Category: Congenital Malformation
Affected System: Central Nervous System
Specific Target Organ: Brain
| Synonyms and related keywords |
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Dandy-Walker syndrome, Dandy-Walker cyst, Dandy-Walker deformity, Luschka-Magendie foramina atresia
| Introduction |
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Definition:
Dandy-Walker malformation is a rare congenital malformation and involves the cerebellum and fourth ventricle. The condition is characterized by agenesis or hypoplasia of the cerebellar vermis, cystic dilatation of the fourth ventricle, and enlargement of the posterior fossa. A large number of concomitant problems may be present, but the syndrome exists whenever these 3 features are found. Approximately 70-90% of patients have hydrocephalus, which often develops postnatally. Dandy-Walker malformation may be associated with atresia of the foramen of Magendie and, possibly, the foramen of Luschka.
Dandy-Walker malformation first was described by Dandy and Blackfan in 1914. Since the original description, additional studies have reported on the various morphologic features of the syndrome. Not until 1954 did Benda first emphasize that atresia of the cerebellar outlet foramina is not an essential feature of the condition and suggested the now widely accepted term Dandy-Walker malformation.
Studies by D'Agostino in 1963 and Hart et al in 1972 further defined the characteristic triad of Dandy-Walker malformation as consisting of (1) complete or partial agenesis of the vermis, (2) cystic dilatation of the fourth ventricle, and (3) an enlarged posterior fossa with upward displacement of lateral sinuses, tentorium, and torcular herophili. The triad typically is found in association with supratentorial hydrocephalus, which should be considered a complication rather than part of the malformation complex.
Dandy-Walker complex
Classically, posterior fossa cystic malformations have been divided into Dandy-Walker malformation, Dandy-Walker variant, mega cisterna magna, and posterior fossa arachnoid cyst. Precisely differentiating the malformations may not be possible using imaging methods. Dandy-Walker malformation, variant, and mega cisterna magna currently are believed to represent a continuum of developmental anomalies on a spectrum that has been termed the Dandy-Walker complex.
Dandy-Walker complex is characterized by an enlarged posterior fossa, high position of tentorium with upward displacement of the lateral sinuses, torcular herophili associated with varying degrees of vermian aplasia or hypoplasia, and a cystic dilatation of the fourth ventricle that nearly fills the entire posterior fossa. Since the vermis is present in posterior fossa arachnoid cyst, this is considered separately from Dandy-Walker malformation.
Dandy-Walker variant
Dandy-Walker variant consist of vermian hypoplasia and cystic dilatation of the fourth ventricle without enlargement of the posterior fossa (see Image below).
Caption: Dandy-Walker malformation. Dandy-Walker variant in a 13-year-old girl with thoracal scoliosis. Sagittal T1-weighted MRI shows agenesis of the corpus callosum and a hypoplastic inferior vermis. The fourth ventricle is enlarged slightly, but the posterior fossa typically is normal in size.Mega cisterna magna
The mega cisterna magna consist of an enlarged posterior fossa, secondary to an enlarged cisterna magna, but a normal cerebellar vermis and fourth ventricle (see Image below).
Caption: Dandy-Walker malformation. Mega cisterna magna. Sagittal T1-weighted MRI shows a large retrocerebellar cerebrospinal fluid collection and a normal fourth ventricle and vermis.Arachnoid cyst
Retrocerebellar arachnoid cysts of developmental origin are uncommon but clinically important. True retrocerebellar arachnoid cysts displace the fourth ventricle and cerebellum anteriorly and show significant mass effect. Differentiation of posterior fossa arachnoid cyst from Dandy-Walker malformation is essential as surgical therapy differs between the two entities (see Image below).
Caption: Dandy-Walker malformation. Posterior fossa arachnoid cyst in a 15-month-old girl with a lumbar pilonidal sinus. Sagittal T1-weighted MRI shows a large posterior fossa cyst that is compressing the cerebellar hemispheres, vermis, fourth ventricle (arrow), and brainstem.Pathophysiology:
Dandy-Walker malformations are formed during embryogenesis. Insults of varying severity to both the developing cerebellar hemispheres and fourth ventricle currently are believed to be the genesis of the anomaly. The nature and cause of the insult are unknown.
Multiple diverse theories have been offered to explain the diffuse manifestations of Dandy-Walker malformation. To date, no single theory has proven satisfactory; therefore, none has been accepted widely. Theories regarding the origin of the malformation have centered on defects in embryogenesis as below.
