Preterm Infants Exhibit Greater Variability in Cerebrovascular Control than Term Infants

Physiology during sleep in preterm infants: Implications for increased risk for the sudden infant death syndrome

Approximately 15 million babies are born preterm (<37 weeks of completed gestation) worldwide annually. Although neonatal and perinatal medicine have contributed to the increased survival rate of preterm newborn infants, premature infants are at increased risk of mortality in the first years of life. Infants born preterm are at four times the risk of Sudden Infant Death Syndrome (SIDS) compared to infants born at term. SIDS is believed to be multifactorial in origin. The Triple Risk hypothesis has been proposed to explain this. The model suggests that when a vulnerable infant, such as one born preterm, is at a critical but unstable developmental period in homeostatic control, death may occur if exposed to an exogenous stressor, such as being placed prone for sleep. The highest risk period is at ages 2-4 months, with 90 % of deaths occurring before 6 months. The final pathway to SIDS is widely believed to involve some combination of immature cardiorespiratory control and a failure of arousal from sleep. This review will focus on the physiological factors which increase the risk for SIDS in preterm infants and how these factors may be identified and potentially lead to effective preventative strategies.

Cerebrovascular responses to a 90° tilt in healthy neonates

BACKGROUND

Tilts can induce alterations in cerebral hemodynamics in healthy neonates, but prior studies have only examined systemic parameters or used small tilt angles (<90°). The healthy neonatal population, however, are commonly subjected to large tilt angles (≥90°). We sought to characterize the cerebrovascular response to a 90° tilt in healthy term neonates.

METHODS

We performed a secondary descriptive analysis on 44 healthy term neonates. We measured cerebral oxygen saturation (rcSO2), oxygen saturation (SpO2), heart rate (HR), breathing rate (BR), and cerebral fractional tissue oxygen extraction (cFTOE) over three consecutive 90° tilts. These parameters were measured for 2-min while neonates were in a supine (0°) position and 2-min while tilted to a sitting (90°) position. We measured oscillometric mean blood pressure (MBP) at the start of each tilt.

RESULTS

rcSO2 and BR decreased significantly in the sitting position, whereas cFTOE, SpO2, and MBP increased significantly in the sitting position. We detected a significant position-by-time interaction for all physiological parameters.

CONCLUSION

A 90° tilt induces a decline in rcSO2 and an increase in cFTOE in healthy term neonates. Understanding the normal cerebrovascular response to a 90° tilt in healthy neonates will help clinicians to recognize abnormal responses in high-risk infant populations.

IMPACT

Healthy term neonates (≤14 days old) had decreased cerebral oxygen saturation (~1.1%) and increased cerebral oxygen extraction (~0.01) following a 90° tilt. We detected a significant position-by-time interaction with all physiological parameters measured, suggesting the effect of position varied across consecutive tilts. No prior study has characterized the cerebral oxygen saturation response to a 90° tilt in healthy term neonates.

Sympathetic nervous system activity and pain-related response indexed by electrodermal activity during the earliest postnatal life in healthy term neonates

Sympathetic nervous system (SNS) undergoes a prolonged period of fetal and neonatal development and maturation during which is vulnerable to a variety of influences (e.g. painful experiences). Thus, we aimed to evaluate SNS activity at rest and in response to stressful stimulus (pain) within the earliest postnatal life in healthy term neonates using electrodermal activity (EDA) measures. In twenty eutrophic healthy term neonates EDA was recorded within the first two hours after birth (measurement 1 - M1) and 72 h after birth (measurement 2 - M2) at rest and in response to pain (M1 - intramuscular K vitamin administration; M2 - heel stick). Evaluated parameters were skin conductance level (SCL), non-specific skin conductance responses (NS.SCRs), skin SCL 10 s before pain stimulus (SCL_10 before pain), skin conductance response (SCR) peak after pain stimulus, SCL 10 s after pain stimulus (SCL_10 after pain), SCR magnitude, latency, SCR rise/decline time, SCR half recovery time. SCL was significantly decreased at rest during M2 compared to M1 (p=0.010). SCL_10 before pain, SCR peak after pain, and SCL_10 after pain stimulus were significantly decreased in M2 compared to M1 (p=0.014, p=0.020, p=0.011, respectively). SCL was significantly decreased and NS.SCRs were significantly higher in the recovery period after the pain stimulus during M2 compared to M1 (p=0.015, p=0.032, respectively). Our results indicate EDA parameters sensitive to detect sympathetic changes during the earliest postnatal life reflecting its potential in early diagnosis of the autonomic maturation - linked pathological states in neonates.

