What Causes Congenital Heart Defects?

Despite much research, in the majority of cases, Congenital Heart Defects (CHD) occur without any known cause. This is one reason the incidence has remained fairly constant at 7 to 8 cases per 1000 live births over decades, while other forms of heart disease have been declining in frequency.

We know the causes of some forms of congenital heart defects (CHD).

Environmental Insults

During pregnancy, some events are known to be associated with an increased risk for CHD.

Rubella infection: This viral infection, also called German Measles, can cause a syndrome of deafness, heart defects, cataracts and microcephaly. The heart defects most often seen are PDA, PS and ASD.

Thalidomide ingestion: Introduced as a drug for morning sickness, this medicine unleashed a horror when fetuses with gross malformations were delivered months later. The drug can cause major limb defects and a spectrum of heart birth defects too.

Lithium therapy: Used in the treatment of mania and a few other behavioral disturbances, lithium can cause fetal tricuspid valve anomalies if taken during pregnancy.

Fetal alcohol syndrome: A well-documented syndrome noticed in fetuses of women who abuse alcohol during pregnancy includes microcephaly, microphthalmia, growth retardation, developmental delay and heart defects. VSD is the most often seen.

Maternal Lupus Erythematosus: SLE in the mother has been associated with heart blocks in the fetus.

Others: In animal experiments, a variety of other factors have been incriminated in causing CHD. They include:

– hypoxia
– deficiency or excess of several vitamins
– drugs of various categories
– ionizing radiation

Genetic Factors Causing Congenital Heart Defects

Genetic aspects of CHD are an area of intense research and study.

Single gene mutations may be causative in familial forms of ASD, MVPS, VSD, congenital heart block, pulmonary hypertension and syndromes like Noonan, LEOPARD, Ellis-van Creveld and Kartagener.

Genes responsible for conditions like Long QT syndrome and Holt Oram syndrome have been mapped. Those likely to cause other conditions including hypertrophic cardiomyopathy, Marfan syndrome and supravalvular AS have been identified.

Chromosome 22 has been implicated in the development of conotruncal malformations.

However, less than 10% of all heart defects can be accounted for by chromosomal and genetic disorders.

Clinical Implications 

* Family studies indicate a 2 to 10 fold increase in incidence of CHD in siblings of affected patients, or in offspring of an affected parent.

* Because the incidence in siblings or offspring is only 2 to 10%, it is seldom wise to discourage the parents of an affected child from having additional children if either parent is free from a cardiovascular defect.

* This low possibility, combined with increasing possibilities for effective treatment for most cardiac defects usually justifies a positive approach to family counseling.

* If two or more members of the family are affected, the recurrence risk is high. A pedigree should be obtained and mendelian inheritance patterns figured out before any recommendation can be made.

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