Ultrasound scans are used routinely for important medical reasons, such as checking a baby’s heartbeat or identifying potential birth defects. They’re also used for trivial purposes, such as making a video of the fetus for grandparents. Whatever its use, though, the technology, which relies on high-frequency sound waves, has been widely assumed to be harmless. But a new study by neuroscientists at Yale University shows that prolonged exposure of prenatal mice to ultrasound can cause neural defects in the area of the brain responsible for complex functions, including those involved in memory, language processing, and consciousness in humans.
Researchers say that, while the study should not scare pregnant women away from sonography appointments, it provides an important reminder that ultrasound scans should be used only when medically necessary. The Yale scientists are now conducting long-term behavioral studies in mice and rhesus macaque monkeys, which represent a much closer model of human gestation and brain development. They hope these further studies will provide a better assessment of the procedure’s safety.
The mouse ultrasound study, led by Pasko Rakic, director of the Kavli Institute of Neuroscience at Yale, specifically found problems in the migration of neurons into the developing cerebral cortex. If misplaced neurons live, and fail to make the right connections, they can cause seizures, delayed language acquisition, or other behavioral problems. These neurons don’t cooperate properly with the neurons around them because they behave as though they’re somewhere else in the brain. Problems with neuron migration have been associated with fetal alcohol syndrome and birth defects that result from pregnant women taking cocaine.
In order to find out whether ultrasound affected the ability of cortical neurons to reach their proper destinations, Rakic’s group injected pregnant mice with a DNA-labelling molecule that becomes incorporated into the chromosomes of rapidly dividing cells. The injection was timed to coincide with the generation of cortical neurons. Over the next three days, when these neurons are known to migrate, the mice were exposed to multiple ultrasound sessions totalling from five to 420 minutes. Ten days after their birth, the mice were killed and the labelled neurons were located. When prenatal mice are exposed to ultrasound for 30 minutes or longer, the Yale researchers found, a small but significant number of neurons fail to reach the appropriate positions. Because the mice were killed after ten days, the researchers don’t know whether the anatomical abnormalities they observed would have led to behavioral differences or seizures.
Researchers have known that ultrasound beams can generate tissue-damaging heat. But Rakic’s study suggests a different possible mechanism by which ultrasound might disrupt tissues: shear stress on cellular walls. The vibrations might make it difficult for migrating neurons to cling to the guides, called glial shafts, that help them reach the proper destination.