Congenital Malformation of the Brain

More than 2000 different congenital malformations of the brain have been described in the literature, and their incidence is reported to be about 1 percent of all live births.1 Since the congenital anomalies of the brain are commonly encountered in day to day practice, it is very important for every radiologist to be familiar with the basic imaging findings of common congenital anomalies to make a correct diagnosis necessary for optimum management of these conditions. Magnetic resonance imaging (MRI) is very useful in studying these malformations. The aim of this chapter is to provide an overview of all important and routinely encountered congenital malformations of the brain.


Introduction
More than 2000 different congenital malformations of the brain have been described in the literature, and their incidence is reported to be about 1 percent of all live births. 1 Since the congenital anomalies of the brain are commonly encountered in day to day practice, it is very important for every radiologist to be familiar with the basic imaging findings of common congenital anomalies to make a correct diagnosis necessary for optimum management of these conditions. Magnetic resonance imaging (MRI) is very useful in studying these malformations. The aim of this chapter is to provide an overview of all important and routinely encountered congenital malformations of the brain.

Normal brain development
Congenital anomalies of the brain are extremely complex and are best studied by correlating with embryological development. Basic events in normal brain development includes the following four stages: 1 Stage 1: Dorsal Induction: Formation and closure of the neural tube -Occurs at 3-5 weeks -Three phases: Neurulation, canalization, retrogressive differentiation -Failure: Neural tube defects (Anencephaly, Cephalocele, Chiari malformations) and Spinal dysraphic disorders.

Classification of congenital malformation of brain
A number of classification systems have been proposed, but with regards to our basic purpose, simplified classification of brain malformations has been taken into account, which is as follows: 1 -Complex anomaly involving skull, dura, brain, spine and the cord -Skull and dural involvement  Luckenschadel (lacunar skull), concave clivus and petrous ridges  Small and shallow posterior fossa with low lying transverse sinuses and torcular herophilli  Hypoplastic tentorium cerebelli with gaping (heart shaped) incisura  Hypoplastic, fenestrated falx cerebri with interdigitating gyri  Gaping foramen magnum - The herniated brain dysgenetic and non-functional -Absence or erosion of the crista galli with enlargement of foramen cecum is a constant feature of a nasal cephalocele. Fig. 4. Occipital cephalocele. Sagittal T1W (a) image shows herniation of severely dysplastic cerebellar tissue and the occipital lobe into a large CSF containing sac through an osseous defect in the occipital bone(thin white arrow). Thin strand of dysplastic brain tissue or septa can be seen traversing the CSF within the sac. Also note small posterior fossa, and deformed brain stem. 2D Time-of-flight venogram (b) demonstrates no herniation of dural venous sinuses in the cephalocele sac. This is important information for the surgeons.

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-Differential diagnosis of a nasal cephalocele includes congenital nasal masses (e.g. dermoid) where crista galli is present but split. -Antenatal ultrasound and MRI are useful in evaluation of content of the sac. MR venography is useful to assess major dural sinuses within the herniated sac, which may be responsible for major bleeding during surgery. CT is useful in demonstrating bony defect. -Associated anomalies  Chiari II and III malformation (seen with occipital cephalocele)  Corpus callosum agenesis, Dandy-Walker malformation (seen with parietal cephalocele)     image shows a small subcutaneous mass (thin white arrow) in high occipital region just external to a small defect in the calvarium. Note that the brain is not entering the cephalocele; instead, a thin strand of fibrous tissue is seen extending across the osseous defect, from the surface of the brain to the subcutaneous mass. Small posterior fossa arachnoid cyst is also seen. 2D TOF venogram (c) shows presence of median procencephalic vein within embryonic falcine sinus(thin white arrow) and absence of sagittal sinus.