Dandy and Blackfan (1914) and Taggart and Walker (1942) believed that the massive dilatation of the fourth ventricle originates in a congenital obstruction of the outlets of Luschka and Magendie. This theory includes the presence of a developmental cerebellar defect that begins before the embryologic differentiation of the fourth ventricle foramina, and results in blockage or atresia of the foramina of Magendie and Luschka. This, in turn, results in cystic transformation of the roof of the fourth ventricle and in an obstructive (noncommunicating) hydrocephalus, in which a cyst arises from compromised absorption of CSF.
The most comprehensive theory concerns dysembryogenesis involving the hindbrain. An insult that leads to developmental arrest in formation of the hindbrain, with lack of fusion of the cerebellum in the midline, can be localized temporally between the 7th and 10th gestational weeks. This results in persistence of the anterior membranous area, which extends and herniates posteriorly. Simultaneous formation of the foramen of Magendie, tentorium, superior longitudinal sinus, straight sinus, torcular herophili, and lateral sinuses helps explain their association with Dandy-Walker malformation.
Differential Diagnosis:
Arachnoid Cyst
Epidermoid, Brain
Other Problems to be Considered:
Isolated fourth ventricle
Mega cisterna magna
Joubert syndrome
Tectocerebellar dysraphia
Differential diagnosis of posterior fossa cystic lesions is summarized below.
Dandy-Walker malformation
Complete or partial cerebellar vermian hypoplasia
Cystic dilatation of fourth ventricle
Large posterior fossa
High torcular and transverse sinuses
Hydrocephalus
Dandy-Walker variant
Varying cerebellar primarily inferior vermian hypoplasia
Varying enlargement of the fourth ventricle
Normal posterior fossa volume
Mega cisterna magna
Normal cerebellar hemispheres and vermis
Large retrocerebellar CSF collection communicates with fourth ventricle
Normal fourth ventricle
Occasionally, quite enlarged posterior fossa
Posterior fossa arachnoid cyst
Noncommunicating posterior fossa cyst
Normal but distorted vermis and cerebellum
Normal but displaced fourth ventricle
Usually enlarged posterior fossa
Diagnosis:
Dandy Walker syndrome is diagnosed with X-rays and a CT (computerized tomography) scan of the brain. CT scanning is a more advanced imaging technique that uses x-rays and computer technology to produces more clear and detailed pictures than a traditional x-ray. Magnetic resonance imaging (MRI) scans of the brain can also be used. MRI scans produce extremely detailed pictures of the inside of the body by using very powerful magnets and computer technology. MRI scans are more detailed and more expensive that CT scans. A rarely used technique to diagnose Dandy Walker syndrome is a ventriculogram. A ventriculogram is a specific x-ray study of the ventricles in the brain. Ventricles are openings in the brain that produce the cushiony fluid that protects the brain and spine.
Signs and Symptoms:
The list of medical symptoms mentioned in various sources for Dandy-Walker Syndrome includes those listed below. Note that Dandy-Walker Syndrome symptoms usually refers to various medical symptoms known to a patient, but the phrase Dandy-Walker Syndrome signs may often refer to those signs that are only noticeable by a doctor:
· Hydrocephalus
· Increased intracranial pressure
· Slow motor development
· Progressive macrocrania (abnormally enlarged skull)
· Symptoms of increased intracranial pressure
- Irritability
- Vomiting
- Convulsions
· Symptoms of cerebellar dysfunction
- Ataxia
- Nystagmus (jerky eyes)
· Increased head circumference
· Bulging occiput (the back of the head)
· Cranial nerve dysfunction
· Abnormal breathing patterns
· Progressive head enlargement
· Vomiting
· Hyperirritability
· Convulsions
· Congested scalp veins
· Bulging anterior fontanelle
· Separated cranial sutures
· Swelling of optic disk
· Slowed heart rate
· Bradypnea
· Long head
· Thinning of occipital squama
· Dilatation of fourth brain ventricle
See also Physical Assessment/Anatomy.
Inheritance:
The etiology is heterogeneous, and familial occurrence also has been reported. A few cases resulting from autosomal recessive genes have been reported, although in most patients, the cause of Dandy-Walker malformation is not known. Genetic counseling is critical to estimate the risk of recurrence of genetic disorders in family members.
