CONGENITAL CNS MALFORMATIONS
Congenital CNS malformations account for ~5-10% of all major malformations in live births.
Embryology: Development of CNS may be broadly divided into three steps (Fig. 18.3), as follows:
a.
Formation of neural tube: CNS develops from the primitive ectoderm, which forms a neural plate during 3rd-4th weeks of gestation, overlying the notocord. Central indentation of this plate leads to formation of neural groove flanked by ridges, i.e. neural folds, which later fuse together in midline to form the neural tube.b. Segmentation of neural tube: Cephalic portion of neural tube develops three saccular segments to form ventricular system and aqueduct, while walls of these dilatations develop into prosencephalon (cerebral
hemispheres), mesencephalon (pons and cerebellum) and Rhombencephalon (medulla). Caudal portion develops into spinal cord. Peripheral nerves, spinal/ autonomic ganglions, leptomeninges and lower cranial nerve ganglions develop from neural crest.
c. Migration of neuronal cells: Functional maturation of CNS involves development of primitive neuroepithelial cells in periventricular region, which then migrate outwards towards the surface of cortical hemispheres. Increasing cellular activity leads to development of sulci and gyri on cerebral surface, to increase the surface area of brain.
CNS malformations may develop either due to defective formation or segmentation of the neural tube or due to abnormalities in migration and maturation of neuronal cells (Table 18.24), some of whom are discussed here.
18.10.1
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