Possible teratogenicity of maternal fever

Is ambient heat exposure levels associated with miscarriage or stillbirths in hot regions? A cross-sectional study using survey data from the Ghana Maternal Health Survey 2007

It is well established that high ambient heat could cause congenital abnormalities resulting in miscarriage or stillbirth among certain species of mammals. However, this has not been systematically studied in real field settings among humans, despite the potential value of such knowledge for estimating the impact of global warming on the human species. This study sought to test the hypothesis that maternal heat exposure during pregnancy in hot regions is associated with increased prevalence of spontaneous abortions or stillbirths and to develop an analytical strategy to use existing data from maternal health surveys and existing data on historical heat levels at a geographic grid cell level. A subsample of the Ghana Maternal Health Survey 2007 was used in this study. This study sample consisted of 1136 women with pregnancy experiences between 2004 and 2007, out of which 141 women had a pregnancy that terminated in miscarriage or stillbirth. Induced-abortion cases were excluded. The linkage between ambient heat exposure and pregnancy outcome followed the epidemiological time-place-person principle, by linking timing of pregnancy outcome with historical data of local area heat levels for each month, as estimated in an international database. Maternal heat exposure level was estimated using calculated levels of the wet-bulb globe temperature (WBGT), which takes into account temperature, humidity, heat radiation, and air movement over the skin (wind speed). The values we used applied to exposure in the shade or in buildings without cooling (no solar heat radiation) and a standard air movement of 1 m/s. We applied two exposure durations: yearly average and monthly average for second month of pregnancy. In one analysis, we restricted the sample to four regions with time-homogeneous ambient heat. Analysis was made using logistic regression. About 12% of the latest pregnancies ended in either miscarriage (9.6%) or stillbirth (2.8%). The odds ratios indicated 12 to 15% increase (OR_crude 1.15, 95% CI 0.92-1.42, and OR_{age adjusted} 1.12, 95% CI 0.90-1.39) in the odds of having a stillbirth or miscarriage with each additional degree increase in WBGT, although this was just outside two-sided statistical significance. The WBGT range was quite narrow (most annual values in the range 24-26 °C, and most monthly values in the range 23-27 °C), which may have hidden any real impacts of high heat levels. The seemingly positive association observed disappeared after adjusting for gravidity. The analyses of the four selected regions indicated 27 to 42% increase in the odds of experiencing miscarriage or stillbirth with every degree increase in WBGT (crude OR 1.42 95% CI 1.00-2.03). This association remained after adjusting for maternal age pregnancy history, although no longer statistically significant (adjusted OR 1.27, 95% CI 0.89-1.81). Environmental heat exposures may be associated with adverse pregnancy outcomes, but this study was inconclusive, possibly because the heat exposure range was small. Historical records of routine observations in existing databases can be used for epidemiological studies on the health effects of heat, although data from properly and purposively designed studies might be more suitable for further studies.

Thermal thresholds for teratogenicity, reproduction, and development

The human embryo and foetus may be especially vulnerable to chemical and physical insults during defined stages of development. In particular, the scheduled processes of cell proliferation, cell migration, cell differentiation, and apoptosis that occur at different times for different organ structures can be susceptible to elevated temperatures. With limited ability to regulate temperature on its own, the developing embryo and foetus is entirely dependent upon the mother's thermoregulatory capacity. As a general rule, maternal core body temperature increases of ∼2°C above normal for extended periods of time, 2-2.5°C above normal for 0.5-1 h, or ≥4°C above normal for 15 min have resulted in developmental abnormalities in animal models. Significant differences in thermoregulation and thermoneutral ambient temperatures make direct extrapolation of animal data to humans challenging, and the above temperatures may or may not be reasonable threshold predictions for adverse developmental effects in humans. Corresponding specific absorption rate (SAR) values that would be necessary to cause such temperature elevations in a healthy adult female would be in the range of ≥15 W/kg (whole body average or WBA), with ∼4 W/kg required to increase core temperature 1°C. However, smaller levels of thermal stress in the mother that are asymptomatic might theoretically result in increased shunting of blood volume to the periphery as a heat dissipation mechanism. This could conceivably result in altered placental and umbilical blood perfusion and reduce heat exchange with the foetus. It is difficult to predict the magnitude and threshold for such an effect, as many factors are involved in the thermoregulatory response. However, a very conservative estimate of 1.5 W/kg WBA (1/10th the threshold to protect against measurable temperature increases) would seem sufficient to protect against any significant reduction in blood flow to the embryo or foetus in the pregnant mother. This is more than three times above the current WBA limit for occupational exposure (0.4 W/kg) as outlined in both IEEE C95.1-2005 and ICNIRP-1998 international safety standards for radiofrequency (RF) exposures. With regard to local RF exposure directly to the embryo or foetus, significant absorption by the mother as well as heat dissipation due to conductive and convective exchange would offer significant protection. However, a theoretical 1-W/kg exposure averaged over the entire 28-day embryo, or averaged over a 1-g volume in the foetus, should not elevate temperature more than 0.2°C. Because of safety standards, exposures to the foetus this great would not be attainable with the usual RF sources. Foetal exposures to ultrasound are limited by the US Food and Drug Administration (FDA) to a maximum spatial peak temporal average intensity of 720 mW/cm(2). Routine ultrasound scanning typically occurs at lower values and temperature elevations are negligible. However, some higher power Doppler ultrasound devices under some conditions are capable of raising foetal temperature several degrees and their use in examinations of the foetus should be minimised.