Near-infrared spectroscopy monitoring of neonatal cerebrovascular reactivity: where are we now?

Cerebrovascular reactivity defines the ability of the cerebral vasculature to regulate its resistance in response to both local and systemic factors to ensure an adequate cerebral blood flow to meet the metabolic demands of the brain. The increasing adoption of near-infrared spectroscopy (NIRS) for non-invasive monitoring of cerebral oxygenation and perfusion allowed investigation of the mechanisms underlying cerebrovascular reactivity in the neonatal population, confirming important associations with pathological conditions including the development of brain injury and adverse neurodevelopmental outcomes. However, the current literature on neonatal cerebrovascular reactivity is mainly still based on small, observational studies and is characterised by methodological heterogeneity; this has hindered the routine application of NIRS-based monitoring of cerebrovascular reactivity to identify infants most at risk of brain injury. This review aims (1) to provide an updated review on neonatal cerebrovascular reactivity, assessed using NIRS; (2) to identify critical points that need to be addressed with targeted research; and (3) to propose feasibility trials in order to fill the current knowledge gaps and to possibly develop a preventive or curative approach for preterm brain injury. IMPACT: NIRS monitoring has been largely applied in neonatal research to assess cerebrovascular reactivity in response to blood pressure, PaCO₂ and other biochemical or metabolic factors, providing novel insights into the pathophysiological mechanisms underlying cerebral blood flow regulation. Despite these insights, the current literature shows important pitfalls that would benefit to be addressed in a series of targeted trials, proposed in the present review, in order to translate the assessment of cerebrovascular reactivity into routine monitoring in neonatal clinical practice.

Stabilizing breathing pattern using local mechanical vibrations: comparison of deterministic and stochastic stimulations in rodent models of apnea of prematurity

Mechanical stimulation has been shown to reduce apnea of prematurity (AOP), a major concern in preterm infants. Previous work suggested that the underlying mechanism is stochastic resonance, amplification of a subthreshold signal by stochastic stimulation. We hypothesized that the mechanism behind the reduction of apnea length may not be a solely stochastic phenomenon, and suggest that a purely deterministic, non-random mechanical stimulation could be equally as effective. Mice and rats were anesthetized, tracheostomized, and mechanically ventilated to halt spontaneous breathing. Two miniature motors controlled by a microcontroller were attached around the abdomen. Ventilation was paused, stimulations were applied, and the time to the rodent's first spontaneous breath (T) was measured. Six spectrally different signals were compared to one another and the no-stimulation control in mice. The most successful deterministic stimulation (D) at reducing apnea was then compared to a pseudo-random noise (PRN) signal of comparable amplitude and frequency. CO2%, CO2 stabilization time (Ts), O2 saturation (SpO2%), and T were also measured. D significantly reduced T compared to no stimulation for medium and high amplitudes. PRN also reduced T, without  a difference between D and PRN. Furthermore, both stimulations significantly reduced Ts with no significant differences between the respective stimulations. However, there was no effect of D or PRN on SpO2%. The lack of differences between D and PRN led to an additional series of experiment comparing the same D to a band-limited white noise (WN) signal in young rats. Both D and WN were shown to significantly reduce T, with D showing statistical superiority in reduction of apnea. We further speculate that both deterministic and stochastic mechanical stimulations induce some form of mechanotransduction which is responsible for their efficacy, and our findings suggest that mechanical stimulation may be effective in treating AOP.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13534-021-00203-x.

