Chapter 162. Malformations of The Brain - Review of Medical Embryology Book

THE TELENCEPHALON FAILS TO SUBDIVIDE into 2 hemispheres and has a single cavity which communicates with the third ventricle by a single opening. Since the neural groove is closed, the cranial vault and skin are normal. However, there is only 1 median eye (cyclopia), usually no hypophysis, and the nasal apparatus is more or less atrophied and often replaced by a trunk with a single median opening instead of 2 nasal openings
THIS MALFORMATION is the result of a defective induction of the prochordal plate

In humans, a chromosomal defect has been suggested as a possible cause
It has been produced experimentally by excision of the prochordal plate, x-rays, and hypervitaminosis A

ETHMOCEPHALY usually results in death; however, some children with minor types of malformations have been known to live for several years

Arrhinencephaly falls into the general category of ethmocephaly and is characterized by dysgenesis of the rhinencephalon and a general atrophy of the telencephalon

MALFORMATIONS OF THE INTERMAXILLARY SEGMENT, whose development is related to that of the rhinencephalon, are also seen and result in

Cleft plate
Agenesis of the nasal septum with a median union of the nostrils and nasal fossa
Hypotelorism

HYPOGENESIS or absence of the rhinencephalon is due to a disorder of the olfactory placodes which fail to produce fibers or induce the formation of the olfactory bulbs

Another cause is a genetic defect or defective induction of the prochordal plate

IT CAN BE THE RESULT of a genetic abnormality, pelvic x-rays during pregnancy, or toxoplasmosis, just to name a few
DEVELOPMENTAL ARREST takes place at late stages of gestation and may involve problems of cellular multiplication or migration. Cellular densities may be abnormally low, or the cortical layers may be less numerous than normal
THIS CONDITION usually is accompanied by mental deficiency and convulsions

IN PORENCEPHALY (collective term for variety of cerebral defects involving cortical tissue) a lateral ventricle communicates with the subarachnoid spaces or cortical tissue

These anomalies are probably the result of a defect of vascularization and often result in debility, convulsions, and cerebral motor disability

AGENESIS OF THE CORPUS CALLOSUM
HEMISPHERIC OR CEREBELLAR AGENESIS OR HYPOGENESIS is rarely seen and is probably also caused by early vascular problems

ROSETTES are small, accessory paraependymal cavities which may or may not be connected to the main cavity. They are surrounded by a germinal layer which can give rise to nervous cells

These disorders vary according to localization and have been produced experimentally by x-rays or vitamin deficiencies

CONGENITAL MYXEDEMA: thyroid deficiency can result in defective cellular migration, resulting in heterotopias that cause problems

Anomalies of the cortical surface are always accompanied by debility or idiocy and are of unknown pathogenesis. They may include lissencephaly or absence of cortical convolutions and micro- or macrogyria (too small or too large convolutions)