Neuroimaging of Dandy-Walker Malformation

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differs from the histologic structure of the arachnoid cyst (AC), which consists of thin delicate arachnoid. The membrane from the Blake's pouch cyst usually ...
ORIGINAL ARTICLE

Neuroimaging of Dandy-Walker Malformation New Concepts Gustavo Gumz Correa, MD,* La´zaro Faria Amaral, MD,Þ and Leonardo Modesti Vedolin, MD, PhD*

Abstract: Dandy-Walker malformation (DWM) is the most common human cerebellar malformation, characterized by hypoplasia of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa with upward displacement of the lateral sinuses, tentorium, and torcular. Although its pathogenesis is not completely understood, there are several genetic loci related to DWM as well as syndromic malformations and congenital infections. Dandy-Walker malformation is associated with other central nervous system abnormalities, including dysgenesis of corpus callosum, ectopic brain tissue, holoprosencephaly, and neural tube defects. Hydrocephalus plays an important role in the development of symptoms and neurological outcome in patients with DWM, and the aim of surgical treatment is usually the control of hydrocephalus and the posterior fossa cyst. Imaging modalities, especially magnetic resonance imaging, are crucial for the diagnosis of DWM and distinguishing this disorder from other cystic posterior fossa lesions. Persistent Blake’s cyst is seen as a retrocerebellar fluid collection with cerebrospinal fluid signal intensity and a median line communication with the fourth ventricle, commonly associated with hydrocephalus. Mega cisterna magna presents as an extraaxial fluid collection posteroinferior to an intact cerebellum. Retrocerebellar arachnoid cysts frequently compress the cerebellar hemispheres and the fourth ventricle. Patients with DWM show an enlarged posterior fossa filled with a cystic structure that communicates freely with the fourth ventricle and hypoplastic vermis. Comprehension of hindbrain embryology is of utmost importance for understanding the cerebellar malformations, including DWM, and other related entities. Key Words: neuroradiology, hindbrain, posterior fossa malformation, cerebellar malformation, magnetic resonance imaging (Top Magn Reson Imaging 2011;22: 303Y312)

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andy-Walker malformation (DWM), the most common human cerebellar malformation, is defined by hypoplasia and upward rotation of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa with upward displacement of the lateral sinuses, tentorium, and torcular.1,2 It was first described by Dandy and Blackfan in 19143 and further reviewed by Taggard and Walker in 1942.4 The eponymous name used nowadays was proposed by Benda5 in 1954. In addition to the classic triad of findings that define it, DWM is associated with many other abnormalities and malformations in the central nervous system (CNS), including agenesis of corpus callosum, heterotopias, occipital meningocele, visual deficits, and epilepsy.6,7 Although common, DWM is poorly understood. However, recent advances in developmental genetics, molecular biology,

From the *Hospital Moinhos de Vento, Porto Alegre; and †Medimagem, Hospital Beneficieˆncia Portuguesa, Sa˜o Paulo, Brazil. Reprints: Gustavo Gumz Correia, MD, Neuroradiology Section, Hospital Moinhos de Vento, Ramiro Barcelos St, 910, Moinhos de Vento, Porto Alegre, RS, Brazil (e-mail: [email protected]). The authors declare no conflict of interest. Copyright * 2013 by Lippincott Williams & Wilkins

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and neuroimaging have led to better understanding of this disorder as well as other abnormalities of the embryonic midbrain and hindbrain,8 which form the adult brainstem and cerebellum. The understanding of congenital malformation of the cerebellum and posterior fossa is critical because of newly discovered genetic mechanisms that regulate hindbrain development, the use of advanced imaging techniques, and the mounting evidence for the role of the cerebellum in higher cortical functions.9Y12 The purposes of this article were to review the embryology of the cerebellum and to discuss the role of the genetic abnormalities linked to DWM. Finally, we reviewed the imaging findings associated with this disorder and other cystic malformations of the posterior fossa included in the differential diagnosis.

EMBRYOLOGY OF THE HD The stages of brain development include gastrulation, dorsal induction, ventral induction, neural proliferation, differentiation and histogenesis, neuronal migration, and axonal myelination.13,14 Those processes are regulated by a complex hierarchy of genes and cellular interactions that regulate the prenatal and postnatal growth of the brain, cerebellum, and brainstem. The development of the CNS begins with the formation of the notochord and somites at the trilaminar embryo, established after the gastrulation process. Both structures underlie the ectoderm and do not contribute directly to the nervous system but are involved with patterning its initial formation.14Y17 The central portion of the ectoderm forms the neural plate that folds to form the neural tube, which will eventually develop the entire CNS. At about the third postconceptional week, the anterior neural tube undergoes 2 constrictions to form the 3 primary brain vesicles, called prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). In the fourth postconceptional week, the rhombencephalon undergoes further division into 8 structures anatomically distinct called rhombomeres (r1Yr8), which are established in the brainstem and become crucial to pattern and organize the hindbrain,18 as they define domains of gene expression, cell lineage, and neuronal differentiation. The first rhombomere gives rise to the cerebellar cortex, and the second rhombomere will develop the deep cerebellar nuclei.19 The lower rhombomeres will eventually become the lower part of the brainstem. The rhombencephalon splays apart dorsally after the closure of the caudal neuropore, and the subsequent stretching of the roof plate leads to the formation of the fourth ventricle. Intense neuroblastic activity of 2 embryonic primordial structures called rhombic lips is initiated in the edges of the fourth ventricle, the alar plate of the rhombencephalon. This neuroblastic activity is the key event that ultimately forms the cerebellar neurons. The rhombic lips consequently fuse in the midline to form the cerebellar commissure in the membranous roof of the fourth ventricle, resulting in the flexion of the rhombencephalon and formation of the pontine flexure.20 The pontine flexure then divides the rhombencephalon in metencephalon rostrally (the future pons and cerebellum) and myelencephalon caudally

