Schizencephaly: correlation between the lobar topography of the cleft(s) and absence of the septum pellucidum

The bipolar defects observed in schizencephalies - clefts in the hemispheri c mantle on the one hand, absent septum pellucidum on the other - without a ny anatomic or functional continuity suggest that there is some sort of com mon specific vulnerability of both structures. A study of the correlation b etween lobar location of the clefts and involvement of the septum pellucidu m was undertaken, considering the hypothesis that the septum pellucidum is the portion of a "medial medullary velum" that corresponds to the frontal l obe, while the psalterium would correspond to the parieto-occipital lobe an d the fimbria corresponds to the temporal lobe. This retrospective study of 16 cases of schizencephaly properly investigated by MR discloses a perfect correlation, all cases with absent septum pellucidum having clefts into th e frontal lobe, all cases with present septum pellucidum having clefts in t he parietal, temporal, and occipital lobes, and only the few instances (3 c ases) of overlapping findings being characterized by clefts in the central area, where the distinction between posterior frontal and anterior parietal lobes is uncertain because of the cortical dysplasia related to the clefts . Partial defects of the septum also proved to correlate closely, topograph ically, with the location of the clefts. Therefore, the facts confirm a seg mental organization of the mantle and septal defects, suggesting a developm ental rather than a destructive mechanism, which could at least be related to a segmental pattern of selective vulnerability.