Imaging: High-resolution MRI is performed soon after birth to define contents of the encephalocele, the severity of the malformation, and the frequent associated anomalies. These include callosal agenesis, anomalies of cortical formation, and variable anomalies of the cerebellum, diencephalon, and brainstem. Although dysplastic nonfunctional neural tissue is usually resected during closure of the encephalocele, major dural venous sinuses are preserved. Therefore, magnetic resonance venography is essential for diagnosing large occipital and midline parietal encephaloceles that may contain dural venous sinuses. Basal encephaloceles containing optic chiasm or nerves or pituitary tissue cannot be closed without sacrificing these structures. Affected patients are at risk for meningitis and cerebrospinal fluid (CSF) leakage into the nasophayrnx. The typical intracranial manifestations of atretic parietal encephaloceles include posterior tenting of the tectal plate, a persistent falcine sinus with or without atresia of the straight sinus, and expansion of posterior interhemispheric CSF. Hydrocephalus is common after closure of large parietal and occipital encephaloceles.

Clinical Presentation: Chiari II malformations are the intracranial manifestations of posterior dysraphic defects such as myelomeningoceles. Neural tube defects have been reported with multiple chromosomal abnormalities, including trisomies 18, 13, and 9; triploidy; unbalanced translocations and deletions; and in Turner, DiGeorge, and velocardiofacial syndromes. The genes in the region of 22q11 have been implicated in the development of neural tube defects, although neural tube defects and the associated Chiari II malformation are probably caused by a combination of genetic polymorphisms and environmental factors, including dietary folate intake and maternal folate metabolism.⁵

The most commonly accepted unifying theory for the Chiari II malformation suggests that the failure of neural tube closure prevents the transient closure of the central canal, which is essential for the distension of the primitive ventricular system;⁶ this results in the premature fusion of the mesenchymal components that form the calvarium, whereas the hindbrain manifestations result from leakage of CSF through the neural tube defect. The Chiari II malformation is associated with a wide range of brain malformations;^1,2,7,8 DTI shows disordered axonal migration, suggesting that the malformation is not explained by mechanical alterations caused by faulty CSF dynamics alone.⁸ The Chiari II malformation is characterized by mesodermal dysplasia, small and dysplastic lower cranial nerve ganglia, deficient tentorium cerebelli, hypoplastic and dysmorphic cerebellum, and thickened basal meninges.⁶ As in holoprosencephaly (HPE), these anomalies are attributable to a defective or deficient mesenchyme, which presumably deprives the skull base, hindbrain, and rhombencephalon of normal inductive effects.⁷ Clinical problems related specifically to the malformed hindbrain include apnea, aspiration, feeding difficulties, and recurrent respiratory infections.

Imaging: The calvarial manifestations of the Chiari II malformation include a bifid frontal bone and luckenschadel, or lacunar, skull. The latter is a manifestation of the mesodermal dysplasia of the membranous skull and is caused by nonossified fibrous bone in the inner and outer tables of the skull, resulting in the apparent cranial scalloping (Fig. 31-9); the affected cranium ossifies and appears normal by 6 months of age. Luckenschadel is not the result of increased intracranial pressure and is not synonymous with “the beaten copper skull.”