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(the future medulla oblongata).14,15 Some genes play a crucial role for the emergence of the cerebellum. The constriction between the mesencephalon and metencephalon marks the position of the developing organizer isthmus, an important organizing center for posterior fossa structures regulated by the expression of Otx/Gbx 2.21 The Wnt1 and Fg f 8, also expressed in the isthmus, promote the expression of other transcription factors.22 The formation of cerebellar cortex is composed of 2 classes of progenitor neurons. The Purkinje cells constitute the major class of efferent neurons in the cerebellum, and their genesis comes from the neuroblasts in the floor of the fourth ventricle. The granule cells originate from the anterior domain of the rhombic lips,11 migrating along the outer edge of the cerebellar cortex to form the external granule layer, and descending past the Purkinje cell layer to form the inner granule layer.19 The gene Atoh-1 (previously Math-1) is a important regulatory gene expressed in rhombic lips and is required for the birth and generation of granule cells.11 Wnt-3 is expressed only in Purkinje cells and is critical to their differentiation and survival.23 Cell contact from both climbing fiber afferents and granule cell is required for the formation of Purkinje cell dendrites,10 and a number of trophic factors such as insulin-like growth factor I promote the survival of granule and Purkinje cells.11 The 3 anatomically distinct cerebellar lobes also develop at different paces. The phylogenetically older flocculonodular lobe develops first, being evident at about 2 months’ gestation. The vermis and paravermian hemispheres rostral to the primary fissure comprise the paleocerebellum, which is present by the fourth month of gestation. The larger cerebellar hemispheres constitute the neocerebellum and develop from the fourth month of embryonic life onward.11 It is important to notice that the development of the cerebellum is not complete by the time of a full-term gestation. During the first 2 years of postnatal life, cell differentiation, granule cell migration, and arborization of the dendrites of Purkinje cells occur, as well as the development of cerebellar folia that grow into the adult pattern.10,24,25 This prolonged postnatal developmental period makes the cerebellum susceptible to a myriad of insults that could disrupt its normal maturation process.

ROLE OF GENETICS IN DWM The DWM is a heterogeneous disorder that has been associated with several malformation syndromes and cytogenetic abnormalities.26 During the past decade, several genetic loci have been implicated in the pathogenesis of DWM and other linked disorders, such as cerebellar vermis hypoplasia (CVH) and mega cisterna magna (MCM). Dandy-Walker malformation, CVH, and MCM are generally classified as different disorders; however they share similarities in appearance, and it is not known whether they represent distinct entities or share a common pathogenesis.27 The low empiric recurrence rate shown in nonsyndromic DWM suggests a polygenic model for the malformation28 (Table 1). The first locus involved in DWM was localized by Grinberg et al29 in 2004. The heterozygous loss of the genes ZIC1 and ZIC4 is the cause of DWM in individuals with deletion of 3q2, and the involvement of this locus with DWM has been reported by other authors.30 It was demonstrated that ZIC1 has a role in the cerebellar development in mice,31 and severely affected mice in both ZIC1 and ZIC4 loci that had a markedly smaller vermis did not survive past weaning.29 The gene Forkhead box 1 (FOXC1), located in chromosome 6p25.3, is also required for a normal cerebellar development and associated with DWM and other cerebellar and posterior fossa malformations.32 FOXC1 is not expressed in the

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TABLE 1. Chromosomal Abnormalities and Genetic Loci Associated With DWM in Humans and Animal Models Chromosomal Abnormality

Author(s) Grinberg et al29 Aldinger et al32

Deletion of 3p2 Deletion or duplication of 6p25.3 Tetrasomy 9p

Malaragno et al; Cazorla Calleja et al35 Deletion of 13q McCormack et al36; Bellarati et al Deletion of 2q36.1 Jalali et al38 Deletion and duplication Liao et al39 of 7p21.3

Gene Associated ZIC1, ZIC4 FOXC1

PAX3 NDUFA4, PHF14

cerebellum itself, but it is expressed in the adjacent posterior fossa mesenchyme early in the CNS formation, and its protein product participates in a broad range of developmental processes, including somatic, cardiovascular, kidney, eye, skull, and cortical development.33 FOXC1 is necessary to direct the normal differentiation and migration of roof plate and rhombic lip derivatives. The specific loss of this gene along with general defects in mesenchymal signaling may result in cerebellar malformation compatible with DWM, CVH, and MCM, suggesting that these entities belong to a single spectrum with shared etiology in some affected individuals.32 Similar posterior fossa malformations along with other CNS and somatic abnormalities were also described in patients with tetrasomy 9p,34,35 deletion of 13q,36,37 and deletion of 2q36.1. The later genetic abnormality may lead to an altered expression of the PAX3 gene, whose genetic lesion in murine is related to severe defects of the neural tube and neural crest derivatives.38 Common deletions and duplication on chromosome 7p21.3 have also been reported, and the cerebellar abnormalities may be originated from haploinsufficiency or overexpression of NDUFA4 and PHF14 genes.39

PATHOLOGICAL FEATURES AND PATHOPHYSIOLOGY Dandy and Blackfan laid down the basis for modern thought regarding brain anatomy, physiology, and the pathogenesis of DWM. The triad first described by these authors included the association of cystic dilation of the fourth ventricle, partial or complete absence of the cerebellar vermis, and hydrocephalus.3 In addition to the features originally described, DWM is also usually associated with the elevation of the transverse sinus, enlargement of the posterior fossa, and lack of patency of the foramina of Luschka and Magendie.2 It was first believed by Dandy, Blackfan, Taggart, and Walker that the disorder resulted from the failure of the foramina of Luschka and Magendie to open and that hypoplasia of the vermis was the effect of chronic compression by the cyst.3,4 Benda5 later proposed that the malformation actually represented a form of cerebellar rachischisis, such as spinal cord myelomeningoceles. A hydrodynamic theory was postulated by Gardner40 in 1977, in which the hypertrophy of the posterior fossa choroid plexus with increased cerebrospinal fluid production should lead to pouching of the anterior membranous area, leading to cystic enlargement of the fourth ventricle and high inserted tentorium. As discussed earlier, DWM results from a dysgenetic development of the roof of the rhombencephalon at the level of the anterior membranous area, and the role of foraminal obstruction in its pathogenesis is more complex. * 2013 Lippincott Williams & Wilkins