Hyperthermia: malformations to chaperones

Hyperthermia has been known to induce malformations in numerous animal models as well being associated with human abnormalities. This was apparent particularly when the hyperthermia exposure was during the early stages of neural development. Although it was recognized relatively early that these exposures induced cell death, the specific molecular mechanism of how a brief heat exposure was translated in to specific cellular functions remains largely unknown. While our understanding of the events that govern how cells react to heat, or stresses in general, has increased, there is much that remains undiscovered. In this brief review, animal and clinical observations are outlined as are some of the scientific explorations that were undertaken to characterize, define, and better understand the morphological, biochemical, and molecular effects of hyperthermia on the developing embryo.

Maternal fever during early pregnancy and the risk of oral clefts

An increased risk of birth defects after hyperthermic exposures has been confirmed in animal studies, but population studies have yielded inconsistent results. Oral clefts are a common birth defect and have been associated with these exposures in some of these studies. In this study, data from the National Birth Defects Prevention Study was used to evaluate the association of maternal report of febrile illness in early pregnancy and the risk of oral clefts. All oral cleft cases born between 1997 and 2004 were compared with nonmalformed controls born in the same geographical region during the same time period. Mothers reporting febrile illness during pregnancy were stratified by fever grade and antipyretic use. Logistic regression models were used to generate crude and adjusted odds ratios for exposure to fever and association with each oral cleft phenotype. The dataset included 5821 controls, 1567 cases of cleft lip with or without cleft palate (CL+/-P) and 835 cases of cleft palate only. A modestly increased risk was observed for isolated CL+/-P (odds ratio, 1.28; 95% confidence interval, 1.01-1.63). Stratification by fever grade (body temperature <101.5 degrees or > or =101.5 degrees F) did not yield significant differences in risk. Risk estimates were higher among women who reported a fever, but did not take antipyretics to control their fever, particularly for nonisolated compared with isolated oral clefts. This finding suggests that adequate control of fever may diminish the deleterious effects of fever in cases of oral cleft.

Delineation of a multiple congenital abnormality syndrome in the offspring of pregnant women affected with high fever-related disorders: a population-based study

Our previous study showed an association between high fever-related maternal diseases during the second and/or third gestational months and a higher risk of multiple congenital abnormalities (MCA) in the population-based large dataset of the Hungarian Case-Control Surveillance of Congenital Abnormalities. The objective of our analysis is to attempt the delineation of the spectrum of the characteristic component defects of an MCA syndrome associated with high fever-related maternal diseases. Of 1349 cases with MCA without recognized genetic and teratogenic syndromes in the total dataset, 181 had a possible association with influenza, common cold with secondary complications, tonsillitis and recurrent orofacial herpes with high fever in the second and/or third gestational months. At the evaluation of component defects in these 181 MCA cases, an association was found between the components of the so-called two schisis-type defects, such as neural-tube defects and orofacial cleft, in addition to microphthalmos, neurogenic limb contractures, and indeterminate sex (i.e. maldevelopment of male external genital organs, such as hypoplasia of the penis and pseudohermaphroditism). In addition, previous findings that showed an association between high fever and facial anomalies (micrognathia and midfacial hypoplasia), microcephaly and defects of external ears, were confirmed in our dataset. Thus, we delineated the maternal high fever-related MCA syndrome, including the above component defects and this MCA syndrome was identified in 38 MCA (21.0%) among 181 MCA babies born to mothers with high fever-related diseases. In the total dataset of 1349 MCA, 2.8% of cases can be associated with high fever.

High fever-related maternal diseases as possible causes of multiple congenital abnormalities: a population-based case-control study

BACKGROUND

Multiple congenital abnormalities (MCAs) represent the most severe category of structural birth defects; therefore, we decided to evaluate the possible etiological factors for MCAs.

METHODS

The population-based large data set of the Hungarian Case-Control Surveillance of Congenital Abnormalities (1980-1996) was evaluated.

RESULTS

We compared the data of 1,349 cases with MCAs, 2,405 matched population controls without any defect, and 21,494 malformed controls with isolated congenital abnormalities. An association was found between a higher risk for MCAs and high fever-related influenza, common cold with secondary complications, tonsillitis, and recurrent orofacial herpes (adjusted ORs with 95% CIs: 2.3, 1.8-2.9). However, the risk for MCAs was reduced by antifever drug therapy (adjusted OR with 95% CI: 1.6, 0.9-2.9).

CONCLUSIONS

An association was found between high fever-related maternal diseases and a higher risk for MCAs; however, a certain portion of these MCAs is preventable by antifever therapy.

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.

