arnold – chiari malformation

the caudal displacement of posterior fossa structures gradually effaces the cisterna magna, such that measurements of this subarachnoid space remain below 2 mm in the transcerebellar plane. in these circumstances, the cerebellum is now closely apposed to the occipital bone, giving rise to the banana” appearance. the displacement also effects a relative shift in the anterior cranial contents, causing scalloping of the frontal bones, known as the “lemon” sign; this describes the outline of the skull when viewed in the transthalamic plane. ventriculomegaly may also be observed; this arises due to herniation of the fourth ventricle, which causes obstruction of the exit foramina draining cerebrospinal fluid from the ventricular system. aqueduct deformities may also occur secondary to ventricular enlargement and compression of the brain stem. the presence of these signs should initiate a careful search for spina bifida.