congregations of dysmorphic neurons or balloon cells in the cortex and subadjacent white matter.295,530,661,863 Focal cortical dysplasia is now the single most important cause of intractable epilepsy in childhood. It typically presents in a child with an otherwise normal neurological examination.47,52 Initial MR imaging is frequently interpreted as normal.470 Upon further review, subtle abnormalities in gyration and cortical thickness, or blurring of the gray/white matter junction may be the only detectable abnormality on MR imaging.530 However, its extent can range from focal involvement of a gyrus, to involvement of multiple gyri, transmantle dysplasia, lobar or hemispheric dysplasia, or even multifocal dyplasia of both hemispheres.530 Changes in white matter signal intensity, as detected on FLAIR images, are more commonly found than those in gray matter signal intensity on T2-weighted images, with both thought to represent the presence of balloon cells.530 A classification system has recently been devised661 based primarily upon the degree of dysplasia and the presence or absence of abnormal balloon cells or large dysmorphic neurons.530 Different histological types show distinct clinical and neuroimaging characteristics.492 Total removal of the lesion and any perilesional epileptigenic focus are needed for a good outcome.396 Some forms of focal cortical dysplasia can be associated with encephalomalacia and periventricular leukomalacia.492 Cortical dysplasia can accompany neuroglial tumors (dysembryonal neuroepithelial tumors),225 gangliogliomas,701 and mesial temporal sclerosis.716

Anomalies of the Hypothalamic–Pituitary Axis

Posterior Pituitary Ectopia

Posterior pituitary ectopia refers to the constellation of (1) absence of the normal posterior pituitary bright spot, (2) absence of the pituitary infundibulum, and (3) an abnormal focus of hyperintense tissue at or near the tuber cinereum on T1-weighted MR images (Fig. 2.4).128 Normally, the posterior lobe of the pituitary gland is hyperintense on T1-weighted MR images, probably because of the chemical composition of the phospholipid vesicles contained in it. It is speculated that, following injury to the infundibulum, the trophic influence of continued antidiuretic hormone/neurophysin secretion at the median eminence causes an abnormal collection of posterior pituicytes to form where the upper infundibulum is normally located. This ectopic cluster of cells seems to function as a normal posterior pituitary gland, so affected patients have isolated anterior pituitary deficiency.128 In patients with optic nerve hypoplasia, the finding of posterior pituitary ectopia implicates the pituitary infundibulum as the primary site of structural dysgenesis responsible for the associated hypopituitarism.128

Posterior pituitary ectopia is often accompanied by optic nerve hypoplasia, but may also accompany isolated pituitary

dwarfism or follow traumatic transection of the pituitary stalk.128,323,468 The concurrence of posterior pituitary ectopia and isolated congenital hypopituitarism is associated with male predominance and breech delivery, indicating probable ischemic injury to the infundibulum.500,560,642,881 In contrast, posterior pituitary ectopia with optic nerve hypoplasia probably reflects hypoplasia of the pituitary infundibulum, because an increased frequency of breech delivery is not found in this setting.119,689 When absence of the infundibulum is unaccompanied by an ectopic pituitary gland, diabetes insipidus is also present.126,836

In patients with optic nerve hypoplasia, it is associated with male predominance but not with breech delivery, suggesting that it may be a sign of infundibular hypoplasia rather than traumatic transection in this setting.128 Posterior pituitary ectopia is seen in about 15% of children with optic nerve hypoplasia.128 We have found it to be a sensitive and specific neuroimaging marker for anterior pituitary hormone deficiency in children with optic nerve hypoplasia.128,610,689 Rarely, posterior pituitary ectopia can herald anterior pituitary deficiency in optic nerve aplasia.122

Empty Sella Syndrome

Empty sella was first described by Busch,150 who, in an autopsy study of the sella turcica, found an incomplete diaphragma sella with an apparently empty sella and a pituitary gland flattened at the bottom in 5.5% of cases.582 The sella may be enlarged or normal in size.582 Whether an incompetent or defective diaphragma sella is an essential prerequisite for this condition is unclear.458 Pituitary surgery, pituitary apoplexy, or irradiation582 and medical treatment with bromocriptine can lead to prolapse of the optic nerves or chiasm into the empty sella, with visual loss.259

Third ventricular enlargement can also produce empty sella with chiasmal prolapse in children with aqueductal stenosis.654 In some patients with visual loss, repositioning of the optic nerves can improve vision.259 More often, it is secondary to elevated intracranial pressure, (usually from idiopathic intracranial hypertension) (Fig. 3.4). Inferior displacement of the chiasm into the sella is not seen in the latter condition. Following normalization of intracranial pressure, the pituitary gland can reexpand to fill the sella in patients with idiopathic intracranial hypertension.17,966 Notwithstanding these associations, idiopathic empty sella syndrome is infrequently seen in children.17

Encephaloceles

Cephaloceles are congenital malformations consisting of herniation of an intracranial structure through a defect in the cranium and dura mater.47 Meningoencephaloceles are cepha-