Paediatric Neuroradiology

Cerebellar Malformations

Cerebellar Hypoplasia

Definition

The cerebellum may be small due to a congenital lack of formation or from subsequent atrophy
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  Causes: infection (especially congenital cytomegalovirus) ▸ inborn errors of metabolism (e.g. glycolysation disorder)
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  Other rarer causes: carbohydrate-deficient glycoprotein syndrome ▸ infantile neuroaxonal dystrophy ▸ pontocerebellar hypoplasia ▸ spinocerebellar atrophies ▸ Friedreich's ataxia

Clinical presentation

Variable hypotonia or ataxia

Radiological features

MRI

Symmetrical atrophy of the cerebellar folia with widened cerebellar fissures ▸ variable cerebellar SI ▸ vermis more frequently affected

Dandy–Walker Complex

Definition

This encompasses a spectrum of cystic posterior fossa malformations from the complete Dandy–Walker malformation to a mega cisterna magna
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  Membranous obstruction to the foramina of Magendie / Luschka causes cystic dilatation of the 4 ^th ventricle
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  All have an apparently focal extra-axial CSF collection which is continuous with the 4 ^th ventricle (with a variable degree of cerebellar hypoplasia)
It is associated with hydrocephalus and other midline abnormalities (e.g. agenesis or a lipoma of the corpus callosum)

Clinical presentation

Developmental delay ▸ seizures ▸ hydrocephalus

Radiological features

Dandy–Walker malformation

There is cystic dilatation of the 4 ^th ventricle (which almost fills an entire enlarged posterior fossa) ▸ the cerebellar vermis is hypoplastic as well as rotated or aplastic ▸ the tentorium and venous confluence of the torcula are elevated

Dandy–Walker variant

This describes cases where the above lesions are less marked and the posterior fossa is not enlarged

Mega cisterna magna

This forms the mildest end of the spectrum and is of dubious clinical significance
It consists of an infracerebellar CSF collection and occasionally an enlarged posterior fossa ▸ the cerebellum and 4 ^th ventricle are normal

Pearl

Differential: a posterior fossa arachnoid cyst (this will not communicate with the 4 ^th ventricle)

Joubert's Syndrome

Definition

Total aplasia of the cerebellar vermis reflecting the failure of formation of the decussation of the superior cerebellar peduncles, a lack of the pyramidal decussations and other anomalies of the midbrain crossing tracts and their nuclei
Occasionally a genetic locus has been identified
Many syndromes with additional features (e.g. renal cysts, ocular abnormalities, liver fibrosis, hypothalamic hamartomas and polymicrogyria) have been classified with this anomaly

Clinical presentation

Tachypnoea ▸ abnormal eye movements ▸ ataxia
Patients may occasionally be clinically normal

Radiological features

MRI

A cleft is present within the vermis ▸ the midbrain is small ▸ the superior peduncles appear enlarged

‘Molar tooth’ appearance: seen on axial images arising from the lack of the superior cerebellar decussation
‘Batwing’ appearance: this is due to an enlarged 4 ^th ventricle with cerebellar hypoplasia, midline vermian cleft and a dysplastic small vermis

Other Cerebellar Malformations

Rhombencephalosynapsis

A very rare cerebellar malformation where the cerebellar hemispheres are fused across the midline and there is hypoplasia or aplasia of the vermis ▸ it is associated with other midline supratentorial anomalies (e.g. absence of the septum pellucidum and corpus callosum, as well as holoprosencephaly)

Lhermitte–Duclos or dysplastic cerebellar gangliocytoma

A developmental mass lesion with enlargement of the cerebellar cortex ▸ this usually affects one hemisphere

MRI

A non-enhancing mass with diffusely enlarged cerebellar folia (± pial enhancement)

	Dandy–Walker malformation	Dandy–Walker variant	Mega cisterna magna
*Posterior fossa*	Enlarged	Normal	Normal or enlarged
*Vermis*	Absent or very hypoplastic	Hypoplastic	Normal
*Hypoplastic cerebellar hemisphere*	Yes	Rare	No
*Hydrocephalus*	75%	25%	Unusual
*Supratentorial abnormalities*	Common	Uncommon	Rare
*Falx cerebelli*	Absent	Present (32%)	Present (63%)
*4 ^th ventricle*	Opens into cyst	Cyst dilatation	Normal
*Prognosis*	Poor	Good	Good