Intermittent Hypoxia and Respiratory Patterns During Sleep of Preterm Infants Aged 3 to 18 Months Residing at High Altitudes

STUDY OBJECTIVES

the aim of this study was to determine the impact of apneas on oxygen saturation and the presence of intermittent hypoxia, during sleep of preterm infants (PTIs) born at high altitudes and compare with full-term infants (FTIs) at the same altitude.

METHODS

PTIs and FTIs from 3 to 18 months were included. They were divided into three age groups: 3-4 months (Group 1); 6-7 months (Group 2) and 10-18 months (Group 3). Polysomnography parameters and oxygenation indices were evaluated. Intermittent hypoxia was defined as brief, repetitive cycles of decreased oxygen saturation. Kruskal-Wallis test for multiple comparisons, t-test or Mann-Whitney U test were used.

RESULTS

127 PTI and 175 FTI were included. Total apnea-hypopnea index (AHI) was higher in PTI that FTI in all age groups (Group 1: 33.5/h vs. 12.8/h, p=0.042; Group 2: 27.0/h vs. 7.4/h, p<0.001 and Group 3: 11.6/h vs. 3.1/h, p<0.001). In Group 3, central-AHI (8.0/h vs. 2.3/h, p<0.001) and obstructive-AHI (1.8/h vs. 0.6/h, p<0.008) were higher in PTI than FTI. T90 (7.0% vs. 0.5, p<0.001), oxygen desaturation index (39.8/h vs. 11.3, p<0.001) were higher in PTI than FTI, nadir SpO2 (70.0% vs. 80.0, p<0.001) was lower in PTI .

CONCLUSION

At high altitude, compared to FTI, PTI have a higher rate of respiratory events, greater desaturation and a delayed resolution of these conditions, suggesting the persistence of intermittent hypoxia during the first 18 months of life. This indicates the need for follow-up of these infants for timely diagnosis and treatment of respiratory disturbances during sleep.

Cerebral oxygen saturation and cerebrovascular instability in newborn infants with congenital heart disease compared to healthy controls

Objective

Infants with Congenital Heart Disease (CHD) are at risk for developmental delays, though the mechanisms of brain injury that impair development are unknown. Potential causes could include cerebral hypoxia and cerebrovascular instability. We hypothesized that we would detect significantly reduced cerebral oxygen saturation and greater cerebrovascular instability in CHD infants compared to the healthy controls.

Methods

We performed a secondary analysis on a sample of 43 term infants (28 CHD, 15 healthy controls) that assessed prospectively in temporal cross-section before or at 12 days of age. CHD infants were assessed prior to open-heart surgery. Cerebral oxygen saturation levels were estimated using Near-Infrared Spectroscopy, and cerebrovascular stability was assessed with the response of cerebral oxygen saturation after a postural change (supine to sitting).

Results

Cerebral oxygen saturation was 9 points lower in CHD than control infants in both postures (β = -9.3; 95%CI = -17.68, -1.00; p = 0.028), even after controlling for differences in peripheral oxygen saturation. Cerebrovascular stability was significantly impaired in CHD compared to healthy infants (β = -2.4; 95%CI = -4.12, -.61; p = 0.008), and in CHD infants with single ventricle compared with biventricular defects (β = -1.5; 95%CI = -2.95, -0.05; p = 0.04).

Conclusion

CHD infants had cerebral hypoxia and decreased cerebral oxygen saturation values following a postural change, suggesting cerebrovascular instability. Future longitudinal studies should assess the associations of cerebral hypoxia and cerebrovascular instability with long-term neurodevelopmental outcomes in CHD infants.

