raises well-considered concerns about the prenatal use of ultrasound. She has long wondered about its potential teratogenic effects, particularly since at least one researcher reported changes in the brain of mice after exposure in 2006, and in her post she presents results of her extensive literature review to determine the current state of knowledge. She does a great job explaining the biology of ultrasound and its possible effects on developing tissues, which we can't and won't repeat here, but do go take a look. What we can do is address the more general issues that the question of the dangers of ultrasound bring up.
Ultrasound is more than an imaging technique. It has therapeutic applications, including being used to speed the healing of bone fractures and enhance the healing of soft tissue damage. It is used to enhance nerve regeneration and to deliver drugs in a targeted way, and high energy ultrasound is used to break calculi such as kidney stones into fragments, among numerous other uses.
Ultrasound is just what the name says -- high frequency sound waves. The sound waves penetrate tissue, and cause alternating compression and expansion, and that, says Emily, is where the problem lies. Water pressure ebbs and flows within cells, and this can cause cells to rupture, and/or can result in abnormal ion transfer into cells, with potentially dangerous results, particularly in a developing embryo. Ultrasound also heats water, and raised temperatures in developing tissues can be dangerous as well.
When first developed, ultrasound was used late in pregnancy, whereas it is now done early and often, with unknown consequences. Emily doesn't say this explicitly in her post, but one of the reasons people have been concerned about prenatal ultrasound is its potential association with autism. Indeed, she has co-authored at least one paper on this possible connection and herself has a history of interest in autism. Studies looking for an association have been done, but have not been definitive. The 2006 study has been criticized, for example, for not representing actual exposure amounts or times.
Why is it proving to be difficult to rule ultrasound in or out as a cause of autism? Or any other developmental disorder? For many of the same reasons that it is difficult to understand any complex trait. In part, it is because there are so many variables that are difficult to control or account for, from frequency of the sound waves (one study found that 40% of ultrasound transducers in hospitals were malfunctioning, meaning that it's impossible to know dose exposure in many cases), to definitions of autism. Indeed, consensus is still out as to whether incidence of autism has increased as dramatically as it seems to have in recent decades, or whether increased incidence is primarily a function of increased diagnosis. Experiments can't be done on humans and animal models may or may not be representative.
And in part, if ultrasound is causal, not every child exposed in utero develops autism, so the effect must be either fairly small, or there is a small window of 'opportunity' during which exposure is more damaging than at other times, depending on whatever is happening developmentally in the brain.
The individual at risk, but risk is a population phenomenon
Whatever the truth about this candidate risk factor -- or many others, for that matter, the problem we face is not just that there are many ways to end up with similar traits, and that these can, may, or even must include at least some genetic factors. But they can interact with each other in ways we can't specify in advance, and there can, may -- or even must? include others that we have not identified.
This is such a common problem, that sooner or later perhaps we will, as the science collective, find better ways to ask the causal question. Meanwhile, every time we can actually identify a cause with notable effect, it is worth addressing it rather than just amassing a list of everything somebody thought might be a cause.
Because each case is different, and the risk to any individual may also be small, one must infer factors like the risk of prenatal ultrasound on a population basis. But the greater the sample you take or need, the more variation in all sorts of variables you will also be including. This makes the evaluation very challenging......and it's the same story with so many multi-causal traits and one of the major problems with genomic studies where most genetic risk factors have individually small effects.
Unfortunately, a cause (if it is one) like ultrasound can, once discovered and clearly documented, present us with a dilemma: the cause of one problem may be a benefit in regard to some other problem. This is likely to be a common fact. Ultrasound has some quite beneficial value. Similarly with diet: eating, say, dietary sugar or fat provides value to the body but in some instances puts it at risk.
What are we supposed to do?
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