The intracranial stigmata of the Chiari II malformation are complex and variable; the hallmarks of the malformation are infratentorial. In the most severe forms, the foramen magnum is enlarged and the posterior cranial fossa is constricted, with effacement of CSF spaces; the cerebellum wraps around the ventral aspect of the brainstem (see Fig. 31-9), and the clivus and petrous ridges are concave. MRI shows caudal displacement of a dysplastic brainstem and a hypoplastic cerebellum into the upper cervical canal with “kinking” of the cervicomedullary brainstem (Fig. 31-10). The fourth ventricle is effaced and caudally displaced. The torcula and transverse sinuses are low lying, which presents a potential surgical hazard during decompressive suboccipital craniectomy, which may be performed for relief of symptomatic hindbrain compression. A “beaked” tectal plate is present, and a variably thickened massa intermedia may be so thick that the thalami appear virtually fused with partial atresia of the third ventricle.^1,7 Common supratentorial abnormities include callosal dysgenesis, neuronal migration anomalies, and hydrocephalus. After ventricular shunt placement, the cerebral hemispheres drop away from the inner table of the skull, allowing interdigitations of the cortex across the midline under the hypoplastic falx and projection of the superior cerebellar vermis superiorly across a widened tentorial incisura. These findings are nonspecific manifestations seen after CSF diversion of severe congenital obstructive hydrocephalus. DTI of Chiari II malformations associated with more severe degrees of cerebellar hypoplasia shows absence of the dorsal transverse fibers at the level of the pons with preservation of the corticospinal tracts and mediolateral lemniscus fibers (Fig. 31-11). The cingulum may be anomalous, crossing the midline above the corpus callosum (Fig. 31-12). These alterations in axonal migration evident by DTI are difficult to appreciate with conventional MRI.

Chari III malformations are high cervical or low occipital encephaloceles.⁹ The term Chiari IV malformation describes severe cerebellar hypoplasia in association with a neural tube defect. A better description of affected patients would be Chiari II malformation with cerebellar hypoplasia or aplasia (e-Fig. 31-13). Defects of neurulation have also been reported to coexist in patients with HPE; the latter malformation is considered a disorder of differentiation of the dorsal neural plate.^10,11 The coexistence of these malformations, traditionally considered disparate in embryologic timing and insult, may be explained, in part, by mutations in genes implicated in both developmental pathways.^12,13

Clinical Presentation: HPE is the most common anomaly affecting the ventral forebrain, resulting from a primary defect in patterning and induction of the basal forebrain expressed around the fifth to sixth week of gestation, and occurs in 1 in 250 embryos and 1 in 8300 to 16,000 live births.1,13,14 HPE is thought to be caused by a deficient or defective prechordal mesoderm, with failure of induction or abnormal fusion of normally paired and separate neo-cortex, caudates, and claustrum. The consequences of the inductive failure of the median and paramedian structures are most pronounced in the ventral forebrain and decline in severity from rostral to caudal and from medial to lateral. HPE is caused by a combination of genetic polymorphisms and environmental factors; the incidence of HPE is increased 200-fold in maternal diabetes mellitus. HPE is notable for its genetic heterogeneity. Mutations in single genes result in syndromic forms of HPE, triploidies of entire chromosomes (e.g., 13 and 18), deletions or duplications of regions of a chromosome, and copy number variants. At least 12 HPE loci that play some role in the midline of the developing nervous system have been identified, including Shh, Otx2, Emx, Pax, Nkx-2.2, and some POU domain genes.¹³ Although 24% to 45% of affected live-born individuals have chromosomal abnormalities, no known correlation exists between the type or severity of the holoprosencephalic defect and the specific mutation. In addition to controlling prosencephalic separation and differentiation, the inductive effects of the prechordal mesoderm also influence the corpus striatum, thalami, eyes, face, and cerebral vasculature.¹³ The spectrum of associated facial anomalies range from flattening of the nasal bridge, hypotelorism without metopic synostosis, a single central maxillary incisor, cleft lip or palate to facial clefting, and cyclopia with a central proboscis.¹⁵ More severe facial anomalies are seen with more severe variants of HPE, but mild facial anomalies may occur in the absence of brain anomalies. Clinical problems associated with HPE include developmental delay, seizures, hypothalamic and brainstem dysfunction with swallowing and respiratory problems, thermal instability, pituitary dysfunction, and erratic sleeping patterns.^13,15

Imaging: The hallmark of HPE is incomplete separation of the forebrain, absence of the anterior interhemispheric falx, and fusion of central gray nuclei. The septum pellucidum is absent. Considerable topographic variation exists in HPE, which is most often characterized as alobar, semilobar, or lobar.13–19 The malformation represents a continuum, ranging from overt hypotelorism, with severe facial anomalies, to mild hypotelorism, with subtle fusion of basal forebrain and central gray nuclei. Hydrocephalus may be present.

