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.
Not all cases of neural tube defects are open lesions. Approximately 20% of spina bifida and 80% of cephaloceles are closed lesions. In this scenario the MSAFP is normal and the diagnosis is made at the time of the ultrasound examination.
Approximately 80 to 90% of children with Chiari II will develop hydrocephaly.
During the second and third trimesters the "classic" sonographic findings that aid in the diagnosis of spina bifida are the lemon and banana signs. Recently, three additional findings have been described: pointed lateral ventricles, beaked tectum, and interhemispheric cyst. In the first trimester, the newly proposed "intracranial translucency" may become an early sign of open spina bifida.
Anomalies of dorsal induction are those that result from failure or abnormal closure of the neural tube. Essentially these anomalies are better known as neural tube defects (NTDs). NTDs result from failure of the neural tube to close during primary neurulation. They 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).1 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 1). Table 5–1 lists the defects that arise as a result of abnormal primary neurulation in decreasing order of severity.
TABLE 5–1.NEURAL TUBE DEFECTS: DEFECTS OF PRIMARY NEURULATION ||Download (.pdf) TABLE 5–1. NEURAL TUBE DEFECTS: DEFECTS OF PRIMARY NEURULATION
|Craniorachischisis totalis |
|Myelomeningocele, Arnold-Chiari malformation |
In the United States, all pregnant women are routinely offered screening with maternal serum alpha-fetoprotein (MSAFP) for neural tube defects at 15 to 18 postmenstrual weeks. MSAFP is one of the components of the quad screen (alpha-fetoprotein, human chorionic gonadotropin, estriol, and inhibin-A), which is commonly used at present for screening of Down syndrome and open NTDs. Among low-risk women, MSAFP screening results in the detection of 80% to 90% of ...