Marshall J. Edwards: Discoverer of maternal hyperthermia as a human teratogen

In a series of animal studies performed over a career spanning 40 years at the University of Sydney, Professor Marshall J. Edwards investigated the hypothesis that maternal hyperthermia during gestation can be teratogenic to the developing fetus. He is one of few investigators to have discovered a known human teratogen primarily through animal studies. In 1970 he earned his Ph.D. from the University of Sydney, writing a doctoral thesis entitled "A Study of Some Factors Affecting Fertility of Animals with Particular Reference to the Effects of Hyperthermia on Gestation and Prenatal Development of the Guinea-Pig." He went on to prove that hyperthermia-induced malformations in animals involve many organs and structures, particularly the central nervous system. Other defects include craniofacial anomalies, heart defects and hypodactyly, cataracts and coloboma, kyphoscoliosis, renal anomalies, dental agenesis, and abdominal wall defects. In a series of carefully planned and executed experiments, he demonstrated that the type of defect is related to the timing of the hyperthermic insult, and analyzed the underlying mechanisms. Cell death, membrane disruption, vascular disruption, and placental infarction were all implicated in causing embryonic damage. This special article reviews the scientific discoveries and personal philosophy of Marshall J. Edwards, the discoverer of maternal hyperthermia as a human teratogen.

Eye anomalies: seasonal variation and maternal viral infections

INTRODUCTION

Eye anomalies, including anophthalmos and microphthalmos (absent eye and small eye), are common features in congenital infection syndromes such as rubella. Influenza, Coxsackie virus, and Parvovirus are less well-established teratogens. We report an ecologic analysis of anophthalmia/microphthalmia prevalence in England in relation to temporal variation in these infections using routine infection data.

METHODS

The national (England) anophthalmos/microphthalmos register holds details of 406 babies (excluding chromosomal anomalies) born 1988-1994. Weekly infection data were obtained from the U.K. Public Health Laboratory Service and Royal College of General Practitioners Weekly Returns Service. We assigned cases and births an estimated exposure based on the population infection counts in the gestational weeks known to be critical for ocular development. We calculated the rate ratio of anophthalmos/microphthalmos prevalence for infection in the 90th percentile compared with the 10th percentile by Poisson regression.

RESULTS

There was a positive association between severe anophthalmos/microphthalmos prevalence and Parvovirus B19 (rate ratio = 1.26; 95% confidence interval = 1.00-1.58) and for influenza (1.41; 1.08-1.84). Coxsackie virus showed a negative association with any anophthalmos/microphthalmos (0.75; 0.58-0.98) and with severe cases (0.66; 0.46-0.95).

CONCLUSIONS

This analysis provides suggestive evidence for a teratogenic role of influenza and Parvovirus in anophthalmos/microphthalmos. The role of maternal infections in congenital anomaly etiology is worthy of further investigation.

SOX2anophthalmia syndrome

Heterozygous, de novo, loss-of-function mutations in SOX2 have been shown to cause bilateral anophthalmia. Here we provide a detailed description of the clinical features associated with SOX2 mutations in the five individuals with reported mutations and four newly identified cases (including the first reported SOX2 missense mutation). The SOX2-associated ocular malformations are variable in type, but most often bilateral and severe. Of the nine patients, six had bilateral anophthalmia and two had anophthalmia with contralateral microphthalmia with sclerocornea. The remaining case had anophthalmia with contralateral microphthalmia, posterior cortical cataract and a dysplastic optic disc, and was the only patient to have measurable visual acuity. The relatively consistent extraocular phenotype observed includes: learning disability, seizures, brain malformation, specific motor abnormalities, male genital tract malformations, mild facial dysmorphism, and postnatal growth failure. Identifying SOX2 mutations from large cohorts of patients with structural eye defects has delineated a new, clinically-recognizable, multisystem disorder and has provided important insight into the developmental pathways critical for morphogenesis of the eye, brain, and male genital tract.

Investigations into the etiology of neural tube defects

Neural tube defects (NTDs) are serious malformations affecting approximately 1 per 1000 births, yet the mechanisms by which they arise are unknown. There have been consistent efforts in many fields of research to elucidate the etiology of this multifactorial condition. While no single gene has been identified as a major independent risk factor for NTDs, candidate genes have been proposed that may modify the effects of maternal and/or embryonic exposures. Folate supplementation effectively reduces the occurrence of NTDs and, consequently, has focused much research on metabolism of folate-related pathways during pregnancy and development. Further understanding of normal development and how teratogens can perturb these orchestrated processes also remains at the fore of modern scientific endeavors. The composite of these factors remains fragmented; the aim of this review is to provide the reader with a summary of sentinel and current works in the body of literature addressing NTD disease etiology.

Association between maternal fever and psychological/behavior outcomes: A hypothesis

BACKGROUND

This study is one of the first to investigate the association between maternal report of fever during middle to late pregnancy and psychological, behavioral, and educational outcomes in offspring. The hypothesis guiding this research was that maternal fever during the second trimester of pregnancy has an adverse effect on the development of the central nervous system (CNS) of the fetus, resulting in abnormalities of psychological development and behavior that can be observed in childhood.

METHODS

Multivariate analyses of a birth cohort compared outcomes for children whose mothers never reported fever during pregnancy and those who reported fever in the second and third trimesters. Children were compared on measures of temperament, behavior, and academic performance in infancy and at five and 12 years of age.

RESULTS

Associations were obtained for second-trimester fever and distress to novelty (p < 0.05) in infancy. Significant associations were also obtained for inhibition (p < 0.01), negative emotionality (p < 0.05), and lack of task persistence (p < 0.01) at age five. Furthermore, school achievement (p < 0.05) and task orientation (p < 0.01) at age 12 were associated with maternal reports of second-trimester fever exposure.

