Fetal magnetoencephalography

The German Gestational Diabetes Study (PREG), a prospective multicentre cohort study: rationale, methodology and design

INTRODUCTION

Even well-treated gestational diabetes mellitus (GDM) might still have impact on long-term health of the mother and her offspring, although this relationship has not yet been conclusively studied. Using in-depth phenotyping of the mother and her offspring, we aim to elucidate the relationship of maternal hyperglycaemia during pregnancy and adequate treatment, and its impact on the long-term health of both mother and child.

METHODS

The multicentre PREG study, a prospective cohort study, is designed to metabolically and phenotypically characterise women with a 75-g five-point oral glucose tolerance test (OGTT) during, and repeatedly after pregnancy. Outcome measures are maternal glycaemia during OGTTs, birth outcome and the health and growth development of the offspring. The children of the study participants are followed up until adulthood with developmental tests and metabolic and epigenetic phenotyping in the PREG Offspring study. A total of 800 women (600 with GDM, 200 controls) will be recruited.

ETHICS AND DISSEMINATION

The study protocol has been approved by all local ethics committees. Results will be disseminated via conference presentations and peer-reviewed publications.

TRIAL REGISTRATION NUMBER

The PREG study and the PREG Offspring study are registered with Clinical Trials (ClinicalTrials.gov identifiers: NCT04270578, NCT04722900).

A review of the characteristics, mechanisms and clinical significance of habituation in foetuses and newborn infants

Habituation has been a topic of interest since the early 20th century. We summarise the characteristics of habituation, the proposed habituation mechanisms, the associated cortical responses and the link between habituation and cognitive development. Behavioural and neuroimaging studies have highlighted the early sensory abilities of foetuses and newborn infants, with preterm newborn infants exhibiting decreased habituation and dishabituation capabilities that increase their environmental vulnerability. Habituation provides a foundation for the learning and cognition on which higher functions are constructed. It has been suggested that it is efficient for predicting cognitive developmental outcomes in term and preterm newborn infants.

Tracking evoked responses to auditory and visual stimuli in fetuses exposed to maternal high-risk conditions

Magnetoencephalography (MEG) has been successfully applied to record fetal auditory (auditory evoked response [AER]) and visual evoked responses (VER). In this study, we report the AER and VER development trajectory by tracking the evoked response detectability and latency from recordings starting at 27 weeks of gestation in pregnancies classified as high risk. Fetal MEG and ultrasound recordings were performed on 158 pregnant women, and the total number of fetal auditory and visual tests conducted was 321 and 237, respectively. The overall evoked response analysis showed 237 AER (73.8%) and 164 VER detections (69.2%). The mean AER latency was 290.7 (SD 125.5) ms and the mean VER latency was 293.7 (SD 114.5) ms. The rate of decrease (95% confidence limits) in average AER and VER first-peak latency between 100-350 ms was 1.97 (-1.86, +5.81) ms/week and 1.35 (-3.83, +6.53) ms/week, respectively. This trend in high-risk fetuses conforms to the general trajectory of decrease in latency with gestational age progression, even though this decrease was non-significant, as reported in the case of normal growing fetuses. Although there was a significant difference in detection rates between male and female fetuses, this was not reflected in either latency values or the sensory modality applied. Furthermore, the main factors that had the most significant effect on response detectability included the presence of intervening layers of adipose tissue between the fetal head and stimulus source and an increase in the maternal body mass index.

Exploring early human brain development with structural and physiological neuroimaging

Early brain development, from the embryonic period to infancy, is characterized by rapid structural and functional changes. These changes can be studied using structural and physiological neuroimaging methods. In order to optimally acquire and accurately interpret this data, concepts from adult neuroimaging cannot be directly transferred. Instead, one must have a basic understanding of fetal and neonatal structural and physiological brain development, and the important modulators of this process. Here, we first review the major developmental milestones of transient cerebral structures and structural connectivity (axonal connectivity) followed by a summary of the contributions from ex vivo and in vivo MRI. Next, we discuss the basic biology of neuronal circuitry development (synaptic connectivity, i.e. ensemble of direct chemical and electrical connections between neurons), physiology of neurovascular coupling, baseline metabolic needs of the fetus and the infant, and functional connectivity (defined as statistical dependence of low-frequency spontaneous fluctuations seen with functional magnetic resonance imaging (fMRI)). The complementary roles of magnetic resonance imaging (MRI), electroencephalography (EEG), magnetoencephalography (MEG), and near-infrared spectroscopy (NIRS) are discussed. We include a section on modulators of brain development where we focus on the placenta and emerging placental MRI approaches. In each section we discuss key technical limitations of the imaging modalities and some of the limitations arising due to the biology of the system. Although neuroimaging approaches have contributed significantly to our understanding of early brain development, there is much yet to be done and a dire need for technical innovations and scientific discoveries to realize the future potential of early fetal and infant interventions to avert long term disease.

