ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 2: performance of targeted neurosonography

Central nervous system (CNS) malformations are some of the most common congenital abnormalities, with an incidence at birth of 14/10 0001. Neural tube defects are the most frequent CNS malformation, with a prevalence in pregnancy of 52/100 0002. The incidence of intracranial abnormalities with an intact neural tube is uncertain, as most of these abnormalities are likely to escape detection at birth and manifest only in later life. Long‐term follow‐up studies suggest, however, that the incidence may be as high as one in 100 births3. During pregnancy, ultrasound screening for CNS malformations is carried out mainly at the time of the mid‐trimester anomaly scan4 and relies on visualization of three axial planes, namely, the transventricular, transthalamic and transcerebellar planes; basic evaluation of the fetal spine is also part of this screening procedure, and has been described in Part 1 of these guidelines5. However, of note is that some malformations may be detectable as early as the first‐trimester scan.The focus of this Guideline is to describe the protocol for the diagnostic ultrasound examination that should be performed in any case in which there is an increased risk of CNS malformation. A detailed list of indications for this targeted fetal neurosonography was published in Part 1 of these guidelines5. It is commonly accepted that targeted fetal neurosonography has a much greater diagnostic potential than does the basic screening examination, and is particularly helpful in the evaluation of complex malformations6, 7. However, this targeted examination of the fetal CNS requires a high level of expertise that is not always available in many ultrasound facilities, since the method has not yet been implemented universally.