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The Independent Advisory Group on Non-Ionising Radiation (AGNIR) published a report in February 2010 [footnote 1] that examined the potential health risks arising from intentional and adventitious exposure to ultrasound and infrasound. Ultrasound is defined as high frequency sound waves with frequencies above 20 kHz (kilohertz) while infrasound is defined as sound waves with frequencies below 20 Hz (hertz).
Following a thorough review of the published scientific literature, the report concludes that overall there is no clear evidence that either the use of diagnostic ultrasound or exposure to infrasound at levels commonly found in the environment are associated with any specific hazard. However, there are some uncertainties regarding ultrasound imaging of the fetus which suggest the possibility of subtle effects on the developing brain cannot be ruled out.
1. Health Protection Agency (HPA) advice
HPA advises that people should not hesitate to continue using ultrasound for diagnostic and other medical purposes, including in pregnancy. Such use has an established track record of safety and is regulated.
However, HPA considers that parents-to-be should be aware of uncertainties regarding ultrasound imaging of the fetus and take these into account when deciding whether to have ultrasound scans that do not have a defined diagnostic benefit and provide only keepsake images or ‘real time’ scans.
HPA advises that ultrasound examinations should be performed by specialist health care professionals, who are well trained in ultrasound safety and can ensure compliance with guideline exposure values.
HPA advises that further research is done to address gaps in knowledge about the health effects of ultrasound and infrasound.
Justin McCracken, Chief Executive of HPA, welcomed the publication of the report saying:
This is another thorough and detailed report from the independent Advisory Group. Overall, there is a track record of safety with diagnostic use of ultrasound, so people should continue using ultrasound for medical purposes. However, there are some uncertainties that need to be clarified through additional research. This should be borne in mind by parents-to-be when considering having extra scans just to produce keepsake images.
Diagnostic ultrasound is widely used in managing pregnancy, providing detailed information to the clinician and health care professional. However, very little scientific information is available with which to assess the impact of exposure to ultrasound on the unborn child. Subtle effects have been reported in studies of brain development in small animals, and some studies in humans indicate changes in neurological functions following in utero exposures. While these data are not considered to provide clear evidence of a specific hazard, the possibility of subtle long-term effects cannot be ruled out. In addition, the evidence on the effects of fetal ultrasound mainly date from some time ago, and used different techniques and lower exposures than are used today: there is little evidence on the safety of modern techniques.
Nevertheless, there would appear to be no reasons to withhold diagnostic imaging during pregnancy at present. Compliance with existing exposure guidelines should ensure that no adverse acute effects can occur; best practice suggests exposure times and acoustic doses should be kept as low as possible.
At the same time, ultrasound scans without any diagnostic purpose are becoming increasingly popular with parents-to-be in order to provide souvenir images or recordings of the unborn as keepsakes. In contrast to the scans performed as part of the clinical management of the pregnancy, where keepsake images are a bi-product of the diagnostic procedure, souvenir scans provide no clinical benefit. For every intervention, a balance must be maintained between benefit and risk, and so it is difficult to justify imaging that provides no clinical benefit.
While there is no clear evidence that such exposures are harmful to the fetus, parents-to-be must decide for themselves whether they wish to have souvenir scans. The desire for the keepsake must be balanced against the possibility of unconfirmed risks to the unborn. In addition, HPA recommends that ultrasound examinations should be performed by specialist health care professionals, who are well trained in ultrasound safety, and who can ensure compliance with exposure guidelines. Such specialists can also offer the appropriate advice and counselling should any abnormality or adverse event be observed. These issues may also inform any decision about souvenir scans.
HPA is aware that research into the safety of ultrasound continues to be carried out both in the UK and abroad. In the light of the uncertainties surrounding the possibility of neurological effects from prenatal exposure, HPA endorses the need for such research, particularly with regard to in utero exposures. HPA will pursue the recommendation for further research with the relevant UK funding bodies. The Agency will continue to monitor the worldwide research in this area and update its advice based on the careful analysis of the latest evidence.
HPA notes that a number of other agencies and professional groups have stated that ultrasound scanning merely for keepsake purposes should not be done, raised ethical concerns about commercial keepsake scanning and/or stressed the need for appropriate training. These include the American Institute for Ultrasound in Medicine [footnote 2], and the British Medical Ultrasound Society [footnote 3], the US Food and Drug Administration [footnote 4], Health Canada [footnote 5] and the European Federation of Societies for Ultrasound in Medicine and Biology [footnote 6].
