CNS MALFORMATIONS EARLY BRAIN DEVELOPMENT, Exams of Neuroanatomy

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CNS MALFORMATIONS

EARLY BRAIN DEVELOPMENT

 (^) About the 15th day of life, ectodermal cells on the surface of the embryo proliferate to form a plate of tissue, the “ primitive streak ”. Rapidly proliferating group of cells, the “ Hensen’s node ” form at one end (cephalic end).  (^) From Hensen’s node, cells that form the notochord migrate rostrally and induce differentiation of the dorsal midline ectoderm into “ neuroectoderm ”.  (^) The plate like condensation of the neuroectoderm is the “ neural plate ”.  (^) At 17 days, lateral aspects of the neural plate thicken and neural folds bend medially, meeting in the midline around 20 days.  As the neural tube closes, the neuroectoderm which will form the CNS separates from the overlying ectoderm, which will become the skin.

CNS DEVELOPMENT

Primary neurulation: 3-4 gw

Prosencephalic development: 2-3 mo

Neuronal proliferation: 3-4mo

Neuronal migration: 3-5 mo

Organization: 5mo-yrs postnatal

Myelination: IU-years postnatal

NORMAL DEVELOPMENT

Primary neurulation (Dorsal induction) -

inductive events in the dorsal aspect of the

embryo resulting in the formation of brain and

spinal cord, excluding segments caudal to

lumbar region.

Secondary neurulation - exclusive to lower

sacral segments of the cord (caudal neural tube

formation) - occurs later

SECONDARY NEURULATION  Formation of caudal NT (lower sacral and coccygeal segments) occurs by sequential canalization (4-7gw) and retrogressive differentiation (7wks - birth+)  At 28-32 days, aggregate of undifferentiated cells at caudal end (caudal cell mass) develop vacuoles - enlarge - coalesce - make contact with central canal; accessory lumina may remain  (^) Remaining structures are ventriculus terminalis (in the conus) and filum terminale

DISORDERS OF INDUCTIVE EVENTS

IN ORDER OF DECREASING SEVERITY:

Craniorachischisis totalis

Anencephaly

Myeloschisis

Encephalocele

Myelomeningocele/Chiari malformation

ANENCEPHALY



Defect is failure of anterior neural tube

closure



Most severe case - defect from level of

lamina terminalis to foramen magnum

(holocrania/holoanencephaly); if defect

does not extend to foramen magnum -

meroacrania/meroanencephaly

ANENCEPHALY

 Most commonly - forebrain, variable amount of upper brainstem involved; exposed neural tissue represented by hemorrhagic, fibrotic, mass of neuroglial tissue - area cerebrovasculosa ; anterior pituitary present, posterior pituitary usually absent  Frontal bones above superciliary ridge, parietal bones, squamous part of occipital absent  (^) Onset no later than 24 days; polyhydramnios  (^) 75% SB; others can have brainstem function.

MECKEL’S SYNDROME

Constellation of occipital

encephalocele, microcephaly,

microphthalmia, cleft lip and palate,

polydactyly, polycystic kidney,

ambiguous genitalia

MYELOMENINGOCELE

Restricted failure of posterior neural

tube closure; 80% in lumbar area

(last area of neural tube to close)

Chiari II, hydrocephalus

Chiari malformation

 Squamous bones of the vault have patches of irregular thickness - craniolacunia; shape does not correspond to gyri, not secondary to >ICP, can disappear.  (^) Falx is short and fenestrated  (^) Shallow posterior fossa, low position of torcular, low insertion of tentorium, tightly crowded brainstem and cerebellum displaced caudally and impacted into the foramen magnum  (^) Herniated cerebellar tissue protrudes into the cervical spinal canal and overrides the dorsal surface of the cord as a peg. Herniation originates from caudal vermis - tissue firm, sclerotic. In others, tonsils may herniate with vermis.

Chiari malformation

 Deformation of the brainstem- caudal shift of dorsal medulla, the 4th ventricle, choroid plexus - dorsal nuclear groups thus much more caudal to the ventral surface. The tissue caudal to the ventricle containing the gracile and cuneate nuclei form a hump which overrides and compresses the cervical cord.  (^) Beaking of the tectum  (^) Cervical spinal roots project cranially instead of taking their usual lateral or descending course  Hydromyelia common in the cord  Hydrocephalus with redundant cortical gyri (polygyria, microgyria, stenogyria), but normal lamination

DANDY-WALKER MALFORMATION  Dandy-Walker malformation - enlarged posterior fossa, high position of the tentorium, hypogenesis or agenesis of the cerebellar vermis, cystic dilatation of the 4th ventricle that fills the posterior fossa, hydrocephalus  Most common anomaly associated is callosal hypogenesis  There may be dysplasias of the brainstem, especially abnormal inferior olives.

Malformations of the cerebellum  Joubert syndrome Agenesis/hypogenesis of the cerebellar vermis with midline clefting, abnormal eye movements, periodic hyperpnea, ataxia, mental retardation  (^) Rhombencephalosynapsis - small cerebellar hemispheres fused in midline with sulci running across, agenesis of vermis  (^) Lhermitte Duclos syndrome (dysplastic cerebellar gangliocytoma): Mass effect, focal area of enlarged cortex, abnormal neurons in granule cell layer, hypermyelinated marginal layer; can be associated with Cowden disease (multiple hamartoma syndrome, other malignancies)