Disorders of diverticulation and cleavage
Holoprosencephaly 1,2,5 ( Figure 14  Lissencephaly. Axial T1W image shows a complete smooth brain with thickened cortex and shallow sylvian fissures(arrows) giving the brain characteristic figure of eight appearance. Fig. 18. Lissencephaly with hemimegalencephaly. Axial (a) and coronal (b) T1W image of brain shows right sided hemimegalencephaly and lissencephaly. Right frontal lobe is particularly enlarged, has a disorganized, thickened, nearly agyric cortex with complete loss of cortico-medullary differentiation (arrow). The anterior interhemispheric fissure is displaced to the opposite side by the hypertrophied frontal lobe; ipsilateral frontal horn is also enlarged.  Heterotopias are isointense to normal gray matter in all pulse sequences and do not enhance on administration of intravenous contrast. They are best appreciated on medium tau inversion recovery sequences.  The differential diagnosis is subependymal nodules (SENs) of tuberous sclerosis. On MRI, SENs are not precisely isointense to gray matter, however, occasionally show enhancement after contrast administration. They are often calcified.  Large dysplastic and disorganized masses of ectopic gray matter may mimic intracranial mass, and produce severe deformity of ipsilateral ventricle.  Subcortical heterotopias are less frequent. -Band or laminar type  A layer of neurons interposed between the ventricle and cortex, seen as alternating layer of gray and white matter band - The cortex overlying the heterotopia is nearly always abnormal with pachygyria or polymicrogyria.  Severe form of open lip schizencephaly has an appearance which is called "basket brain".  Closest differential is porencephalic cyst in which CSF space is lined by gliotic white matter, in contrast to gray matter lined schizencephaly.
-Associated anomalies: heterotopias, septo-optic dysplasia, absence of septum pellucidum and callosal dysgenesis  MR shows enlargement of a part or whole of one cerebral hemisphere, ipsilateral ventricle is frequently dilated and the frontal horn is stretched. The cortex is affected by diffuse migration anomaly (polymicrogyria, pachygyria) and the underlying white matter is gliotic and dysmyelinated -Rarely, associated enlargement and dysplasia of ipsilateral cerebellar hemisphere and brain stem may be present, a condition referred as total hemimegalencephaly -Heterotopias may be present -Associated anomalies: Epidermal nevus syndrome, Klippel-Trenaunay-Weber syndrome, Neurofibromatosis type 1        MR image demonstrates a large laminated-appearing T2W/FLAIR hyperintense and T1weighted hypointense mass involving the right cerebellar hemisphere. Note gross thickening of the cerebellar folias (arrow). No perilesional edema present. However, mass effect on the fourth ventricle with moderate hydrocephalus can be seen. Proton MR spectroscopy (d) reveals normal metabolites peak.

Disorders of histogenesis
"Neurocutaneous syndromes" or "Phakomatoses" constitute a group of congenital malformations which are characterized by cutaneous lesions associated with CNS anomalies. Some of the common neurocutaneous syndromes are described below. -Also known as Von Recklinghausen disease or peripheral neurofibromatosis -Accounts for > 90% of all NF cases -Incidence = 1:2000 to 3000 live births -Diagnostic criteria: two or more of the following findings are present  Six or more café-au-lait spots(≥5mm in pre-pubertal children and ≥15mm in postpubertal period)  One plexiform neurofibroma or two or more neurofibromas of any type  Two or more pigmented iris hamartomas(Lisch nodules)  Optic nerve glioma  Axillary or inguinal freckling  Osseous lesions such as dysplasia of greater wing of sphenoid, pseudoarthrosis  First degree relative with NF-1 -CNS lesions present in 15-20% cases. These include  Optic nerve glioma (most common CNS lesion), may extend to involve the optic chiasma, optic tract, optic radiation and the lateral geniculate bodies.  Nonoptic gliomas may involve the brain stem, tectum, and periaqueductal region.  Plexiform neurofibroma is a hallmark of NF-1. It is an unencapsulated neurofibroma along the path of major cutaneous nerve of the scalp and neck, which commonly involves the first (ophthalmic) division of trigeminal nerve. It is often associated with dysplasia of sphenoid bone and bony orbit.  Non-neoplastic hamartomatous lesions (80%) of basal ganglia and white matter. Majority of lesions show no mass effect or contrast enhancement. These lesions may increase in size or number in early childhood, diminishes with age and rarely observed into adulthood.  Other intracranial lesions include astrocytic proliferation of the retina, intracranial aneurysms, vascular ectasia and a progressive cerebral arterial occlusion disease akin to moya-moya pattern.  Spinal lesions may include cord astrocytoma / hamartoma, dural ectasia and lateral/anterior intrathoracic meningoceles.  Skeletal dysplasias may include hypoplasia of sphenoid bone and bony orbit, kyphoscoliosis, scalloping of posterior aspect of the vertebral bodies

