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Abstract

The phenotypic expression of autism, according to the Triple Hit Hypothesis, is determined by three factors: a developmental time window of vulnerability, genetic susceptibility, and environmental stressors. In utero exposure to thalidomide, valproic acid, and maternal infections are examples of some of the teratogenic agents which increase the risk of developing autism and define a time window of vulnerability. An additional stressor to genetically susceptible individuals during this time window of vulnerability may be prenatal ultrasound. Ultrasound enhances the genesis and differentiation of progenitor cells by activating the nitric oxide (NO) pathway and related neurotrophins. The effects of this pathway activation, however, are determined by the stage of development of the target cells, local concentrations of NO, and the position of nuclei (basal versus apical), causing consequent proliferation at some stages while driving differentiation and migration at others. Ill-timed activation or overactivation of this pathway by ultrasound may extend proliferation, increasing total cell number, and/or may trigger precipitous migration, causing maldistribution of neurons amongst cortical lamina, ganglia, white matter, and germinal zones. The rising rates of autism coincident with the increased use of ultrasound in obstetrics and its teratogenic/toxic effects on the CNS demand further research regarding a putative correlation.
Potential teratogenic effects of ultrasound on corticogenesis:
Implications for autism
q
E.L. Williams, M.F. Casanova
*
Department of Psychiatry and Behavioral Sciences, University of Louisville, Louisville, KY, USA
article info
Article history:
Received 12 January 2010
Accepted 20 January 2010
Available online xxxx
summary
The phenotypic expression of autism, according to the Triple Hit Hypothesis, is determined by three fac-
tors: a developmental time window of vulnerability, genetic susceptibility, and environmental stressors.
In utero exposure to thalidomide, valproic acid, and maternal infections are examples of some of the ter-
atogenic agents which increase the risk of developing autism and define a time window of vulnerability.
An additional stressor to genetically susceptible individuals during this time window of vulnerability
may be prenatal ultrasound. Ultrasound enhances the genesis and differentiation of progenitor cells by
activating the nitric oxide (NO) pathway and related neurotrophins. The effects of this pathway activa-
tion, however, are determined by the stage of development of the target cells, local concentrations of
NO, and the position of nuclei (basal versus apical), causing consequent proliferation at some stages while
driving differentiation and migration at others. Ill-timed activation or overactivation of this pathway by
ultrasound may extend proliferation, increasing total cell number, and/or may trigger precipitous migra-
tion, causing maldistribution of neurons amongst cortical lamina, ganglia, white matter, and germinal
zones. The rising rates of autism coincident with the increased use of ultrasound in obstetrics and its ter-
atogenic/toxic effects on the CNS demand further research regarding a putative correlation.
Ó2010 Published by Elsevier Ltd.
‘‘Would you like to see a picture of your baby?” The first words
uttered by an obstetrician to my daughter at 12 weeks of gesta-
tion. – Manuel F. Casanova, M.D.
Autism and the Triple Hit Hypothesis
Idiopathic autism is a group of multifactorial conditions charac-
terized by a profile of deficits in gestalt informational processing
(e.g., socialization, language, executive functions) but often accom-
panied by preserved or enhanced processing and orientation to de-
tail (e.g., hypersensitivity to sensory stimuli, savant abilities) [1].
Although it was once thought of as a rare condition affecting 4 in
every 10,000 children, it is now considered a spectrum ranging
from low- to high-functioning, with approximately 1 out of every
100 school age children receiving an autistic spectrum diagnosis
[2]. While debate as to whether this increase reflects a true rise
in occurrence still continues, a 25-fold increase in diagnostic rates
with a <100% monozygotic concordance [3,4] suggests that factors
other than genetics and improved medical and public awareness
may be at play.
Because environment appears to play some role in the develop-
ment of autism, in answer to this Casanova [5] has suggested a
‘Triple Hit Hypothesis of Autism’ in which the determinants of
the condition’s phenotypic expression are the confluence to vari-
ous degrees of: ‘‘(i) a critical period of brain development, (ii) an
underlying vulnerability [i.e., genetic factors], and (iii) exogenous
stressor(s)” (p. 423). A number of environmental factors in utero
are known to increase the risk of developing autism (e.g., valproic
acid, thalidomide, maternal infections), and these stressors suggest
a time window of vulnerability to the condition falling within the
first and second trimesters [6–8]. Genetics also play a considerable
role in predisposition and, while the conditions are not considered
necessarily ‘‘inherited” when contrasting concordance rates
amongst dizygotic twins, they are highly ‘‘heritable” and present
with high concordance in monozygotes [9]. Considering the genet-
ic predisposition in autism, the general vulnerability of the infant
during embryogenesis and early fetal development, and known
teratogens which have been shown to play a role in the etiology
of some cases of autism, other exogenous factors which target this
time period of vulnerability continue to be investigated [10]. One
agent, ultrasound, has received considerable attention due to its
toxic effects on tissue generation at nondiagnostic levels (with tox-
icity defining the delineation between diagnostic and nondiagnos-
tic cutoffs). However, investigations as to the potential teratogenic
effects of ultrasound on developing central nervous system (CNS)
are currently lacking. This article reviews research suggesting that
0306-9877/$ - see front matter Ó2010 Published by Elsevier Ltd.
doi:10.1016/j.mehy.2010.01.027
q
This work was supported by funding from the National Institutes of Health,
Grant Nos. R01 MH86784 and R01 MH88893.
*Corresponding author. Address: 500 S Preston St. Bldg. 55A Rm 217, Louisville,
KY 40202, USA. Tel.: +1 502 852 4077; fax: +1 502 852 4078.
E-mail address: m0casa02@louisville.edu (M.F. Casanova).
Medical Hypotheses xxx (2010) xxx–xxx
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Medical Hypotheses
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ultrasound can act as a CNS teratogen. It is hypothesized that this
exogenous stressor, coupled with genetic susceptibility, may give
rise to a portion of cases of autism if applied during susceptible
periods of development.