Effects of tilt on cerebral hemodynamics measured by NeoDoppler in healthy neonates

BACKGROUND

Today, there are conflicting descriptions of how neonates respond to tilt. Examining physiologic responses of cerebral blood flow velocities (BFVs) in challenging situations like a tilt requires equipment that can cope with positional changes. We aimed to characterize how healthy term neonates respond to mild cerebral hemodynamic stress induced by a 90° tilt test using the recently developed NeoDoppler ultrasound system.

METHODS

A small ultrasound probe was fixated to the neonatal fontanel by a cap, and measured cerebral BFV in healthy neonates during and after a 90° head-up tilt test, five min in total, at their first and second day of life. Unsupervised k-means cluster analysis was used to characterize common responses.

RESULTS

Fifty-six ultrasound recordings from 36 healthy term neonates were analyzed. We identified five distinct, immediate responses that were related to specific outcomes in BFV, heart rate, and pulsatility index the next two min. Among 20 neonates with two recordings, 13 presented with different responses in the two tests.

CONCLUSIONS

Instant changes in cerebral BFV were detected during the head-up tilt tests, and the cluster analysis identified five different hemodynamic responses. Continuous recordings revealed that the differences between groups persisted two min after tilt.

IMPACT

NeoDoppler is a pulsed-wave Doppler ultrasound system with a probe fixated to the neonatal fontanel by a cap that can measure continuous cerebral blood flow velocity. Healthy neonates present with a range of normal immediate cerebral hemodynamic responses to a 90° head-up tilt, categorized in five groups by cluster analysis. This paper adds new knowledge about connection between immediate responses and prolonged responses to tilt. We demonstrate that the NeoDoppler ultrasound system can detect minute changes in cerebral blood flow velocity during a 90° head-up tilt.

Exposures influencing the developing central autonomic nervous system

Autonomic nervous system function is critical for transition from in-utero to ex-utero life and is associated with neurodevelopmental and neuropsychiatric outcomes later in life. Adverse prenatal and neonatal conditions and exposures can impair or alter ANS development and, as a result, may also impact long-term neurodevelopmental outcomes. The objective of this article is to provide a broad overview of the impact of factors that are known to influence autonomic development during the fetal and early neonatal period, including maternal mood and stress during and after pregnancy, fetal growth restriction, congenital heart disease, toxic exposures, and preterm birth. We touch briefly on the typical development of the ANS, then delve into both in-utero and ex-utero maternal and fetal factors that may impact developmental trajectory of the ANS and, thus, have implications in transition and in long-term developmental outcomes. While many types of exposures and conditions have been shown to impact development of the autonomic nervous system, there is still much to be learned about the mechanisms underlying these influences. In the future, more advanced neuromonitoring tools will be required to better understand autonomic development and its influence on long-term neurodevelopmental and neuropsychological function, especially during the fetal period.

The Use of Near-Infrared Spectroscopy (NIRS) to Measure Cerebral Oxygen Saturation During Body Position Changes on Infants Less than One Year Old

OBJECTIVE

To describe the state of the literature for near-infrared spectroscopy (NIRS) to measure cerebral oxygen saturation during body position changes on infants <1 year old.

INTRODUCTION

Although regional cerebral oxygen saturation is commonly used in critically ill populations, it is not usual practice to tailor care based on differences in the cerebral oxygen saturation during measurements in different body positions. We believe that alterations in cerebral oxygen saturation during position changes can also inform clinicians regarding brain health, such as the regulation of brain blood flow.

INCLUSION CRITERIA

We included studies in infants <1 year old; who had cerebral oxygen saturation measured in varying positions (e.g. supine versus side-lying).

METHODS

On March 30, 2019, we searched Medline, Embase, Cochrane CENTRAL, CINAHL, and Web of Science for studies written in English with no restriction on publication dates. We selected studies that involved infants <1 year old and measured cerebral oxygen saturation during varying body positions.

