The term "neural tube defects" refers to a biologically interrelated series of malformations of the central nervous system and overlying tissues. There is an abnormality in morphogenesis of the brain and/or spinal cord, associated with varying degrees of absence or malformation of the meninges, bone, muscle, and skin. In addition to morphologic similarities, these lesions are related epidemiologically and probably genetically.
The terminology is complex, but the lesions can be placed in a simple framework. The Table outlines the most common defects.
The malformation occurs early in embryonic development, since the gross structure of the central nervous system has been completed by the 26th gestational day. There are numerous theories of pathogenesis and there is a good deal of subprimate teratologic research, but no single etiologic hypothesis is widely accepted or rigorously proven. Despite this, clinical risk factors have been clearly defined and empiric techniques for prenatal diagnosis have been developed.
FREQUENCY AND EPIDEMIOLOGY
Neural tube defects represent one of the most common groups of major congenital malformations. Each year approximately 6,000 to 8,000 infants are born with these conditions in the United States. Incidence rates vary among different groups from 0.6 to 4.1/1,000 total births for anencephaly. The risk is not uniformly spread over the entire population; a number of factors are related to initial occurrence and recurrence.