Etiologic heterogeneity and low recurrence risk in siblings (1-5%) for Dandy-Walker malformation have been reported. Increased frequency of an association with congenital heart disease, cleft palate, and neural tube defects appears to exist. An unusual case of an infant with both Ellis-van Creveld and Dandy-Walker syndromes and with homozygosity for an unusually long heterochromatic segment of the long arm of chromosome 9 (9qh+) was reported. An extensive tabulation of single gene disorders, chromosomal aberrations, teratogen-induced conditions, sporadic forms, or forms with undetermined inheritance associated with Dandy-Walker malformation also were reported. In a large series, an abnormal karyotype was found in 5 of 17 patients.
Predisposing factors:
Predisposing factors include gestational (first trimester) exposure to rubella, cytomegalovirus, toxoplasmosis, warfarin (Coumadin), alcohol, and isotretinoin.
Frequency:
In the US: The incidence of Dandy-Walker malformation is 1 case per 25,000-35,000 live births. Dandy-Walker malformation accounts for approximately 1-4% of hydrocephalus cases.
Mortality/Morbidity:
Overall mortality rates of 12-50% have been reported in Dandy-Walker malformation in the pediatric neurosurgical literature. Associated congenital anomalies contributed to 83% of postnatal deaths. Mortality rates have improved significantly over the last 30 years as a result of better anesthesia and shunting devices and the reduction of posterior fossa exploration. Sudden and unexpected death is an uncommon but well-recognized occurrence in patients with Dandy-Walker malformation.
The prognosis is difficult to formulate. The prognosis is only moderately favorable, even when hydrocephalus is treated early and correctly. In one study, 3 patients with isolated Dandy-Walker cysts with hydrocephalus diagnosed in utero were treated at birth with shunting, and all 3 had normal outcomes. An extreme range of severity is seen in this malformation. The presence of multiple congenital defects may affect survival adversely. Some people have Dandy-Walker variant their entire lives without any symptoms. Some infants may have it in association with other syndromes, resulting in severe complications or death.
Race:
DWM has not been shown to be more frequent in any particular ethnic group or race. About 85% of babies born with DWM have one or more other congenital malformations, or some type of recognizable syndrome.
Sex:
Dandy-Walker malformation occurs more frequently in females than in males. The male-to-female ratio was 1:3 in one Spanish series.
Age: Depending on the time of onset and degree of hydrocephalus, the age at diagnosis varies from birth to older childhood. Presentation in adulthood has been reported but is unusual. Patients with Dandy-Walker variant are more likely to present in adulthood than in infancy or childhood.
| Clinical |
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History:
Patients with history of gestational (first trimester) exposure to rubella, cytomegalovirus, toxoplasmosis, warfarin (Coumadin), alcohol, and isotretinoin are at risk of developing the disorder.
Physical Assessment/Anatomy:
Most definitions of Dandy-Walker malformation have included the following 3 features: (1) dysgenesis of the vermis, (2) cystic dilatation of the fourth ventricle, and (3) enlargement of the posterior fossa with elevation of the tentorium/torcula.
Dandy-Walker malformation consists of a malformation of the roof of the fourth ventricle and of the cerebellum. The cerebellum is poorly developed, displaced upwards and laterally. The enlarged fourth ventricle balloons out backward (see Image A, Image B below).
Caption: Image A. Dandy-Walker malformation. Sagittal T1-weighted MRI in a 5-year-old girl shows a large posterior fossa cyst elevating the torcular herophili and sinus rectus (small arrow). The hypoplastic vermis is everted over the posterior fossa cyst (large arrow). The cerebellar hemispheres and brainstem (b) are hypoplastic. Thinned occipital squama is seen (arrowheads).
Caption: Image B. Dandy-Walker malformation. Sagittal T1-weighted MRI in an 11-day-old boy shows agenesis of the corpus callosum, hypoplastic brainstem (b), elevated the torcular herophili (lambdoid-torcular inversion, large arrow), a large fourth ventricle, and a markedly hypoplastic vermis that is rotated superiorly (small arrow).The fourth ventricle is grossly misshapen and is a membrane-wrapped cyst that extends into the foramen magnum. This cyst may lift and displace the posterior portion of the brain, as well as cause an internal obstruction of normal CSF flow with resultant hydrocephalus (see Image C, Image D, Image E).