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Although not included in the classic triad of features in DWM, hydrocephalus develops in a large percentage of patients affected by the disorder. Hydrocephalus is often absent at birth, and atresia of foramina of Luschka and Magendie is frequent in normal brain; therefore, the initial hypothesis of atresia of foramina as essential factor in its pathogenesis is questionable. In DWM, the outlet foramina of the fourth ventricle may be patent, partially or even completely obstructed.41,42 Control of the hydrocephalus is often one of the treatment’s main goals, and this subject is thoroughly discussed later in this article. The midline cyst open freely in the fourth ventricle is one of the key points in DWM, and its assessment is one of the fundamental points for telling apart other cystic lesions of the posterior fossa. Although in magnetic resonance imaging (MRI), the cyst wall is not usually noticeable, in our experience the sequence FIESTA allows us to define the wall totally or partially. Nevertheless, the wall in posterior fossa cystic lesions has some important pathological features distinguishing these entities. It is necessary to examine the whole fixed brain under water to adequately investigate posterior fossa cysts in necropsy because the delicate cyst floats out away from the specimen with this maneuver. The cyst wall in DWM contains arachnoid, glial, and ependymal cells as well as neurons but lacks choroid plexus. It differs from the histologic structure of the arachnoid cyst (AC), which consists of thin delicate arachnoid. The membrane from the Blake’s pouch cyst usually presents arachnoid, glial, and ependymal cells and, differently from DWM cyst, also includes choroid plexus. Because the histology of the cyst wall is usually not known, it has been recommended that the radiologist should describe the cyst by its location, secondary effects, and associated abnormalities, for some terms may confuse the assisting physician and affect the management of patients.

CLINICAL ASPECTS Although its incidence is 1 per 25 to 30.000 live births,25 the DWM is the most common cerebellar malformation.1 Males and females are equally affected, and the presence of this disorder in more than 1 sibling is unusual. Familial cases are rare.43 In addition to the previously discussed genetic disorders related to DWM, this condition is also associated with Klippel-Feil syndrome,44 Coffin-Siris syndrome, Mohr syndrome, MeckelGruber syndrome, and congenital infections such as cytomegalovirus, rubella, and toxoplasmosis, the use of Coumadin and alcohol, as well as maternal diabetes.25,28 The clinical picture of children affected by DWM is very variable and depends mainly on the severity of other CNS malformations associated. The neurological development may range from normal to severely retarded.43 Intelligence is normal in up to 75% of patients.25 The severity of cerebellar dysgenesis also accounts for a worse prognosis, with an abnormal vermian lobulation being associated with poor intellectual outcome.45,46 The association of brain malformations with DWM varies from 45% to 68% in autopsy cases and clinical series.43,47,48 The most frequent are agenesis of corpus callosum, aqueductal stenosis, rachischisis, ectopic brain or cerebellar tissue, holoprosencephaly, and neural tube defects.28,49,50 Dandy-Walker malformation is responsible for 1% to 4% of all cases of hydrocephalus,43,48,50,51 and approximately 80% to 90% of the children present symptoms in the first year of life,43,48,50,51 most often macrocrania. The lambdoid suture disproportionately widened for the presence of a very large posterior fossa is sometimes observed.52 Delayed achievement of motor and cognitive milestones is usually found in children after their first year of life and most commonly observed as difficulties in walking and impaired * 2013 Lippincott Williams & Wilkins

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coordination. Nystagmus, truncal ataxia, cranial nerve palsies, dysmetria and explosive speech, symptoms related to cerebellar or brainstem dysfunction are relatively uncommon, as are focal neurological deficits.50 Later in life, DWM can present itself with symptoms of increased intracranial pressure, being indistinguishable from a posterior fossa tumor.42,53 These include occipital headache, ataxic gait, mental status changes, cranial nerve palsies, vomiting, and pyramidal tract signs. Seizures are one of the most common symptoms presented by patients with DWM, being frequently associated with the presence of other brain malformations, such as heterotopia. The concomitant existence of visual and hearing problems correlates with a poor intellectual outcome.54 Calvarium defects and occipital meningoceles have also been reported in patients with DWM, and the existence of these abnormalities may be related to the increased pressure in posterior fossa during fetal life.55 Several articles also describe the association of DWM with syringomyelia,56Y58 which may be originated from the herniation of the lower pole of the cyst into the foramen magnum, altering the cerebrospinal fluid (CSF) flow dynamics by mechanisms similar to tonsillar herniation in Chiari malformation.56 It is interesting to notice that the treatment of hydrocephalus and the cyst dilation frequently leads to the resolution of the syringomyelia.57 Hydrocephalus has an important role in the development of symptoms in DWM, and the control of hydrocephalus and the posterior fossa cyst is the goal of treatment in this disorder.52 More than 80% of patients with DWM develop hydrocephalus, mostly in the first 3 months of life,43 and it is believed that the abnormality is multifactorial. The impairment of the CSF flow may be distal to the outlets of the fourth ventricle, with obstruction of the cisterna magna or the perimedullar cisterns due to an inflammatory process40,59,60 or abnormal development of the subarachnoid space as part of the malformation itself.61 Aqueductal stenosis, although absent in typical DWM, may be a consequence of primary developmental defect, or it may be secondary to herniation of the vermis or cyst through the tentorial notch,62 as a ‘‘functional aqueductal stenosis.’’63 Venous hypertension could also play a role in the development of hydrocephalus, as the malformation leads to lengthening of the venous sinuses with direct compression of the posterior fossa cyst.52 Surgical treatment aims to control the hydrocephalus and the posterior fossa cyst. Several options have been proposed; however, contemporary management is still controversial. The first surgical approach proposed was membrane excision in the posterior fossa cyst just below the foramen magnum, to communicate the cyst with the surrounding subarachnoid spaces.3,4 This procedure is currently not considered the initial choice because of its invasiveness and associated significant morbidity and mortality.64 The most common surgical procedure performed nowadays is shunt placement, although there is some discussion on the optimal position for the proximal end of the shunt, which can be placed in the lateral ventricle (ventriculoperitonial [VP] shunt), fourth ventricle (cyst-peritonial shunt), or both locations. The VP shunts have a relatively low incidence of malposition and migration52 and are recommended as first-line therapy to obtain early decompression of the supratentorial compartment.42,62,65 The cystperitonial shunt usually drains both the cyst and the ventricle and is considered the more logical placement because it maintains a physiologic downstream flow in the aqueduct.52 Nevertheless, there are some complications associated with overdrainage of the cyst, including posterior fossa hematomas,54 brainstem tethering,66 and chronic cerebral herniation. The combined shunts are www.topicsinmri.com

