mercoledì 22 marzo 2006
Agenesis of the corpus callosum
Noncontrast CT of the head demonstrating absence of the corpus callosum with widely spaced parallel lateral ventricles, and dilatation of the trigones and occipital horns of the lateral ventricles (Figure 3).
Diagnosis: Agenesis of the corpus callosum
Agenesis of the corpus callosum is a congenital abnormality affecting approximately 0.7% of all births. Patients may present with refractile seizures, macrocephaly, mental retardation, hypothalamic dysfunction, or may be asymptomatic and of normal intelligence. The development of the corpus callosum begins at approximately the eighth week of gestation. Formation is initiated at the anterior genu and proceeds through the posterior splenium. The rostrum is the final portion to form at about week 20. Callosal abnormalities can vary from hypogenesis to complete agenesis. When hypogenetic, the structures that are formed the latest are the areas that are affected, including the posterior body, splenium, and rostrum.
There are several characteristic radiographic findings with agenesis of the corpus callosum. The corpus callosum is a relatively firm structure due to the compact nature of the axons within it, and thus in the normal brain, gives the ventricles their shape. In its absence, only loose white matter surrounds the ventricles and they expand. Anteriorly, the firm caudate heads maintain the shape of the frontal horns. Therefore, in callosal agenesis, only the posterior portions of the lateral ventricles enlarge, manifested by dilatation of the trigones and occipital horns. This is referred to as colpocephaly. In addition, the bodies of the lateral ventricles appear straight, widely spaced, and parallel. The lateral ventricles may also appear crescent-shaped. The white matter fiber tracks that would normally cross at the corpus callosum are redirected and run parallel to the interhemispheric fissure along the medial aspect of the lateral ventricles. The tracks leave an impression along the medial borders of the lateral ventricles, giving them a crescent shape. These white matter tracks are referred to as the longitudinal callosal bundles of Probst. With callosal agenesis, the third ventricle is often dilated and described as “high-riding,” as its location is more superior, lying between the lateral ventricles. Also, when the corpus callosum fails to form, the cingulate gyri, which lie just superiorly, do not invert and there is subsequent absence of the cingulate sulcus.
Agenesis of the corpus callosum may be associated with other midline abnormalities, including an interhemispheric cyst or pericallosal lipoma. There are many syndromes in association with agenesis of the corpus callosum. Aicardi syndrome is an x-linked dominant disorder that includes agenesis of the corpus callosum, mental retardation, seizures that are often resistant to anticonvulsants, and abnormalities of the retina. Corpus callosal agenesis is also associated with Chiari malformations, Dandy-Walker syndrome, and Trisomies 13, 15, and 18. In addition, it is the most common brain abnormality seen in fetal alcohol syndrome.