loceles in which the protruding structure contains leptomeninges, brain, and CSF.625 The terms meningoencephalocele and encephalocele are often used interchangeably. The most common anatomical location of encephaloceles varies according to geographic distribution, with occipital encephaloceles most common in Europe and North America and frontoethmoidal encephaloceles most common in Russia and southeast Asia.249,625 Most encephaloceles occur on a sporadic basis and are not associated with syndromes.249 The embryology is complex and may vary according to location.900

Transsphenoidal Encephalocele

Transsphenoidal encephalocele is a rare midline congenital malformation in which a meningeal pouch, often containing the chiasm and adjacent hypothalamus, protrudes inferiorly through a large round defect in the sphenoid bone (Fig. 11.30). Children with this occult basal meningocele have a wide head, flat nose, mild hypertelorism, midline notch in the upper lip and, sometimes, a midline cleft in the soft palate. The meningocele protrudes into the nasopharynx, where it may obstruct the airway. Associated brain malformations include agenesis of the corpus callosum and posterior dilatation of the lateral ventricles. Most affected children have no overt intellectual or neurological deficits, but panhypopituitarism is common. Surgery for transsphenoidal encephalocele is considered by many authorities to be contraindicated, because herniated brain tissue may include vital structures, such as the hypothalamic–pituitary system, optic nerves and chiasm, and anterior cerebral arteries, and because of the reported high postoperative mortality rate – especially in infants.963

A variety of optic disc dysplasias, particularly the morning glory disc anomaly, occur in association with transsphenoidal encephalocele.144,161 The combination of a V- or tongue-shaped zone of infrapapillary retinochoroidal depigmentation with optic disc dysplasia may be a retinal marker for transsphenoidal encephalocele.132

Orbital Encephalocele

Orbital encephalocele is a rare congenital abnormality caused by a defect of the cranio-orbital bones that usually manifests soon after birth as a soft, cystic fullness in the superomedial canthal area with associated exophthalmos (Fig. 11.30).867 The globe may pulsate synchronously with the heartbeat, and crying or coughing may increase the degree of proptosis. The encephalocele can herniate through a bony orbital defect or, in some cases, through a natural opening such as the optic foramen or orbital fissures. Surgical treatment usually requires a combined orbital and intracranial approach, with use of dural flaps and bone grafts to close the defect. Most

orbital encephaloceles are isolated anomalies that do not preclude normal mental and physical development; however, posterior orbital encephaloceles may be associated with neurofibromatosis.867

Occipital Encephalocele

Occipital encephaloceles account for 80% of encephaloceles in the white population of Europe and North America.59 Occipital encephaloceles may be associated with callosal anomalies, cerebral migration anomalies, Chiari malformations, and Dandy–Walker malformations.47,59,89 In children with multisystem disease, the diagnoses of Meckel–Gruber syndrome or Walker–Warburg syndrome should be considered. Occipital encephalocele is accompained by high myopia and retinal detachments in the Knobloch syndrome. The size of the defect is highly variable, ranging from a few millimeters in diameter to encephaloceles that contain most of the brain (Fig. 11.30).249 Hydrocephalus may affect the entire ventricular system or may be limited to the extracranial portion of the ventricles.625 The occipital lobes and cerebellar hemispheres may be included partially or totally in the herniated sac and show extensive vascular lesions in the form of old and recent infarction.452 The finding of meningomyelocele in 7% of children with occipital encephalocele suggests that occipital encephaloceles are related to defects in neural tube closure.625 Histopathological examination of brains with large, herniated occipital encephalocele reveals atrophic anterior visual pathways. In some cases, the optic nerves are stretched and the chiasm is postfixed, suggesting posterior traction on the anterior visual pathways as one mechanism of injury.452

The diagnosis is usually obvious clinically, and MR imaging is obtained to determine whether other severe brain abnormalities are present and whether the dural venous sinuses course within the encephalocele.59 The clinical outcome depends on the size, presence, or absence of brain herniation and the presence or absence of associated brain malformations.625 Children with small occipital meningomyeloceles do well, while those with larger lesions associated with brain herniation are totally dependent.586 Surgical correction is performed to protect the child from ulceration of the sac, which could lead to infection or hemorrhage, to prevent the sac from expanding, and to improve the cosmetic appearance.625

Cerebellar Malformations

As the cerebellum is increasingly recognized for its role in cognition as well as motor control, formal classification of the cerebellar malformations has become much more

Fig. 11.30  Encephaloceles. (a) Transsphenoidal encephalocele. Arrow denotes lower margin of meningeal pouch just above hard palate. Optic chiasm is split and herniated downward into defect. (b) Occipital encephalocele in a patient with modified Dandy–Walker malformation.

