	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

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 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

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 > 5mm of descent below the foramen magnum (children between 5 and 15 years can have normal tonsillar descent of up to 6mm)

• 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 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 a small 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

CEREBRAL MALFORMATIONS

DISORDERS OF DORSAL INDUCTION

DEFINITION

• These are malformations related to the formation of the neural tube (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

• 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

DISORDERS OF VENTRAL INDUCTION

DEFINITION

• These are malformations related to 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)

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

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. cyclopia¹ 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