Ultrasound sends neurons down wrong path
Study raises questions over prenatal procedure.
The type of ultrasound used to scan babies in the womb disturbs brain cells in mouse fetuses, say researchers. The finding fuels a debate about the safety of the technique for unborn babies.
Babies in the womb are routinely scanned using high-frequency sound waves. The scans allow doctors to check on growth rates and spot developmental abnormalities.
Pasko Rakic of Yale Medical School in New Haven, Connecticut and his team were similarly scanning experimental mice, to help inject dye into embryos. When later studying the brain development of these mice, the team noticed that certain neurons in the growing cortex were not behaving normally.
Rakic discussed his preliminary results at the Society for Neuroscience meeting in 2004 (see ' Ultrasound scans accused of disrupting brain development'). But he says he wanted more results to be convinced now, 335 mice later, he is.
Rakic says that he has no evidence that ultrasound scanning disrupts the brains of human fetuses. The affected mice in his study were exposed to continuous ultrasound for 30 minutes or more; a baby's brain would be exposed for only a fraction of this time during a 30-minute scan of its entire body. And a narrow ultrasound beam will hit and affect far more of a small mouse brain than a larger human one.
Some studies have hinted that ultrasound causes subtle brain changes in humans that increases the chance of a child being left-handed or developing speech problems. Others suggest that the risk is minimal. A 2004 study found that fetuses exposed to five ultrasound sessions rather than one are more likely to grow poorly, but that this difference disappears after the first year1.
At the same time, Rakic says that it is sensible to avoid the lengthy ultrasound scans that some parents now choose in order to make videos of their baby. The US Food and Drug Administration and other medical groups also discourage unnecessary ultrasound scans.
Neurons are born deep inside an embryo's brain cortex and then migrate to the outer edge. This migration creates a series of parallel zones running through the cortex, like rock strata, each of which has specific functions in communicating to other areas of the brain.
These layers are vital for the processes directed by the cortex such as memory and language. In animals, alcohol, cocaine and viral infections can also disrupt the neurons' migration and affect behaviour.
Rakic marked some of these migrating neurons with a dye and then exposed the growing mice to between 5 minutes and 7 hours of ultrasound over the final three days of gestation, using a machine identical to that used for humans. He then examined the brains of the young mice a few days after birth.
In those exposed to 30 minutes or more of ultrasound, not all of the neurons destined for the outer layers of the cortex completed their journey. Instead, some remained scattered in interior layers of cortex, or even in neighbouring white matter. The more ultrasound the mice received, the more scattered their neurons. The results are reported in Proceedings of the National Academy of Sciences2.
The team also found that neuronal migration was disrupted in a group of animals that went through the experimental procedures for 7 hours, but were not exposed to ultrasound. This suggests that the stress caused by an extended period of handling could also affect the brains of a mother's babies. But stress can't explain all the results, says Rakic's team.
The scientists are not sure how the ultrasound waves disrupt neuron migration. One idea is that they make the neurons vibrate, preventing them from sticking to adjacent cells.
Rakic is repeating his experiments in monkeys, and looking for behavioural changes in the offspring. He says he does not yet have results.
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- Newnham J.P., et al. Lancet, 364. 2038 - 2044 (2004).
- Ang, E.S.B.C., et al. PNAS, http://www.pnas.org/cgi/doi/10.1073/pnas.0605294103 (2006).