Prenatal diagnosis of central nervous system abnormalities

Abstract Fetal anomalies have been the subject of innumerable publications both in the prenatal and neonatal literature. This has significantly increased in the last 10 years, mainly because of the advent of high-resolution ultrasound equipment and improvement of scanning techniques. In addition, guidelines issued by professional organizations involved in prenatal diagnosis have encouraged a more universal approach to the imaging and documentation of prenatal findings. The fetal central nervous system is the most frequently investigated organ system, mainly because of its easy accessibility and prominence even in the early stages of embryologic development. The biparietal diameter was the first fetal measurement to be widely used in determining gestational age. As investigators gained more experience, the appearance of ultrasound images achieved the resolution that allows direct comparisons with gross specimens and more recent sophisticated techniques of computed tomography and magnetic resonance imaging. Now endovaginal ultrasound can document early first trimester development and compare it to known embryologic landmarks. Interest in demonstrating the ultrasound counterpart of central nervous system structures in the early stages of development has resulted in a plethora of articles proving the unique ability of ultrasound in imaging the developing fetus. In view of all these developments, the beginning ultrasound specialist is faced with the challenge and responsibility not only of being familiar with the literature but also of the mastery of scanning techniques that allow accurate prenatal diagnosis. It is therefore helpful to review key developmental milestones in embryologic life and correlate them with the corresponding prenatal ultrasound appearance. In addition, the changing appearance of the developing fetus has created a need for a systematic approach in the evaluation of structures so routine protocols can be established. This has been the subject of other publications that allow the novice to draw from the cumulative experience of different centers around the world. It is important to pay attention to the specifics described in the literature when duplicating results in one's laboratory. The frustration of not being able to reproduce results is common, especially when technical limitations prevent imaging under ideal conditions. This is especially true in patients who are first seen in the later third trimester with no prior prenatal care. Fetal lie, maternal body habitus, and suboptimal equipment can lead to failure in visualizing the key findings described in the literature. Under these circumstances, ultrasound limitations have to be acknowledged, and complementary studies should be considered. Occasionally, endovaginal definition of intracranial anatomy in a low-lying fetal head may be helpful. Sometimes, when management decisions depend on the prenatal diagnosis, computed tomography or magnetic resonance imaging limited to the area of concern can be done. As a matter of caution, some published observations have been generated in the early stage of gestation. As such, their application later in pregnancy may be invalid or unknown. The details of imaging or differential diagnosis of every central nervous system anomaly cannot be covered in any review, no matter how comprehensive. We consolidate the fundamentals recommended in current literature and encourage the reader to seek the original articles for clarification of specific problems of diagnosis. As more investigation is conducted, certain findings do not seem to follow established criteria. A critical evaluation of the reasons behind such cases usually results in newer literature that improves, modifies, or altogether changes currently accepted methods of examination.