Magnetoencephalography and the infant brain

Magnetoencephalography (MEG) is a non-invasive neuroimaging technique that provides whole-head measures of neural activity with millisecond temporal resolution. Over the last three decades, MEG has been used for assessing brain activity, most commonly in adults. MEG has been used less often to examine neural function during early development, in large part due to the fact that infant whole-head MEG systems have only recently been developed. In this review, an overview of infant MEG studies is provided, focusing on the period from birth to three years. The advantages of MEG for measuring neural activity in infants are highlighted (See Box 1), including the ability to assess activity in brain (source) space rather than sensor space, thus allowing direct assessment of neural generator activity. Recent advances in MEG hardware and source analysis are also discussed. As the review indicates, efforts in this area demonstrate that MEG is a promising technology for studying the infant brain. As a noninvasive technology, with emerging hardware providing the necessary sensitivity, an expected deliverable is the capability for longitudinal infant MEG studies evaluating the developmental trajectory (maturation) of neural activity. It is expected that departures from neuro-typical trajectories will offer early detection and prognosis insights in infants and toddlers at-risk for neurodevelopmental disorders, thus paving the way for early targeted interventions.

Heart rate variability parameters and fetal movement complement fetal behavioral states detection via magnetography to monitor neurovegetative development

Fetal behavioral states are defined by fetal movement and heart rate variability (HRV). At 32 weeks of gestational age (GA) the distinction of four fetal behavioral states represented by combinations of quiet or active sleep or awakeness is possible. Prior to 32 weeks, only periods of fetal activity and quiesence can be distinguished. The increasing synchronization of fetal movement and HRV reflects the development of the autonomic nervous system (ANS) control. Fetal magnetocardiography (fMCG) detects fetal heart activity at high temporal resolution, enabling the calculation of HRV parameters. This study combined the criteria of fetal movement with the HRV analysis to complete the criteria for fetal state detection. HRV parameters were calculated including the standard deviation of the normal-to-normal R–R interval (SDNN), the mean square of successive differences of the R–R intervals (RMSSD, SDNN/RMSSD ratio, and permutation entropy (PE) to gain information about the developing influence of the ANS within each fetal state. In this study, 55 magnetocardiograms from healthy fetuses of 24–41 weeks’ GA were recorded for up to 45 min using a fetal biomagnetometer. Fetal states were classified based on HRV and movement detection. HRV parameters were calculated for each state. Before GA 32 weeks, 58.4% quiescence and 41.6% activity cycles were observed. Later, 24% quiet sleep state (1F), 65.4% active sleep state (2F), and 10.6% active awake state (4F) were observed. SDNN increased over gestation. Changes of HRV parameters between the fetal behavioral states, especially between 1F and 4F, were statistically significant. Increasing fetal activity was confirmed by a decrease in PE complexity measures. The fHRV parameters support the differentiation between states and indicate the development of autonomous nervous control of heart rate function.

Habituation of visual evoked responses in neonates and fetuses: a MEG study

In this study we aimed to develop a habituation paradigm that allows the investigation of response decrement and response recovery and examine its applicability for measuring the habituation of the visually evoked responses (VERs) in neonatal and fetal magnetoencephalographic recordings. Two paradigms, one with a long and one with a short inter-train interval (ITI), were developed and tested in separate studies. Both paradigms consisted of a train of four light flashes; each train being followed by a 500Hz burst tone. Healthy pregnant women underwent two prenatal measurements and returned with their babies for a neonatal investigation. The amplitudes of the neonatal VERs in the long-ITI condition showed within-train response decrement. An increased response to the auditory dishabituator was found confirming response recovery. In the short-ITI condition, neonatal amplitude decrement could not be demonstrated while response recovery was present. In both ITI conditions, the response rate of the cortical responses was much lower in the fetuses than in the neonates. Fetal VERs in the long-ITI condition indicate amplitude decline from the first to the second flash with no further decrease. The long-ITI paradigm might be useful to investigate habituation of the VERs in neonates and fetuses, although the latter requires precaution.