CONCLUSIONS

Much of the gestation/hyperthermia research has focused on the relationship between hyperthermia exposure and profoundly teratogenic outcomes. In this study we investigated subtler psychological/behavioral associations that may not be observable until later in development. Although the current study was hampered by technical limitations, the results support the need for more rigorously controlled research into a possible association between gestational fever and psychological/behavioral outcomes.

Thermal environment and human birth weight

Human birth weight is known to be influenced by several factors, including maternal energy supply, maternal stature, disease status, smoking status and gestation length. This article proposes that the thermal environment may be a further factor influencing birth weight. Experimental animal studies demonstrate clear effects of thermal stress on placental function and birth weight, but may have limited relevance for humans due to between-species differences in pregnancy physiology. Observational studies suggest an inverse relationship between environmental temperature and birth weight within and between human populations. Variation in maternal size, body fatness, pregnancy weight gain and heat production is predicted to influence maternal thermoregulatory capacity, as are the size and composition of the foetus. These associations generate the hypothesis that low birth weight in hot environments may in part represent an adaptation to environmental heat stress.

Ocular colobomata

Ocular colobomata present diagnostic and therapeutic challenges in patients of all ages, but especially in young children. The "typical" coloboma, caused by defective closure of the fetal fissure, is located in the inferonasal quadrant, and it may affect any part of the globe traversed by the fissure from the iris to the optic nerve. Ocular colobomata are often associated with microphthalmia, and they may be idiopathic or associated with various syndromes. Types and severity of complications vary depending on the location and size of the colobomata. This article reviews the pathogeneses, categorization, genetic bases, differential diagnoses and management of ocular coloboma.

Identification of a growth arrest specific (gas 5) gene by differential display as a candidate gene for determining susceptibility to hyperthermia-induced exencephaly in mice

Neural tube defects (NTDs) are among the most common congenital malformations, affecting approximately 1 per 1,000 liveborn infants in the United States [Nakano, 1973; Richards et al., 1972]. Maternal exposure to hyperthermia, either through recreational sources or due to an infectious agent, is thought to account for approximately 10% of observed NTD cases. The specific genes conferring susceptibility or resistance to hyperthermia-induced NTDs have not been identified. This study used differential display-polymerase chain reaction (DD-PCR) to characterize alterations in gene expression in the anterior embryonic neural tube of two highly inbred murine strains (SWV/Fnn, LM/Bc/Fnn) known to differ in their genetically determined susceptibility to heat-induced NTDs. Herein, we report the neural tube-specific differential expression of the growth arrest specific (gas 5) gene in the highly susceptible SWV/Fnn strain during neural tube closure (NTC). Although the expression of gas 5 did not appear to be altered by the teratogenic heat treatment, its spatial and strain-specific pattern of expression makes it an excellent candidate gene responsible for the observed genetic differences in NTD susceptibility between these two inbred murine strains.

Teratogen update: gestational effects of maternal hyperthermia due to febrile illnesses and resultant patterns of defects in humans

This review has covered the pertinent literature concerning the teratogenic effects of hyperthermia in man and experimental animals. This is the first teratogen that was initially discovered in animals and then subsequently found to be a cause for concern in humans when similar patterns of defects were observed. Hyperthermia is a physical agent with a dose-response curve for abortions and malformations, but these effects can be mitigated in some circumstances by the heat shock response (HSR). We have reviewed the known functions of HSR and provided some insight into why embryos have some protection following an initial dose of heat, if it is sufficient to initiate the response. Thus, by reviewing the effects of hyperthermia in experimental animals, as well as malformative and protective mechanisms of teratogenesis, we have attempted to understand the effects of human hyperthermia teratogenesis.

Geographical variation in anophthalmia and microphthalmia in England, 1988-94

OBJECTIVE

To investigate the geographical variation and clustering of congenital anophthalmia and microphthalmia in England, in response to media reports of clusters.

DESIGN

Comparison of pattern of residence at birth of cases of anophthalmia and microphthalmia in England in 1988-94, notified to a special register, with pattern of residence of all births. Three groups studied included all cases, all severe cases, and all severe cases of unknown aetiology.

OUTCOME MEASURES

Prevalence rates of anophthalmia and microphthalmia by region and district, and by ward population density and socioeconomic deprivation index of enumeration district grouped into fifths. Clustering expressed as the tendency for the three nearest neighbours of a case to be more likely to be cases than expected by chance, or for there to be more cases within circles of fixed radius of a case than expected by chance.

RESULTS

The overall prevalence of anophthalmia and microphthalmia was 1.0 per 10 000 births. Regional and district variation in prevalence did not reach statistical significance. Prevalence was higher in rural than urban areas: the relative risk in the group of wards of lowest population density compared with the most densely populated group was 1.79 (95% confidence interval 1.15 to 2.81) for all cases and 2.37 (1.38 to 4. 08) for severe cases. There was no evidence of a trend in risk with socioeconomic deprivation. There was very little evidence of localised clustering.

CONCLUSIONS

There is very little evidence to support the presence of strongly localised environmental exposures causing clusters of children to be born with anophthalmia or microphthalmia. The excess risk in rural areas requires further investigation.

