Chapter 161. Malformations of The Brain - Review of Medical Embryology Book

Introduction: malformations of the brain occur in about 0.51% of live births and in about 3.0% of stillbirths. In females, malformations of the nervous system are more numerous than in other organ systems, whereas in males, the incidence of malformations of the nervous system fall somewhat between those involving the cardiac and digestive systems

Exogenous causes: nutritional, physical (especially x-rays), viral, parasitic, chemical, and medications
Endogenous causes: hereditary (often poorly defined in some cases)
Interaction of exogenous and endogenous causes: these have been particularly shown in experimental animals and are a result of differences in sensitivity to exogenous factors as a result of species and animal strain

TERATOGENIC FACTORS can exert their influence for a very long time since brain development takes place over an extended period of time

Malformations of squamous occipital bone: the most frequently affected bone of the skull is the squamous occipital, which may be partly or totally missing and the resultant opening confluent with the foramen magnum

IF THE SKULL OPENING IS SMALL, only meninges bulge through forming a meningocele
IF THE DEFECT IS LARGE, part of the brain also bulges, forming a meningoencephalocele
IF THE DEFECT IS VERY LARGE, part of the brain and even ventricle may penetrate through the opening into the meningeal sac to form a meningohydroencephalocele

ANENCEPHALY: common abnormality (1/1,000); 4 times more frequent in the female and 4 times more frequent in whites; results from a failure of the cephalic part of the neural groove to close in the brain. It is a disorder of early embryogenesis and is caused by defective induction of the prochordal plate or the parachordal mesoderm or poor receptivity of the competent neural plate

The overall brain structure is disturbed, and at birth the brain is a mass of degenerated tissue exposed to the surface
The nervous tissue, as in typical spina bifida, is not covered with bone or skin since the cranial vault differentiates only under the influence of a normal neural tube
The skin is in continuity with the nervous tissue, as in the neural groove stage
Normal histogenesis takes place until the beginning of neocortical differentiation, but several weeks before term, vascular problems lead to general necrosis so that only choroid plexus, some nerves, and degenerating nervous tissue persist
Anencephaly results in neonatal death, can be recognized by x-rays since the skull vault is missing, and in the last 2 months of pregnancy is characterized by hydramnios since the fetus lacks the control mechanism for swallowing
Craniorachischisis is a particularly extreme form of anencephaly. The neural groove remains open throughout its length

ENCEPHALOCELE is the most serious type of defect involving closure of the encephalic neural groove. Part of the brain is herniated under the skin through a hole in the skull. It is most often untreatable
MENINGOCELE is a hernia of the meninges, is compatible with life, and can be treated surgically. As a result of its origin, it is usually midline

Localization of the groove defect enables the bone to form normally in its lateral parts. Similarly, the small extent of the opening allows the nervous tissue and the skin at the edges to join more or less at a late stage; however, the skin is always abnormal, fine parchmentlike, and hairless. In some cases, the abnormality of closure is evident
Meningoceles have been produced experimentally by x-rays, anoxia, nutritional deficiencies, hypervitaminosis A, drugs, et