Removal of interference from fetal MEG by frequency dependent subtraction

Fetal magnetoencephalography (fMEG) recordings are contaminated by maternal and fetal magnetocardiography (MCG) signals and by other biological and environmental interference. Currently, all methods for the attenuation of these signals are based on a time-domain approach. We have developed and tested a frequency dependent procedure for removal of MCG and other interference from the fMEG recordings. The method uses a set of reference channels and performs subtraction of interference in the frequency domain (SUBTR). The interference-free frequency domain signals are converted back to the time domain. We compare the performance of the frequency dependent approach with our present approach for MCG attenuation based on orthogonal projection (OP). SUBTR has an advantage over OP and similar template approaches because it removes not only the MCG but also other small amplitude biological interference, avoids the difficulties with inaccurate determination of the OP operator, provides more consistent and stable fMEG results, does not cause signal redistribution, and if references are selected judiciously, it does not reduce fMEG signal amplitude. SUBTR was found to perform well in simulations and on real fMEG recordings, and has a potential to improve the detection of fetal brain signals. The SUBTR removes interference without the need for a model of the individual interference sources. The method may be of interest for any sensor array noise reduction application where signal-free reference channels are available.

Increasing the efficiency of neonatal MEG measurements by alternating auditory and tactile stimulation

OBJECTIVE

To evaluate the possible effect of intervening auditory stimulation on somatosensory evoked magnetic fields in newborns.

METHODS

We recorded auditory and tactile evoked responses with magnetoencephalography (MEG) from two groups of healthy newborns. One group (n=11) received only tactile stimuli to the index finger, the other (n=11) received alternating tactile and auditory (vowel [a:] with 300-ms duration) stimuli. The interval between subsequent tactile stimuli was always 2 s. We analyzed the equivalent current dipoles (ECDs) of the main auditory and somatosensory responses.

RESULTS

The ECDs of the tactile responses agreed with activation of the primary somatosensory cortex at ∼60 ms and the secondary somatosensory region at ∼200 ms. The source of the auditory response (∼250 ms) was clearly distinct from those to tactile stimulation and in line with auditory cortex activation. The intervening auditory stimulation did not affect the strength, latency, or location of the ECDs of the tactile responses.

CONCLUSIONS

Auditory and tactile MEG responses from newborns can be obtained in one measurement session.

SIGNIFICANCE

The alternating stimulation can be used to shorten the total measurement time and/or to improve the signal to noise ratio by collecting more data.

Early development of brain responses to rapidly presented auditory stimulation: a magnetoencephalographic study

BACKGROUND

The processing of rapidly presented stimuli has been shown to be a precursor for the perception of speech in infants, long before they learn to speak. However, the onset and early development of rapid temporal processing (RTP) skills is not yet well understood. The main goal of this study was to assess the development of RTP skills during the prenatal and early postnatal stages of life.

METHODOLOGY

Tone pairs were presented in two difficulties (long and short) and event-related magnetic fields were recorded using MEG. Pregnant women (22) (gestational ages between 29 and 38 weeks') participated in the fetal study and 15 returned for a neonatal follow-up study between 2 and 38 days after delivery or 38 and 44 weeks gestational age (GA).

RESULTS

In the postnatal follow-up study, a trend towards two peaks with increasing chronological and gestational age was observed in the longer tone pair. However, no such trend was evident in neonatal responses to the short tone pairs or in fetal recordings.

CONCLUSIONS

Neonates showed a gradual trend to successful processing of the longer tone pair with increasing age. By 22 days of chronological age, the infants processed this tone pair successfully, as indicated by two-peak waveforms. Therefore, the first 3 weeks of life could be critical for the development of RTP.

SIGNIFICANCE

This study is a first approach towards the assessment of early RTP development. The results provide promising indications for future studies, which might lead to an early detection of deficits in speech perception and therefore prevent further language impairments.