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sometimes used as first-line treatment to equalize the pressure across the tentorium and decrease the ‘‘functional aqueductal stenosis’’ after VP shunt alone.51,67 Endoscopic third ventriculostomy has also been performed with satisfactory results,43,68 as a minimally invasive surgical procedure with reduced risk of postoperative adhesion compared with cyst fenestration and being a useful alternative to shunt placement in selected patients.52 The presence of mental symptoms has also been associated with DWM and its related disorders; however, the spectrum of these abnormalities varies greatly between cases, ranging from psychotic to cognitive symptoms. These include schizophrenia-like psychosis, mania, as well as bipolar affective and obsessive-compulsive disorders.69Y72 The varieties of structural abnormality of the brain in the cases reported might partly contribute to the myriad of mental symptoms described. However, morphometric and functional neuroimaging studies have demonstrated a link between the cerebellum and mood symptoms in patients with bipolar disorder,73Y75 suggesting a causal relationship between the pathophysiology of some psychiatric disorders and cerebellar vermian hypoplasia. Although uncommon, asymptomatic DWM can also be found incidentally in necropsy or imaging studies performed for other reasons,42,53 and the absence of associated fatal congenital anomalies increases life expectancy in such cases.76 It is not well understood how long-term asymptomatic DWM becomes symptomatic in some patients; however, Kumar et al77 suggest that degenerative chances in communication channels could lead to unbalance in valvular channels maintaining CSF circulation through posterior fossa cyst membrane and surrounding basal cisterns and patients who maintain good CSF communication may remain asymptomatic throughout their life.76

Imaging Findings in DWM The diagnosis of DWM can be made by ultrasonography as early as 14 weeks of gestation,78 and it may also be discriminated from the other cystic posterior fossa lesions by its imaging findings (Fig. 1). The common ultrasonographic findings include the presence of a large posterior fossa cyst, absent cerebellar vermis, and splayed cerebellar hemispheres.79 Intrauterine evolution of a posterior fossa fluid collection can be

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useful in differentiating DWM from Blake’s pouch cyst and mega cisterna magna because the cystic component in the former usually enlarges as the gestation progresses, whereas the others frequently disappear.79 The relative accuracy of prenatal ultrasound versus MRI has been debated,80,81 and some authors found that MRI rarely adds significant information when compared with a well-performed ultrasonography.79,81 Nevertheless, MRI may allow a better visualization of the position of the torcular, which is important to recognize in DWM.79 Although MRI is now being considered the criterion standard to investigate posterior fossa lesions, plain films of the skull may demonstrate some characteristic features of DWM, such as enlargement of the posterior fossa and elevation of the groove for the transverse sinus and torcular.52 The advantages of MRI over computed tomography (CT) include the possibility of multiplanar imaging acquisition, better spatial resolution, and absence of ionizing radiation, which is a matter of concern especially in the pediatric population. MRI is also more efficient in the detection of associated brain malformations, the assessment of the degree of cerebellar dysgenesis, and the analysis of patency of the cerebral aqueduct.82 The main imaging findings in DWM are an enlarged posterior fossa associated with a cystic dilation of the fourth ventricle and a superiorly rotated hypoplastic caudal cerebellar vermis (Fig. 2). The cystic component of the fourth ventricle may occupy the majority of the posterior fossa, compressing the cerebellar hemispheres against the petrous bone. The cyst can also mechanically hinder normal fetal caudal migration of torcular, with the torcular remaining elevated with a high sloping tentorium. Cyst wall is often difficult to discern even in MRI; however, in our experience, the use of FIESTA may allow characterization of the cyst wall at some extent. As previously discussed, although DWM can be found as an isolated malformation, its association with other CNS abnormalities is not uncommon. These include callosal dysgenesis, heterotopia, polymicrogyria, schizencephaly, encephalocele, intracranial lipoma, and other conditions (Fig. 3). A compressed and anteriorly displaced fourth ventricle lacking a communication with a cystic extraaxial area is usually the hallmark for the distinction between DWM and other cystic

FIGURE 1. Axial fetal ultrasound image (A) shows an enlarged posterior fossa containing an anechoic cystic structure (arrow) and vermian hypoplasia (arrowhead). Note the high-grade hydrocephalus. The axial T2-weighted fetal MRI (B) demonstrates the vermian hypoplasia (arrow) and the posterior fossa cyst as well as the dilated temporal horns of the lateral ventricles (arrowhead).

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FIGURE 2. Sagittal 3-dimensional (3D) CISS MRI (A) shows vermian hypoplasia, fourth ventricle dilatation, and an enlarged posterior fossa in a female patient with DWM. The torcular, straight sinus, and tentorium are displaced superiorly (arrows). The axial T2-weighted MRI of the same patient (B) clearly demonstrates the vermian hypoplasia.

lesions in the posterior fossa. The differential diagnosis includes mega cisterna magna as well as persistent Blake’s pouch and ACs. These entities are better discussed hereafter.

Arachnoid Cyst Arachnoid cysts are cystic dilatations lined by arachnoid cells that develop within the arachnoid layer, first described by Bright in 1831. The communication of the ACs with the subarachnoid spaces may or may not exist, and these structures are filled up with fluid almost identical to CSF in its composition. Arachnoid cyst may be congenital or acquired. During the early embryonic period, the neural tube is surrounded by a loose layer of connective mesenchyme, the primitive meninx, which forms the pia and arachnoid matter. Cerebrospinal fluid starts to circulate at approximately the 15th week of gestation, and its flow contributes to the development of the trabeculae that shape the structure of the subarachnoid space. Disorders in this process could result in fluid being trapped within the arachnoid and leading to the development of an AC.83 Acquired ACs may result from arachnoid adhesions formed after

inflammatory processes in traumatic, tumoral, infectious, and iatrogenic conditions.84Y86 Although usually asymptomatic, ACs may lead to symptoms depending on their size and location, the commonest being headache. Other clinical manifestations include focal neurological signs, macrocephaly, psychomotor delay, seizures, as well as ocular, endocrine, and cerebellar symptoms.87 Arachnoid cysts can develop anywhere in the posterior fossa, and they represent up to 13% of all intracranial mass lesions.88 Arachnoid cysts are not usually associated with hydrocephalus; however, there are reports of the coincidence of the 2 entities.89,90 Arachnoid cysts can be recognized as sharply defined extraaxial fluid collections, with same density or signal intensity when compared with CSF and do not enhance after endovenous administration of contrast media (Fig. 4). FLAIR demonstrates complete suppression of signal intensity, and the lack of restriction of diffusion of water molecules allows to distinguish ACs from epidermoid cysts, which classically present restricted diffusion on diffusion-weighted imaging. There may be scalloping of the occipital bone and distortion and compression on the cerebellum depending on their sizes. Arachnoid cysts should not