Caption: Image C. Dandy-Walker malformation. Axial T2-weighted MRI shows hydrocephalus, a large cerebrospinal fluid cyst in the posterior fossa, thinned occipital bone (arrows), and hypoplastic cerebellar hemispheres with a winged appearance (c).
Caption: Image D. Dandy-Walker malformation. Axial T1-weighted MRI shows ventriculomegaly and superiorly displaced posterior fossa cyst.
Caption: Image E. Dandy-Walker malformation. Axial CT scan in a 7-year-old girl with hydrocephalus shows a large cerebrospinal fluid cyst in the posterior fossa and hypoplastic cerebellar hemispheres with a winged appearance (c).Dandy-Walker malformation may be associated with atresia of the foramen of Magendie and, possibly, the foramen of Luschka.
Common findings of Dandy-Walker malformation include the following:
· Enlarged posterior fossa
· Varying degrees of cerebellar and vermian hypoplasia or complete vermian absence (see Images below)
· Cyst formation in the posterior fossa (see Images below)
· Vermian remnant is everted above the posterior fossa cyst (See Images below)
· Hypoplastic cerebellar hemispheres winged anterolaterally (outward) in front of the cyst (see Images below)
· Absence of the foramina of Luschka and Magendie
· Obstructive hydrocephalus secondary to cystic dilatation of the fourth ventricle (70-90%; see Images below)
· Abnormally high position of the straight sinus, torcular herophili, and tentorium (see Images below)
Caption: Dandy-Walker malformation. Axial T1-weighted MRI shows an elevated anteriorly displaced torcular herophili (arrow) and superiorly displaced posterior fossa cyst.· Sinus confluence and lateral sinuses elevated above the lambdoid sutures (high tentorial insertion = lambdoid-torcular inversion; see Images below)
· Aqueductal obstruction is an important component since it may affect the need for supratentorial decompression.
· If callosal agenesis coexists (20-25%), development of dilatation of the occipital horns (colpocephaly; see Images below)
· Brainstem possibly compressed and hypoplastic; degree of pontine hypoplasia related directly to degree of cerebellar hypoplasia (see Images below)
· Thinning and bulging of occipital bones (see Images below)
Associated central nervous system (CNS) abnormalities of Dandy-Walker malformation are reported in 70% of children.
· Dysgenesis of corpus callosum (20-25%; see Images below)
· Lipoma of corpus callosum
· Holoprosencephaly (25%)
· Porencephaly
· Dysplasia of cingulate gyrus (25%)
· Schizencephaly
· Polymicrogyria/gray matter heterotopia (5-10%)
· Cerebellar heterotopia
· Occipital encephalocele (7%)
· Microcephaly
· Dermoid cysts
· Malformation of cerebellar folia (25%)
· Malformation of inferior olivary nucleus
· Hamartoma of tuber cinereum
· Syringomyelia
· Klippel-Feil deformity
· Spina bifida
· Lumbosacral meningoceles
· Spinal lipoma
Non-CNS–associated malformations are reported in 20-33% of children.
· Orofacial deformities and cleft palate (6%)
· Polydactyly and syndactyly
· Cardiac anomalies
· Urinary tract abnormalities (polycystic kidneys)
· Cataracts, retinal dysgenesis, and choroid coloboma
· Facial hemangioma
· Hypertelorism
· Meckel-Gruber syndrome
· Neurocutaneous melanosis
Clinical Details:
Patients with Dandy-Walker malformation present with developmental delay, enlarged head circumference, or signs and symptoms of hydrocephalus. The clinical presentation depends to some extent on the combination of the developmental anomalies in the infant.
An estimated 80% of patients had normal ventricles at birth, and by age 1 year, 80% had ventriculomegaly. Hydrocephalus is present in approximately 90% of patients at the time of diagnosis.
If no other anomalies are present, the only symptom can be an abnormal enlargement of the head. Typical signs of increased intracranial pressure seen in older children and adults may be absent in infants secondary to the ability of the head to increase in size. Macrocrania usually is the consequence of hydrocephalus, but in some patients, it results from massive enlargement of the posterior fossa by the posterior fossa cyst. In this situation, macrocrania precedes development of hydrocephalus, giving the skull a characteristic dolichocephalic shape with bulging of the occiput.
