[Congenital heart defects in Icelandic twins.]

Several investigations have been published on the incidence of congenital heart defects (CHD), showing the incidence to be 0.8-1.1%. Few studies have dealt with the incidence of CHD in twins, but they indicate higher incidence of CHD among twins compared to singletons. The purpose of this study was to investigate the incidence of CHD and the different types of defects among twins born in Iceland over a 10 year period and compare the data to the incidence of CHD in the Icelandic population.

Information was obtained on all twins born alive in Iceland from 1986-1995 at the Icelandic Statistical Bureau. That list was compared to a list of all children born in Iceland over the same period and diagnosed with CHD, thus finding all twins with diagnosed CHD. The incidence of CHD for the period of 1986-1990 and 1991-1995 was studied separately. Causes of death, whether the patients needed treatment or not, and the results of treatment were studied. Chi-squared test for statistical analysis was used when appropriate.

Incidence: Of 1089 liveborn twins 35 had CHD or 3.21% compared to an incidence rate of 1.0% among singletons, which is a statistically significant difference (p<0.001). The incidence was 2.13% in the years 1986-1990 and 3.78% in the years 1991-1995 (p<0.005). Types of defects: Of 35 twins with CHD 20 had minor defects (no treatment needed) whereas 15 had major defects. Ventricular septal defects were the most common defects encountered, or 40%, atrial septal defects were 20%, patent ductus arteriosus 9%, complex defects 9% and tetralogy of Fallot 6%. Other defects were less common. Sex ratio: The male/female ratio was 0.591, whereas the ratio in the control population was 1.1. However, this did not reach statistical difference (p>0.05). Twin types: Of the twins with CHD 34% were male-female pairs, 26% were male-male pairs and 40% were female-female pairs. Age at diagnosis: Thirteen patients were diagnosed in the first week of life and 20 from one week to five months of age. After six months of age only two patients have been diag notnosed with CHD.

Twelve patients have been operated for CHD, one patient was treated in an interventional cardiac catheterization and 10 patients received medical treatment. Mortality: Six twins died as a consequence of their heart defect (17%). The mortality rate was significantly higher among twins with major CHD compared to the control population (p<0.005).

Congenital heart defects are more common among twins compared to the control group. They were also more common in the latter half of the investigation period. The mortality rate is also higher among the twins. In 1991 in titero fertilization was started in Iceland resulting in increased number of twins and at the same time increase in incidence of CHD is seen. Further studies are required to clarify this.