Basic consciousness of the newborn

The newborn shows several signs of consciousness, such as being awake and aware of him/herself and mother. The infant processes olfactory and painful inputs in the cortex, where consciousness is believed to be localized. Furthermore, the newborn expresses primary emotions such as joy, disgust, and surprise and remember rhymes and vowels to which he or she has been exposed during fetal life. Thus, the newborn infant fulfills the criteria of displaying a basic level of consciousness, being aware of its body and him/her-self and somewhat about the external world. Preterm infants may be conscious to a limited degree from about 25 weeks, when the thalamocortical connections are established.

Cerebral blood flow and metabolism in the developing fetus

The inaccessibility of the human fetal brain to studies of perfusion and metabolism has impeded progress in the understanding of the normal and abnormal systems of oxygen substrate supply and demand. Consequently, current understanding is based on studies in fetal animals or in the premature infant (ex utero fetus), neither of which is ideal. Despite promising developments in fetal magnetic resonance imaging (MRI) and Doppler ultrasound, major advances in fetal neurodiagnostics will be required before rational and truly informed brainoriented care of the fetus becomes feasible.

Fetal neurological assessment using noninvasive magnetoencephalography

SQUID Array for Reproductive Assessment is a unique magnetoencephalography device designed for the noninvasive recording of fetal brain activity. In this article, we provide a general overview of the technology and its potential application to fetal medicine. A large number of studies that have been conducted and published describing this device since it was brought into operation are referenced throughout the article.

Fetal MEG evoked response latency from beamformer with random field theory

Analysis of fetal magnetoencephalographic brain recordings is restricted by low signal to noise ratio (SNR) and non-stationarity of the sources. Beamformer techniques have been applied to improve SNR of fetal evoked responses. However, until now the effect of non-stationarity was not taken into account in detail, because the detection of evoked responses is in most cases determined by averaging a large number of trials. We applied a windowing technique to improve the stationarity of the data by using short time segments recorded during a flash-evoked study. In addition, we implemented a random field theory approach for more stringent control of false-positives in the statistical parametric map of the search volume for the beamformer. The search volume was based on detailed individual fetal/maternal biometrics from ultrasound scans and fetal heart localization. Average power over a sliding window within the averaged evoked response against a randomized average background power was used as the test z-statistic. The significance threshold was set at 10% over all members of a contiguous cluster of voxels. There was at least one significant response for 62% of fetal and 95% of newborn recordings with gestational age (GA) between 28 and 45 weeks from 29 subjects. We found that the latency was either substantially unchanged or decreased with increasing GA for most subjects, with a nominal rate of about -11 ms/week. These findings support the anticipated neurophysiological development, provide validation for the beamformer model search as a methodology, and may lead to a clinical test for fetal cognitive development.

The emergence of human consciousness: from fetal to neonatal life

A simple definition of consciousness is sensory awareness of the body, the self, and the world. The fetus may be aware of the body, for example by perceiving pain. It reacts to touch, smell, and sound, and shows facial expressions responding to external stimuli. However, these reactions are probably preprogrammed and have a subcortical nonconscious origin. Furthermore, the fetus is almost continuously asleep and unconscious partially due to endogenous sedation. Conversely, the newborn infant can be awake, exhibit sensory awareness, and process memorized mental representations. It is also able to differentiate between self and nonself touch, express emotions, and show signs of shared feelings. Yet, it is unreflective, present oriented, and makes little reference to concept of him/herself. Newborn infants display features characteristic of what may be referred to as basic consciousness and they still have to undergo considerable maturation to reach the level of adult consciousness. The preterm infant, ex utero, may open its eyes and establish minimal eye contact with its mother. It also shows avoidance reactions to harmful stimuli. However, the thalamocortical connections are not yet fully established, which is why it can only reach a minimal level of consciousness.

Delayed maturation of auditory-evoked responses in growth-restricted fetuses revealed by magnetoencephalographic recordings

OBJECTIVE

The purpose of this study was to investigate fetal brain development of growth-restricted fetuses with auditory evoked responses (AER) that were recorded by the noninvasive magnetoencephalographic technique.

STUDY DESIGN

Serial fetal recordings that started at 27 weeks of gestation were conducted on a fetal magnetoencephalographic device that was especially designed for obstetric assessment. Fifteen normotrophic fetuses were compared with 14 hypotrophic fetuses. After birth, 10 of the hypotrophic fetuses were diagnosed with asymmetric growth restriction; 4 fetuses were classified as symmetrically small for gestational age.

