for now

Suggestions that ultrasound could damage male fetuses has led to calls for continuous assessment of the technique.

By | December 20, 2001

For as long as medicine has used ultrasound — beginning in the early 1950s — there have been questions about whether the technique, particularly when used on pregnant women, is entirely beneficial or might pose threats to health in some circumstances. The consensus for decades has been that it is safe — the high frequency sound waves have about as much energy as a torch beam — but should be constantly reviewed.

Despite the publication in November of research findings that seemed to suggest that ultrasound scans during pregnancy might cause brain damage in men, that is still the case. Members of the British Medical Ultrasound Society — who coincidentally were holding their annual meeting in Edinburgh (12-14 December 2001) — took the opportunity to discuss the findings that gave rise to the concern, and decided that the findings do not justify changing advice on the use of ultrasound during pregnancy.

The furore began when Helle Kieler from Uppsala University in Sweden and her colleagues reported in the November 2001 Epidemiology that they had observed what appears to be a 30% increased likelihood of being born left handed in males exposed to ultrasound in the womb. In their discussion in the paper, the authors pointed out that the male brain develops in utero over a longer period and thus might be more vulnerable to ultrasound.

Clearly being left handed is not itself a problem, but if the fetus was in fact genetically right handed, being born left handed could — only could — be the result of a pathological change. The authors raised the stakes by writing; "We believe that a positive association between prenatal ultrasound scanning and left handedness, if it is causal, could result from lesions to the left or to both hemispheres." Though the authors say that such lesions do not necessarily interfere with other aspects of brain development, they do then argue that there should be studies for other effects of ultrasound. Throughout the paper there are enough comments about brain development to leave non expert readers quite reasonably with the impression that there is a possibility that ultrasound might be associated with brain damage.

Not surprising, then, that the study, being by a reputable group publishing in a peer reviewed journal, should have caught the eye of mainstream medical and science correspondents. And also not surprising that experts in the field would then have been dismayed by the distillation of the research findings — which in themselves are only a small part of an ongoing area of study — into short news stories. "The risk has been blown up by the press," said Kjell Salvesen from Trondheim University in Norway, and the first to report (in the British Medical Journal in 1993) an association between left-handedness and ultrasound scans during pregnancy.

Kieler's current study is the largest to confirm Salvesen's observations. Her team undertook a cohort study of men born in Sweden between 1973 and 1978. The men were selected because they were all tested for handedness when they enrolled for military service (The Swedish army's rifles are designed for right handed people). The study includes 6858 men born in a hospital where ultrasound scans were standard practice and 172,537 men born in hospitals without ultra scanning programmes.

The team found that for men born between 1973 and 1975 (when routine practice was one ultrasound scan at week 28) there was no difference between the exposed and unexposed groups. In men born between 1976 and 1978 (when routine practice was a scan at week 19 and week 32), there was a 30% higher chance that the man would be left handed.

"Taking these numbers at their face value, this is a significant effect," said Frances Duck, a medical physicist and outgoing chair of the European Committee for Safety in Medical Ultrasound (ECSUM). But the authors themselves point out that one can't be sure of whether those in the exposed and unexposed groups were actually exposed or not. Some mother's, for example, might not have been scanned, even though the hospital where the child was born had a scanning programme — and vice versa.

Further, said Duck, it is not clear how valid a test of handedness the Swedish Army screening programme for left handed recruits is. Such considerations influenced last week's decision not to change advice about ultrasound and pregnancy.

Others, too, have misgivings about the study. Salvesen, who is now the newly appointed chair of ECSUM, is cautious about the significance that should be given to the findings, because they are based on a cohort study. Such studies, he points out, are notoriously hard to control for confounding factors.

Salvesen's own study in the BMJ was a more rigorous randomized controlled trial. It showed a 32% likelihood that those exposed to ultrasound in the womb would be non right handed (as in left handed or ambidextrous).

"If you look for associations with enough things, you are bound to find one," say Salvesen, "and that is how the association with left handedness was explained at first." Then came another random controlled study by Kieler in 1998 also showing a correlation. "This is the third to show the 30% increased likelihood of left handedness following ultrasound exposure in the womb," he said.

And so, despite his reservations about the methodology, Salvesen is concerned. "Can you hear it in my voice?" he asks. "On the one hand, I am convinced that clinicians are not harming babies by ultrasound scans, on the other hand, we are seeing something, but we don't know what it is or what it means."

Perhaps equally important in providing guidance in the face of these unknowns are the other findings to emerge from 1993 study. Salvesen did not find any evidence of neurological damage associated with ultrasound. "There were 2000 children. I tested 600 children myself for impacts on IQ, hearing, dyslexia, etc. against ultrasound exposure." The only other association he found was that those exposed had a greater chance of improved visual acuity.

Salvesen argues that women should not be discouraged from routine scans or scans when there is a medical need, but should perhaps not have scans if it is to simply to get a picture of the baby in the womb.

"The findings leave us in an unsatisfactory position," said Duck, "and, yes, if we could find a way to do a definitive epidemiological study, that would be a good thing, but it would be hard. My personal opinion is that we should do more basic research looking at possible biological mechanisms of damage caused by ultrasound."

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