The neurobiology of autism
An important finding to date in the etiology of autism is an in-
crease in the number of neocortical minicolumns and a decrease in
peripheral neuropil between those units [11,12]. While there ap-
pears to be an increase in minicolumnar numbers in autism, the
number of cells contained within those columns is not affected
[13]. As might be expected in order to sustain interconnectivity
amongst a greater number of cells, an enhancement of local con-
nectivity accompanies this increase [14–16]. Smaller volumes of
commissural tracts are also observed, perhaps due to constriction
of the gyral window [17] and/or an economy of wiring compensat-
ing for the increase in local arcuate fibers [18,19].
Cortical heterotopias and laminar irregularities have been doc-
umented in the autism literature [20–22], although only minor
attention having been paid to them given the heterogeneity of
the abnormalities observed. However, despite that the specifics
of the neuronal maldistribution vary per individual, the occurrence
of disturbed cortical cell migration is still common to the condi-
tion. Subcortical heterotopias have also been noted [20,21], includ-
ing ectopic clusters within the periventricular zone [21] indicating
possible neural migratory arrestment prior to exiting the subven-
tricular stratum.
In lieu of the increased head circumference noted in early aut-
ism, particular attention has been paid to neurotrophins in recent
years, including the family of Fibroblast Growth Factors (FGFs),
which are intimately involved in progenitor cell proliferation. Be-
cause of the increase in neocortical cell number in autism, FGFs
have come under scrutiny, the subject of investigations of aberra-
tion along these pathways [23,24]. Brain-derived Neurotrophic
Factor (BDNF), involved in differentiation, and timing and location
of neural migration [25], has also been intimated in the etiology of
autism [26–28], although its role in the condition is less clear.
Time window of vulnerability
Progenitors of neuronal lineages divide within the ventricular
and subventricular zones forming an epithelium near the surface
of the cerebral ventricle. Many cells migrate radially towards the
expanding cortical plate along a radial glial scaffolding, eventually
settling in an inside-out fashion forming the vertical minicolumns
which typify the microstructure of the neocortex [29–31]. Glial
scaffolding subserves cortical neurons, acting as anchors during ra-
dial migratory locomotion and forming guide-like bridges along
which these postmitotic cells migrate to their final destinations
[30]. By gestational day 40 in human and non-human primates,
cells within the periventricular zone have completed the majority
of their symmetrical divisions [31].
As discussed, autism is typified by an increase in the number of
neocortical minicolumns, which subsequently gives rise to in-
creases in local arcuate fibers and decreases in long-range cortico-
cortical tracts [18]. Because the total complement of minicolumns
is determined by the number of progenitor cells, environmental
factors which specifically target the stage of symmetrical division
may alter the number of minicolumns. A number of teratogens
have already been shown to increase the risk of developing autism.
During this early stage of development, anticonvulsants taken dur-
ing pregnancy can cause Fetal Anticonvulsant Syndrome [32], with
congenital and cognitive abnormalities typically varying by drug,
and increased embryopathology often associated with combined
exposure to two or more anticonvulsants [33].In utero exposure
to valproate (VPA) in particular, and to a lesser extent carbamaze-
pine, has been associated with up to a 10-fold increase in expres-
sion of autistic-like symptoms [7,34–36]. This and related
research has in turn prompted the design of a successful VPA rat
model of autism [37].Why this medication in particular is more
teratogenic to the CNS than other anticonvulsants is unknown.
However, VPA crosses the placental barrier and has a tendency to
accumulate in the embryo’s blood stream; therefore, due to the
slower rate at which the medication is metabolized, valproate be-
haves pharmacokinetically different in the embryo than it does in
the mother, with greater dosage effect seen in the child [32].
Thalidomide is another teratogen associated with increased risk
of autism, with the height of its teratogenicity most typically seen
between 20 and 36 days postconception [6]. Despite the drug’s use
since the 1950s, autism was not associated with its exposure until
the 1990s when Strömland et al. [6] investigated a cohort of 100
children with thalidomide embryopathology, finding that of the 5
who presented with considerable learning difficulties, 4 met the
criteria for autism. While they did not investigate the remaining
95 children for signs of autism, the study showed that at least 4%
of these children were autistic, a 4-fold increase over present day
statistics [2].
Maternal infections have also been linked to increased rates of
psychiatric, neurological, and medical conditions in the offspring
[38–40]. Congenital rubella has been associated with the develop-
ment of schizophrenia [41], mental retardation [42], diabetes
mellitus [43], deafness, optic pathology, low birth weight, cardio-
vascular diseases, and both skeletal and muscular defects [44].
Following the last major outbreaks in the US in the 1960s, a
connection was noted between rubella and autism: Chess [45],in
following a cohort of 243 children diagnosed with congenital
rubella, found that 4% had co-occurring autism while a further
3% exhibited a ‘‘partial syndrome” (p. 33). While 7% were deemed
autistic or autistic-like in 1971, given the broadening of autism cri-
teria since that time, it is possible this percentage would be
deemed greater today.
Medications and maternal infections may exhibit enhanced ter-
atogenicity during embryogenesis and early fetal development, not
just because they are capable of affecting cell division and neural
topography, but because alterations in proliferation, differentia-
tion, and migration are foundational elements, altering widespread
modular relationships and synaptic mapping in the adult [8].
Therefore, when investigating the teratogenicity of ultrasound
and other agents, the question of ‘‘When?” is just as important as
‘‘How?” and ‘‘How much?”
Ultrasound as an exogenous stressor
It is well established in the literature that diagnostic levels of
ultrasound induce bone formation and have therefore been used
therapeutically for a number of years for the treatment of bone
fractures [46–48]. Ultrasound has also been shown to stimulate
chondrogenesis of mesenchymal (cartilage) stem cells [49]. Osteo-
and chondrogenic research into the method’s therapeutic mecha-
nisms has considerable implications, specifically whether the
waveform is capable of similar effects on developing CNS.