Difficulty with balance, spasticity, and poor fine motor control are common. The degree of developmental delay appears to be related to the level of control of hydrocephalus and to the extent of supratentorial anomalies. Interference with respiratory control centers in the brainstem may cause respiratory failure. Seizures occur in 15-30% of patients.
Hearing or visual difficulties, systemic abnormalities, and CNS abnormalities are associated with poor intellectual development. Subnormal intelligence (intelligence quotient <83)>
Causes:
In most cases, the cause of Dandy Walker syndrome is unknown. It has been suggested that abnormalities in chromosomes may cause some cases of Dandy Walker syndrome. Chromosomes are structures in a person's cells that contain genes. Genes are units of material that contain coded instructions as for how certain bodily characteristics (such as eye color) will develop. All of a person's genes come from his/her parents. Some genes are normal whereas others may be abnormal. Abnormal genes can cause diseases.
It has also been suggested that Dandy Walker syndrome can be caused by exposure to an anti-acne (anti-pimple) medication called isotretinoin during the first three months that the baby is developing inside the mother. Other suggested causes are exposure to rubella, cytomegalovirus, toxoplasmosis, warfarin (Coumadin), and alcohol.
| Tests/Studies |
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Preferred Examination:
Dandy-Walker malformation is diagnosed best with the help of US and MRI. US may be the initial examination performed since it can be done portably, without sedation, and allows multiplanar imaging.
The introduction of modern imaging techniques, specifically MRI, has radically changed the evaluation of symptoms related to the posterior fossa.
MRI usually is performed for detailed evaluation of Dandy-Walker malformation lesions and complications after the diagnosis is suspected using computed tomography (CT) and US. MRI can best define the relationship between the cyst and the fourth ventricle and can detect vermian rotation and the presence of signs of vermian dysgenesis.
MRI allows surgeons to view the cerebellum and associated structures accurately and to determine which form the malformation has taken and to what extent the malformation has progressed. MRI also demonstrates which space should be shunted first. Recently, MRI has been used frequently for diagnosis of fetal craniospinal anomalies.
CT scanning also is useful in Dandy-Walker malformation, since it can distinguish between hydrocephalus associated with Dandy-Walker and hydrocephalus with other etiologies.
Classic abnormal findings of Dandy-Walker malformation described on cranial CT and MRI also can be demonstrated on cranial sonography. US is used routinely during the antenatal period as a screening method and is used in particular for postnatal follow-up studies of hydrocephalus. US evaluation of posterior fossa cystic abnormalities in the newborn is best accomplished via a posterolateral fontanelle approach or through the cisterna magna posteriorly.
In recent years, plain radiography has been used primarily in the evaluation of shunt malfunction and for diagnosis of associated anomalies.
Limitations of Techniques:
Plain radiographs have diagnostic importance in the evaluation of shunt malfunction and for evaluation of bone abnormalities.
CT is an effective diagnostic method but exposes the infant to ionizing radiation. Clearly distinguishing subtypes of the Dandy-Walker complex on axial CT images is difficult. In addition, evaluating subtle supratentorial pathologies and associated abnormalities on CT images may not be easy because its routine use is constrained by the axial plane.
MRI is relatively expensive. High-quality MRI images require patient cooperation or sedation.
US is limited because it is heavily operator dependent. US does not image well such abnormalities as the gyral, dural, tentorial, and skull anomalies accompanying Dandy-Walker malformations.
Radiograph
Findings:
On conventional radiographs, the posterior fossa is enlarged disproportionately, with characteristic thinning and bulging of the occiput. Increased pressure from the massively dilated fourth ventricle, along with prominent CSF pulsations through the cyst fluid, causes widening of the diastatic lambdoid sutures and erosive scalloping of the occiput. Torcular herophili and lateral sinus grooves are located high above lambdoid angle (torcular-lambdoid angle inversion).
Degree of Confidence:
Plain radiographs have diagnostic importance in imaging bony abnormalities and in the evaluation of ventriculoperitoneal shunt malfunction.
CT Scan
Findings:
CT is used to diagnose Dandy-Walker malformation and to follow ventricular shunt function in shunted patients. Nonenhanced CT examination successfully delineates multiple components of Dandy-Walker malformation. The components include partial or complete absence of the cerebellar vermis, posterior fossa cyst contiguous with the fourth ventricle, small and widely separated cerebellar hemispheres, anterior and lateral displacement of hypoplastic cerebellar hemispheres, anterior displacement of pons, elevated imprint of the transverse sinuses with thinning and bulging of the bones of the posterior fossa, scalloping of petrous pyramids, and hydrocephalus (see Images below).