RESULTS

We found 24 primary studies on 694 infants. The authors investigated whether brain oxygen saturation was influenced by body position. A majority of the studies found a statistically significant difference between cerebral oxygen saturation in various body positions.

CONCLUSIONS

More research needs to be performed on variations in brain oxygen saturation during body position changes and the correlation with outcomes. Knowledge of brain oxygen saturation can provide clinicians an understanding of the infant's brain health. Healthcare providers may adapt care specifically to improve brain health with NIRS-based brain oxygen saturation monitoring.

Regional tissue oxygenation monitoring in the neonatal intensive care unit: evidence for clinical strategies and future directions

Near-infrared spectroscopy (NIRS)-based monitoring of regional tissue oxygenation (rSO₂) is becoming more commonplace in the neonatal intensive care unit (NICU). While increasing evidence supports rSO₂ monitoring, actual standards for applying this noninvasive bedside technique continue to evolve. This review highlights the current strengths and pitfalls surrounding practical NIRS-based monitoring in the neonatal population. The physiologic background of rSO₂ monitoring is discussed, with attention to understanding oxygen delivery/consumption mismatch and its effects on tissue oxygen extraction. The bedside utility of both cerebral and peripheral rSO₂ monitoring in the NICU is then explored from two perspectives: (1) disease/event-specific "responsive" monitoring and (2) "routine," continuous monitoring. Recent evidence incorporating both monitoring approaches is summarized with emphasis on practical applicability in the NICU. Finally, a future paradigm for a broad-based NIRS monitoring strategy is presented, with attention towards improving personalization of neonatal care and ultimately enhancing long-term outcomes.

Neonatal circumcision and prematurity are associated with sudden infant death syndrome (SIDS)

BACKGROUND

Sudden infant death syndrome (SIDS) is the most common cause of postneonatal unexplained infant death. The allostatic load hypothesis posits that SIDS is the result of cumulative perinatal painful, stressful, or traumatic exposures that tax neonatal regulatory systems.

AIMS

To test the predictions of the allostatic load hypothesis we explored the relationships between SIDS and two common phenotypes, male neonatal circumcision (MNC) and prematurity.

METHODS

We collated latitudinal data from 15 countries and 40 US states sampled during 2009 and 2013. We used linear regression analyses and likelihood ratio tests to calculate the association between SIDS and the phenotypes.

RESULTS

SIDS mortality rate was significantly and positively correlated with MNC. Globally (weighted): Increase of 0.06 (95% CI: 0.01-0.1, t = 2.86, p = 0.01) per 1000 SIDS mortality per 10% increase in circumcision rate. US (weighted): Increase of 0.1 (95% CI: 0.03-0.16, t = 2.81, p = 0.01) per 1000 unexplained mortality per 10% increase in circumcision rate. US states in which Medicaid covers MNC had significantly higher MNC rates (χ̄ = 0.72 vs 0.49, p = 0.007) and male/female ratio of SIDS deaths (χ̄ = 1.48 vs 1.125, p = 0.015) than other US states. Prematurity was also significantly and positively correlated with MNC. Globally: Increase of 0.5 (weighted: 95% CI: 0.02-0.086, t = 3.37, p = 0.004) per 1000 SIDS mortality per 10% increase in the prematurity rates. US: Increase of 1.9 (weighted: 95% CI: 0.06-0.32, t = 3.13, p = 0.004) per 1000 unexplained mortalities per 10% increase in the prematurity rates. Combined, the phenotypes increased the likelihood of SIDS.

CONCLUSIONS

Epidemiological analyses are useful to generate hypotheses but cannot provide strong evidence of causality. Biological plausibility is provided by a growing body of experimental and clinical evidence linking aversive preterm and early-life SIDS events. Together with historical and anthropological evidence, our findings emphasize the necessity of cohort studies that consider these phenotypes with the aim of improving the identification of at-risk infants and reducing infant mortality.