Ultrasound acts on osteoblasts within the bone, prolonging the
proliferative periods of stromal cells, preventing them from osteo-
clast differentiation [50]. Reher et al. [51] found that with diagnos-
tic exposure, obsteoblasts produced nitric oxide (NO), a free radical
which is a fundamental signal transduction molecule in vertebrate
organisms expressed in multiple cells types including the central
nervous system [52]. Likewise, other studies have noted increased
nitric oxide synthase (NOS) and NO activity in other tissue types
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following ultrasound exposure [53,54]. NO’s involvement in neuro-
genesis is evident in neuronal nitric oxide synthase (nNOS) knock-
out mice, which present with decreased numbers of progenitor
cells and subsequent reduction in olfactory neurons arising from
the subventricular zone in adulthood [55]. During the early embry-
onic stage and following neuronal insult, NO acts as a neuroprotec-
tive proliferative agent during a time when certain target-derived
growth factors are minimal or absent [56,57]. At other stages of
development, NO can have the opposite effect, driving differentia-
tion and migration of cells [58]. Not only is timing of NO pathway
activation key, studies also suggest that absolute concentration of
NO is a determining factor in the molecule’s biphasic activity
[59]. The location of a cell’s nucleus (basal versus apical) may also
determine the outcome of NO pathway activation [60]. Given its
involvement in cellular growth, differentiation, and apoptosis,
multiple studies have begun to investigate NO’s role in autism.
While the research is still new and its relevance uncertain, these
studies have consistently found significantly increased levels of
NO in the serum of patients with autism, possibly indicating abnor-
mal activation of this pathway in general [61–63].
Other research by Reher et al. [64] and Doan et al. [65] has
shown that ultrasound exposure produces increased levels of basic
Fibroblast Growth Factor (bFGF). Within the CNS, bFGF utilizes the
NO pathway to signal its trophic effects [66]. bFGF is responsible
for cellular proliferation within various germinal zones, including
the ventricular and subventricular zones [67,68]. Activation of
the NO pathway during this critical time period of development
triggers proliferative effects on progenitor cell division and aids
in determining total pyramidal cell number [55,69]. Because of
the increased cranial size in early childhood in autism [70] and
the condition’s tendency towards macrocephaly [71,72], recent
studies have targeted factors affecting neural growth, such as bFGF.
Genetics studies have also found linkage peaks on chromosome 10
in autism [73], a section of chromosome which is proximal to
genes encoding for the bFGF receptor, intimating the trophic fac-
tor’s possible role in development of the condition [69].
Other trophic factors have implications in autism, particularly
those involved in differentiation and migration [20,22]. Of signifi-
cance, BDNF plays a key role in stimulating these two events. NO
takes part in a positive feedback loop with BDNF determining both
the timing of differentiation and the location of migration [25],
with abnormal activation disrupting neuronal distribution
amongst lamina [74] and creating other nuclear and cortical het-
erotopias [75]. Ang et al. [76] have shown in a mouse model of
ultrasound exposure that an increased heterotopic effect is present
with increasing exposure times at diagnostic levels. It is suggestive
that while exposure to ultrasound can activate the NO pathway,
timing of exposure is integral in determining which trophic factors
are subsequently expressed. In addition, in progenitors, the precise
expression of neurotrophin receptors can likewise determine the
action of ultrasound on that cell [77]. Some research has already
linked autism with abnormalities in BDNF expression and accumu-
lation in serum [26–28] and heterotopias have been identified
postmortem in multiple cases of the condition [20,22]. Further
study is still needed, however, to elucidate BDNF’s involvement
in autism and how timing, concentration, and nuclear location play
integral roles in actions of the neurotrophin.
While atemporal model of neurotrophin action in autism is
suggestive, no behavioral animal models have been designed to
adequately illustrate the effects of ultrasound on earlier embryo-
genesis. For the behavioral animal models which have been used,
the vast majority of research has studied fetal development rather
than embryogenesis. For instance, in fetal mouse models behavior
was frequently affected, in particular certain types of learning and
memory [78–81] and locomotor activity [78–80]. But because no
model has been designed to illustrate effects on corticogenesis, it
is difficult to extrapolate to a condition such as autism. Neverthe-
less, the fact that ultrasound is capable of altering outward behav-
ior is strong evidence of its teratogenic effects on CNS in general. It
should be cautioned, however, that the behaviors seen in these
mice may be quite different from behaviors resultant from earlier
exposure, since stimulation of the NO pathway has stage-depen-
dent activation [56–58].
Despite the increased use of ultrasound in obstetrics, only a
small number of studies have investigated its immediate and
long-term effects during development. Of the studies which have
been performed, links have been proposed between increased
exposure and handedness [82–84], neurological deficits such as
speech delay [85], and low birth size/weight [86,87]. One study
also investigated the immediate effects of pulsed ultrasound on fe-
tal activity, observing increased gross body movements of the fetus
following exposure [88]. More pertinent to autism, however, Gre-
ther et al. [89] performed a retrospective case-control analysis of
ultrasound exposure and risk of autism in children born between
1995 and 1999. The authors looked mostly at autism simplex fam-
ilies (families without multiple autism incidences), and analyzed
both the number of exposures and the timing (trimester) in which
the infant had been exposed. While they found no consistent rela-
tionship indicating ultrasound as a risk factor, given the design of
the study a type II error may have occurred: while the significance
of ultrasound in autism may not solely be related to number of
exposures but an issue of timing, this study was unable to deter-
mine exposure according to precise landmarks of embryo- and fe-
tal-genesis but instead grouped exposure times within a broad
timescale, i.e., trimesters. Notably, the study used a mix of ap-
proaches to estimate gestational age at the time of scan, including
various algorithms and reports of the mother’s last menstrual per-
iod. Because differences of only a few days may be sufficient to al-
ter teratogenic outcome, imprecise methodology may cloud
significant group variation. And given the small but significant in-
crease in rates of autism in relation to other teratogenic exposures
such as thalidomide and VPA in which the rise in occurrence may
only be as great as 4-fold that of the general population, a percent-
age increase such as this may have been statistically overlooked in
the Grether et al. study [89].