Axial T2WI (A) and sagittal T1WI (B) in a patient with the Dandy–Walker syndrome showing an enlarged posterior fossa with a high tentorium, and a large fluid-filled 4 th ventricle–cisterna magna complex in association with vermian hypoplasia. †

Child with Joubert's syndrome. (A) Typical batwing appearance to the 4 th ventricle (arrow) and (B) prominent superior cerebellar peduncles with failure of the normal midline decussation (arrow). This gives the typical ‘molar tooth’ appearance. The midbrain is hypoplastic in this condition. *

Dandy–Walker malformation. (A, B) The 4 th ventricle opens into a large posterior fossa cyst. There is associated hydrocephalus. (C) The cerebellum is hypoplastic and a thin rim of cerebellar tissue is seen forming the wall of the posterior fossa cyst (arrow). The vein of Galen, straight sinus and venous confluence are elevated above the level of the lambdoid suture. *

Chiari Malformations

Definition

Chiari malformation: A group of abnormalities characterized by dislocation of the hindbrain into the spinal canal

Chiari I Malformation (Cerebellar Ectopia)

Definition

A form of hindbrain deformation rather than a true malformation characterized by tonsillar descent through a normal-sized foramen magnum ▸ it may be an acquired condition due to raised intracranial pressures, lowered intraspinal pressures or diminished posterior fossa volumes (e.g. basilar invagination)
It is often an isolated hindbrain abnormality of little consequence
It is not related to the Chiari II or III malformations

Clinical presentation

There are usually no symptoms during childhood unless there is an associated syringomyelia or hydrocephalus
Clinical symptoms are more likely when there is >5 mm of descent below the foramen magnum (children between 5 and 15 years can have normal tonsillar descent of up to 6 mm)
Symptoms may include a cough-induced headache, cranial nerve palsies and a disassociated peripheral anaesthesia

Radiological features

MRI

It is usually associated with ‘peg-like’ cerebellar tonsils ▸ an elongated medulla oblongata can be seen with a kink sometimes forming on its posterior surface

Associations

Syringohydromyelia (50%) ▸ basilar invagination (30%) ▸ hydrocephalus (25%) ▸ Klippel–Feil anomaly (10%)
It is not associated with a myelomeningocele

Chiari Ii Malformation

Definition

A congenital malformation of the hindbrain (with a dysplastic cerebellum) that is almost always associated with a neural tube defect (usually a lumbosacral myelomeningocele)
The inferior vermis is everted (rather than inverted) so that the nodulus becomes its most inferior aspect and the 4 ^th ventricle is reduced to a coronal cleft (the cerebellar herniation consists mainly of the cerebellar vermis) ▸ the medulla is invariably elongated and kinked

Clinical presentation

Affected children usually present with hydrocephalus following repair of a myelomeningocele after birth
Other symptoms: upper airway problems ▸ feeding problems ▸ dysphagia

Radiological features

CT/MRI

Inferior displacement of the cerebellum, pons, medulla oblongata and cervical cord ▸ medullary kinking ▸ a small slit-like 4 ^th ventricle (which is inferiorly displaced and elongated) and a small posterior fossa ▸ scalloping of the clivus ▸ flattening of the ventral pons and a low attachment of the tentorium ▸ the falx is partially absent or fenestrated with consequent interdigitation of the gyri across the midline ▸ the foramen magnum is enlarged and ‘shield-shaped’

‘Tectal beaking’: this follows fusion of the midbrain colliculi into a single beak pointing posteriorly
‘Towering cerebellum’: the tentorial incisura is enlarged and the cerebellum herniates superiorly into the supratentorial space
‘Batwing’ configuration of the frontal horns (coronal view): this is due to impressions from prominent caudate nuclei
‘Hourglass ventricle’: a small biconcave 3 ^rd ventricle due to a large massa intermedia
‘Cervicomedullary kink’: herniation of the medulla posterior to the spinal cord
‘Banana’ sign: the cerebellum is wrapped around the posterior brainstem (seen during obstetric US)

Associations

Lacunar membranous skull dysraphism (‘luckenschadel’) ▸ disorders of neuronal migration ▸ malformation of the corpus callosum ▸ a dorsal midline cyst ▸ absence of the septum pellucidum ▸ colpocephaly (occipital horn enlargement)

Complications

Hydrocephalus ▸ an isolated 4 ^th ventricle ▸ hydro-syringomyelia ▸ compression of the craniocervical junction