Caption: Dandy-Walker malformation. Axial CT scan in a 1-day-old boy shows a large posterior fossa cyst, separation of the lambdoid sutures (large arrows), and concavity of the petrous ridges (small arrows).Three-dimensional CT and CT angiography studies can be used to evaluate osseous malformation and the relationships between vascular and bony structures. For these studies, 1-mm thickness, an axial plane and 3D reconstruction are used.
Degree of Confidence:
CT scans show the malformation relatively well, but a slice may miss the relevant presence of the vermis. Clearly distinguishing Dandy-Walker complex subtypes using axial CT images is difficult.
False Positives/Negatives:
CT scans show the malformation relatively well, but a slice may miss the relevant presence of the vermis. Clearly distinguishing Dandy-Walker complex subtypes using axial CT images is difficult.
MRI
Findings:
Malformations of the CNS are best delineated using MRI. The diagnosis is straightforward when typical MRI findings are present. MRI usually is required for better anatomic resolution prior to surgical intervention.
Nonenhanced routine cranial MRI can image the altered anatomy and provide excellent images in all projections (sagittal, axial, coronal), of which the sagittal view is one of the most useful (see Images below).
Caption: Dandy-Walker malformation. Dandy-Walker variant in a 13-year-old girl with thoracal scoliosis. Sagittal T1-weighted MRI shows agenesis of the corpus callosum and a hypoplastic inferior vermis. The fourth ventricle is enlarged slightly, but the posterior fossa typically is normal in size.Hypoplasia or absence of the cerebellar vermis is detected best using thin-sectioned midline sagittal T1-weighted images (see Image above). Wide separation of the cerebellar hemispheres without intervening vermis can be seen in axial or coronal images. In some patients, the inferior lobules of the vermis appear to be hypoplastic, while in others, they appear to be intact. Remnants of the dysplastic upper vermis are rotated anterosuperiorly, compressed, and attached to the tentorium (see Image below).
Axial images alone may be misleading in revealing the upwardly rotated vermis.
Enlarged posterior fossa, cyst formation in the posterior fossa, anterolaterally winged cerebellar hemispheres (winged outward), absence of falx cerebelli, and scalloping of petrous pyramids are well demonstrated on T1-weighted images (see Images below).
Sagittal MRI can help evaluate an abnormally high position of the straight sinus, torcular herophili, tentorium, high tentorial insertion (lambdoid-torcular inversion), hypoplastic and compressed brainstem, and obstructive hydrocephalus secondary to cystic dilatation of the fourth ventricle in patients with Dandy-Walker malformation (see Images above).
CSF flow and cine MRI techniques are valuable imaging methods with which to demonstrate patency of CSF pathways at the level of the incisura or aqueduct of Sylvius. Shunting of the cyst alone in the presence of aqueductal obstruction may result in downward transincisural herniation of the cerebrum and atria of the lateral ventricles. On the contrary, shunting of the lateral ventricles alone when the aqueduct is obstructed may lead to an upward herniation of the posterior fossa cyst resulting in a characteristic snowman appearance in the sagittal plane in the absence of communication. Frequently the hydrocephalus in patients with Dandy-Walker is communicating in type with patency of the aqueduct.
Assessing the presence of associated supratentorial anomalies is important, since the prognosis for patients is much better in the absence of the anomalies (see Images below).
MRI affords an accurate and objective detailed identification of supratentorial anomalies. Recently, MRI (as well as US) has been used as a diagnostic tool during the antenatal period.
Degree of Confidence:
MRI is reliable and is the diagnostic method of choice in differentiating Dandy-Walker malformation from other posterior fossa pathologies.
False Positives/Negatives:
MRI findings in Dandy-Walker malformation may be confused with mega cisterna magna, arachnoid cyst, isolated fourth ventricle, and Joubert syndrome (see Images below).
Caption: Dandy-Walker malformation. Joubert syndrome in an 8-month-old boy. Axial CT scan obtained near the pontomesencephalic junction shows a batwing configuration of the fourth ventricle and unusual definition of the superior cerebellar peduncles at this level (arrows). Vermis is dysgenetic, and the 2 cerebellar hemispheres appose each other in the midline.