Clinical Presentation: Septo-optic dysplasia (SOD) is considered along the continuum of disorders of ventral forebrain differentiation.²⁰ Patients typically present in early childhood with nystagmus, optic nerve atrophy, short stature because of growth hormone deficiency, or panhypopituitarism without diabetes insipidus. Affected patients are at risk for Addisonion crisis from subclinical adrenal insufficiency, which may become clinically apparent only during illness or severe stress. Most cases of isolated SOD are sporadic.

Imaging: SOD is characterized by absence of the septum pellucidum and variable optic nerve hypoplasia (e-Fig. 31-19). The neurohypophysis is often ectopic or may be absent, in which case the pituitary stalk may be interrupted. However, agenesis of the septum pellucidum may be isolated with normal optic pathways and intact neuroendocrine function. Absence of the septal leaves results in downward displacement of the fornices into the third ventricle. SOD is associated with multiple malformations, such as schizencephaly, neuronal migration anomalies, and neural tube defects.

Imaging: The corpus callosum is the largest of the three telencephalic commissures, which include the anterior and hippocampal commissures. The AC is a variably-sized tract embedded in the cranial aspect of the lamina terminalis, which demarcates the anterior wall of the third ventricle and connects the anterior temporal lobes. The hippocampal commissure is a thin sheet of white matter connecting the two crura of the fornices and is not routinely visualized on MRI of the normal brain. Absence of the corpus callosum, AC, and hippocampal commissure is described as “complete commissural agenesis,” although some portion of the corpus callosum may be preserved (Fig. 31-20).^1,2,22,23 Preservation of the AC in the absence of the corpus callosum and hippocampal commissure is “callosohippocampal agenesis” (e-Fig. 31-21), whereas the anterior and hippocampal commissures may be preserved, which results in “isolated callosal agenesis” (e-Fig. 31-22). Callosohippocampal agenesis is the most common malformation, although it might be argued these classifications are somewhat academic. The AC may be enlarged. On MRI, the sulci over the mesial surface of the frontal lobes have a radial configuration, and the lateral ventricles have a characteristic parallel orientation. Absence of the temporal segments of the cingulum results in dilatation of the temporal horns. Hypoplasia of association fibers connecting the occipital and temporal lobe allows dilatation of trigones of the lateral ventricles referred to as colpocephaly. When the axons fail to cross the midline and remain in their hemisphere of origin, they course along the medial aspect of the lateral ventricle, forming the longitudinal callosal bundles of Probst, which are thought to function as association fibers connecting regions of cortex within the same hemisphere (e-Fig. 31-23). Callosal agenesis may be associated with an interhemispheric diencephalic pseudocyst (e.g., high-riding third ventricle), or interhemispheric cysts that do not communicate with the ventricles (see e-Fig. 31-23). By DTI, the rudimentary cingulum and fornices are fused to the bundles of Probst (Fig. 31-24). In patients with preservation of a portion of the corpus callosum, diffusion tractography suggests the existence of aberrant connections between regions of cortex and subcortical structures in some patients, and heterotopic hemispheric connections that exist in such a manner that a frontal lobe may be connected with the contralateral parietal or occipital lobe (e-Fig. 31-25). These aberrant callosal fibers have been termed “asymmetric sigmoid bundles” and have also been reported in patients with a fully formed but abnormally shaped corpus callosum (e-Fig. 31-26).^24,25