Chiari Iii Malformation

Definition

This is the most severe abnormality (and unrelated to the Chiari II or III malformations)
It consists of a high cervical or low occipital meningoencephalocele (in addition to an intracranial Chiari II malformation)

Chiari Iv Malformation

Definition

Extremely rare severe cerebellar hypoplasia

Chiari II malformation. (A) Imaging through the level of the midbrain reveals tectal beaking (arrow) with the cerebellum wrapping around the posterolateral aspect of the midbrain. The cerebellum is towering between the leaves of the tentorium. (B) Imaging through the supraventricular region shows interdigitation of the gyri (arrows) with hypoplasia of the falx. +

Chiari II malformation. (A) The posterior fossa is enlarged and ‘shield shaped’. The 4 th ventricle is small and slit-like. (B) The cerebellum towers superiorly through the tentorium and there is interdigitation of a cerebral gyrus through the fenestrated falx (arrow). *

Chiari III malformation. T1WI shows downward herniation of the cerebellar tonsils associated with an occipital encephalocele (arrow). There is associated agenesis of the corpus callosum and maximal hydrocephalus. +

Chiari II malformation with tectal beaking, a cord syrinx and a treated myelomeningocele. +

Chiari II malformation. The 4 th ventricle, which should normally be small and slit-like in this condition, is enlarged, indicating hydrocephalus. There is cascading tonsillar tissue herniating through the foramen magnum (white arrow). Beaking of the tectal plate is also seen (black arrow), as well as a cervical spinal cord syringomyelic cavity. *

Cerebral Malformations

Disorders of Dorsal Induction

Definition

These are malformations considered to be consequences of abnormalities of dorsal induction (also including Chiari II malformations)

Anencephaly

The most common fetal cerebral malformation which is incompatible with life ▸ there is no cerebral cortex present (unlike gross hydrocephalus)

Cephalocele

An extracranial protrusion of intracranial structures through a congenital defect of the skull and dura mater ▸ unlike a spinal myelomeningocele there is usually no skin defect ▸ they tend to occur in the occipital and frontal regions and may be pulsatile ▸ mainly midline
Meningocele: containing leptomeninges and CSF only
Encephalocele: containing leptomeninges, CSF and neural tissue
Encephalocystocele: containing leptomeninges, CSF, neural tissue and part of the ventricle

Radiological Features

MRI

This can establish: the presence of any neural tissue ▸ other intracranial malformations or hydrocephalus ▸ whether there is any ischaemia within the herniated neural tissue

Parieto-occipital cephalocele with herniation of the brain and meninges through a calvarial defect. Most of the herniated component is in the form of a cerebrospinal fluid-containing meningocele. *

Disorders of Ventral Induction

Definition

These are malformations following the formation of the neural tube

Holoprosencephaly

A midline malformation of ventral induction of the anterior brain, skull and face (resulting from the failure of the embryonic prosencephalon to undergo segmentation and cleavage into two separate cerebral hemispheres) ▸ it is associated with chromosomal abnormalities, facial clefting and various teratogenic factors (including maternal diabetes)

Holoprosencephaly

	Alobar form Severe form (often fatal)	Semilobar form Intermediate form	Lobar form Mild form
*Cleavage into two hemispheres*	None (a ‘cup’-shaped brain)	Partial posterior cleaving	Complete
*Facial abnormalities (e.g. cyclopia* ¹ *and hypotelorism* ² )	Severe	Intermediate	None
*Lateral ventricles*	U-shaped monoventricle with a dorsal cyst	Partial anterior fusion (with partial occipital and temporal horns)	Normal (the frontal horns may be ‘squared’)
*Falx and corpus callosum*	Absent	Absent anteriorly	Normal (they may be incomplete or dysplastic)
*Thalami*	Fused	Partial separation	Normal
*Septum pellucidum*	Absent	Absent	Absent

a single eye the eyes are too close together

Malformations of Commissural and Related Structures

Definition

Agenesis of the septum pellucidum

This may be associated with septo-optic dysplasia, agenesis of the corpus callosum, holoprosencephaly, Chiari II malformation, schizencephaly and other migration disorders

Septo-optic dysplasia (de Morsier's syndrome)

A triad of: (1) hypopituitarism, (2) hypoplasia of the optic nerves, (3) absence of the septum pellucidum (the frontal horns have a typical ‘box-like’ configuration)

Agenesis or dysgenesis of the corpus callosum

Fibres that normally travel across the corpus callosum now run in longitudinal bundles along the medial walls of the lateral ventricles (the bundles of Probst)