Ultrasound deregulation and poor practices
Growing concern over ultrasound includes not only its in-
creased use in obstetrics since the 1960s, but its deregulation in
the 1990s [90]. Because image clarity is determined by frequency
and the degree of attenuation of the sonic wave, denser tissue
necessitates higher intensity output in order to penetrate deeply
enough to produce adequate image resolution [91]. Because of
the need to accommodate its use on a range of body sizes, the
FDA has loosened regulations on the maximum intensity allowed
for clinical use, provided each device comes equipped with thermal
and cavitational (mechanical) indicators to prevent the patient
from being subjected to dangerous levels of exposure. However,
thermal, cavitational, and other direct effects of sonography (i.e.,
acoustic streaming and radiation force) are still not well under-
stood, making it difficult to predict experimental and clinical out-
comes. In addition, earlier studies determining the safety of
ultrasound were performed using devices with considerably lower
output than what is available today [90].
But not only is equipment variability cause for concern, poor
practitioner education is alarming. Certification by the American
Registry for Diagnostic Medical Sonography (ARDMS) requires only
a 2-year course in an allied health education program (e.g., occupa-
tional therapist, physical therapist, registered nurse) and 1 year of
full-time practical experience [92]. However, while certification is
available, licensure is neither required nor available, stressing the
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lack of governmental regulation. General physician and obstetri-
cian awareness of equipment and safety procedures also seems
to be lacking: in a survey of 130 ultrasound end users, of whom
81 were physicians, only 17.7% demonstrated adequate under-
standing of the term ‘‘thermal index”, while even fewer, 3.8%, gave
the correct response explaining ‘‘mechanical index” [93]. And most
alarming of all, almost 80% of respondents did not know where to
find the acoustic indices (measurements of thermal and mechani-
cal outputs) nor that they were available on the monitor during
use. Awareness and understanding of these two safety indices
are vital in monitoring output in order to prevent patient exposure
to ultrasound levels known to be dangerous.
The advent of ultrasound use in the private sector is more cause
for concern, and despite that the FDA has issued official warnings
against the unwarranted use of ultrasound for ‘‘keepsake images”
[94] the general public appears largely unaware of the method’s
potential for harm (Table 1). Because nondiagnostic intensities of
ultrasonic waves are potentially toxic to tissue, even at diagnostic
levels ultrasound should be used with caution and in moderation.
In this sense, it is not unlike the earlier use of X-ray in obstetrics,
when the prevailing opinion of the past medical establishment be-
lieved that X-ray conferred negligible risks to the fetus. It was not
until later work by MacMahon [97] linking X-ray exposure to
childhood leukemia that its mutagenic effects were realized.
According to a recent report from the United States Office of the
Inspector General (OIG) [98] reviewing the year of 2007, a number
of high-use counties issued insurance claims for ultrasound ser-
vices amounting to approximately one sixth of total US claims
($336 million of a total of $2.1 billion). While a number of these
exhibited ‘‘questionable characteristics” (p. ii) indicative of false
insurance claims, these high-use counties in general received twice
as many ultrasound services than observed in other parts of the
country and the ratio of ultrasound providers to beneficiaries
was considerably increased. While this report focused on Medicare
claims (meaning that the total number of obstetric ultrasound ser-
vices would have been minimal given the demographic of typical
Medicare beneficiaries), it may be indicative of a general increase
in ultrasound use across various fields of medicine (for example,
see Ref. [99], p. 46). While increased use of ultrasound may be
reflective of FDA deregulation, the OIG report [98] also noted the
low cost of ultrasound machines as compared to other imaging
equipment, with used machines sometimes costing less than
$5000. And not only are ultrasound machines available at low cost
to practitioners, machines are available to anyone on websites such
as Amazon.com and eBay.com. However, while inexpensive easy
access to ultrasound equipment may facilitate the rise in diagnos-
tic and therapeutic use, doctor’s fear of legal reprisal may play a
considerable role in the growing trend:
Of particular concern to all parents is the risk of an abnormality
in their baby. Consumer demand for reassurance in this regard
is becoming overwhelming and the birth of an undetected
abnormal child may often be followed by attempts at litigation.
Failure to perform an ultrasound, cardiotocograph or other
medical tests at an appropriate time are commonly cited in
writs against doctors, midwives and hospitals ([99], p. 46).
Therefore, while inexpensive access to sonographic equipment
has allowed a greater number of professionals to practice and
while deregulation has given these practitioners more leeway,
the patient has sometimes played an active insistent role, not nec-
essarily understanding the risk she and her baby are undertaking
by unnecessary exposure.
Links proposed between autism and ultrasound are hardly new,
although their appearance has been minimal in peer-reviewed
works [100]. Nevertheless, the increased use of ultrasound in
obstetrics and the increase in autism diagnostic rates have not
gone unnoticed. And while correlation is not necessarily indicative
of causation, research is needed to determine what relationship, if
any, ultrasound and autism may have. But regardless of this rela-
tionship, current practice is in need of renewed regulations on
ultrasound use and practitioner/public availability, increased train-
ing, appropriate licensure, and a greater understanding of the bio-
physics of diagnostic ultrasound in order to ensure appropriate
patient safety.
