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[Prevalence and diagnosis of neural tube malformations in Iceland 1972-1991.].

OBJECTIVE
The incidence and types of central nervous system (CNS) defects diagnosed in fetuses and newborns over a 20-year period, 1972-1991, were investigated. Concurrently the effect of prenatal ultrasound screening on the rates of delivered babies with neural tube defects were evaluated.


MATERIAL AND METHOD
All CNS defects diagnosed in newborn babies and in miscarried or legally aborted fetuses in Iceland during the study period were searched for using the Icelandic Birth and Abortion Registries. For ascertainment maternity case records, hospital records and autopsy and pathology reports were scrutinized. The number and types of defects were noted for each year and the prevalence evaluated for five- and 10-year periods as well as for the time after introduction of organised obstetric ultrasound screening.


RESULTS
There was considerable yearly variation in the incidence of CNS defects. In the first five-year period no CNS defect was diagnosed prenatally, but thereafter the number of cases diagnosed before birth rose, partly because of improved identification of more subtle defects. In spite of an increase in the number of cases to 2.2/1000 births, the incidence of babies born with CNS defects was more than halved to 0.8/1000 births. Organised nationwide ultrasound screening in and after 1984 had a marked lowering effect on the number of births with these defects. In 60% of cases the diagnosis was obtained before birth, most often before 22 weeks gestation. Most severe malformations were diagnosed, except for spina bifida, where after 1984 only 40% were diagnosed early enough to allow termination of pregnancy.


CONCLUSION
The incidence of CNS defects has risen, probably because of improved recognition of these malformations during pregnancy and to some extent after birth. Organised obstetric ultrasound allowed diagnosis of most severe defects, although 1/3 of defects leading to severe handicap went undiagnosed until after birth, particularly in smaller hospitals and health centers outside Reykjavik. Improving the diagnostic possibilities through training of staff handling obstetric screening is important, but the introduction of nationwide biochemical screening must also be considered.