The general term, spinal dysraphism is used to refer to open and closed defects, which can be induced experimentally. An alternative mechanism of rupture of the neural tube has been proposed, and this occurs at a slightly later time in development. Anencephaly is absence of the cranial vault and brain and is incompatible with life. Encephalocoele is protrusion of the cranial contents and is associated with high perinatal mortality or severe physical and intellectual disability in survivors. Meningomyelocoele is a cystic protrusion of the spinal cord and results in defective function of the cord below the level of the lesion with paraplegia and incontinence.
Associated hydrocephalus causes specific learning difficulties.
Incidence rates for these abnormalities have varied throughout the ages. There are world-wide trends, and strange parallels have been noted in seasonal peaks and sex ratios. Perinatal mortality figures for New South Wales are available from 1958 to 1990 and show a downward trend which is partly due to true decrease in incidence which has been observed world-wide, and partly to changes in diagnostic procedures which identify cases before 20 weeks of pregnancy, thus facilitating early termination which is not notifiable to Australian Bureau of Statistics. True incidence figures from multiple data sources are available from 1965 to 1990, and show a less marked downward trend. Epidemiological studies have shown variations by race, country, area, social class, and parental occupation.