FETAL CEREBRAL VENTRICULOMEGALY - MISIDENTIFICATION OF THE TRUE MEDIAL BOUNDARY OF THE VENTRICLE AT US

PURPOSE: To investigate the implications of mistaking the medial surfa ce of the cerebral hemisphere for the medial wall of the lateral ventr icle at antenatal ultrasonography (US) and to identify US clues that m ight help avoid this interpretive error. MATERIALS AND METHODS: In 50 second-and third-trimester fetuses, a directed attempt was made to dem onstrate 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 l ateral 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 diamet er of 6.2 mm. This value was greater than or equal to 10 mm (the gener ally accepted upper Limit of normal for the ventricular diameter) in a ll 15 third-trimester fetuses and in 16 (46%) of 35 second-trimester f etuses. The parietal occipital fissure was demonstrated along the medi al surface of the cerebral hemisphere in 36 (72%) of 50 fetuses, and t he medial surface of the cerebral hemisphere could be traced posterior ly around the occipital lobe in 45 (90%). CONCLUSION: When ventriculom egaly is suspected, the examiner should make a direct attempt to find the medial wall of the ventricle and distinguish it from the medial bo undary of the cerebral hemisphere. Correct identification of the anato mic interfaces is facilitated by demonstrating that the cerebral inter face contains the parietal occipital fissure and can be traced posteri orly around the occipital lobe.