Specific Imaging Findings

Most Chiari 2 malformations are today diagnosed by sonography before birth and encountered abnormalities include: the lemon sign which is due to concavities involving the frontal bones, the banana sign which is due to an abnormal shape of the cerebellar hemispheres with effacement of the cisterna magna, lacunar skull and myelomeningocele (MMC). CT and MR are the imaging methods of choice, but MRI shows the abnormalities to a better extent. On sagittal images the main findings are: herniation of the vermis through the foramen magnum which is large or normal in size, a smooth cerebellar vermis, absent fourth ventricle, small brainstem, beaked tectal plate, large massa intermedia, small third ventricle, variable dilation of the lateral ventricles and dysgenesis of the corpus callosum and stenogyria involving predominantly the occipital lobes. Hippocampi are commonly abnormal, with atypical sulcation of the adjacent mesial temporal cortex. Infratentorially axial images show that the cerebellar hemispheres wrap around the brainstem and that the tentorium is incompletely formed allowing superior protrusion of the cerebellum in the shape of a heart. Axial supratentorial images show an absent falx cerebri with variable interdigitation of the cortical gyri across the midline. Other findings include hydrocephalus, as well as a variety of neuronal migration anomalies.