ECMUS Safety Committee Report - Can Antenatal Exposure to Ultrasound Affect Neuronal Migration in Mice?

Table of Contents

Gail ter Haar

In view of its potential implications for the safe use of ultrasound in obstetrics, the EFSUMB safety committee (ECMUS) has taken a close look at a paper published by Ang and colleagues (Ang E, Gluncic V, Duque A, Schafer M, Rakic P. Prenatal exposure to ultrasound waves impacts neuronal migration in mice. Proceedings of the National Academy of Science 2006), and have produced the following critique:

This study indicates that exposure of the embryonic mouse to ultrasound for 30 minutes or longer can affect neuronal migration in the cerebral cortex and thereby prevent some neurons from attaining their final proper position.

Methods

Pregnant mice were injected on embryonic day 16 with a DNA-replication marker to label dividing proliferative zone cells destined for superficial cortical layers 2 and 3. Within the next 3 days, while these cells were migrating across the cerebral wall, the animals were exposed to multiple sessions of ultrasound. The total exposure ranged from 5 to 420 min. Control mice were subjected to identical procedures but without exposure to ultrasound. The pregnancies were brought to term (embryonic day 19), and euthanized on postnatal day 10. The brains of the pups were fixed, sectioned and stained. The labelled cells were located and counted by investigators who were blind to the experimental conditions.

Results

Analysis of over 335 animals revealed that, when exposed to ultrasound for a total of 30 min or longer, a small but statistically significant number of neurons failed to reach their proper position in the brain. The amount of faulty dispersion of labelled neurons increased with duration of exposure to ultrasound. However, the “dose-response” was not completely linear; there was a slightly smaller effect following 210 min exposure than following 60 min exposure. On the other hand, there was also an increase in abnormal cell migration in animals exposed to a 420-min sham experiment over that in normal controls. This might be due to the effect of stress experienced by pregnant animals during prolonged exposure to the experimental procedure. Shorter durations of sham exposure had no effect on cell migration compared to normal controls. Thus, humorally mediated stress resulting from the exposure of pregnant mothers to the experimental procedure, appears to play a role only in extended exposures (420 min).

In an independent sham-exposure experiment, ultrasound exposure did not affect oxygenation or body core temperature in pregnant mice, and thus the authors speculate that the mechanism for the disturbed neuronal migration resulting from ultrasound may be a nonthermal, noncavitational, mechanically mediated effect, perhaps involving radiation force or microstreaming, or shear effects on cellular walls. These mechanical effects might interfere with the delicate adhesion between the migratory neurons and the surface of migratory substrates, such as the radial glial shafts, which serve as guides. Ultrasound may also disturb exocytosis, essential for the extension of the leading tip of migrating neurons, or disrupt the cytoskeletal rearrangement essential for the translocation of the nucleus within its leading process.

What is the relevance of these mouse experiments to cortical development in humans? Data supporting relevance for the human fetus

• The characteristics of the ultrasound beams and the ultrasound intensities used in the experiment are similar to those used in ultrasound examinations of human fetuses. The shortest exposure time (30 min) that resulted in a statistically significant effect on neuronal migration time is similar to that used in some medically indicated scans. Souvenir scans may last longer than 30 min. Moreover, during souvenir scanning, the examiner often concentrates on getting good images of the fetal face, and the ultrasound beam may therefore be directed towards the fetal head for rather long times.

• Ultrasound may have a similar or even greater impact on neuronal migration in the human fetal brain than in the mouse brain.

  - Firstly, migrating neurons in the human forebrain are only slightly larger than in the mouse, and the amount of energy absorbed within a comparable small volume of tissue during the ultrasound exposure may be the same as in the mouse embryo.

  - Secondly, in the human brain, the number of neurons migrating is much larger and their routes are more complex, thus increasing the chance of a cell going astray from its proper migratory course.

  - Thirdly, the settling pattern of neurons in primates is more precise than in rodents and so the tolerance to malpositioning may be less.

Data supporting lack of relevance for the human fetus

• There are huge differences in the number of neurons and the size of the cerebral cortex between mice and humans.

• The distance between the exposed cells and ultrasound transducer in the mice experiments is much shorter than for human ultrasound examinations, and the attenuation of ultrasound is likely to be much greater in the human.

• In the mouse experiment, the probe was held still, and so the same part of the mouse embryo – probably the whole mouse embryo – was continuously exposed to ultrasound. In a clinical examination of the human fetus, the ultrasound probe is continuously moved, and different parts of the fetus are exposed to ultrasound, the duration of the exposure of one particular part of the fetus being relatively short.

• The duration of neuronal production and the migratory phase of cortical neurons in the human fetus lasts at least18 times longer than in mice (migration in humans occurs over 18 weeks, i.e. between 6 and 24 weeks of gestation, with the peak occurring between 11 and 15 weeks as compared with over 1 week (between days 11 and 18) in the mouse). Thus, an exposure time of 30 min represents a much smaller proportion of the time dedicated to the development of the cerebral cortex in the human than in the mouse and, thus, could have a lesser overall effect, making human corticogenesis less vulnerable to ultrasound.

Future work needed

It is essential to examine the possible effects of ultrasound exposure on cortical development in non-human primates, where the duration of embryogenesis and the size and complexity of migratory pathways are more similar to those in humans.

Conclusion

It is not known whether, or to what extent, ultrasound exposure affects migrating neurons in developing humans, but there are numerous human neuropsychiatric disorders that are thought to be the result of the misplacement of cells as a consequence of abnormal neuronal migration. These results in pregnant mice support the EFSUMB warnings about the use of non-medically indicated, or commercial, prenatal ultrasound videos.

Prof Gail ter Haar

Chairman ECMUS Committee

This review, and those of the other papers recently reviewed by ECMUS1–4 are to be found on the ECMUS area of EFSUMB’s website.

References

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• 04 Williams AR . Wiggins RC . Wharram BL . Goyal M . Dou C . Johnson KJ . Miller DL . Nephron injury induced by diagnostic ultrasound imaging at high mechanical index with gas body contrast agent.  Ultrasound Med Biol.. 2007;  33 1336-1344