I. General Considerations
The differentiation of the nervous system continues for a very long period of time. In fact, differentiating cells are still present in the cerebellum at approximately one year post-natal age. This long period of differentiation provides the justification for considering "developmental defects" as disorders of growth resulting from interruption of the orderly sequence of development at any stage by any agent or disease category. Developmental defects are not only "developmental" in origin, but can arise from a vascular, traumatic, metabolic, toxic, nutritional, neoplastic or infectious etiology. Etiologic agents can either lead to interruptions of normal development or result in tissue destruction.
The etiology is actually unknown in most cases. Factors known to be associated with central nervous system malformations include:
1. Chromosomal defects: The severity of malformations with the various chromosomal defects varies from case to case. In no instance is it really clear why specific abnormalities are associated with mental retardation
2. Certain infectious diseases are associated with specific sets of central nervous system malformations or disordered development. These include AIDS, syphilis, and the TORCH infections (toxoplasma, rubella, cytomegalovirus-CMV, herpes viruses) rubella, toxoplasmosis, syphilis, and cytomegalic inclusion virus.
3. Teratogenic drugs which induce developmental disorders include chemotherapeutic agents, medications such as anticonvulsant, and drugs of abuse (particularly alcohol - fetal alcohol syndrome).