OBJECTIVE: Visualization of the cavum septi pellucidi (CSP) is considered an integral part of the prenatal second-and third-trimester sonographic evaluations of the fetal neural axis. We have noted that another anatomic structure, the columns of the fornix, can be mistaken for the CSP and result in the missed diagnosis of agenesis of the corpus callosum. METHODS: We describe a case in which the columns of the fornix were misinterpreted as representing the CSP during a sonographic evaluation at 18 weeks' gestation. After a follow-up sonogram at 35 weeks, agenesis of the corpus callosum was detected. A retrospective evaluation of the prevalence of forniceal columns was then performed in 100 consecutive sonograms of pregnancies between 18 and 24 weeks' gestation. A prospective study was then performed in 20 patients to determine the prevalence of visualization of the columns of the fornix. RESULTS: In 86 of 100 patients, the columns of the fornix were retrospectively identified as discrete and separate structures from the CSP. When additional views were obtained prospectively in the forebrain, the fornix could be identified in all 20 patients. CONCLUSIONS: The columns of the fornix may simulate the appearance of the CSP on second- and third-trimester sonograms. The distinction between these structures can be made because the columns of the fornix will show a linear reflection (3 lines) at their interface, whereas the CSP will appear as a fluid-filled triangular or rectangular fluid-filled space without a central line.
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