Molecular/cellular biology of the heat stress response and its role in agent-induced teratogenesis

Available data indicate that heat shock proteins act as chaperones under non-stress conditions by assisting in: (1) the folding of newly synthesized proteins, (2) the intracellular translocation of proteins, and (3) the function of other proteins. As we gain additional information concerning cellular physiology, we may find that heat shock proteins play a key role in many additional cellular functions. When cells experience thermal or chemical stress, heat shock proteins take on a new role, conserved from bacteria to humans, of protecting cells from the detrimental effects of stress. This latter role takes on added significance for the embryo in which the developmental program must be read linearly, with little opportunity to cycle backward to complete a missed segment of the program. Although circumstantial evidence clearly implicates heat shock proteins in protecting embryos from thermal stress, definitive evidence is still lacking. The challenge for the future is to obtain such definitive data. Ideally, such information will lead to new therapeutic paradigms that will afford protection to the human embryo/fetus exposed to thermal/chemical stress.

The importance of prenatal factors in childhood blindness in India

The causes of visual loss in 1411 children attending schools for the blind in different geographical areas in India are described. Ninety-three percent (1318) of the children were severely visually impaired (SVI) or blind (i.e. corrected acuity in the better eye of <20/200 [<6/60]). In 60% of SVI/blind children vision loss was attributable to factors operating in the prenatal period, in 47% the prenatal factors were known and definite, and in 13% prenatal factors were the most probable causes. Hereditary retinal dystrophies and albinism were seen in 19% of SVI/blind children and 23% had congenital ocular anomalies. There were variations in the relative importance of different causes by state. The observed pattern of causes of visual loss is intermediate between those seen in industrialised countries and in the poorest developing countries. This suggests that strategies to combat childhood blindness in India need to address concurrently both preventable and treatable causes. The need for aetiological studies, particularly on anophthalmos and microphthalmos, is highlighted.

Growth and behavioral changes in mice prenatally exposed to methylmercury and heat

Postnatal and behavioral changes in mice exposed prenatally to methylmercury and heat were investigated. Pregnant ICR mice were immersed in water at 37 degrees C or 42 degrees C for 10 min once or twice daily from day 12 through day 15 of gestation. Two hours after the heat exposure on day 12 of age, mice were injected s.c. with 5 mg Hg/kg of methylmercury (MeHg, as chloride) or saline. Prenatal exposure to heat significantly induced inactivity in an open field test (OPF) in males and retarded walking ability in both males and females. Prenatal exposure to MeHg caused significant inactivity in the OPF in females. Although heat did not enhance the effect of MeHg on physical growth or the behavior of pups and vice versa, there were some interactions between the effects of these two agents. Thus, the difference in walking ability in both sexes caused by heat was more distinctive in the saline-treated groups than in the MeHg-treated groups; the difference in locomotion in OPF caused by MeHg in females was more distinctive in the normothermic group than among the hyperthermic groups. The mechanisms underlying these behavioral changes need to be further examined.

Skeletal development following heat exposure in the rat

The effects of gestation day (GD) 10 heat exposure in the rat were studied to determine the temperature-response relationship for the induction of skeletal and other defects. Conscious pregnant rats (Experiment 1) were exposed to various temperatures in a warm air chamber. Body temperature was measured using a rectal probe, and these measurements were confirmed as representing core body temperature in separate animals using telemetric procedures. Those animals whose core body temperature was raised to 41-41.9 degrees C had over 90% malformed pups (examined at postnatal day (PND) 3), and a 25% reduction in the percent of live pups per litter. Animals whose temperature was raised to 39.2-40.9 degrees C had a low incidence of pups with similar types of malformations. The primary types of malformations were of the axial skeleton, consisting of fusions and other abnormalities of the ribs and vertebral elements, and a decrease in the total number of ribs and centra. The acute maternal effects of these temperature increases were signs of heat exhaustion during and 1-2 hr after exposure, but there were no permanent changes in weight gain or other signs. When temperatures were raised to > or = 42 degrees C, all maternal animals died. In a second study (Experiment 2), pregnant rats (from a different supplier) were anesthetized to determine the effect of reducing maternal stress and were exposed to heat as in Experiment 1. Those animals whose core body temperature was raised to 42-42.5 degrees C for 5 min had pups with similar responses to those in Experiment 1 at 41-41.9 degrees C, although the reduction in litter size was not as great. Animals whose temperature was raised to 41 degrees C had a much lower incidence of pups with similar defects, and animals whose temperature was raised to 43 degrees C did not survive. A more detailed analysis of the skeletal defects in Experiment 2 showed rib and vertebral malformations that appear to be related to the stage of somite development at the time of exposure.

Teratogenic effect of hyperthermia during early organogenesis period in mice

Pregnant Swiss albino mice were subjected to 41, 42, or 43 degrees C temperature for 10 minutes on day 6.5 of gestation. Another group of animals treated at 37 degrees C was used as control. All animals were killed on the 18th day of gestation and fetuses were examined for prenatal mortality, growth retardation, and microphthalmia incidence. Results indicated a dose dependent increase in the mortality rates with a 42% death in the 43 degrees C group. Treatment with the higher temperatures (42 and 43 degrees C) resulted in a significant increase in the number of growth retarded fetuses and in the incidence of microphthalmia. Reduction in head length and decrease in brain weight were observed in the group exposed to 43 degrees C, particularly in the growth retarded fetuses. However, the percent brain weight(g)-body weight(g) ratio did not show any significant difference from the control values.