Discussion
In the framework of nitric oxide pathway activation, the pres-
ence of increased neurogenesis and abnormal neuronal migration
in autism suggests an exogenous factor(s) which may target a win-
dow of vulnerability during corticogenesis, with disparate effects
determined by precise timing of exposure. Given the multiple
exposures pregnant women are subjected to nowadays [101] as
well as deregulation for the last two decades, a prime candidate
for this environmental agent is ultrasound. However, not only is
the timing of NO activation vital in determining its specific effects
on neurogenesis (hit #1 of the Triple Hit Hypothesis), but genetics
likely play a combinatorial role in that ultrasound may exponen-
tially perturb an already over-activated pathway (hits #2 and #3)
[102].
While the concept of autism as a spectrum has been generally
well accepted by professional and lay communities alike, in spite
of the spectrum’s commonalities the heterogeneity of the condi-
tion is undeniable. Not necessarily in the sense that Autistic Disor-
der should be considered separate from Asperger’s Syndrome, but
that even individuals falling under the same diagnostic label exhi-
bit not only behavioral but genetic [103] and neuroanatomical and
-physiological heterogeneity [20]. A uniting factor when consider-
ing this diversity may be the action of certain teratogens, such as
ultrasound, and the distinct effects they have based on timing,
number, and strength of exposure. Also, the diverse heterotopias
observed in autism may be explained by timing of exposure and
the distinct effects it has on cellular proliferation, differentiation,
and migration.
Research addressing the teratogenicity of ultrasound on the
developing CNS has been sparse. Given the more recent deregula-
tion, poor practitioner training, and the fact that ultrasound’s mech-
anisms of action, even for therapeutic use, are unknown, highlight
the need for further study. When investigating CNS teratogens, mi-
totic and heterotopic abnormalities may be subtle and not readily
apparent in histological samples, allowing significant disparities
Table 1
Clinical Practice Guidelines. These are the recommendations of the Alberta Medical Association regarding the use of diagnostic ultrasound during the first trimester [95]. See also
the American College of Radiology’s practice guidelines [96] for additional information.
Routine use of ultrasound during the first trimester is NOT recommended To diagnose pregnancy
For dating when last menstrual period and physical examination are concordant
Use of obstetric ultrasound during the first trimester IS indicated To confirm ectopic pregnancy, hydatidiform mole, or pelvic mass
Prior to planned termination of pregnancy
Where accurate dating is not available and medical intervention is anticipated
During invasive diagnostic procedures where guided visualization is required
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to be easily overlooked. Additionally, insult to the CNS, especially in-
sult to the neocortex and higher-order functions, may not be real-
ized until well beyond the postnatal period at a time when the
causative factor is no longer a consideration. While autism is not
likely caused by ultrasound alone, ultrasound may still be a deter-
mining factor in some cases, either playing a role in development
of the condition or worsening the severity of a condition already
present. Intrauterine exposure to other environmental agents, such
as thalidomide, valproate, maternal infections, and even maternal
stress [104,105], has already been shown to increase the risk of
developing autism. Given the similarities in exposure timing and
outcome of these different environmental agents (medications,
infection, and stress), they may activate similar or related pathways
as ultrasound during embryogenesis. Like ultrasound, infection and
maternal stress have both been found to activate the NO pathway
[106,107]. Thalidomide and valproate, on the other hand, inhibit
this pathway which, depending on the timing and duration of expo-
sure, may halt neurogenesis and trigger early differentiation, or may
inhibit differentiation and migration, prolonging cell proliferation
[108,109]. Regardless of these agents’ precise mechanisms of action
and unapparent commonalities, however, they each have the poten-
tial to behave as a teratogen, the third component of the Triple Hit
Hypothesis of Autism. It is this third component in the idiopathic
condition, combined with underlying genetic susceptibility and a
developmental time window of vulnerability, which ultimately
determines the autism phenotype.
Conflicts of interest statement
None declared.
Acknowledgments
We would like to thank Eugenius Ang and his colleagues for
their pioneering work into the effects of ultrasound as a CNS
teratogen, as well as Pasko Rakic for his lifetime body of work elu-
cidating corticogenesis. We would also like to thank David Whit-
lock for providing his expertise on the nitric oxide pathway, and
for bringing our attention to its possible involvement in autism.
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6E.L. Williams, M.F. Casanova / Medical Hypotheses xxx (2010) xxx–xxx
ARTICLE IN PRESS
Please cite this article in press as: Williams EL, Casanova MF. Potential teratogenic effects of ultrasound on corticogenesis: Implications for autism. Med
Hypotheses (2010), doi:10.1016/j.mehy.2010.01.027
... Studies in rodents 6,[13][14][15] have demonstrated that moderate amounts of prenatal ultrasound exposure can adversely affect neuronal migration and postnatal behavior, with a dose-response relationship caused by ultrasound-induced hyperthermia and/or mechanical cellular perturbation. Despite more than 10 years of speculation about a correlation between ultrasonography and ASD, [9][10][11][16][17][18] few studies have assessed this association 14, 19 and even fewer have evaluated effects of the current use of ultrasonography. Our study explored whether an association exists between fetal ultrasound exposure and later ASD in a population of children born at Boston Medical Center (BMC), an academic safety-net medical center serving a diverse population of families, many with low socioeconomic status. ...
... Multiple articles have stressed the need to investigate further the role that prenatal ultrasound exposure, particularly during the first trimester, may play in later development of autism. 7,10,11,18,22 Of note, the mean number of total scans in our study was more than 5, considerably more than the 1 to 2 scans recommended by the American Congress of Obstetrics and Gynecology in low-risk pregnancies. 23,24 Use of screening prenatal ultrasonography in low-risk pregnancies offers no improvement in neonatal outcomes compared with prenatal ultrasonography used only when medically necessary. ...
... Exposure to adverse environmental effects during all stages of pregnancy can influence neuronal migration in the brain. Animal studies demonstrate that prenatal ultrasound exposure may activate the differentiation of neural progenitor cells and result in extended proliferation and erroneous distribution of neurons within the cortical layers, increasing the risk for neurodevelopmental abnormalities, including ASD. 9,17,18 Outside obstetrics, energy generated by ultrasound can produce focused tissue ablation, taking advantage of the modality's hyperthermic properties. 27 Hyperthermia is a known teratogen during pregnancy, and even small increases in fetal tissue temperature owing to ultrasonography can be sufficient to cause tissue damage. ...
