Generally, causal factors of hypoplastic right heart syndrome (HRHS) are not identifiable. Medical science cannot tie the condition to genetic mutations or inheritance, though a very marginal risk of having more than one child with HRHS exists. Getting insufficient folic acid or taking some medications or substances are linked to a greater risk of fetal heart defects, including hypoplastic right heart syndrome. A few multi-system genetic disorders also may have a higher HRHS incidence.
In fetal development, hypoplastic right heart syndrome occurs as a result of improper development of the pulmonary valve and artery. When these are underdeveloped, they don't allow proper circulation of blood through the right side of the heart. This circulation is needed to mature the heart's structures.
The result is an undersized right ventricle, and a tricuspid valve and atrium that are too small. While still in utero the fetus gets oxygen from the mother, and this defect is often survivable. Severe problems occur after birth because the heart isn't able to support blood circulation to the lungs, and the body becomes oxygen-deprived quickly.
HRHS is not presently curable, though a staged series of surgeries or a heart transplant are options. Survival well into adulthood is now expected for many babies born with this condition, but the actual defects to the heart aren't reparable. More research on surgical repair is warranted, but understanding what causes hypoplastic right heart syndrome would be helpful, too.
Presently, there has been no consistent identification of a set of genes that affects the pulmonary valve and fetal right heart development. A few conditions with a genetic basis occasionally, but don't always, produce HRHS. These include Ivemark's syndrome, which has related spleen disorders, dextrocardia, and a variety of right-sided defects. Down syndrome is associated with heart defects, and may cause hypoplastic right heart syndrome in some, but not all, individuals.
A number of medications and substances have been identified as potentially causing hypoplastic right heart syndrome and other heart defects. Many of the selected serotonin reuptake inhibitors and serotonin norepinephrine reuptake inhibitors have been reliably linked to increased heart defects in children. Mood stabilizers like valproic acid, carbamazepine, and lithium have similar risks. Cocaine and amphetamines also appear to cause heart defects and other birth defects.
Lack of appropriate folic acid levels has been linked to a variety of birth defects for some time. It now appears that insufficient levels may make it more likely that conditions like hypoplastic right heart syndrome or other heart defects will occur. Ideally, folic acid use should begin prior to conception to reduce this risk.
Parents with kids with HRHS are often troubled because there is no explanation for these serious and dangerous defects. A mother may do "everything right" and still have a baby with hypoplastic right heart syndrome. While climbing survival rates encourage families, they may remain discouraged that researchers cannot reliably identify a clear cause.