(c) Frontoethmoidal encephalocele pushing the right globe laterally (d) CT scan showing bony defect in the superior orbit with herniation of brain into the medial orbit

important.47 It has recently been recognized that cortical dysplasia and subcortical heterotopia can be identified in the cerebellum, as they can in the cerebrum.680 In general terms, cerebellar malformations can be classified into cerebellar hypoplasia (focal or general), and cerebellar dysplasia, which can also be focal (as in isolated vermian hypoplasia, molar tooth malformation, rhombencephalosynapsis, Lhermitte–Duclos syndrome, or focal cerebellar cortical dysplasia or heterotopia), or generalized (as in autosomal recessive lissencephaly with the RELN mutation, lissen­

cephaly with agenesis of the corpus callosum and cerebellar dysplasia, or diffuse cerebral polymicrogyria).402,680,755

While cerebellar hypoplasia is usually distinguished from atrophy on the basis of progression, the neuroimaging finding of a small, well-formed cerebellum is suggestive of hypoplasia, while enlarged fissure with accentuated arborization of the individual lobes within the vermis suggests atrophy.219 Congenital ocular motor apraxia and Joubert syndrome are associated with hypoplasia of the cerebellar vermis, while ataxia telangectasia and the spinocerebellar ataxias are

associated with atrophy. Hypoplasia of the cerebellar vermis tends to preferentially involve the inferior vermis, as is often seen in congenital ocular motor apraxia.471 The major pediatric neuro-ophthalmologic disorders associated with cerebellar vermian hypoplasia are congenital ocular motor apraxia and Joubert syndrome. All involve the ocular motor system predominantly but Joubert syndrome and spinocerebellar ataxia (SCA7) may have sensory visual loss as a salient feature, as can the rare syndrome of autosomal recessive cerebellar hypoplasia with tapetoretinal degeneration.258

Molar Tooth Malformation

Patients with Joubert syndrome have a small, dysplastic cerebellar vermis with midline clefting, dysplasias and heterotopias of the cerebellar nuclei, near total absence of the pyramidal decussations.680 They also have anomalies in the structure of the inferior olivary nuclei, descending trigeminal tract, solitary fascicle and dorsal column nuclei,47,315 with probable absence of decussation of the superior cerebellar peduncles and central pontine tracts as well.955 That midline structures of the brainstem are disordered both structurally and functionally in Joubert syndrome suggests that the underlying developmental abnormality may be an inability of posterior fossa axons to cross the midline.769 Although once thought to be pathognomonic for Joubert syndrome, recent studies have shown that the molar tooth sign is seen with many associated anomalies that encompass many syndromes.347,782

The fact that some patients with Joubert syndrome have mutations on chromosome 9q34.3,766 while others do not93 further demonstrates that this finding is not diagnostic for a specific condition.47 Some patients with identical neuroimaging findings have renal anomalies (juvenile nephronophthiasis or multicystic dysplastic kidney), some have ocular anomalies (retinal dysplasias and colobomas), some have hepatic fibrosis and cysts, and some have hypothalamic hamartomas.47 A number of genetic syndromes, including Dekaban–Arima syndrome, COACH syndrome, Senior– Löken syndrome,782 Varadi–Papp syndrome,347 Joubertpolymicrogyria syndrome,347 and others,201 have been reported with identical neuroimaging findings.47 Therefore, patients with molar tooth syndrome should not be monosynaptically diagnosed with Joubert syndrome. Rather, they should be screened for supratentorial anomalies (hypothalmic hamartoma, polymicrogyria, or other malformations of cortical development) and for ocular, hepatic, and renal disease.347

On axial MR imaging, the superior cerebellar peduncles appear large, clearly horizontal, and do not cross in the dorsal midbrain (Fig. 7.7).464 The midbrain is small in its ante- rior–posterior diameter, particularly in its midline, probably

because of the absence of decussation of the superior cerebellar peduncles. Sagittal images show a small vermis that is situated abnormally high and a fourth ventricle that is abnormally thin. The vermian folial pattern may also be abnormal. Coronal images show a cleft in the vermian midline, yielding a triangular, “bat wing” shaped fourth ventricle.

Rhombencephalosynapsis

Rhombencephalosynapsis is characterized by the absence or hypoplasia of the cerebellar vermis and fusion of the cerebellar hemispheres, dentate nuclei, and cerebellar peduncles (Fig. 11.31). Its cause is unknown. Clinical presentation ranges from mild truncal ataxia and normal cognition to severe cerebral palsy with epilepsy and mental retardation.873 Neuro-ophthalmologic abnormalities are primarily confined to the ocular motor system, consisting of prenuclear disturbances such as A-pattern strabismus with primary superior oblique overaction, and skew deviation.687 Imaging studies show dorsal fusion of the cerebellar hemispheres, absence of the vermis, and fusion of the superior cerebellar peduncles – usually in association with ventriculomegaly. Cortical malformations, hydrocephalus, abscesses of the septum pellucidem, and cranial suture synostosis can coexist.

Fig. 11.31  Rhombencephalosynapsis. T2-weighted MR image shows fusion of the cerebellar hemispheres with no intervening vermis. Note the horizontal orientation of the folia across the cerebellar midline. With permission from Phillips PH et al.687