FIGURE 3. Axial T2-weighted MRI (A) demonstrates the vermian hypoplasia (arrowhead) and an enlarged posterior fossa. Coronal T2-weighted MRI of the same patient (B) clearly shows dysgenesis of the corpus callosum (arrow). Right cleft palate is also noted in the CT 3D surface rendering image (C). * 2013 Lippincott Williams & Wilkins

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Blake’s pouch cyst is seen as a fluid collection with density and signal intensity similar to CSF posterior and inferior to the fourth ventricle (Fig. 6). The fourth ventricle may be enlarged or presenting usual dimensions, and there is a free communication between the fourth ventricle and the cyst. The choroid plexus of the fourth ventricle frequently extends inferiorly along the superior wall of the cyst. The cerebellar vermis is normal and may be superiorly rotated, depending on the cyst’s size. The normal cerebellum permits distinguishing DWM from a persistent Blake’s cyst.93,97,98

Mega Cisterna Magna

FIGURE 4. Axial T2-weighted MRI shows a retrocerebellar cystic structure compressing the cerebellum anteriorly. Although the posterior fossa presents large dimensions, the cerebellar vermis is normal (arrow), and there is no dilation of the fourth ventricle (arrowhead).

demonstrate enhancement or only demonstrate enhancement after a delay in CT cisternography. Arachnoid cyst also do not have communication with the fourth ventricle; large lesions located posterior to the cerebellar vermis usually are associated with anteriorly compressed cerebellum and fourth ventricle and a normal vermis, allowing differentiation from the posterior fossa cyst in DWM. In difficult cases, high-resolution sequences (CISS, FIESTA) may be used to a better delineation of the cyst wall and adjacent anatomic structures91 (Fig. 5).

The cisterna magna is the subarachnoid space located between the medulla oblongata and the inferior surface of the cerebellum, first described by Liliequist in 1952.99 This structure originates from the permeabilization of the Blake’s pouch, allowing the CSF to flow outside from the fourth ventricle. Inferiorly, the cisterna magna communicates with the perimedullary subarachnoid spaces. The concept of MGM was introduced by Gonsette et al,100 and it is characterized by an enlarged cisterna magna associated with a normal fourth ventricle, cerebellar hemispheres, and vermis.101 MGM is a relatively common condition, representing nearly half of all cystic posterior fossa malformations. It is usually asymptomatic and discovered incidentally; however, there are reports of psychiatric disorders associated with the presence of MCM.99 The theory of a link between MCM and psychiatric disorders finds support in the description of a subgroup of patients with psychotic symptoms associated with mild medianline brain malformations.99,102 The developmental outcome of children with isolated MCM is favorable, but there are data which suggest that higher cognitive functions, language abilities, and executive functions may be impaired in adults.103 The brainstem and cerebellum are normal in MCM, and the CSF flows freely between the surrounding subarachnoid spaces. Hydrocephalus is not usually associated with MCM, and there are no formal indications for shunt surgery for MCM.97

Blake’s Pouch Cyst Blake’s pouch is an inferior protrusion of the fourth ventricle located in the posterior membranous area.1 In 1900, Blake92 described a midline evagination of the roof of the embryonic human fourth ventricle lined by ependyma, which extended posterior and upward into the primitive meninx inferior to the cerebellum and carried in its walls histologic elements that included glia, ependyma, and choroid plexus.93 Between 7 and 8 postovulatory weeks, this evagination ruptures and forms a midline aperture from the fourth ventricle to the subarachnoid space, located adjacent and caudal to the inferior surface of the cerebellar vermis and constituting the foramen of Magendie. When this communication fails to develop, CSF in the fourth ventricle may cause this evagination to expand, eventually leading to the formation of a retrocerebellar cyst, and later this structure was termed Blake’s pouch.94 Therefore, in normal embryogenesis, the Blake’s pouch is a transient structure that initially does not communicate with the surrounding subarachnoid space, and its persistence is also called Blake’s pouch cyst. Similar cysts may arise in relation to Luschka’s foramen when it does not open into the subarachnoid space, and these structures are histologically related to Blake’s pouch cysts.93,95,96

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FIGURE 5. Sagittal 3D CISS MRI of the same patient showed in Figure 4 demonstrates a large AC enlarging the posterior fossa and compressing a normal cerebellum anteriorly (arrow). Also note supratentorial hydrocephalus in this case. * 2013 Lippincott Williams & Wilkins

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FIGURE 6. Sagittal 3D CISS (A) and axial T2-weighted MRIs (B) of a 2-year-old female patient show a retrocerebellar cyst associated with an enlarged posterior fossa and fourth ventricle dilation as well as anterior displacement of the brainstem. Topography and morphology of the cerebellar vermis are preserved. There is supratentorial hydrocephalus.

In imaging modalities, MGM share some radiological features with persistent Blake’s pouch, but the latter is usually associated with hydrocephalus. The cerebellar hemispheres and vermis are intact, and so is the brainstem. Absence of compression of the cerebellum helps to distinguish MCM from a retrocerebellar AC97,98 (Fig. 7).

CONCLUSIONS The DWM is a heterogeneous disorder characterized by hypoplasia and upward rotation of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa with upward displacement of the lateral sinuses, tentorium, and torcular. It is the most cerebellar malformation in humans, and the association of DWM and other CNS abnormalities, including ectopic brain and cerebellar tissue, dysgenesis of corpus callosum, and holoprosencephaly is not uncommon. Its pathogenesis is not completely understood; however, recent advances

in molecular genetics are leading to the discovery of several genetic loci in which abnormalities may result in DWM. Indepth knowledge of the embryology of the hindbrain is critical for a better understanding of cerebellar malformations, both related and unrelated to DWM. Imaging modalities play a fundamental role in the diagnosis of DWM, its related disorders, and associated CNS abnormalities as well as recurrent analysis of the degree of hydrocephalus and its surgical treatment. Prenatal ultrasonography and MRI may successfully demonstrate the hallmarks of DWM from the early gestation’s second semester. In infants and older children, CT may show vermian hypoplasia and the fourth ventricle communicating with a large posterior fossa cyst. MRI, nevertheless, stands as the criterion standard for investigating posterior fossa lesions because of its better tissue contrast and multiplanar imaging capacity, allowing finer distinction between the differential diagnoses for the lesions of that location.