Update of sporadic microphthalmos and coloboma. Non-inherited anomalies

The majority of patients with microphthalmos and colobomas have genetic disorders. This is a survey of non-inherited, sporadic microphthalmos. Such cases may occur in the Goldenhar, CHARGE and VATER associations; it may also be due to teratological agents, for instance maternal ingestion of drugs, maternal infection, fever or irradiation. In these cases it is possible that neural crest cell development is abnormal. Some drugs, for instance retinoic acid are regulators of Hox genes which control an ordered sequence of differentiation; coloboma or microphthalmos may appear if deregulation occurs. Large choristomas of the anterior segment of the eye may be associated with microphthalmos or anophthalmos. Encephaloceles or orbital tumours may deform the growing eye and are another cause of non-inherited microphthalmos.

An update on microphthalmos and coloboma. A brief survey of genetic disorders with microphthalmos and coloboma

This is a brief survey of more than 100 syndromes in which microphthalmos and/or coloboma appear. It is suggested that the disorders be assessed systematically by consecutive discussions of their aetiology, morphology and embryology.

Ocular teratology

The principles of teratology are described, and animal models for research in abnormal ocular development and clinical studies of human teratogens are surveyed. A review is made of presumed ocular teratogenic agents: radiation; external environmental teratogens; maternal conditions such as infections, diabetes, and epilepsy; alcohol and drugs such as thalidomide, retinoic acid, and coumarin anticoagulants; and other agents, such as cigarettes.

On the possible effects of homeostatic shifts in human embryonic development

Possible etiological factors of congenital malformations as well as of human trisomies are considered in the framework of the repressor hypothesis. In this approach gene expression is envisaged from the point of view of the functional variations of the total activation energy for normal gene expression in homeostatic equilibrium. We restrict our attention to variations of the total activation energy under the effect of temperature gradients. We discuss the evidence that hyperthermia may be an etiological factor for trisomies in humans.

Embryonic resistance to chemical and physical factors: manifestation, mechanism, role in reproduction and in adaptation to ecology

Chemical and physical factors may adversely affect embryonic development. As an example of chemical factors, the effects of diabetic metabolic factors on embryonic development in mammals was reviewed. The existence of a stage-dependent reaction of embryos was found. At preimplantation stages diabetic metabolic factors are embryotoxic and lethal, and the blastocysts reacted by an "all-or-none" response. Early somite embryos showed a higher resistance to the effects of diabetic metabolic factors resulting in various types of malformations. Both groups of embryos showed a very high sensitivity to the effects of combined diabetic metabolic factors. Congenital defects in term foetuses were lower than those observed during middle phases of pregnancy because some of the severely malformed embryos resorb during gestation. The effects of temperature on embryonic development were presented as an example of physical influences. In man, hyperthermia in pregnancy seems to correlate with defects in the development of the nervous and skeletal systems. In domestic animals, changes in environmental temperature correlated with depressions of reproduction rate. In laboratory animals, hyperthermia caused the development of congenital malformations. Stage-dependent as well as genetic differences in embryonic susceptibility to hyperthermia were found. Critical periods in sensitivity of embryos to hyperthermic influences were also observed. It has been shown that, in spite of similar external manifestations of the reaction of embryos to effects of diabetes and hyperthermia, the mechanism of these reactions was different. High resistance of early reptile and bird embryos to influences of temperature was considered as an example of morphofunctional adaptations in early embryogenesis of vertebrates to their development in terrestrial conditions.

Interference with gastrulation during the third week of pregnancy as a cause of some facial abnormalities and CNS defects

During the third week of pregnancy the human embryo undergoes a major developmental process, gastrulation, during which the two-layered embryo is converted into a three-layered embryo. At the same time, the upper epiblast layer is induced to form the neural plate. Evidence is presented which suggests that interference with this process by genetic, physical, or chemical agents can cause a range of CNS abnormalities and facial abnormalities, including those described as characteristic of the FAS.

Hirschsprung disease in the offspring of mothers exposed to hyperthermia during pregnancy

Interview data for events of pregnancy on 40 infants with Hirschsprung disease showed a significant relationship to a history of hyperthermia in the mother in the first trimester, during the time of embryonic development and migration of ganglion cells along the intestine. In order to eliminate maternal recall bias, a control group consisted of 41 malformed infants with congenital limb deficiency. Hyperthermia may be one factor contributing to the cause of some cases of Hirschsprung disease.

Possible effects of electric blankets and heated waterbeds on fetal development

Seasonal patterns were seen in fetal growth and in abortion rate for families using electrically heated beds. These patterns could be attributed to the seasonal use of heated beds. The fact that such seasonal patterns were seen only in users, and not in nonusers, of electrically heated beds suggests that electrical bed heating may have a direct effect on fetal development. The effect could be due to excessive heat or to electromagnetic field exposure.

