Advertisement

SMFM Fetal Anomalies Consult Series #3: Intracranial Anomalies

Published:August 20, 2020DOI:https://doi.org/10.1016/j.ajog.2020.08.041

      Introduction

      Intracranial anomalies are among the most common congenital birth defects. They range from severe and lethal malformations, which are often amenable to prenatal diagnosis during the first to early second trimesters of pregnancy, to more subtle anomalies that may only be detected during the second to third trimesters of pregnancy.
      The diagnosis of intracranial anomalies depends on 3 important factors: (1) accurate determination of the gestational age of the fetus, (2) knowledge of the developmental embryology of the central nervous system at various gestational ages, and (3) use of a systematic approach for the evaluation of the fetal brain.
      During the first trimester of pregnancy, measuring the nuchal translucency and assessing the presence or absence of the nasal bone have become common practices; the resulting images of the median plane allow the assessment of the developing brain (Figure 1). Most anomalies discussed in this document can be diagnosed or suspected during this scan. In low-risk pregnancies, the fetal brain is evaluated as part of the anatomy scan during the standard second-trimester ultrasound examination, which is typically performed between 18 and 22 weeks of gestation. In this study, the lateral ventricles, choroid plexus, midline falx, cavum septi pellucidi, cerebellum, and cisterna magna are assessed (Figure 2). This study also includes biometric measurements of the biparietal diameter and head circumference (Figure 3). In high-risk pregnancies, a detailed obstetrical ultrasound examination that includes a more extensive evaluation of the intracranial anatomy may be performed, usually by an expert in fetal imaging, such as a maternal-fetal medicine subspecialist. Additional structures imaged include the integrity and shape of the cranial vault and brain parenchyma; images of the third and fourth ventricles and the corpus callosum are obtained as clinically indicated (Figures 4 and 5). Additional biometry includes the transcerebellar diameter. Both the standard second-trimester ultrasound examination and the detailed ultrasound examination include the basic fetal brain survey and are typically performed using transabdominal ultrasonography in the 3 axial planes: transventricular, transthalamic, and transcerebellar.
      Figure thumbnail gr1
      Figure 1The fetal brain at 12 weeks of gestation
      A, Transvaginal median sagittal plane demonstrating the intracranial anatomy. B, Transabdominal median plane with the pertinent anatomy labeled. C, Color Doppler reveals the pericallosal artery (arrow). D, Transventricular planes demonstrating the echogenic CP; the anechoic LV and the interhemispheric fissure (arrow) divide the brain into the right and left hemispheres.
      4V, fourth ventricle; BS, brainstem; CM, cisterna magna; CP, choroid plexus; LV, lateral ventricle; M, mandible; NB, nasal bone; NT, nuchal translucency; P, palate.
      SMFM. SMFM Anomalies Consult Series #3. Am J Obstet Gynecol 2020.
      Figure thumbnail gr2
      Figure 2Transabdominal axial scan
      A, Transventricular plane. B, Transcerebellar plane.
      SMFM. SMFM Fetal Anomalies Consult Series #3. Am J Obstet Gynecol 2020.
      Figure thumbnail gr3
      Figure 3Biometry
      A, The biparietal diameter and head circumference are measured in the transthalamic plane. B, The lateral ventricle is measured in the transventricular plane at the level of the parietooccipital groove (arrow). C, In the transcerebellar plane, the cerebellum and cisterna magna are measured.
      SMFM. SMFM Anomalies Consult Series #3. Am J Obstet Gynecol 2020.
      Figure thumbnail gr4
      Figure 4Transabdominal scan showing additional brain structures
      SMFM. SMFM Anomalies Consult Series #3. Am J Obstet Gynecol 2020.
      Figure thumbnail gr5
      Figure 5Transabdominal median section of the brain
      A, Grayscale. B, Color Doppler.
      SMFM. SMFM Anomalies Consult Series #3. Am J Obstet Gynecol 2020.
      The next step in the evaluation of the fetal brain is the fetal neurosonogram, which is usually performed by an imaging specialist, such as a maternal-fetal medicine subspecialist or a neuroradiologist. It is distinguished from the previous 2 scans in that it (1) is often performed using transvaginal ultrasonography (if the fetus is in a cephalic presentation) to improve imaging, (2) adds the coronal and sagittal planes, (3) uses color Doppler to clarify the nature of cystic structures or document brain vessels, and (4) adds 3-dimensional ultrasonography (if available) to obtain planes and reconstructions not possible with 2-dimensional imaging. It is important to note that evaluation of the fetal spine in the axial, coronal, and sagittal views is part of the assessment of the fetal nervous system. The spine is discussed in a separate section.
      This Consult Series reviews the ultrasonographic diagnosis, genetic evaluation, and potential treatment and outcome of the following intracranial abnormalities:
      • Exencephaly-anencephaly sequence
      • Posterior encephalocele
      • Holoprosencephaly
      • Agenesis of the corpus callosum
      • Absent cavum septi pellucidi
      • Vein of Galen aneurysmal malformation
      • Ventriculomegaly
      • Intracranial hemorrhage
      • Dandy-Walker malformation
      • Intracranial cysts
      • Blake's pouch cyst

      Coding

      When coding for fetal intracranial anomalies, the Society for Maternal-Fetal Medicine Coding Committee recommends utilizing the International Classification of Diseases, 10th revision, code series O35.0xx1.

      Acknowledgments

      The authors wish to acknowledge Mary E. Norton, MD, Jeffrey A. Kuller, MD, and Angie C. Jelin, MD, for providing a review of the genetics content and Joseph Wax, MD, for providing a general review of this Consult.

      Linked Article