... Studies in rodents 6,[13][14][15] have demonstrated that moderate amounts of prenatal ultrasound exposure can adversely affect neuronal migration and postnatal behavior, with a dose-response relationship caused by ultrasound-induced hyperthermia and/or mechanical cellular perturbation. Despite more than 10 years of speculation about a correlation between ultrasonography and ASD, [9][10][11][16][17][18] few studies have assessed this association 14, 19 and even fewer have evaluated effects of the current use of ultrasonography. Our study explored whether an association exists between fetal ultrasound exposure and later ASD in a population of children born at Boston Medical Center (BMC), an academic safety-net medical center serving a diverse population of families, many with low socioeconomic status. ...
... Multiple articles have stressed the need to investigate further the role that prenatal ultrasound exposure, particularly during the first trimester, may play in later development of autism. 7,10,11,18,22 Of note, the mean number of total scans in our study was more than 5, considerably more than the 1 to 2 scans recommended by the American Congress of Obstetrics and Gynecology in low-risk pregnancies. 23,24 Use of screening prenatal ultrasonography in low-risk pregnancies offers no improvement in neonatal outcomes compared with prenatal ultrasonography used only when medically necessary. ...
... Exposure to adverse environmental effects during all stages of pregnancy can influence neuronal migration in the brain. Animal studies demonstrate that prenatal ultrasound exposure may activate the differentiation of neural progenitor cells and result in extended proliferation and erroneous distribution of neurons within the cortical layers, increasing the risk for neurodevelopmental abnormalities, including ASD. 9,17,18 Outside obstetrics, energy generated by ultrasound can produce focused tissue ablation, taking advantage of the modality's hyperthermic properties. 27 Hyperthermia is a known teratogen during pregnancy, and even small increases in fetal tissue temperature owing to ultrasonography can be sufficient to cause tissue damage. ...
Article
Importance The prevalence of autism spectrum disorder (ASD) has been increasing rapidly, with current estimates of 1 in 68 children affected. Simultaneously, use of prenatal ultrasonography has increased substantially, with limited investigation into its safety and effects on brain development. Animal studies have demonstrated that prenatal ultrasonography can adversely affect neuronal migration. Objective To quantify prenatal ultrasound exposure by the frequency, timing, duration, and strength of ultrasonographic scans in children with later ASD, developmental delay, and typical development. Design, Setting, and Participants This case-control study included 107 patients with ASD, 104 control individuals with developmental delay, and 209 controls with typical development. Participants were identified from medical records based on prenatal care and delivery at Boston Medical Center, a diverse, academic, safety-net medical center, from July 1, 2006, through December 31, 2014, with a gestational age at birth of at least 37 weeks. Data were analyzed from May 1, 2015, through November 30, 2017. Exposures Ultrasonographic exposure was quantified by the number and timing of scans, duration of exposure, mean strength (depth, frame rate, mechanical index, and thermal index), and time of Doppler and 3- and 4-dimensional imaging. Main Outcomes and Measures Among participants with ASD and controls with developmental delay and typical development, ultrasound exposure was quantified and compared per trimester and for the entire pregnancy, with adjustment for infant sex, gestational age at birth, and maternal age. Results A total of 420 participants were included in the study (328 boys [78.1%] and 92 girls [21.9%]; mean age as of January 1, 2016, 6.6 years; 95% CI, 6.5-6.8 years). The ASD group received a mean of 5.9 scans (95% CI, 5.2-6.6), which was not significantly different from the 6.1 scans (95% CI, 5.4-6.8) in the developmental delay group or the 6.3 scans (95% CI, 5.8-6.8) in the typical development group. Compared with the typical development group, the ASD group had shorter duration of ultrasound exposure during the first (290.4 seconds [95% CI, 212.8-368.0 seconds] vs 406.4 seconds [95% CI, 349.5-463.3 seconds]) and second (1687.6 seconds [95% CI, 1493.8-1881.4 seconds] vs 2011.0 seconds [95% CI, 1868.9-2153.1 seconds]) trimesters but no difference in the number of scans. The ASD group had greater mean depth of ultrasonographic penetration than the developmental delay group in the first trimester (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.1-12.1 cm]). The ASD group had greater mean depth than the typical development group during the first (12.5 cm [95% CI, 12.0-13.0 cm] vs 11.6 cm [95% CI, 11.3-12.0 cm]) and the second (12.9 cm [95% CI, 12.6-13.3 cm] vs 12.5 cm [95% CI, 12.2-12.7 cm]) trimesters. Conclusions and Relevance This study found significantly greater mean depth of ultrasonographic penetration in the ASD group compared with the developmental delay group in the first trimester and compared with the typical development group in the first and second trimesters. Further research is needed to determine whether other variables of ultrasound exposure also have adverse effects on the developing fetus.
... The severity of ASD varies from mild to severe. Casanova explains this spectrum in the "triple hit" hypothesis [73,74] wherein the development of autism is determined by the different interaction of three factors: the vulnerable stage of brain development, genetic susceptibility, and environmental impact, especially during the first trimester. Williams and Casanova [74] hypothesized that the severity of ASD depends on the timing, duration, and intensity of P-USG scanning to the embryo/fetus. ...
... Casanova explains this spectrum in the "triple hit" hypothesis [73,74] wherein the development of autism is determined by the different interaction of three factors: the vulnerable stage of brain development, genetic susceptibility, and environmental impact, especially during the first trimester. Williams and Casanova [74] hypothesized that the severity of ASD depends on the timing, duration, and intensity of P-USG scanning to the embryo/fetus. ...
