Fetal cerebral ventriculomegaly: misidentification of the true medial boundary of the ventricle at US.

PURPOSE

To investigate the implications of mistaking the medial surface of the cerebral hemisphere for the medial wall of the lateral ventricle at antenatal ultrasonography (US) and to identify US clues that might help avoid this interpretive error.

MATERIALS AND METHODS

In 50 second- and third-trimester fetuses, a directed attempt was made to demonstrate the medial surface of the cerebral hemisphere and the medial wall of the lateral ventricle on images that depicted the lateral wall of the ventricle. In each fetus, measurements of the diameter of the false ventricular atrium were compared with the true diameter of the lateral ventricle to assess the potential magnitude of error.

RESULTS

The average diameter measured with the medial surface of the cerebral hemisphere was 10.7 mm, compared with the true mean ventricular diameter of 6.2 mm. This value was greater than or equal to 10 mm (the generally accepted upper limit of normal for the ventricular diameter) in all 15 third-trimester fetuses and in 16 (46%) of 35 second-trimester fetuses. The parietal occipital fissure was demonstrated along the medial surface of the cerebral hemisphere in 36 (72%) of 50 fetuses, and the medial surface of the cerebral hemisphere could be traced posteriorly around the occipital lobe in 45 (90%).

CONCLUSION

When ventriculomegaly is suspected, the examiner should make a direct attempt to find the medial wall of the ventricle and distinguish it from the medial boundary of the cerebral hemisphere. Correct identification of the anatomic interfaces is facilitated by demonstrating that the cerebral interface contains the parietal occipital fissure and can be traced posteriorly around the occipital lobe.