The ΔT thermal dose concept 1: in vivo teratogenesis

Arrhenius thermodynamics and birth defects: chemical teratogen synergy. Untested, testable, and projected relevance

This article addresses the issue of hyperthermia-induced birth defects with an accompanying additional teratogen, be it a chemical or a physical agent (i.e., a simultaneous "combinational" exposure to two teratogens, one of which is hyperthermia). Hyperthermia per se is a recognized human and animal teratogen. An excellent example of such combinational exposures is an epileptic woman who becomes pregnant while taking valproic acid (VPA) to control seizures. VPA is a recognized chemical teratogen, and fever (hyperthermia) is not an uncommon event during pregnancy. While VPA also may occasionally induce fever as a side effect, we are concerned here with fevers arising from other, unrelated causes. There is a small but internally consistent literature on these combinational-teratogen exposures involving hyperthermia plus a chemical teratogen; in each instance, the effect level has been observed to be synergistically elevated above levels induced by the separate teratogenic components. The data were empirical. The observed synergy is, however, consistent with Arrhenius thermodynamics, a well-known chemical rate equation. The need for information about combinational teratogen exposures is acute; fever is a common occurrence during pregnancy; and there are many instances whereby there is also the simultaneous presence of some other teratogen(s). Given that the rate of autism spectrum disorders in the United States was recently presented as 1 in 88 births, it seems reasonable to suspect that such combinational regimens are much more prevalent than previously thought. Our hypothesis is that synergistic birth defect levels from combinational regimens are consistent with Arrhenius thermodynamics.

Ultrasound bioeffects and safety

The main mechanisms by which ultrasound can induce biological effects as it passes through the body are thermal and mechanical in nature. The mechanical effects are primarily related to the presence of gas, whether drawn out of solution by the negative going ultrasound pressure wave (acoustic cavitation), a naturally occurring gas body (such as lung alveoli), or deliberately introduced into the blood stream to increase imaging contrast (microbubble contrast agents). Observed biological effects are discussed in the context of these mechanisms and their relevance to ultrasound safety is discussed.

Fetal thermal effects of diagnostic ultrasound

Processes that can produce a biological effect with some degree of heating (ie, about 1 degrees C above the physiologic temperature) act via a thermal mechanism. Investigations with laboratory animals have documented that pulsed ultrasound can produce elevations of temperature and damage in biological tissues in vivo, particularly in the presence of bone (intracranial temperature elevation). Acoustic outputs used to induce these adverse bioeffects are within the diagnostic range, although exposure times are usually considerably longer than in clinical practice. Conditions present in early pregnancy, such as lack of perfusion, may favor bioeffects. Thermally induced teratogenesis has been shown in many animal studies, as well as several controlled human studies; however, human studies have not shown a causal relationship between diagnostic ultrasound exposure during pregnancy and adverse biological effects to the fetus. All human epidemiologic studies, however, were conducted with commercially available devices predating 1992, that is, with acoustic outputs not exceeding a spatial-peak temporal-average intensity of 94 mW/cm2. Current limits in the United States allow a spatial-peak temporal-average intensity of 720 mW/cm2 for fetal applications. The synergistic effect of a raised body temperature (febrile status) and ultrasound insonation has not been examined in depth. Available evidence, experimental or epidemiologic, is insufficient to conclude that there is a causal relationship between obstetric diagnostic ultrasound exposure and obvious adverse thermal effects to the fetus. However, very subtle effects cannot be ruled out and indicate a need for further research, although research in humans may be extremely difficult to realize.

A proposal to clarify the relationship between the thermal index and the corresponding risk to the patient

The thermal index (TI) displayed on the screens of most modern diagnostic ultrasound machines is linearly proportional to the absorbed power or, equivalently, to the in-situ intensity or temperature rise. Users are instructed to interpret the TI as a "relative indication of bioeffect risk." The thermal dose is a well-known empirical relationship between the temperature T of a biological system and the time t needed for that temperature to induce a deleterious effect. For any two temperatures, T1 and T2, and the corresponding times t1 and t2, required to produce the same level of effect, this general relation holds: t1/t2=RT2-T1, where R is the thermal normalization constant. Hence, it is experimentally determined that the rate of induction, or risk, of a thermal effect increases exponentially with temperature. Because exponential relationships are not intuitive to many users, there is a significant potential for underestimation of the thermal risk associated with exposure to diagnostic ultrasound. To better quantify this risk and thereby make the displayed information more useful, the current linear display of the calculated value of the thermal index, i.e., of TIcur, should be altered to an exponential form based on the thermal dose and representing the excess risk associated with the exposure: TInew=(RTIcur-1)/(R-1). This expression has the advantage that for the usual choice of R=4 for T<or=43 degrees C, TInew approximately TIcur in the range most often seen onscreen, i.e., TIcur<1.2, minimizing any confusion during a transition from TIcur to TInew. For the relatively rare but potentially much more serious circumstances when TIcur>3.5, the displayed TInew>>TIcur, consistent with empirical observations of the likelihood of harm. Additional advantages also obtain.

