PRENATAL-DIAGNOSIS OF CENTRAL-NERVOUS-SYSTEM ABNORMALITIES (REPRINTEDFROM CURRENT PROBLEMS IN DIAGNOSTIC-RADIOLOGY, VOL 23, PG 69, 1994)

Fetal anomalies have been the subject of innumerable publications both in the prenatal and neonatal literature. This has significantly incre ased in the last 10 years, mainly because of the advent of high-resolu tion ultrasound equipment and improvement of scanning techniques. In a ddition, guidelines issued by professional organizations involved in p renatal diagnosis have encouraged a more universal approach to the ima ging and documentation of prenatal finding's. The fetal central nervou s system is the most frequently investigated organ system, mainly beca use of its easy accessibility and prominence even in the early stages of embryologic development. The biparietal diameter was the first feta l measurement to be widely used in determining gestational age. As inv estigators gained more experience, the appearance of ultrasound images achieved the resolution that allows direct comparisons with dress spe cimens and more recent sophisticated techniques of computed tomography and magnetic resonance imaging. Now endovaginal ultrasound can docume nt early first trimester development and compare it to known embryolog ic landmarks. Interest in demonstrating the ultrasound counterpart of central nervous system structures in the early stages of development h as resulted in a plethora of articles proving the unique ability of ul trasound in imaging the developing fetus. In view of all these develop ments, 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 prenat al diagnosis. It is therefore helpful to review key developmental mile stones in embryologic life and correlate them with the corresponding p renatal 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 frus tration of not being able to reproduce results is common, especially w hen 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 habitu s, and suboptimal equipment can lead to failure in visualizing the key findings described in the literature. Under these circumstances, ultr asound limitations have to be acknowledged and complementary studies s hould be considered. Occasionally, endovaginal definition of intracran ial anatomy in a low-lying fetal head may be helpful. Some times, when management decisions depend on the prenatal diagnosis, computed tomog raphy or magnetic resonance imaging limited to the area of concern can be done. As a matter of caution, some published observations have bee n generated in the early stage of gestation. As such, their applicatio n later in pregnancy may be invalid or unknown. The details of imaging or differential diagnosis of every central nervous system anomaly can not be covered in ally review, no matter how comprehensive. We consoli date 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 fi ndings do not seem to follow established criteria. A critical evaluati on of the reasons behind such cases usually results in newer literatur e that improves, modifies, or altogether changes currently accepted me thods of examination.