FIGURE 7. Axial T2-weighted (A) and sagittal 3D CISS MRIs (B) demonstrate an enlarged cisterna magna with dura mater septations visible in its posterior aspect (arrow). The cerebellar vermis has its morphology preserved, and there is no compression on the fourth ventricle. * 2013 Lippincott Williams & Wilkins

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REFERENCES 1. Patel S, Barkovich AJ. Analysis and classification of cerebellar malformations. AJNR Am J Neuroradiol. 2002;23:1074Y1087. 2. Hart MN, Malamud N, Ellis WG. The Dandy-Walker syndrome. A clinicopathological study based on 28 cases. Neurology. 1972;22: 771Y780. 3. Dandy W, Blackfan K. Internal hydrocephalus: an experimental, clinical and pathological study. Am J Dis Child. 1914:406Y482. 4. Taggart TK, AE W. Congenital atresia of foramina of Luschka and Magendie. Arch Neurol Psychiatry. 1942;583Y612. 5. Benda CE. The Dandy-Walker syndrome or the so-called atresia of the foramen Magendie. J Neuropathol Exp Neurol. 1954;13:14Y29. 6. Maria BL, Zinreich SJ, Carson BC, et al. Dandy-Walker syndrome revisited. Pediatr Neurosci. 1987;13:45Y51. 7. Jinkins JR, da Costa Leite C. Congenital/Developmental Pathology, in Neurodiagnostic Imaging: Pattern Analysis and Differential Diagnosis. Vol I. Philadelphia, PA: Lippincot-Raven; 1998:5Y65. 8. Barkovich AJ, Millen KJ, Dobyns WB. A developmental and genetic classification for midbrain-hindbrain malformations. Brain. 2009; 132(pt 12):3199Y3230. 9. Hatten ME, Alder J, Zimmerman K, et al. Genes involved in cerebellar cell specification and differentiation. Curr Opin Neurobiol. 1997;7:40Y47. 10. Hatten ME, Heintz N. Mechanisms of neural patterning and specification in the developing cerebellum. Ann Rev Neurosci. 1995;18:385Y408. 11. Goldowitz D, Hamre K. The cells and molecules that make a cerebellum. Trends Neurosci. 1998;21:375Y382. 12. Boltshauser E. Cerebellar imagingVan important signpost in paediatric neurology. Childs Nerv Syst. 2001;17:211Y216. 13. Kumar R. Textbook of Human Embryology. New Delhi, India: IK International Publishing House; 2008:217Y244. 14. Moore K. The Developing Human Clinically Oriented Embryology. Philadelphia, PA: WB SaundersYElsevier; 2008:380Y418. 15. Fotos J, Olson R, Kanekar S. Embryology of the brain and molecular genetics of central nervous system malformation. Semin Ultrasound CT MR. 2011;32:159Y166. 16. Humphrey T. Embryology of the central nervous system: with some correlations with functional development. Ala J Med Sci. 1964;1:60Y64. 17. Sarnat HB. Embryology and neuropathological examination of central nervous system malformations. Handb Clin Neurol. 2008;87:533Y554.

&

Volume 22, Number 6, December 2011

25. Altman NR, Naidich TP, Braffman BH. Posterior fossa malformations. AJNR Am J Neuroradiol. 1992;13:691Y724. 26. Chitayat D, Moore L, Del Bigio MR, et al. Familial Dandy-Walker malformation associated with macrocephaly, facial anomalies, developmental delay, and brain stem dysgenesis: prenatal diagnosis and postnatal outcome in brothers. Am J Med Genet. 1994;52:406Y415. 27. Millen KJ, Gleeson JG. Cerebellar development and disease. Curr Opin Neurobiol. 2008;18:12Y19. 28. Murray JC, Johnson JA, Bird TD. Dandy-Walker malformation: etiologic heterogeneity and empiric recurrence risks. Clin Genet. 1985;28:272Y283. 29. Grinberg I, Northrup H, Ardinger H, et al. Heterozygous deletion of the linked genes ZIC1 and ZIC4 is involved in Dandy-Walker malformation. Nat Genet. 2004;36:1053Y1055. 30. Tohyama J, Kato M, Kawasaki S, et al. Dandy-Walker malformation associated with heterozygous ZIC1 and ZIC4 deletion: report of a new patient. Am J Med Genet A. 2011;155A:130Y133. 31. Aruga J, Minowa O, Yaginuma H, et al. Mouse Zic1 is involved in cerebellar development. J Neurosci. 1998;18:284Y293. 32. Aldinger KA, Lehmann OJ, Hudgins L, et al. FOXC1 is required for normal cerebellar development and is a major contributor to chromosome 6p25.3 Dandy-Walker malformation. Nat Genet. 2009;41:1037Y1042. 33. Zarbalis K, Siegenthaler JA, Choe Y, et al. Cortical dysplasia and skull defects in mice with a Foxc1 allele reveal the role of meningeal differentiation in regulating cortical development. Proc Natl Acad Sci U S A. 2007;104:14002Y14007. 34. Melaragno MI, Brunoni D, Patricio FR, et al. A patient with tetrasomy 9p, Dandy-Walker cyst and Hirschsprung disease. Ann Genet. 1992;35:79Y84. 35. Cazorla Calleja MR, Verdu A, Felix V. Dandy-Walker malformation in an infant with tetrasomy 9p. Brain Dev. 2003;25:220Y223. 36. McCormack WM Jr, Shen JJ, et al. Partial deletions of the long arm of chromosome 13 associated with holoprosencephaly and the Dandy-Walker malformation. Am J Med Genet. 2002;112:384Y389. 37. Ballarati L, Rossi E, Bonati MT, et al. 13q Deletion and central nervous system anomalies: further insights from karyotype-phenotype analyses of 14 patients. J Med Genet. 2007;44:e60. 38. Jalali A, Aldinger KA, Chary A, et al. Linkage to chromosome 2q36.1 in autosomal dominant Dandy-Walker malformation with occipital cephalocele and evidence for genetic heterogeneity. Hum Genet. 2008;123:237Y245.

18. Schneider-Maunoury S, Gilardi-Hebenstreit P, Charnay P. How to build a vertebrate hindbrain. Lessons from genetics. C R Acad Sci III. 1998;321:819Y834.

39. Liao C, Fu F, Li R, et al. Dandy-walker syndrome and microdeletions on chromosome 7 [in Chinese]. Zhonghua Yi Xue Yi Chuan Xue Za Zhi. 2012;29:48Y51.