... Several authors have hypothesized that the two are linked. 3,30 Similarly, however, over the same period, there has been tremendously increased use of cell phones, personal computers, frozen food, and many other "life improvements." Folic acid has been used, in relatively high doses, as a pregnancy supplement to prevent neural tube defects since the 1990s, and it too has been conjectured as a possible offender. ...
... Ultrasound has been mentioned as one such external factor. 3,30 It has been suggested that the window of vulnerability to external insults is, in fact, quite narrow and very early in gestation, with injuries to the central nervous system occurring during or just after neural tube closure. 82 Such injuries will cause a selective loss of neurons. ...
... hypothesis71,72 wherein the development of autism is determined by the different interaction of three factors: the vulnerable stage of brain development, genetic susceptibility, and environmental impact such as ultrasonic exposure primarily during the first trimester. Casanova hypothesized that the severity of ASD depends on the timing, duration, and intensity of PUS scanning to the embryo/fetus.In July 2016, Webb et al.73 retrospectively analyzed a modification of Casanova's hypothesis by analyzing a possible association between ASD severity and PUS exposure within the first trimester of pregnancy with fetuses genetically predisposed to ASD. ...
Preprint
For the past several decades, abdominal prenatal ultrasonography has been the most significant technology in obstetrics with a long-established application. However, the frequency, exposure time, thermal and cavitation exposure indices, and increased acoustic output of the ultrasonic waves may be harmful to the embryo/fetus and might increase susceptibility to Autism Spectrum Disorder (ASD). The increase in the prevalence of ASD is associated with an affluent ethnicity, high socioeconomic status, and high parental education where prenatal ultrasonography is readily available and affordable. Enhanced biophysical adverse effects may link the analogous increase in prenatal ultrasonography and autism, and prenatal ultrasonography may emerge as a risk factor for autism. Radiography usage provides historical evidence for this fact: the predominant past opinion was that exposure to X-rays during pregnancy caused no significant risk to a fetus. However, the association between X-ray exposure and childhood leukemia was only established 40 years after X-ray use began. This review focuses on excessive PUS usage and ASD development. Public Abstract Advancements in medical technology over the past several decades have made prenatal ultrasound more frequently accessible to expecting mothers during their pregnancy, especially for the affluent. A parallel development in health care is the increase in autism diagnoses (Autism Spectrum Disorder, or ASD) in children of affluent families. There is a general lack of studies of the impact of prenatal ultrasound on fetuses, especially around varying attributes such as frequency, duration of exposure, and thermal and cavitation indices. There is also a historical precedent set, where exposing fetuses to X-rays was not found to be harmful until it was linked to the development of childhood leukemia decades later. This paper seeks to establish a need to further study these attributes of prenatal ultrasound overuse and their possible impact on a developing fetus, with a special focus on the occurrence of Autism.
... [12,13] Other reviews said that there were still concerns about the diagnostic US's overall safety and its potential correlation to increased risk of autism spectrum disorders. [14][15][16][17] In general, the significant effects are not well understood on humans. ...
... Some researchers have speculated that certain insults or toxins play a role in causing autism in a subset of individuals with the disorder ( Landrigan et al., 2018;Williams & Casanova, 2010;Ye et al., 2017). For example, studies have documented proximity relationships between autism and major highways (Volk et al., 2011;von Ehrenstein, 2014) and open fields spread with pesticides (Shelton et al., 2014). ...
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This article provides an overview and discussion on many of the major topics currently discussed in the autism research community. These topics include: prevalence, male-to-female ratio, diagnosis, genetics, environmental factors, neurology, medical comorbidities, sensory processing, and behavior. The article concludes with a discussion of future research in the field of autism.
... In contrast, Casanova 3 hypothesized that ASD is a multifactorial disorder resulting from the convergence of a genetically vulnerable system exposed to an exogenous stressor during a critical period of neurodevelopment. 3 In the triple-hit model, an environmental hit contributes to an ASD outcome when delivered to a genetically vulnerable fetus during a vulnerable period of brain development (eg, first trimester). Our work finds a positive correlation between ASD symptom severity when ultrasonographic exposure occurs during the first trimester in male children with ASD and a genetic copy number variation, 5 consistent with risk based on genetic loci and sex 1 and the presence of additional environmental exposures. ...
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Autism spectrum disorder (ASD) is a behaviorally diagnosed disorder reflecting clinically significant and persistent impairments in social communication and restricted and repetitive behaviors, interests, and activities. Autism spectrum disorder is a heterogeneous disorder, with variability in severity of impairment, intellectual, and language ability and comorbid behavioral and medical concerns. The diagnostic category has undergone historical shifts over time, changing the boundaries of inclusion and categorization. As noted by Chaste et al,¹ our understanding of the etiology of ASD is complex and has also evolved through time. Currently, ASD, as with other psychiatric disorders, is thought to reflect a complex set of genetic and environmental contributions.¹ The most replicated findings of increased nongenetic risk for ASD include pregnancy-related complications and perinatal conditions.² Unfortunately, the focus on other environmental causes has often progressed down less mechanistically justifiable pathways.
... 5 Ultrasound has been identified as potentially one of the environmental factors that may influence the incidence and severity of autism. 6 To date, analysis of in utero ultrasound exposure in humans has failed to show harmful effects in neonates or children, particularly in school performance, attention disorders, and behavioural changes. 2 There is no independently confirmed peer-reviewed published evidence that a cause-effect relationship exists between in utero exposure to clinical ultrasound and development of ASD in childhood from recent high quality studies. [7][8][9] The recent study by Webb et al. involves a case series endeavouring to assess factors influencing ASD severity rather than ASD incidence. ...
Article
Lay summary: This Keynote Lecture, delivered at the 2016 meeting of the International Society for Autism Research, discusses evidence from human epidemiologic studies of prenatal factors contributing to autism, such as pesticides, maternal nutrition and her health. There is no single cause for autism. Examples highlight the features of a high-quality epidemiology study, and what comprises a compelling case for causation. Emergent research directions hold promise for identifying potential interventions to reduce disabilities, enhance giftedness, and improve lives of those with ASD.
