Neural tube closure disorders Disorders of closure of rostral neural pore Disorders of closureof caudal neural pore
Cranial defects Meningocele, meningoencephalocele, and meningohydroencephalocele are all caused by an ossification defect in the bones of the skull. The most frequently affected bone is the squamous part of the occipital bone, which may be partially or totally lacking. If the opening of the occipital bone is small, only meninges bulge through it (meningocele), but if the defect is large, part of the brain and even part of the ventricle may penetrate through the opening into the meningeal sac
The latter two malformations are known as meningoencephalocele and meningohydroencephalocele
Anencephaly Exencephaly is characterized by failure of the cephalic part of the neural tube to close. As a result, the vault of the skull does not form, leaving the malformed brain exposed. Later this tissue degenerates, leaving a mass of necrotic tissue. This defect is called anencephaly, although the brainstem remains intact. Since the fetus lacks the mechanism for swallowing, the last 2 months of pregnancy are characterized by hydramnios. The abnormality can be recognized on a radiograph, since the vault of the skull is absent.
Defects of caudal neuropore Neural tube defects (NTDs), may involve the meninges, vertebrae, muscles, and skin. Spina bifida is a general term for NTDs affecting the spinal region. It consists of a splitting of the vertebral arches and may or may not involve underlying neural tissue. Two different types of spina bifida occur:
◦ Spina bifida occulta is a defect in the vertebral arches that is covered by skin and usually does not involve underlying neural tissue. It occurs in the lumbosacral region (L4 to S1) and is usually marked by a patch of hair overlying the affected region.◦ Spina bifida cystica is a severe NTD in which neural tissue and/or meninges protrude through a defect in the vertebral arches and skin to form a cyst like sac.
Occasionally the neural folds do not elevate but remain as a flattened mass of neural tissue (spina bifida with myeloschisis or rachischisis).
Cerebral cleavage and neuralmigration defects Child with delayed devolopment and seizures Especially when child is dysmorphic MRI is the investigaton of choice After the imaging we have to see the MR in a orderly fashion ◦ Midline structures ◦ Cerebral cortex and cortico white matter junctions ◦ White matter ◦ Basal ganglia and ventricular system and posterior fossa structures
Midline structures Cerebral commisures are the most common anomolies Hypothalamus and pituatary Midline leptomeninges Large csf spaces in posterior fossa
Cerebral cortex Is 2-3mm thick Too thick polymicrogyria and pachygyria Greywhite matter junction irregular polymicrogyria or cobble stone cortex Abnormally thin perinatal or ishaemic injury
White matter Myelination appropriate or not Diffuse layer of hypomyelination or amyelination associated heterotopia or polymicrogyria suggestive of CMV Absent myelination may be localised to a gyrus or may extend towards the ependymal layer transmantle sign characteristic of FCD
Posterior fossa Brain stem and cerebellum 4 th V and vermis Size of the pons with cerebellum
Callosal dysgenesis Partial or complete absence corpus callosum and hippocampal commisures Atrium or occipital horn dilatation colpocephaly Probst bundles Vertical or posterior course ACAs Most common presentation seizures devolopmental delay and cranial deformity and hypertelorism
HVR ◦ Variable vermian hypoplasia BPC ◦ Open 4th V communicates with cyst MCM ◦ Enlarged pericerebellar cisterns communicate with sub arachnoidspace ◦ Vermis and 4 th V normal
Rhombencephalosynapsis Etiology ◦ Unknown: 2 major theories Failure of vermian differentiation Vermian agenesis allowing hemisphere continuity Features ◦ Congenital continuity (lack of division) of cerebellar hemispheres ◦ Usually with fusion of dentate nuclei and superior cerebellar peduncles
◦ Small, single hemisphere cerebellum with continuous white matter (WM) tracts crossing midline ◦ Diamond or keyhole-shaped 4th ventricle ◦ Absent primary fissure Clinical features ◦ Variable neurological signs ◦ Ataxia, gait abnormalities, seizures ◦ Developmental delay ◦ RES discovered in near-normal patients at autopsy
Molar Tooth Malformations(Joubert) Hindbrain anomaly characterized by dysmorphic vermis, lack of decussation of superior cerebellar peduncle, central pontine tracts, corticospinal tracts "Molar tooth" appearance of midbrain on axial images Etiology ◦ result from mutations of ciliary/centrosomal proteins that can affect cell migration, axonal pathway
Clinical features ◦ Most common signs/symptoms: Ataxia, developmental delay, oculomotor and respiratory abnormalities
Holoprosencephaly Features ◦ Failure to delineate normal prosencephalic midline with absent/incomplete hemispheric and basal cleavage ◦ Single ventricle ◦ Azygous ACA ◦ associated facial defects
Clinical features Most common signs/symptoms ◦ Facial malformation (hypotelorism +++) ◦ Seizures and developmental delays ◦ Hypothalamic/pituitary malfunction (75%, mostly diabetes insipidus), poor body temperature regulation ◦ Dystonia and hypotonia: Severity correlates with degree of BG nonseparation
Heterotopic Gray Matter Arrested/disrupted migration of groups of neurons from periventricular germinal zone (GZ) to cortex Ectopic nodule or ribbon, isointense with gray matter (GM) on every MR sequence Periventricular, subcortical/transcerebr al, molecular layer Band heterotopia
Lissencephaly Features ◦ Disorders of cortical formation caused by arrested neuronal migration, resulting in thick 4-layer cortex and smooth brain surface ◦ "Hourglass" or "figure eight" shape of cerebral hemispheres ◦ 3 layers Outer cellular layer → may be relatively thin, smooth Intervening cell-sparse layer Deeper thick layer of arrested neurons mimicking band heterotopia
Schizencephaly Features ◦ Transmantle gray matter lining clefts ◦ Ca++ when associated with CMV Pathology ◦ Can be result of acquired in utero insult affecting neuronal migration ◦ Infection (CMV), vascular insult, maternal trauma, toxin Types ◦ Closed lip ◦ Open lip
Hemimegalencephaly Features ◦ Hamartomatous overgrowth of part/all of hemisphere ◦ Abnormal proliferation, migration, and differentiation of neurons ◦ Embryology Insult to developing brain causes development of too many synapses, persistence of supernumerary axons, and potential for white matter overgrowth Localized epidermal growth factor (EGF) in cortical neurons and glial cells may lead to excessive proliferation
◦ Large cerebral hemisphere, hemicranium◦ Posterior falx and occipital pole "swing" to contralateral side◦ Lateral ventricle is large with abnormally shaped frontal horn
Polymicrogyria Features ◦ Malformation due to abnormality in late neuronal migration and cortical organization ◦ Result is cortex containing multiple small sulci that often appear fused on gross pathology and imaging ◦ Neurons reach cortex but distribute abnormally forming multiple small undulating gyri
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