Hyperthermia in utero due to maternal influenza is an environmental risk factor for schizophrenia

A hypothesis is presented that the association between maternal influenza and other causes of fever during the second trimester of pregnancy and the subsequent development of schizophrenia in the child is due to the damage caused by hyperthermia to the developing amygdalohippocampal complex and associated structures in the fetal brain. Hyperthermia is a known cause of congenital defects of the central nervous system and other organs after sufficiently severe exposures during early organogenesis. The pathogenic mechanisms include death of actively dividing neuroblasts, disruption of cell migration and arborization and vascular damage. In experimental studies, hyperthermia during later stages of central nervous system development also caused damage to the developing brainstem that was associated with functional defects. This damage usually results in hypoplasia of the parts undergoing active development at the time of exposure. Recent studies have shown no evidence of direct invasion of the fetus by the influenza virus. Factors that might interact with hyperthermia include familial liability to schizophrenia, season of birth, maternal nutrition, severe stress and medications used to alleviate the symptoms of fevers. The time of the development of the fetal amygdalohippocampal complex and the changes found in its structure and associated areas of the brain are compatible with the known effects of hyperthermia.

Hemorrhage near fetal rat bone exposed to pulsed ultrasound

Ultrasound-induced hemorrhage near the fetal rat skull was investigated to determine if the damage could be correlated with temporal-average intensity. A 0.92-MHz f/1 spherically focused transducer (5.1-cm focal length) was used to expose the skull of 18- to 19-day gestation exteriorized Sprague-Dawley rat fetuses (n = 197). There were four ultrasound-exposed groups (n = 36 each), one sham exposed group (n = 36) and one cage control group (n = 17). Three of the ultrasound-exposed groups had the same peak compressional (10 MPa)/peak rarefactional (6.7 MPa) pressure but different spatial-peak temporal-average intensities (I(TA)) of 1.9, 4.7 and 9.4 W/cm(2); the pulse repetition frequency (PRF) was varied (100, 250 and 500 Hz, respectively). The fourth ultrasound-exposed group had a peak compressional (6.7 MPa)/peak rarefactional (5.0 MPa) pressure and corresponding I(TA) of 4.6 W/cm(2); PRF was 500 Hz. Hemorrhage occurrence increased slightly with increasing I(TA), as well as peak rarefactional pressure and PRF, but the hemorrhage area did not correlate with any of the exposure parameters.

Quantification of risk from fetal exposure to diagnostic ultrasound

Biomedical ultrasound may induce adverse effects in patients by either thermal or non-thermal means. Temperatures above normal can adversely affect biological systems, but effects also may be produced without significant heating. Thermally induced teratogenesis has been demonstrated in many animal species as well as in a few controlled studies in humans. Various maximum 'safe' temperature elevations have been proposed, although the suggested values range from 0.0 to 2.5 degrees C. Factors relevant to thermal effects are considered, including the nature of the acoustic field in situ, the state of perfusion of the embryo/fetus, and the variation of sensitivity to thermal insult with gestational stage of development. Non-thermal mechanisms of action considered include acoustic cavitation, radiation force, and acoustic streaming. While cavitation can be quite destructive, it is extremely unlikely in the absence of stabilized gas bodies, and although the remaining mechanisms may occur in utero, they have not been shown to induce adverse effects. For example, pulsed, diagnostic ultrasound can increase fetal activity during exposure, apparently due to stimulation of auditory perception by radiation forces on the fetal head or auditory structures. In contrast, pulsed ultrasound also produces vascular damage near developing bone in the late-gestation mouse, but by a unknown mechanism and at levels above current US FDA output limits. It is concluded that: (1) thermal rather than nonthermal mechanisms are more likely to induce adverse effects in utero, and (2) while the probability of an adverse thermal event is usually small, under some conditions it can be disturbingly high.

Review: Hyperthermia and fever during pregnancy

An episode of hyperthermia is not uncommon during pregnancy. The consequences depend on the extent of temperature elevation, its duration, and the stage of development when it occurs. Mild exposures during the preimplantation period and more severe exposures during embryonic and fetal development often result in prenatal death and abortion. Hyperthermia also causes a wide range of structural and functional defects. The central nervous system (CNS) is most at risk probably because it cannot compensate for the loss of prospective neurons by additional divisions by the surviving neuroblasts and it remains at risk at stages throughout pre- and postnatal life. In experimental animals the most common defects are of the neural tube, microphthalmia, cataract, and micrencephaly, with associated functional and behavioral problems. Defects of craniofacial development including clefts, the axial and appendicular skeleton, the body wall, teeth, and heart are also commonly found. Nearly all these defects have been found in human epidemiological studies following maternal fever or hyperthermia during pregnancy. Suggested future human studies include problems of CNS function after exposure to influenza and fever, including mental retardation, schizophrenia, autism, and cerebral palsy.