19. Niesen CE. Malformations of the posterior fossa: current perspectives. Semin Pediatr Neurol. 2002;9:320Y334.

40. Gardner WJ. Hydrodynamic factors in Dandy-Walker and Arnold-Chiari malformations. Childs Brain. 1977;3:200Y212.

20. Kollias SS, Ball WS Jr, Prenger EC. Cystic malformation of the posterior fossa: differential diagnosis clarified through embryologic analysis. Radiographics. 1993;1211Y1231.

41. Takami H, Shin M, Kuroiwa M, et al. Hydrocephalus associated with cystic dilation of the foramina of Magendie and Luschka. J Neurosurg Pediatr. 2010;5:415Y418.

21. Guthrie S. Patterning the hindbrain. Curr Opin Neurobiol. 1996;6:41Y48.

42. Fischer EG. Dandy-Walker syndrome: an evaluation of surgical treatment. J Neurosurg. 1973;39:615Y621.

22. Joyner AL. Engrailed, Wnt and Pax genes regulate midbrain-hindbrain development. Trends Genet. 1996;12:15Y20.

43. Hirsch JF, Pierre-Kahn A, Renier D, et al. The Dandy-Walker malformation. A review of 40 cases. J Neurosurg. 1984;61:515Y522.

23. Salinas PC, Fletcher C, Copeland NG, et al. Maintenance of Wnt-3 expression in Purkinje cells of the mouse cerebellum depends on interactions with granule cells. Development. 1994;120:1277Y1286.

44. Karaman A, Kahveci H. Klippel-Feil syndrome and Dandy-Walker malformation. Genet Couns. 2011;22:411Y415.

24. Loeser JD, Lemire RJ, Alvord EC Jr. The development of the folia in the human cerebellar vermis. Anat Rec. 1972;173:109Y113.

310

www.topicsinmri.com

45. Boddaert N, Klein O, Ferguson N, et al. Intellectual prognosis of the Dandy-Walker malformation in children: the importance of vermian lobulation. Neuroradiology. 2003;45:320Y324.

* 2013 Lippincott Williams & Wilkins

Copyright © 2013 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

Topic in Magnetic Resonance Imaging

&

Volume 22, Number 6, December 2011

46. Klein O, Pierre-Kahn A, Boddaert N, et al. Dandy-Walker malformation: prenatal diagnosis and prognosis. Childs Nerv Syst. 2003;19:484Y489. 47. Hanigan WC, Wright R, Wright S. Magnetic resonance imaging of the Dandy-Walker malformation. Pediatr Neurosci. 1985;12:151Y156. 48. Pascual-Castroviejo I, Velez A, Pascual-Pascual SI, et al. Dandy-Walker malformation: analysis of 38 cases. Childs Nerv Syst. 1991;7:88Y97. 49. Golden JA, Rorke LB, Bruce DA. Dandy-Walker syndrome and associated anomalies. Pediatr Neurosci. 1987;13:38Y44. 50. Sawaya R, McLaurin RL. Dandy-Walker syndrome. Clinical analysis of 23 cases. J Neurosurg. 1981;55:89Y98. 51. Osenbach RK, Menezes AH. Diagnosis and management of the Dandy-Walker malformation: 30 years of experience. Pediatr Neurosurg. 1992;18:179Y189. 52. Spennato P, Mirone G, Nastro A, et al. Hydrocephalus in Dandy-Walker malformation. Childs Nerv Syst. 2011;27:1665Y1681. 53. Lipton HL, Preziosi TJ, Moses H. Adult onset of the Dandy-Walker syndrome. Arch Neurol. 1978;35:672Y674. 54. Bindal AK, Storrs BB, McLone DG. Management of the Dandy-Walker syndrome. Pediatr Neurosurg. 1990;16:163Y169. 55. Bindal AK, Storrs BB, McLone DG. Occipital meningoceles in patients with the Dandy-Walker syndrome. Neurosurgery. 1991;28:844Y847. 56. Cinalli G, Vinikoff L, Zerah M, et al. Dandy-Walker malformation associated with syringomyelia. Case illustration. J Neurosurg. 1997;86:571.

Neuroimaging of Dandy-Walker Malformation

70. Gan Z, Diao F, Han Z, et al. Psychosis and Dandy-Walker complex: report of four cases. Gen Hosp Psychiatry. 2012;34:102 e7Y102 e11. 71. Papazisis G, Mastrogianni A, Karastergiou A. Early-onset schizophrenia and obsessive-compulsive disorder in a young man with Dandy-Walker variant. Schizophr Res. 2007;93:403Y405. 72. Turan T, Besirli A, Asdemir A, et al. Manic episode associated with mega cisterna magna. Psychiatry Investig. 2010;7:305Y307. 73. DelBello MP, Strakowski SM, Zimmerman ME, et al. MRI analysis of the cerebellum in bipolar disorder: a pilot study. Neuropsychopharmacology. 1999;21:63Y68. 74. Cecil KM, DelBello MP, Sellars MC, et al. Proton magnetic resonance spectroscopy of the frontal lobe and cerebellar vermis in children with a mood disorder and a familial risk for bipolar disorders. J Child Adolesc Psychopharmacol. 2003;13:545Y555. 75. Mills NP, Delbello MP, Adler CM, et al. MRI analysis of cerebellar vermal abnormalities in bipolar disorder. Am J Psychiatry. 2005;162:1530Y1532. 76. Jha VC, Kumar R, Srivastav AK, et al. A case series of 12 patients with incidental asymptomatic Dandy-Walker syndrome and management. Childs Nerv Syst. 2012;28:861Y867. 77. Kumar R, Jain MK, Chhabra DK. Dandy-Walker syndrome: different modalities of treatment and outcome in 42 cases. Childs Nerv Syst. 2001;17:348Y352. 78. Achiron R, Achiron A. Transvaginal ultrasonic assessment of the early fetal brain. Ultrasound Obstet Gynecol. 1991;1:336Y344. 79. Gandolfi Colleoni G, Contro E, Carletti A, et al. Prenatal diagnosis and outcome of fetal posterior fossa fluid collections. Ultrasound Obstet Gynecol. 2012;39:625Y631.

57. Hammond CJ, Chitnavis B, Penny CC, et al. Dandy-Walker complex and syringomyelia in an adult: case report and discussion. Neurosurgery. 2002;50:191Y194.

80. Levine D, Barnes PD, Robertson RR, et al. Fast MR imaging of fetal central nervous system abnormalities. Radiology. 2003;229:51Y61.