Callosal agenesis may be an isolated anomaly or associated with a myriad of other malformations, including aqueductal stenosis, Chiari II malformations, and malformations of cortical, brainstem, and cerebellar development.21–23 Numerous syndromes are associated with complete or partial agenesis of the corpus callosum. Aicardi syndrome is a rare sporadic X-linked dominant malformation seen in females, with an incidence of less than 1 in 200,000 births. Affected males have Klinefelter syndrome (XXY). Clinical findings include profound developmental delay, infantile spasms, micro-ophthalmia, coloboma, short philtrum, flat nose, and large ears. Imaging shows callosohippocampal agenesis, interhemispheric cysts, gray matter heterotopias, polymicrogyria (PMG), and cerebellar malformation. Ocular abnormalities include chorioretinic lacunae and coloboma. CRASH syndrome (corpus callosum agenesis, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus) is caused by mutations in the L1 cell adhesion molecule gene that codes for a transmembrane cell adhesion protein involved in axonal migration.

Clinical Presentation: Patients with microlissencephaly have severe microcephaly at birth, with dysmorphic craniofacial features, abnormal genitalia, and arthrogryposis.²⁶ These patients suffer from seizures and have developmental delay that is pervasive and profound. The mode of inheritance is thought to be autosomal recessive, related in some patients to mutations of the RELN gene. Congenital infections are known to cause microlissencephaly, especially CMV.

Imaging: The diagnosis of is made on the basis of a diffusely thinned cerebral mantle, with abnormal sulcation consisting of agyria-pachygyria (Fig. 31-30), variable callosal agenesis, and variable hypoplasia of the brainstem and cerebellum. Type A (Norman-Roberts syndrome) is microlissencephaly without infratentorial anomalies, and type B (Barth syndrome) is microlissencephaly with severe cerebellar and brainstem hypoplasia. Milder forms of microlissencephaly are referred to as microcephaly with a simplified gyral pattern; the cortex is usually of normal thickness.

Clinical Presentation: Hemimegalencephaly is hamartomatous overgrowth of a cerebral hemisphere or lobe of the brain.1,2,28 It occurs as an isolated anomaly or with neurocutaneous syndromes such as epidermal nevus syndrome, Proteus syndrome, unilateral hypomelanosis of Ito, neurofibromatosis type I, Klippel-Trenaunay syndrome, and tuberous sclerosis. The malformation is thought to arise from faulty neuronal proliferation prior to radial neuroblast migration. Hemispherectomy is required for relief of the severe intractable epilepsy.

Imaging: MRI shows diffusely enlarged hemisphere or lobe (Fig. 31-31), with abnormal sulcation pattern that may include PMG, lissencephaly, and gray matter heterotopias. The regional white matter is thickened and gliotic. A hallmark is enlargement of the ipsilateral ventricle that becomes more pronounced with age with expansion of the overlaying hemicranium.

Imaging: Identification of FCD requires high-resolution MRI, using T1-weighted, T2-weighted, and FLAIR (fluid-attenuated inversion recovery) sequences (Fig. 31-32); the diagnostic accuracy of MRI is increased with scrutiny of thin sections of the region of brain suspected of harboring the epileptogenic focus. Findings of type I FCD include focal cortical thickening, often within the depth of a sulcus, and blurring of the gray–white junction. FCD may be clinically occult and found on MRI performed for reasons other than seizures or may be associated with seizures and variable cognitive impairment. The most common histopathologic finding in surgical specimens removed from patients with cryptogenic epilepsy (e.g., epilepsy in which no lesion is identifiable by imaging) is type I FCD.^29,30

Type II FCD is more readily apparent with MRI, especially in the presence of balloon cells. MRI shows increased T2 signal within the subcortical white matter underlying abnormal gyri and is often best seen on T2-weighted and FLAIR images. The radial extension of balloon cells and ectopic neurons into the deep white matter constitutes the ”transmantle sign” of FCD and may be the sole visible evidence of FCD.²⁹ The differential diagnosis of type II FCD is low-grade tumor. Although type II FCD may be suggested on the basis of MRI findings, overlap exists in imaging findings between type I and II, and the ultimate diagnosis is based on histopathology.³⁰

Imaging: MRI shows variable-sized nonenhancing nodules along the ependymal surface of the lateral ventricles (see Fig. 31-33) that have the same signal intensity as the cortex on all sequences.³³ The nodules may be single, multiple, isolated, or associated with other malformations.