Associations

Chiari II malformation ▸ Dandy–Walker malformation ▸ interhemispheric lipoma ▸ abnormalities of neuronal migration and organization ▸ dysraphic anomalies ▸ encephaloceles ▸ septo-optic dysplasia ▸ ocular anomalies ▸ midline facial anomalies

Radiological features

MRI

Parallel widely spaced lateral ventricles with the 3 ^rd ventricle elevated and seen between them (the interhemispheric cyst frequently seen is thought to originate from the 3 ^rd ventricle but has lost continuity with this structure) ▸ vertically oriented sulci extend right down to the ventricle with no horizontally running cingulate sulcus ▸ small frontal horns (‘bull's horn’ appearance) with colpocephaly (large occipital horns)

Pearl

The anterior part (the posterior genu and anterior body) of the corpus callosum is formed before the posterior part (the posterior body and splenium) ▸ thus a small or absent genu or body, with an intact splenium and rostrum, indicates secondary destruction rather than abnormal development

Callosal agenesis. (A) Axial T2WI shows separated ventricles with a parallel orientation. The superior part of the 3 rd ventricle is just seen. (B) Sagittal T1WI through the midline confirms callosal agenesis. There is no cingulate sulcus and the vertically oriented cerebral sulci extend right down to the 3 rd ventricle. This finding is associated with other midline anomalies such as a fronto-ethmoidal cephalocele (arrow). (C) The optic chiasm is absent. *

Malformations of Neuronal Migration and Cortical Organization

Schizencephaly

Definition

A cleft lined by grey matter and leptomeninges (cf. a transmantle infarction in which the defect is lined by white matter) ▸ it involves the complete cerebral mantle and connects the calvarium and the outer surface of the brain with the lateral ventricles

Location

It is typically central (involving the pre- and postcentral gyri) ▸ it is also found within the parasagittal, frontal or occipital sites (with mild clinical manifestations)

Clinical presentation

There are variable features depending on the site and size involved: severe seizures ▸ spasticity ▸ severe mental and psychomotor developmental delay (with bilateral clefts)

CT/MRI

The adjacent thickened cortex demonstrates polymicrogyria ▸ it is associated with subependymal heterotopias (within the contralateral hemisphere) and subependymal or parenchymal calcification

‘Open lip’: a wide open defect
‘Closed lip’: if the cleft is closed but lined with grey matter entirely into the ventricle

Lissencephaly–Agyria–Pachygyria

Lissencephaly (a ‘smooth’ brain) (type 1)

Very few or no gyri are present
Opercularization (development of the sylvian fissures) is abnormal with shallow sylvian fissures ▸ it is associated with agenesis or hypoplasia of the corpus callosum and septum pellucidum

Cobblestone lissencephaly (type 2)

This follows neuronal overmigration with thick meninges adherent to the smooth cortical surface ▸ heterotopias are prominent and there is often a delay in myelination ▸ it may be seen in congenital muscular dystrophies ▸ posterior fossa usually abnormal

Agyria (complete lissencephaly)

Total absence of the gyri and sulci ▸ posterior fossa spared
There are wide and vertically orientated sylvian fissures

Pachygyria (incomplete lissencephaly)

The gyri are relatively few and unusually broad and flat
There is an AP gradation of gyral development

Grey Matter Heterotopias

Definition

Grey matter found in an abnormal position anywhere from subependymal layer to cortical surface

MRI

This is isointense with cortical grey matter demonstrated on all imaging sequences ▸ T1WI + Gad: there is no enhancement

Subependymal heterotopia

This is smooth and ovoid with its long axis typically parallel to the ventricular wall

Subependymal hamartomas seen in tuberous sclerosis

These are irregular and have their long axis perpendicular to the ventricular wall ▸ they are also more heterogeneous and may enhance

Focal subcortical heterotopia

The overlying cortex is thin with shallow sulci and it is associated with variable motor and intellectual impairment ▸ it may coexist with schizencephaly, microcephaly, polymicrogyria, dysgenesis of the corpus callosum, or absence of the septum pellucidum

Band heterotopia or ‘double cortex’

This is located parallel to the ventricular wall and is seen as a homogeneous band of grey matter between the lateral ventricle and the cerebral cortex (separated from both by a layer of white matter) ▸ the overlying cortex is usually of normal thickness but has shallow sulci ▸ partial heterotopias predominantly affect frontal lobes
It is commonly seen in girls with variable developmental delay or seizures

Polymicrogyria

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