58. Milhorat TH, Capocelli AL Jr, Anzil AP, et al. Pathological basis of spinal cord cavitation in syringomyelia: analysis of 105 autopsy cases. J Neurosurg. 1995;82:802Y812.

81. Malinger G, Lev D, Lerman-Sagie T. Is fetal magnetic resonance imaging superior to neurosonography for detection of brain anomalies? Ultrasound Obstet Gynecol. 2002;20:317Y321.

59. Glasauer FE. Isotope cisternography and ventriculography in congenital anomalies of the central nervous system. J Neurosurg. 1975;43:18Y26.

82. Naidich TP, Altman NR, Gonzalez-Arias SM. Phase contrast cine magnetic resonance imaging: normal cerebrospinal fluid oscillation and applications to hydrocephalus. Neurosurg Clin N Am. 1993;4:677Y705.

60. Udvarhelyi GB, Epstein MH. The so-called Dandy-Walker syndrome: analysis of 12 operated cases. Childs Brain. 1975;1:158Y182. 61. Opitz JM, Gilbert EF. CNS anomalies and the midline as a ‘‘developmental field’’. Am J Med Genet. 1982;12:443Y455. 62. Carmel PW, Antunes JL, Hilal SK, et al. Dandy-Walker syndrome: clinico-pathological features and re-evaluation of modes of treatment. Surg Neurol. 1977;8:132Y138. 63. Raimondi AJ, Samuelson G, Yarzagaray L, et al. Atresia of the foramina of Luschka and Magendie: the Dandy-Walker cyst. J Neurosurg. 1969;31:202Y216. 64. Matson DD. Prenatal obstruction of the fourth ventricle. Am J Roentgenol Radium Ther Nucl Med. 1956;76:499Y506. 65. Marinov M, Gabrovsky S, Undjian S. The Dandy-Walker syndrome: diagnostic and surgical considerations. Br J Neurosurg. 1991;5:475Y483. 66. Liu JC, Ciacci JD, George TM. Brainstem tethering in Dandy-Walker syndrome: a complication of cystoperitoneal shunting. Case report. J Neurosurg. 1995;83:1072Y1074. 67. Tal Y, Freigang B, Dunn HG, et al. Dandy-Walker syndrome: analysis of 21 cases. Dev Med Child Neurol. 1980;22:189Y201.

83. Naidich TP, McLone DG, Radkowski MA. Intracranial arachnoid cysts. Pediatr Neurosci. 1985;12:112Y122. 84. Choi JU, Kim DS. Pathogenesis of arachnoid cyst: congenital or traumatic? Pediatr Neurosurg. 1998;29:260Y266. 85. Martinez-Lage JF, Poza M, Lopez F. Arachnoid cyst as a complication of ventricular shunting. Childs Nerv Syst. 1991;7:356Y357. 86. Talamonti G, D’Aliberti G, Picano M, et al. Intracranial cysts containing cerebrospinal fluid-like fluid: results of endoscopic neurosurgery in a series of 64 consecutive cases. Neurosurgery. 2011;68:788Y803. 87. Oberbauer RW, Haase J, Pucher R. Arachnoid cysts in children: a European co-operative study. Childs Nerv Syst. 1992;8:281Y286. 88. Pascual-Castroviejo I, Roche MC, Martinez Bermejo A, et al. Primary intracranial arachnoidal cysts. A study of 67 childhood cases. Childs Nerv Syst. 1991;7:257Y263. 89. Martinez-Lage JF, Perez-Espejo MA, Almagro MJ, et al. Hydrocephalus and arachnoid cysts. Childs Nerv Syst. 2011;27:1643Y1652.

68. Cinalli G. Alternatives to shunting. Childs Nerv Syst. 1999;15: 718Y731.

90. Kurabe S, Sasaki O, Mitsuhashi D, Koike T. Growing posterior fossa arachnoid cyst causing tonsillar herniation and hydrocephalus. Arch Neurol. 2011;68:1606Y1607.

69. Turner SJ, Poole R, Nicholson MR, et al. Schizophrenia-like psychosis and Dandy-Walker variant. Schizophr Res. 2001;48:365Y367.

91. Osborn AG. Arachnoid Cyst. 2nd ed. Osborn AG, ed. Altona, Manitoba, Canada: Amirsys; 2010.

* 2013 Lippincott Williams & Wilkins

www.topicsinmri.com

Copyright © 2013 Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.

311

Topic in Magnetic Resonance Imaging

Correa et al

92. Blake JA. The roof and lateral recesses of the fourth ventricle, considered morphologically and embryologically. J Comp Neurol. 1900;79Y108. 93. Nelson MD Jr, Maher K, Gilles FH. A different approach to cysts of the posterior fossa. Pediatr Radiol. 2004;34:720Y732. 94. Nelson SH. A case of chronic internal hydrocephalus due to ependymitis granularis. J Neurol Psychopathol. 1926;7:117Y124.

&

Volume 22, Number 6, December 2011

98. Blaser SL. Dandy-Walker Continuum. Osborn AG, ed. Altona, Manitoba, Canada: Amirsys; 2010. 99. Langarica M, Peralta V. Psychosis associated to megacisterna magna [in Spanish]. An Sist Sanit Navar. 2005;28:119Y121. 100. Gonsette R, Potvliege R, Andre-Balisaux G, et al. Mega-cisterna magna: clinical, radiologic and anatomopathologic study [in French]. Acta Neurol Psychiatr Belg. 1968;68:559Y570.

95. Nakase H, Ohnishi H, Touho H, et al. Large ependymal cyst of the cerebello-pontine angle in a child. Am J Med Genet. 1994;16:260Y263.

101. Kollias SS, Ball WS Jr, Prenger EC. Cystic malformations of the posterior fossa: differential diagnosis clarified through embryologic analysis. Radiographics. 1993;13:1211Y1231.

96. Monaco P, Filippi S, Tognetti F, et al. Glioependymal cyst of the cerebellopontine angle. J Neurol Neurosurg Psychiatry. 1995;58:109Y110.

102. Harrison PJ. The neuropathology of schizophrenia. A critical review of the data and their interpretation. Brain. 1999;122(pt 4):593Y624.

97. Shekdar K. Posterior fossa malformations. Semin Ultrasound CT MR. 2011;32:228Y241.

312

www.topicsinmri.com

103. Bolduc ME, Limperopoulos C. Neurodevelopmental outcomes in children with cerebellar malformations: a systematic review. Dev Med Child Neurol. 2009;51:256Y267.

* 2013 Lippincott Williams & Wilkins

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