As they grow up, children with heart defects often suffer from learning and attention problems, as well as other cognitive and motor troubles. Now, a team of doctors has used sophisticated imaging on newborns with heart disease and found delayed brain development akin to what's seen in premature babies--a phenomenon that helps explain why infants with heart problems are at such high risk of brain injury. The work sheds new light on an emerging challenge in treating children with congenital heart disease.
Heart surgery performed in the first weeks of life was initially eyed as the culprit behind learning, attention, and other cognitive problems. But a large study in Boston showed that even when a lower-risk form of cardiac bypass was used, babies still grew up to suffer cognitive deficits. Some studies have detected brain lesions or neurological abnormalities in newborns before heart surgery, but it wasn't clear whether these might account entirely for later cognitive deficits, either.
To learn more, pediatric neurologist Steven Miller, now at the University of British Columbia in Vancouver and the University of California, San Francisco (UCSF), and Patrick McQuillen, a UCSF pediatric critical care specialist, along with their colleagues, performed sophisticated brain-imaging tests on 41 babies with congenital heart defects. They relied on traditional magnetic resonance imaging (MRI); magnetic resonance spectroscopy, which shows brain metabolism; and diffusion tensor imaging, which shows the brain's microstructure. The results were compared with those from 16 healthy, full-term babies.
Fourteen of the babies with heart disease had a brain injury visible on MRI, something other studies have recorded too. But the sophisticated imaging efforts revealed that even babies without obvious brain problems before surgery had detectable abnormalities in their brain structure and metabolism, the team reports tomorrow in The New England Journal of Medicine. In particular, the researchers found lower levels of a marker, N-acetylaspartate, that's associated with mature brain cells.
In essence, says Miller, the brains of the babies with severe heart problems closely resembled those of infants born prematurely. In premature babies, the brain's white matter, the myelinated tissue that helps transmit nerve impulses, isn't mature and is much more susceptible to injury, likely accounting for the cognitive deficits these children grow up to suffer. And that, say Miller and McQuillen, is what seems to be happening in these babies with heart disease. MRIs performed after the infants' heart surgeries found nine new cases of white-matter injury, compared to four presurgery, suggesting that the babies were especially vulnerable to it.
The brain of a baby with heart defects "is susceptible to the same type of injury that the preterm or very-low-birth-weight baby might have," says William Mahle, a pediatric cardiologist at Emory University in Atlanta, Georgia, who has detailed white-matter injury before and after surgery in babies with heart defects. "We're moving to an understanding" that much of this happens before birth.
The next question, say physicians, is why. The heart defects studied by the UCSF group are known to cause the heart to send less-oxygenated blood to the brain during development than it would normally get. Miller and the UCSF team now hope to perform MRIs on women carrying fetuses with heart defects to better define when delays in brain development first arise.
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