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Studies of brain evolution have moved rapidly in recent years, building on the pioneering research of Harry J. Jerison. This book provides reviews of primate (including human) brain evolution. The book is divided into two sections, the first gives new perspectives on the developmental, physiological, dietary and behavioural correlates of brain enlargement. It has long been recognized, however, that brains do not merely enlarge globally as they evolve, but that their cortical and internal organization also changes in a process known as reorganization. Species-specific adaptations therefore have neurological substrates that depend on more than just overall brain size. The second section explores these neurological underpinnings for the senses, adaptations and cognitive abilities that are important for primates. With a prologue by Stephen J. Gould and an epilogue by Harry J. Jerison, this is an important reference work for all those working on brain evolution in primates.
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Ultrasound accelerates enzymatic fibrinolysis in vitro and in animal models and may be used as an adjunct to thrombolytic therapy. Ultrasound can also affect vascular tone directly, and we have now investigated the effect of ultrasound on tissue perfusion in a rabbit model of acute muscle ischemia to characterize the magnitude and temporal course of vasodilation and determine its mechanism. After ligation of the femoral artery of rabbits, tissue perfusion in the gracilis muscle as determined using a laser Doppler probe declined by 53% from 13.7 +/- 0.3 U to 6.4 +/- 0.2 U. The tissue became acidotic as pH declined from normal to 7.05 +/- 0.2. Application of 40 kHz ultrasound at intensities from 0.25 to 0.75 W/cm(2) progressively improved perfusion over 60 min and reversed acidosis, but, these effects were both completely blocked by pre-treatment with the nitric oxide synthase inhibitor L-NAME. Nitric oxide synthase activity in muscle was measured using an assay based on the conversion of radiolabeled L-arginine to L-citrulline and demonstrated an increase of 3.6-fold following ultrasound exposure. This effect was greatest at locations close to the transducer and declined progressively away from it. Histologic examination showed greater capillary circumference in ultrasound exposed muscle compared to unexposed tissue with no other histologic changes. We conclude that the application of 40 kHz at low intensity improves perfusion and reverses acidosis in acutely ischemic muscle through a nitric oxide dependent mechanism.
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A neuropathological study of autism was established and brain tissue examined from six mentally handicapped subjects with autism. Clinical and educational records were obtained and standardized diagnostic interviews conducted with the parents of cases not seen before death. Four of the six brains were megalencephalic, and areas of cortical abnormality were identified in four cases. There were also developmental abnormalities of the brainstem, particularly of the inferior olives. Purkinje cell number was reduced in all the adult cases, and this reduction was sometimes accompanied by gliosis. The findings do not support previous claims of localized neurodevelopmental abnormalities. They do point to the likely involvement of the cerebral cortex in autism.
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BACKGROUND—Anticonvulsants taken in pregnancy are associated with an increased risk of malformations and developmental delay in the children. To evaluate the pattern of abnormalities associated with prenatal anticonvulsant exposure further, we undertook a clinical study of 57 children with fetal anticonvulsant syndromes. METHODS—Fifty two children were ascertained through the Fetal Anticonvulsant Syndrome Association and five were referred to the Aberdeen Medical Genetics Service. Pregnancy and medical history were obtained through a standardised questionnaire and interview and the children were examined. RESULTS—Thirty four (60%) were exposed in utero to valproate alone, four (7%) to carbamazepine alone, four (7%) to phenytoin alone, and 15 (26%) to more than one anticonvulsant. Forty six (81%) reported behavioural problems, 22 (39%) with hyperactivity or poor concentration of whom four (7%) had a diagnosis of attention deficit and hyperactivity disorder. Thirty four (60%) reported two or more autistic features, of whom four had a diagnosis of autism and two of Asperger's syndrome. Forty four (77%) had learning difficulties, 46 (81%) had speech delay, 34 (60%) had gross motor delay, and 24 (42%) had fine motor delay. Nineteen (33%) had glue ear and 40 (70%) had joint laxity involving all sizes of joints. Of 46 who had formal ophthalmic evaluation, 16 (34%) had myopia. CONCLUSIONS—Speech delay, joint laxity, glue ear, and myopia are common in the fetal anticonvulsant syndromes and autistic features and hyperactivity form part of the behavioural phenotype. Keywords: fetal anticonvulsant syndrome; fetal valproate syndrome; teratogen; birth defects
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Background: We tested the hypothesis that maternal infections during pregnancy are associated with the subsequent development of schizophrenia and other psychoses in adulthood. Methods: We conducted a nested case-control study of 27 adults with schizophrenia and other psychotic illnesses and 54 matched unaffected control subjects (matched for sex, ethnicity, and date of birth) from the Providence, RI, cohort of the Collaborative Perinatal Project. We retrieved stored blood samples that had been obtained from these mothers at the end of pregnancy. These samples were analyzed for total class-specific immunoglobulins and for specific antibodies directed at recognized perinatal pathogens capable of affecting brain development.
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This report summarizes knowledge accumulated in a long-term study of congenital and maternal cytomegalovirus (CMV) infection in Sweden. Some new findings are included. We considered diagnostic methods, sources of maternal infection (including occupational risks), roles of primary and secondary maternal infections, transmission to foetuses, incidence, symptoms and prognosis of established congenital infection and relative importance of such infection in infantile sensorineural deafness, microcephaly and type 1 diabetes mellitus. Virus isolation testing was done 1977-1985 on 16,474 newborns. 76 (0.5%) congenitally infected infants were found, 22/76 (29%) with transient neonatal symptoms and 11/60 (18%) with neurological symptoms by the age of 7 y. Type of maternal CMV infection was serologically determined in 62/76 cases (30 primary, 32 secondary). CNS disturbances in the infants occurred after both primary (all trimesters) and secondary maternal infections. The negative potential of secondary maternal infe...