diagnosis is made at the time of the ultrasound examination.
In the first trimester, open spina bifida can be diagnosed by direct observation of the spinal defect or by using several posterior brain “clues” such as non-visualization of the intracranial translucency (IT) and cisterna magna (CM); increased brainstem (BS) diameter; decreased brainstem to occipital bone (BSOB) diameter as well as increased BS–BSOB ratio.
During the second and third trimesters, diagnosis of spina bifida can be made by direct observation of the spinal defect or by the “classic” intracranial sonographic signs, namely the “lemon” and “banana” signs.
Anencephaly is a lethal malformation. The outcome of cephalocele is dependent on the size and presence of associated anomalies or genetic syndromes. Open spina bifida and Chiari II malformation are not lethal anomalies, although they are associated with a significant amount of morbidity and mortality.
Recurrence risk for neural tube defects (NTDs) ranges between 2% and 5%.
Neural tube defects (NTDs) result from failure of the neural tube to form and close during primary neurulation. These abnormalities are characterized by the presence of a cerebral, spinal, or combined cerebral/spinal defect or dysraphia.
The normal appearance of the fetal central nervous system (CNS) results from primary neurulation or dorsal induction; this in turn results in the formation of the brain and spinal cord exclusive of those segments caudal to the lumbar area. During primary neurulation, fusion of the neural fold occurs first in the dorsal region of the lower medulla at approximately 22 days after conception. Fusion does not proceed continuously in a caudal to rostral fashion as previously believed (the “zipper” theory).i1 There are two fusion sites, and closure occurs bidirectional with the anterior neuropore closing before the rostral neuropore.2,3 Secondary neurulation, or caudal neural tube formation, occurs approximately between 26 and 32 postovulatory days and results in the formation of the lower sacral and coccygeal segments.4 It is during secondary neurulation that canalization occurs (see Chapter 2). Table 6–1 lists the defects that arise as a result of abnormal primary neurulation in decreasing order of severity.
Table 6–1.NEURAL TUBE DEFECTS: DEFECTS OF PRIMARY NEURULATION ||Download (.pdf) Table 6–1. NEURAL TUBE DEFECTS: DEFECTS OF PRIMARY NEURULATION
|Craniorachischisis totalis |
|Myelomeningocele, Arnold-Chiari malformation |
Over the last 30 years, pregnant women have been routinely ...