2. Background to the AGNIR report
In April 2005, the then Chief Executive Officer of HPA received a ‘Direction to the HPA’ from the Department of Health requesting that HPA should provide advice to the government on ultrasound and infrasound. It was decided that this advice should be developed by a sub-group of AGNIR, largely consisting of acknowledged experts in the fields of ultrasound and infrasound.
Ultrasound has been used in medical practice internationally for at least 50 years for diagnostic scanning, ultrasound therapy and the ablation or destruction of unwanted tissues. Industrial applications of ultrasound include sonochemistry, emulsification, welding, cleaning and non-destructive testing. Ultrasound has also found applications in consumer products. Although clinical ultrasound has an excellent safety record, some uncertainties remain, particularly regarding effects on the fetus.
3. Summary and main conclusions of the AGNIR report
The AGNIR report examines the basic principles involved in the production, propagation and physical interaction of ultrasound with matter. Direct effects on tissues may occur through heating and acoustic cavitation and indirect effects may occur through radiation pressure. The report highlights the main sources of exposure to ultrasound in medical, industrial and domestic applications. However these exposures have not been well quantified except for those from medical devices.
The report reviews the evidence for biological effects of ultrasound by reviewing experimental studies using cells, animals and volunteers. It concludes that high levels of exposure have well-recognised acute adverse effects, and are capable of producing permanent damage to biological tissues through heating and mechanical interactions; these are utilised in therapeutic applications. However, at lower levels of exposure, notably for diagnostic ultrasound, there is no established evidence of specific hazards. Nevertheless there are animal data that suggest that the fetal brain and nervous system may be affected; in particular that diagnostic ultrasound may disrupt the normally well ordered pattern of neuronal migration within the cerebral cortex of the developing mouse brain in utero. In humans, pulsed ultrasound may affect fetal behaviour and cause increases in movement and heart rate. Also the possibility exists that diagnostic ultrasound may affect apoptosis and the normal progression of epithelial cells in the small intestine of adult animals through the cell cycle.
The report also examines the evidence from randomised trials and observational studies: these are mainly concerned with examinations of the fetus with imaging ultrasound. The report concludes that although there is no strong evidence that prenatal exposure to ultrasound currently causes any adverse effect on outcomes such as perinatal mortality and childhood malignancies, there are a few suggestions of possible neurological effects with prenatal exposure. In particular, fetal exposures may increase non-right handedness (ie left handedness and ambidexterity) and one study has reported that fetal ultrasound may be associated with restricted intrauterine growth while another reported delayed speech development.
The report concludes that confounding is likely to be the explanation for changes in handedness seen in some observational studies. It notes that there are deficiencies in the existing research, particularly with regard to non-obstetric uses of ultrasound, and using exposures more typical of current clinical practice, which leave open the possibility of effects.
The report gives a number of recommendations for further research to fill the gaps in knowledge regarding ultrasound. These include further development of dosimetric quantities that relate more closely to biological responses, and improved knowledge of ultrasound exposures outside of medical uses. Confirmation of the study suggesting subtle histological changes in the developing brain of animals is recommended as a high priority.
The report also considered it desirable to determine the variation in number, mode, intensity and duration of ultrasound examinations during pregnancy. To address gaps in knowledge, long-term follow-up of prenatally-exposed children should be considered and include evaluations of growth, development and intellectual performance.
The report also considers the health effects of exposure to infrasound. Man-made sources of infrasound include explosions, machinery, low speed fans and building sway; natural sources include thunderstorms, wind and waves.
Effects of infrasound have been studied using animal models and using volunteers, principally using acute, intense exposures, and investigating effects on hearing and balance and on the cardiovascular system. The report concludes that while very high levels of infrasound may produce acute effects, such as aural pain and body vibration, no ill-effects have been established using levels commonly experienced in the everyday environment. The report notes the lack of useful epidemiological and clinical data and highlights that there have been few studies on the effects of longer-term exposure to infrasound.
The report concludes that the general lack of adverse effects with low levels of infrasound does not suggest that further studies should be given a high priority.
European Federation of Societies for Ultrasound in Medicine and Biology. Statement on the use of diagnostic ultrasound for producing souvenir images or recordings in pregnancy. ↩