RESULTS

Fetal AER latencies in both groups showed an average developmental decrease of 12.74 msec/wk (P = .0035). Hypotrophic fetuses had longer age-adjusted latencies compared with normotrophic fetuses, with a difference of 73.5 msec (P = .034). The subgroup of symmetrically growth-restricted fetuses showed the longest latencies for age, with a difference from the normotrophic fetuses of 120.0 msec (P = .045).

CONCLUSION

The results indicate that biomagnetically recorded AER can be used to monitor functional brain development in growth-restricted fetuses.

Neonatal and fetal response decrement of evoked responses: a MEG study

OBJECTIVE

To investigate the response decrements of visual evoked responses (VER) in newborns and assess the applicability of this paradigm to fetuses in magnetoencephalographic (MEG) recordings.

METHODS

Twelve newborns with no known risks or complications participated at chronological ages between 6 and 22days. They constituted the follow-up group to a prenatal study conducted on a sample of 25 fetuses whose gestational age (GA) varied between 29 and 37weeks at the time of recording. Trains of four light flashes with an interstimulus interval of 2s followed by 10s without stimulation were delivered to record VER.

RESULTS

Nine of the 12 newborns responded to the stimulation and showed response decrements in amplitude from the first to the last light flash. Furthermore, the response latency increased significantly from the first to the last stimulus. The remaining three recordings were discontinued early. Even though the prenatal visual evoked response rate was only 29%, the fetuses exhibited a response decrement after the first stimulus.

CONCLUSIONS

The amplitude of VERs can be used to elicit a response decrement in newborns and, with limitations, even in fetuses. This paradigm might be a useful tool for a direct non-invasive assessment of neonatal and prenatal brain development and CNS functioning.

SIGNIFICANCE

The proposed method might be a first step towards an early detection of developmental deficits in newborns and fetuses.

Fetal brain monitoring: future applications

Future application of fetal brain monitoring is explored by selecting and analysing articles for information on types of brain damage that can be monitored, where in the brain this can be done, how long after the risk exposure, and with what method of investigation. A limited number of--mainly--case histories reported that early (cell death and oedema) and late (gliosis) effects of brain damage can be demonstrated before birth with multiplanar ultrasound and magnetic resonance imaging, and that hypoxic ischaemic injury or infection can induce local or widespread brain injury, occurring as transient or longer-lasting changes in age-related predilection areas for which normal features are known. The antenatal role of risk factors inducing abnormal brain development can be studied longitudinally with ultrasound and magnetic resonance imaging. A multidisciplinary approach will facilitate the introduction of various techniques with adequate know-how of underlying processes, to evaluate the predictive value on neurological outcome and prevent premature introduction into clinical application.

Functional MRI of the newborn

In order to provide accurate prognosis and developmental intervention to newborns, new methods of assessing cerebral functions are needed. The non-invasive technique of functional magnetic resonance imaging (fMRI) can be considered as the leading technique for functional exploration of the infant's brain. Several studies have previously applied fMRI in both healthy and diseased newborns with different sensory and cognitive tasks. In this chapter, the methodological issues that are proper to the use of fMRI in the newborn are detailed. In addition, an overview of the major findings of previous fMRI studies is provided, with a focus on notable differences from those in adult subjects. More specifically, the functional responses and the localization of cortical activations in healthy and diseased newborns are discussed. We expect a rapid expansion of this field and the establishment of fMRI as a valid clinical diagnostic tool in the newborn.

Magnetoencephalography of the newborn brain

This paper reviews the use of event-related magnetic fields (ERFs) in infants; ERFs can be derived from magnetoencephalography by means of averaging. Basic perceptive skills are important prerequisites for the infant's later development. The automatic cortical processes related to processing auditory, somatosensory and visual stimuli can be addressed by using responses recorded directly from the brain. The traditional method, the event-related potential (ERP), has recently been accompanied by ERFs. Similarly to ERPs, higher processes related to short-term memory, stimulus comparisons, and attention allocation can also be studied with ERFs. Further, since addressing the neonatal higher cognitive and social capabilities is challenging using only behavioural means, ERFs provide information on these important functions at a very early stage immediately after birth or in some cases even before birth. The main advantage of ERFs, compared to ERPs, is detection of the signals with high accuracy both with respect to the noise level and estimation of the spatial location.