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Open Neural Tube Defects (NTD)

Open neural tube defects are the result of failure of the neural tube to close during embryologic development.  The location of the opening determines the type of ONTD: spina bifida is due to an opening lower in the spine and anencephaly is due to an opening higher in the spine affecting the brain.  The general population incidence in Canada is about 1 in 2000.  This does not vary with maternal age.  However, there are several risk factors that increase the chance of having an affected child: non-gestational diabetes mellitus, some types of anticonvulsant medications, a family history of NTD and significant prolonged maternal hyperthermia.
Anencephaly is lethal.
Most babies with spina bifida survive and may have problems ranging from hydrocephalus, paralysis and learning/ intellectual disabilities to no physical or mental disabilities.
Prenatal ultrasound findings: ‘lemon sign’ (refers to the inwardly scalloped appearance of the frontal bones of the skull), ‘banana sign’ (refers to anterior curvature of the cerebellum and associated obliteration of the cisterna magna – Chiari malformation), ventriculomegaly, club feet
References:
Thomas, M., 2003. Signs in imaging: the lemon sign. Radiology 228: 206-207.
Fong, K.W., Toi, A., Salem, S., Hornberger, L.K., Chitayat, D., Keating, S.J., McAuliffe, F., Johnson, J. 2004. Detection of fetal structural abnormalities with US during early pregnancy. RadioGraphics 24(1): 157-174.
SOGC Clinical Practice Guideline: Fetal Soft Markers in Obstetric Ultrasound. J Obstet Gynaecol Can 2005;27(6):592–612.