RELEVANCE FOR PATIENTS

Preterm birth and neonatal circumcision are associated with a greater risk of SIDS, and efforts should be focused on reducing their rates.

Autonomic nervous system development and its impact on neuropsychiatric outcome

The central autonomic nervous system (ANS) is essential for maintaining cardiovascular and respiratory homeostasis in the newborn and has a critical role in supporting higher cortical functions. At birth, the central ANS is maturing and is vulnerable to adverse environmental and physiologic influences. Critical connections are formed early in development between the ANS and limbic system to integrate psychological and body responses. The Polyvagal Theory, developed by Stephen Porges, describes how modulation of the autonomic vagal impulse controls social responses and that a broad range of neuropsychiatric disorders may be due to impaired vagal balance, with either deficient vagal tone or excessive vagal reactivity. Under additional circumstances of prematurity, growth restriction, and environmental stress in the fetus and newborn, the immature ANS may undergo "dysmaturation". Maternal stress and health as well as the intrauterine environment are also quite important and have been implicated in causing ANS changes in the infant and neuropsychiatric diseases in children. This review will cover the aspects of ANS development and maturation that have been associated with neuropsychiatric disorders in children.

The Critical Role of the Central Autonomic Nervous System in Fetal-Neonatal Transition

The objective of this article is to understand the complex role of the central autonomic nervous system in normal and complicated fetal-neonatal transition and how autonomic nervous system dysfunction can lead to brain injury. The central autonomic nervous system supports coordinated fetal transitional cardiovascular, respiratory, and endocrine responses to provide safe transition of the fetus at delivery. Fetal and maternal medical and environmental exposures can disrupt normal maturation of the autonomic nervous system in utero, cause dysfunction, and complicate fetal-neonatal transition. Brain injury may both be caused by autonomic nervous system failure and contribute directly to autonomic nervous system dysfunction in the fetus and newborn. The central autonomic nervous system has multiple roles in supporting transition of the fetus. Future studies should aim to improve real-time monitoring of fetal autonomic nervous system function and in supporting typical autonomic nervous system development even under complicated conditions.

Autonomic nervous system depression at term in neurologically normal premature infants

BACKGROUND

Premature infants are vulnerable to destructive brain injury and disturbed neurological development. Prematurity may alter maturation of the central autonomic nervous system (ANS).

AIMS

To compare ANS function (using heart rate variability; HRV) between preterm infants with normal neuroimaging at term equivalent age and low-risk term controls. Study design, subjects. We performed a case-control study of preterm infants born ≤28 weeks gestational age that had normal brain imaging and archived continuous EKG data at term equivalent age. We documented other factors thought to influence ANS maturation (e.g. infection, ventilation days, and postnatal steroids). Controls were low-risk term gestational age newborns from uncomplicated pregnancies/deliveries. We characterized HRV metrics using frequency-(Welch periodogram) and time-domain (detrended fluctuation) analyses. Sympathetic tone was characterized by α1, root mean square analysis (RMS1 and RMS2), low-frequency (LF) power, and normalized LF (nLF) and parasympathetic tone was characterized by high-frequency (HF) power and normalized HF (nHF). α2 characterized ultraslow changes in heart rate. We used ANCOVA to compare HRV metrics between groups. Outcome measures, results. HRV from 26 preterm infants were compared to 55 controls. Analyzed HRV data for preterm infants were recorded at median (range) gestational age of 39 (36-39) weeks and for controls at 39 (37-41) weeks gestational age. α1, RMS2, LF and HF were significantly higher in control infants and remained significant after controlling for infection, ventilator days, and postnatal steroids (P < .005).

CONCLUSIONS

Autonomic maturation is impaired in a premature extrauterine environment. In the absence of destructive brain injury, our data suggest an important role for disturbed programming in this impaired autonomic development.