Hyperthermia as a teratogen: a review of experimental studies and their clinical significance

Although hyperthermia is teratogenic in birds, all the common laboratory animals, farm animals, and primates and satisfies defined criteria as a teratogen, its study as a human teratogen has been neglected. Homeothermic animals, including humans, can experience body temperature elevations induced by febrile infections, heavy exercise and hot environments which exceed the thresholds (1.5-2.5 degrees C elevation) which are known to cause a syndrome of embryonic resorptions, abortions, and malformations in experimental animals. Hyperthermia is particularly damaging to the central nervous system, and if a threshold exposure occurs at the appropriate stages of embryonic development, exencephaly, anencephaly, encephalocoele, micrencephaly, microphthalmia, neurogenic talipes, and arthrogryposis can be produced in a high proportion of exposed embryos, the incidence and type of defect depending on the species and strain within species, the stage of development, and the severity of hyperthermic exposure. Other defects which can be induced experimentally include exomphalos, hypoplasia of toes and teeth, renal agenesis, vertebral anomalies, maxillary hypoplasia, facial clefting, cataract, coloboma, and heart and vascular defects. Proliferating cells are particularly sensitive to temperature elevations, resulting in arrest of mitotic activity and immediate death of cells in mitosis with threshold elevations (1.5-2.5 degrees C) and delayed death of cells probably in S phase with higher elevations (3.5 degrees C). In general, lower temperature elevations (2.5 degrees C) require longer durations of elevation to cause defects than a simple spike at a higher elevation (4.5 degrees C). The death of cells is largely confined to the brain and in the day 21 guinea pig embryo to the alar regions of the brain. Cell death probably accounts for most of the defects in the central nervous system, but microvascular disturbances leading to leakage, oedema and haemorrhage, placental necrosis, and infarction are other known effects of hyperthermia; and these are probably involved in the pathogenesis of many defects of the heart, limbs, kidneys, and body wall. Recent experiments have demonstrated protection of rat embryos in culture against a known teratogenic exposure by a brief nonteratogenic exposure given at least 15 min earlier. This protection is associated with the synthesis of heat-shock proteins, and temporary arrest of the cell proliferative cycle. Hyperthermia appears to be capable of causing congenital defects in all species and may act alone or synergistically with other agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Similarity of effects--experimental hyperthermia as a teratogen and maternal febrile illness associated with oromandibular and limb defects

Experimental hyperthermia in pregnancy causes oromandibular and limb anomalies. In humans, hyperthermia has been suspected of being teratogenic. Two patients were investigated; both had limb defects and one had oromandibular anomalies. Their mothers had a febrile illness at about the 10th wk of gestation. The similarity of defects produced by experimental hyperthermia and those reported here is striking.

Hyperthermia as a teratogen: parameters determining hyperthermia-induced head defects in the rat

This study determined the relationship between the duration and extent of temperature elevation, during a critical period of rat embryonic development, and the induction of congenital malformations. Pregnant Sprague-Dawley rats, at 9 days 12 hours gestation (gastrulation stage), were partially immersed in a water bath until their core temperature, monitored by a rectal thermistor probe, was elevated to a nominated temperature. Seven temperatures were tested from 40.5 degrees C to 43.5 degrees C, elevations of 2.0-5.0 degrees C in core temperature. Various durations at each of these temperatures were tested for potential teratogenicity. A single elevation of 5.0 degrees C or 4.5 degrees C needed only a "spike" in duration to be teratogenic, 4.0 degrees C was teratogenic within 5 minutes, 3.5 degrees C within 10 minutes, 3.0 degrees C within 20 minutes, and 2.5 degrees C within 1 hour. An elevation of 2.0 degrees C for 8 hours was not teratogenic. Microphthalmia was the most common malformation at all teratogenic temperatures and was frequently the only malformation seen at the shortest time exposure for a particular temperature. Encephalocele, facial clefting, and maxillary hypoplasia were the other frequently seen malformations. Five control rats were placed in the water bath for 2 hours at 38 degrees C so that their core temperature was not elevated. All the control fetuses were normal. An elevation of 2.5 degrees C for 1 hour was the threshold combination for teratogenesis. As the temperature increased above a 2.5 degrees C elevation the necessary duration of exposure for teratogenesis decreased.

The induction of microphthalmia, encephalocele, and other head defects following hyperthermia during the gastrulation process in the rat

The aim of this study was to ascertain whether there is a period during early embryonic development of the rat that is particularly sensitive to hyperthermia. Pregnant Sprague-Dawley rats were partially immersed in a water bath at 43.5 degrees C until their core temperatures, monitored by a rectal thermistor probe, were elevated to 43.5 degrees C. The procedure was repeated 6 hours later. The regimen of two heatings was performed over a range of development from early gastrulation (8 days 18 hours) to about the 12 somite stage (10 days 18 hours). The rats were killed on days 17-19 and the fetuses were examined. Each group contained a minimum of five litters. The main teratogenic effect of the hyperthermia was the induction of one or more head defects, notably microphthalmia, encephalocele (either a single, large, parietal encephalocele or multiple small protuberances), and maxillary hypoplasia. Microphthalmia was the most common defect with approximately 90% of surviving fetuses having small eyes when heating occurred between 9 days 6 hours and 10 days 0 hours (9.06 and 10.00). Encephaloceles were induced by heating between 9.00 and 10.00 with a peak sensitivity between 9.12 and 9.18 when 57% of surviving fetuses were affected. Maxillary hypoplasia resulted from heating between 9.06 and 10.06 with up to 20% of surviving fetuses being affected. Control rats were exposed to the same experimental procedure in a water bath at 38 degrees C on 9.12 and 9.18, the gestational time most sensitive to hyperthermia induced malformations. There were no abnormal fetuses in the controls. The critical period identified spans 9 days 6 hours to 10 days 0 hours gestational age. In developmental terms this includes a large proportion of the gastrulation process.