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-CNS lesions present in 100% cases. These include  Bilateral acoustic schwannomas, hallmark of NF-2  Schwannomas of other cranial nerves. Trigeminal nerve is next most frequently involved nerve, albeit, any cranial nerve may be affected (with the exception of the olfactory and optic nerves).  Meningiomas, often multiple  Choroid plexus calcification  Spinal lesions include cord ependymomas, meningiomas, or multilevel bulky schwannomas of exiting roots Fig. 33. Neurofibromatosis type 1: Opticochiasmatic-hypothalamic pilocytic astrocytomas. Axial T2-weighted (a) and coronal FLAIR (b) MR image shows enlargement of bilateral optic chiasma (thin black arrows) and ill-defined hyperintensity involving the hypothalamus(thin white arrow) and adjacent brain. Coronal T1-weighted post contrast image (c) demonstrates mild to moderate enhancement of the optic chiasma/hypothalamus but marked enhancement of the lesions involving the adjacent brain parenchyma. Moderate obstructive hydrocephalus is also present. FLAIR coronal images (d-f) of the same patient shows further extension of the optic pathway glioma to involve bilateral medial temporal lobes, basal ganglia region, mid brain and pons (thin white arrow). These lesions appear as ill-defined areas of high signal intensity on Flair images. The enlarged optic chiasma (thin black arrow) and obstructive hydrocephalus are also seen in these images.        . Right schwannoma appears as a large homogenous enhancing right CP angle mass with intracnalicular extension and the left one is seen as a small intracanalicular enhancing mass. Multiple meningiomas(thin black arrows) are also present seen as enhancing extra-axial masses in right medial temporal and bilateral frontal regions. Right optic nerve meningioma is also seen completely filling the intraconal space. Non-contrast sagittal T1W(d) and coronal T2W image(e) of whole spine of the same patient demonstrates low cervical region meningioma(d) and multiple rounded lumbar region nerve root schwannomas(e), best appreciated on MR myelogram(f).   Tram-track or gyriform pattern of cortical calcification underlying the leptomeningeal angioma is diagnostic of the syndrome. The calcification is unusual before two years of age. Calcifications are best seen on plain CT, T2W and GRE image.

Summary
Congenital malformations of the brain are both complex and multiple. The neuroradiologic diagnosis of such anomalies requires a basic understanding of normal brain development and pathogenesis. The aetiologies associated with development anomalies may result from a variety of insults from genetic to environmental. Abnormalities associated with the neural tube and the neural plate generally occur within the first 28 days of gestation. On the other hand, abnormalities associated with cellular proliferation and migration in the CNS generally occur after the 28th day of gestation. This chapter will cover malformations associated with both of these periods. Congenital anomalies of the brain are commonly encountered in day to day practice. Nevertheless, diagnosing it correctly is of paramount importance. Imaging plays an important role in reaching the correct diagnosis necessary for optimum management of these unfortunate conditions. It is as important for every radiologist to be familiar with basic imaging findings of common congenital anomalies, as it is for the paediatrician.