Clinical Presentation: The hallmarks of LIS1 are profound mental retardation, with a normal-small head size, intractable seizures, hypertonia, and hyperreflexia. Patients with isolated lissencephaly have normal facial structures, whereas patients with MDS have LIS1 with abnormal facies: wrinkled skin over the glabella and frontal suture, prominent occiput, narrow forehead, downward slanting palpebral fissures, and small nose and chin. Males with DCX mutation have classic LIS1 as seen on MRI, whereas females with DCX mutation have variable cognitive delay without seizures and subcortical band heterotopias (SBH) as delineated by MRI. The presence of an unexplained seizure disorder or cognitive problems in the mother of a male child with lissencephaly should trigger a screening MRI of the mother for possible SBH. Females with ARX mutation have ambiguous genitalia without lissencepahy; males have lissencephaly with ambiguous genitalia (XLAG) syndrome. Lissencephaly caused by RELN mutation is associated with congenital lymphedema.33–37

Imaging: The phenotypic expression of lissencephaly caused by LIS1 is similar in males and females and consists of a markedly thickened cerebral cortex, which may be diffuse (Fig. 31-34) or most pronounced posteriorly (Fig. 31-35); and Sylvian fissures fail to occur.^1,2,36–38 The thickened cortex shows a highly organized pattern, as shown by DTI, because of the persistence of the radial pattern of the neurons in arrested migration.³⁹ The lissencephalic pattern in males with DCX mutation is similar to L1S1 and more severe anteriorly. Females with DCX mutation have SBH or “double cortex.”^37,38 MRI shows bands of heterotopic neurons within the cerebral white matter between a normal-appearing cortex and the ventricles (Fig. 31-36).⁴⁰ MDS and the ARX mutation are associated with more severe lissencephaly posteriorly. With LIS1 mutations of MDS and DCX, the pons may be normal or mildly hypoplastic; severe cerebellar and pontine hypoplasia suggest a RELN mutation. MRI abnormalities described in the rare mutations of the TUBA1A gene include classic lissencephaly, with brainstem dysplasia and cerebellar hypoplasia.

Clinical Presentation: The clinical presentation of the cobblestone lissencephalies is congenital muscular dystrophy (CMD), which comprises a heterogeneous group of inherited disorders presenting with diffuse symmetric hypotonia at birth or in infancy.1,41 CMD is categorized as (1) CMD without CNS abnormalities, and (2) CMD with CNS abnormalities; the latter includes Fukuyama (FCMD), muscle-eye-brain disease (MEBD), and Walker-Warburg syndrome (WWS).⁴¹ Depending on the specific syndrome, affected patients also have psychomotor retardation, seizures, micro-ophthalmia, optic nerve hypoplasia, and colobomas. Scoliosis and contractures develop during early childhood. Death often results from respiratory failure, caused by chest wall rigidity and weakness of the diaphragm or aspiration, or from cardiomyopathy. The specific diagnosis is made on the basis of clinical findings, serum creatine kinase, neuroimaging, muscle and skin biopsy, and molecular genetic testing.⁴¹

FCMD is an autosomal-recessive disorder most common in Japan, where the incidence approaches 3 in 100,000 individuals.⁴¹ The syndrome results from mutations in the FKTN gene on chromosome 9, which codes for the protein fukutin, which interacts with α-dystroglycan in the extracellular matrix. MEB disease is an autosomal-recessive disorder, which is rare outside of Finland and is caused by mutations in the POMT1, POMT2, POMGnT1, fukutin, and FKRP genes; phenotypes may overlap with FCMD. WWS is the most severe form of CMD with CNS manifestations and is also associated with mutations in the POMT1, POMT2, fukutin, and fukutin-related protein genes.⁴¹