Hyperthermia and the induction of neural tube defects in mice

The aim of this study was to examine some parameters determining the teratogenicity of hyperthermia. Pregnant inbred (C57B1/6J and CBA/Ca/T6) and outbred (QS) mice were partially immersed in a water bath at 43-43.5 degrees C until their core temperature, monitored by a rectal thermister probe, was elevated to 43 degrees C for the inbred mice and 43.5 degrees C for the outbred mice. The procedure was repeated 6 h later. The regimen of 2 heatings was performed over a range of development from pregastrulation to about the 24 somite stage. The mice were killed on day 19, and the fetuses were examined. Exencephaly was the common malformation seen in each strain. It occurred at a maximum incidence of 34% in the QS mice, 14% in the CBA strain, and 20% in C57B1 mice. For the CBA and QS mice 4 to 11-15 somites was the most heat sensitive stage of development for the induction of exencephaly while in the C57B1 mice the preceding 24 hr of development were most sensitive. Control mice were exposed to the same experimental procedure in a water bath at body temperature at the most sensitive stage of development for the strain. There were no instances of exencephaly in the controls. The sex of all fetuses was determined and revealed a high female/male ratio among the fetuses with exencephaly (2.1 for the QS mice, 1.7 for the CBA mice, and 3.1 for the C57B1 mice) compared with a ratio of approximately 1.0 for all fetuses. Analysis in the QS mice indicated that the predominance of female exencephalics was probably due to prenatal loss of male embryos.

Retardation of brain growth of guinea pigs by hyperthermia: effect of varying intervals between successive exposures

Guinea pigs were exposed to a temperature of 42.5-43.5 degrees C on three occasions between days 20 and 23 of pregnancy. In the first experiment, groups of mothers were exposed at intervals of 18-30 hr. Each exposure ended when the deep rectal temperature had been over 43 degrees C for 6 min and mean temperatures were 43.2-43.4 degrees C. Micrencephaly was found in 78% of heated newborn offspring, the mean brain weights of all groups being significantly less than controls. In the heated groups, the brain weights were reduced significantly as the interval between exposures decreased. Abnormalities other than micrencephaly were found in 10% of heated offspring and included exomphalos, clubfoot, and hypodactyly. In the second experiment, groups of mothers were exposed for 1 hour at intervals of 6-20 hr. The mean temperatures of heated groups were 42.6-42.9 degrees C. The mean brain weights of all groups of heated newborn were significantly reduced and micrencephaly was found in 61% of newborn. Brain weights were reduced significantly as mean maternal temperature increased. There was a significant interaction between the level of temperature elevation and the interval between exposures.(ABSTRACT TRUNCATED AT 250 WORDS)

Embryonic stress hypothesis of teratogenesis

Cells and tissues of essentially all eukaryotes respond uniformly to a variety of stressful situations. Immediately following the onset of several types of environmental insult (e.g., hyperthermia), genes for the so-called heat-shock proteins become unusually active; simultaneously, other genetic loci that were engaged in transcription at the onset of the insult become relatively less active. The biologic significance of the heat-shock response is unknown, as is its role, if any, in maintaining human health. In fact, the heat-shock response seems not to have been invoked previously to explain any aspect of human health or disease. Herein, the proposal is made that induction of the heat-shock response in the mammalian embryo during the critical period of organogenesis can alter the established program of activation and inactivation of genetic loci essential for normal intrauterine development, the result being anatomic malformation. By this hypothesis, induction of the heat-shock response provides a common pathway by which diverse environmental agents can result in any of a variety of developmental abnormalities, the precise period during gestation when the response is induced determining the nature of the abnormalities.

Detection of human teratogenic agents

Criteria for definition of a human teratogen are similar to Koch's postulates and include (1) presence of the agent during the critical period of development, (2) production of congenital defects by the agent in an experimental animal, and (3) evidence that the agent acts directly on the embryo or fetus. Examples of how teratogens have been identified through specific syndrome identification and by application of experimental animal information are given. Hyperthermia, an example of the latter, is discussed in some detail. Certain bias factors that confound epidemiologic studies are discussed. A system for detection an prevention of human teratogenicity is proposed. The system is based on the integration of three data bases: (1) the exposed parent-child pair, (2) the chemical and biologic effects of the agent, and (3) identification of specific exposure syndromes.

Syndromes with cephaloceles

Encephaloceles may occur as isolated malformations or together with other anomalies making up various syndromes or associations. With the possible exception of the aberrant tissue band syndrome and the Meckel syndrome, little attention has been paid to syndromes with encephaloceles as a group. This paper discusses syndromes of known genesis, including the aberrant tissue band syndrome, Chemke syndrome, cryptophthalmos syndrome, dyssegmental dwarfism, frontonasal dysplasia, Knobloch syndrome, Meckel syndrome, pseudo-Meckel syndrome, von Voss syndrome, and warfarin syndrome. Associations include a wide range of anomalies that may occur with encephalocele. Some abnormalities are secondary to encephalocele formation; others represent primary noncontiguous embryonic malformations. Associated anomalies include absent corpus callosum, orofacial clefting, craniostenosis, Dandy-Walker defect, Arnold-Chiari defect, ectrodactyly, hemifacial microsomia, hypothalamic-pituitary dysfunction, Klippel-Feil anomaly, iniencephaly, and myelomeningocele. Finally, several other conditions with encephalocele are discussed. Some may represent true low-frequency associations; others are spurious.