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Ultrasound diagnosis of neural tube defects

Authors
Ana Monteagudo, MD
Ilan E Timor-Tritsch, MD
Section Editors
Louise Wilkins-Haug, MD, PhD
Deborah Levine, MD
Deputy Editor
Vanessa A Barss, MD, FACOG

INTRODUCTION

Central nervous system (CNS) malformations are the second most frequent category of congenital anomalies, after congenital heart disease. Ultrasound examination is an effective modality for the prenatal diagnosis of these anomalies. An accurate fetal diagnosis depends upon a precise description of the sonographic appearance of the CNS and careful evaluation for associated malformations, which are often present [1].

A thorough understanding of the normal sonographic appearance of the CNS at different gestational ages is crucial for accurate diagnosis because the presence or absence of a structure may be deemed normal or abnormal depending upon the age of the fetus.

As an example, a sonogram of the fetal brain at 14 weeks of gestation cannot detect agenesis of the corpus callosum since this structure does not become sonographically apparent until 18 to 20 weeks of gestation and does not acquire its final form until 28 to 30 weeks. In another example, a population-based study of 55,226 pregnancies reported 143 infants with CNS defects, 85 of whom had a prenatal sonogram at 16 to 20 weeks of gestation [2]. The diagnosis of CNS abnormality was made prenatally in 64 (75 percent), not made in 17 (20 percent), and was questionable in 4 (5 percent). Poor timing of the examination, rather than poor sensitivity, was the most important factor in failing to detect a CNS defect.

Three-dimensional (3D) ultrasound plays an important role in the evaluation of brain anomalies since it has the potential for further characterization of these defects. Multiplanar imaging of the brain, as well as use of a variety of display modalities, such as tomographic imaging, inversion, maximum-mode, surface rendering, and volume scanning, allows the sonologist to obtain planes and sections not easily obtainable with conventional two-dimensional (2D) sonography. The pediatric neurologist or neurosurgeon can use this additional information when counseling parents about prognosis and clinical management options.

Fast magnetic resonance imaging (MRI) can also be used to image the fetus. However, we feel that ultrasonography continues to be the modality of choice in the evaluation of a fetus at risk for a neural tube defect (NTD), given its high detection rates (see below). Prenatal screening and diagnosis of NTDs, as well as prevention, are discussed separately. (See "Prenatal screening and diagnosis of neural tube defects" and "Folic acid supplementation in pregnancy".)

         

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