Imaging: MRI abnormalities are less severe in FCMD than in WWS and MEBD.⁴¹ In mild cases of FCMD, the cerebrum may be relatively normal or have a simplified gryal pattern. More severe malformations have variable pachygyria and PMG; the white matter is gliotic with cysts. Neurons may be seen lining the ependymal surface of the ventricle, contrasted on T2-weighted images by hyperintense signal of the CSF and the abnormal white matter (e-Fig. 31-37). The lateral ventricles are enlarged. The cerebellum consistently shows small cysts with variable PMG. The most severe cerebellar hypoplasia is seen with WWS, which is also associated with occipital encephalocele (Fig. 31-38). The brainstem is usually normal in FCMD, hypoplastic in MEBD, and small and kinked posteriorly in WWS. Hydrocephalus is rare in FCMD, common in MEBD, and almost invariable in WWS; the hydrocephalus may mask the malformation.⁴¹

Clinical Presentation: PMG results from disruption of normal terminal neuronal migration and organization and is defined by excessive convolutions of the cerebral cortex.42–45 PMG may be unilateral and focal or bilateral and diffuse; bilateral patterns are most often frontoparietal and peri-Sylvian. The severity of seizures and developmental impairment correlates with the location and severity of the malformation. Focal PMG may be clinically occult or associated with seizures that are often medically uncontrollable. Bilateral peri-Sylvian PMG is associated with epilepsy, delayed development, strabismus, dysphagia and speech problems, and paresis. PMG may be isolated or associated with known disorders, including callosal agenesis, Aicardi syndrome, Joubert syndrome, FCMD, and Zellweger spectrum. Bilateral peri-Sylvian PMG is X-linked, whereas frontoparietal PMG maps to chromosome 16. At least 11 mutations in the G protein–coupled receptor 56 (GPR56) gene have been identified in bilateral PMG. PMG may also occur from CMV infection and intrauterine ischemic injury.^42,43

Imaging: MRI shows increased cortical thickness and irregularity at the cortex–white matter junction (Fig. 31-39). The pattern of the PMG appears to differ with location.⁴⁴ The appearance of the PMG may change with brain maturation, and microgyri obvious in unmyelinated brain may be less conspicuous as myelination progresses.

Schizencephaly results from congenital clefts in the cerebral hemisphere, which extend from the pial surface to the ventricles.⁴⁶ Affected patients may present with seizures, motor deficits, or both or may be asymptomatic. Larger or bilateral defects are associated with poorer neurodevelopmental outcome.^1,2

Imaging: The lips of the defects may be in apposition (closed lip) or separated (open lip). The clefts are lined with dysplastic gray matter, which extends the full length of the cleft (Fig. 31-40). Discontinuity of the ventricular ependyma and the subpial membrane in open-lip schizencephaly results in communication between the lateral ventricles and the subarachnoid space. The pulsations of the CSF may cause remodeling and outward expansion of the calvarium overlaying the open cleft. The septum pellucidum is absent in most cases of schizencephaly involving the frontoparietal regions. Unilateral schizencephaly is often associated with contralateral peri-Sylvian cortical dysplasia.⁴⁶

Clinical Presentation: Joubert syndrome is a rare malformation that is usually autosomal recessive; it involves one of multiple genes involved in ciliary function integral to chemical signaling during neuronal and axonal migration. The clinical syndrome is characterized by hypotonia, episodic hypoventilation, and hyperventilation, which tend to improve with age; truncal ataxia, which develops during early childhood; and oculomotor apraxia, nystagmus, pigmentary retinopathy, endocrinopathies, and abnormal facial features, including ptosis, hypertelorism, and low-set ears. Joubert syndrome and related disorders (JSRD) describes the constellation of clinical problems associated with Joubert syndrome, which are diverse. The marked clinical variability probably results in underdiagnosis.

