corpus callosum – agenesis

agenesis of the corpus callosum may be directly observed, or more commonly, inferred from its effect on adjacent structures. direct demonstration may be optimally made using sagittal views; the characteristic echolucent structure of the corpus callosum is normally seen superior to the cavum septi pellucidi and cavum vergae in the median sagittal plane, running an anteroposterior course. the anterior cerebral artery may be observed in front of the genu. agenesis results in a loss of this echolucent zone, and depending on the degree of caval agenesis, the anterior cerebral artery will be seen to reside directly on the cavum septi pellucidi in this plane. coronal views may also be helpful in providing more information. these views are best attained using a transvaginal, transfontanelle approach where a cephalic presentation exists. even in a dedicated centre, however, a successful detection rate of only 65% may be anticipated. indirect confirmation of agenesis of the corpus callosum is thus the most common approach, certainly in a routinely screened population. observations are obtained from the transverse planes, and include the following: marked separation of the lateral ventricles, with attenuation of the frontal horns and anterior bodies in conjunction with prominence of the atria and occipital horns (colpocephaly). the resultant transventricular view displays a bull’s horn” appearance. other variants of dilation occur if there is associated obstructive hydrocephaly, cortical heterotopia, or focal loss of brain substance. dilatation and elevation of the third ventricle, with variable degrees of interposition between the lateral ventricles. this characteristic transthalamic view has a “leaping frog” appearance. observation of echogenic choroid plexus within the roof of the midline cystic structure aids diagnosis. associated central nervous system anomalies include the dandy-walker and arnold-chiari type ii malformations, porencephaly, agyria, midline lipomas, encephaloceles, arachnoid cysts, microcephaly, cerebellar hypoplasia, holoprosencephaly, septo-optic dysplasia, and aqueductal stenosis. agenesis of the corpus callosum can be found in a number of well-defined genetic disorders, especially the aicardi (xr), andermann (ar), acrocallosal (ar), lissencephaly (ar, or with 17p13 deletion) and shapiro (ar) syndromes. other reports implicate the apert (ad), rubenstein-taybi (16p13.3 deletion) and oral facial-digital i (xd) syndromes, and frontonasal dysplasia. it may also be found in association with trisomies 8, 13, and 18, so it is important that a detailed assessment of the fetal anatomy be carried out in addition to fetal karyotyping. the importance of the accurate diagnosis of isolated agenesis of the corpus callosum is related to its benign prognosis. in fact, once these associated disorders and malformations are excluded, the parents may be reassured and unnecessary obstetric intervention avoided. prenatal diagnosis of callosal agenesis has been made as early as 20 weeks.”