Sonographic prenatal diagnosis of central nervous system abnormalities.

INTRODUCTION: Over the past 20 years, the spectrum of neonatal neurological malformations has changed due to the diffusion of ultrasound, performed either routinely or as required by maternal alpha-fetoprotein screening or history.

DISCUSSION: We review and illustrate the potential of ultrasound for the prenatal diagnosis of abnormalities in size or shape of the skull (macrocephaly, microcephaly, craniostenosis), neural tube defects, ventriculomegaly, hydrocephalus, posterior fossa defects (abnormalities in the size of the cisterna magna, cerebellar abnormalities), midline abnormalities (holoprosencephaly, abnormalities of the corpus callosum), ischemic lesions and hemorrhage, tumours, and focalized hyperechogenic images. The limits of fetal ultrasound screening and of the various diagnostic strategies implemented when a fetal brain abnormality is suspected are discussed. Overall, gross lethal abnormalities such as anencephaly or major hydrocephaly are accessible to prenatal sonographic screening, and nearly always result in termination of the pregnancy. However, hydrocephaly may progress late in gestation and remain undiscovered unless a third trimester ultrasound is performed. A majority of cases with myelomeningocele are diagnosed prenatally, resulting either in termination of the pregnancy or in neonatal management. A growing number of more subtle abnormalities, including midline or posterior fossa abnormalities can be spotted by fetal ultrasound, but their postnatal outcome cannot always be predicted accurately, despite the use of fetal magnetic resonance imaging. In such cases, a trans-disciplinary approach involving perinatologists, pediatric radiologists, neuropathologists, neurosurgeons or neurologists familiar with neonates is crucial to counseling the parents. Some brain abnormalities are still extremely difficult or even impossible to diagnose in utero despite advances in sonographic imaging. This is due to the fact that severe neurological impairment may result from conditions that do not affect substantially affect the morphology of the brain, and that major structural abnormalities may develop late in gestation, and thus remain undetected at second trimester ultrasound.

CONCLUSION: Ultrasound screening identifies a growing number of central nervous system abnormalities, resulting in substantial changes in the neonatal presentation of neurological congenital abnormalities.