Imaging: The diagnosis of Joubert syndrome is made on the basis of characteristic MRI findings, which include hypoplasia of the superior vermis, with enlargement of the superior aspect of the fourth ventricle and the superior cerebellar peduncles.48–50 The molar tooth sign (Fig. 31-41), seen on an axial image through the pontomesencephalic junction, is also seen in a number of cerebello-oculo-renal disorders. DTI of Joubert syndrome shows variable absence of transverse pontine fibers; the decussating fibers of the superior cerebellar peduncles are absent.⁵⁰ In addition to the clinical variability of JSRD, considerable variability also exists in the appearance of the brainstem.

Clinical Presentation: Horizontal gaze palsy with progressive scoliosis (HGPPS) is the expression of mutation of the ROBO3 gene that codes for proteins involved in axonal guidance and neuronal migration.51,52 Expression of the defective gene appears to be limited to decussating fibers in the brainstem. Patients with HGPPS tend to be cognitively normal but have congenital nystagmus and are unable to perform lateral eye movements because of lack of decussating fibers in the brainstem. Progressive scoliosis develops during early childhood.

Imaging: MRI shows hypoplasia of the pons, which is partially divided by a midsagittal cleft (e-Fig. 31-42). Facial colliculi are absent, the inferior olivary nuclei are prominent, and the medulla lacks the normal dorsal convexities. DTI of HGPPS shows hypoplasia of the ventral transverse pontine fibers and absence of the dorsal transverse pontine fibers, small middle and superior cerebellar peduncles, and decussating fibers of the superior cerebellar peduncle within the midbrain.⁵¹ The appearance of the brainstem by diffusion imaging in HGPPS is similar to that of other less well-characterized brainstem malformations.

Imaging: MRI shows pontine hypoplasia; the ventral surface is flattened, and ectopic tissue protrudes dorsally into the fourth ventricle, which is referred to as the tegmental “cap.” The superior cerebellar vermis is hypoplastic, which, along with the elongated and laterally misplaced superior cerebellar peduncle, results in a molar tooth–like deformity. As shown by DTI, the tegemental cap is composed of transversely oriented ectopic pontine fibers. DTI also shows the absence of the normal decussations of the superior and middle cerebellar peduncles.⁵⁴ The pattern of white matter tract abnormalities, as shown by DTI, is similar to those reported in some cases of JSRD.

Imaging: The Chiari I malformation is defined by cerebellar tonsils greater than 6 mm below the posterior lip of the foramen magnum. In symptomatic patients, the low-lying tonsils disrupt CSF flow across the foramen magnum, which predisposes to syringomyelia in the thoracic or cervical spinal cord (Fig. 31-43). Flow of CSF across the foramen magnum is further impaired when the odontoid is elongated and dorsally angulated or with any significant basilar invagination. The dynamic significance of the low-lying tonsils is best depicted using single-slice, multiple-phase, peripheral, or cardiac-gated two-dimensional phase contrast in the sagittal and axial planes with a velocity encoding of 5 centimeters per second (cm/s). Obstruction of CSF flow results in abnormal bidirectional movement of the hindbrain across the foramen magnum, which is seen as changes in signal intensity on the phase contrast sequence. In patients with Chari I malformations, increased intracranial pressure, caused by a tumor or hydrocephalus resulting in secondary tonsillar herniation, should be excluded with screening images through the brain (Table 31-1).

Imaging: The cerebellar hemispheres are fused (Fig. 31-50). Associated brainstem abnormalities include fusion of the colliculi, atresia or dilatation of the fourth ventricle, and abnormalities of lower cranial nerves and the medulla. The corpus callosum may be hypoplastic or absent, and failure of formation of diencephalic structures with thalamic fusion, hydrocephalus, and malformations of cortical development may exist.