Endothelial Colony-Forming Cells in Young Adults Born Preterm: A Novel Link Between Neonatal Complications and Adult Risks for Cardiovascular Disease

BACKGROUND

Preterm birth is linked to cardiovascular risks and diseases. Endothelial progenitor cells play a critical role in vascular development and repair. Cord blood endothelial progenitor cells of preterm-born infants, especially endothelial colony-forming cells (ECFC), show enhanced susceptibility to prematurity-related pro-oxidant stress. Whether ECFC dysfunction is present in adulthood following preterm birth is unknown.

METHODS AND RESULTS

This cross-sectional observational study includes 55 preterm-born (≤29 gestational weeks) young adults (18-29 years old, 38% male) and 55 sex- and age-matched full-term controls. ECFC were isolated from peripheral blood; cell proliferative and vascular cord formation capacities were assessed in vitro. Daytime systolic blood pressure was higher, whereas glucose tolerance and body mass index were lower in preterm-born subjects. ECFC colonies grew in culture for 62% of full-term- and 58% of preterm-born participants. Preterm-born participants have formed ECFC colonies later in culture and have reduced proliferation compared with controls. Only in preterm-born individuals, we observed that the later the ECFC colony grows in culture, the worse was overall ECFC function. In addition, in preterms, elevated systolic blood pressure significantly correlated with reduced ECFC proliferation (r_S=-0.463; P=0.030) and numbers of branches formed on matrigel (r_S=-0.443; P=0.039). In preterm-born subjects, bronchopulmonary dysplasia was associated with impaired ECFC function, whereas exposure to antenatal steroids related to better ECFC function.

CONCLUSIONS

This study is the first to examine ECFC in preterm-born adults and to demonstrate ECFC dysfunction compared with full-term controls. In the preterm-born group, ECFC dysfunction was associated with bronchopulmonary dysplasia, the major prematurity-related neonatal morbidity, and with increased systolic blood pressure into adulthood.

Waking up too early - the consequences of preterm birth on sleep development

Good quality sleep of sufficient duration is vital for optimal physiological function and our health. Sleep deprivation is associated with impaired neurocognitive function and emotional control, and increases the risk for cardiometabolic diseases, obesity and cancer. Sleep develops during fetal life with the emergence of a recognisable pattern of sleep states in the preterm fetus associated with the development, maturation and connectivity within neural networks in the brain. Despite the physiological importance of sleep, surprisingly little is known about how sleep develops in individuals born preterm. Globally, an estimated 15 million babies are born preterm (<37 weeks gestation) each year, and these babies are at significant risk of neural injury and impaired brain development. This review discusses how sleep develops during fetal and neonatal life, how preterm birth impacts on sleep development to adulthood, and the factors which may contribute to impaired brain and sleep development, leading to altered neurocognitive, behavioural and motor capabilities in the infant and child. Going forward, the challenge is to identify specific risk factors for impaired sleep development in preterm babies to allow for the design of interventions that will improve the quality and quantity of sleep throughout life.

Cardiovascular autonomic dysfunction in sudden infant death syndrome

A failure of cardiorespiratory control mechanisms, together with an impaired arousal response from sleep, are believed to play an important role in the final event of sudden infant death syndrome (SIDS). The 'triple risk model' describes SIDS as an event that results from the intersection of three overlapping factors: (1) a vulnerable infant, (2) a critical developmental period in homeostatic control and (3) an exogenous stressor. In an attempt to understand how the triple risk hypothesis is related to infant cardiorespiratory physiology, many researchers have examined how the known risk and protective factors for SIDS alter infant cardiovascular control during sleep. This review discusses the association between the three components of the triple risk hypothesis and major risk factors for SIDS, such as prone sleeping, maternal smoking, together with three "protective" factors, and cardiovascular control during sleep in infants, and discusses their potential involvement in SIDS.

A "Wear and Tear" Hypothesis to Explain Sudden Infant Death Syndrome

Sudden infant death syndrome (SIDS) is the leading cause of death among USA infants under 1 year of age accounting for ~2,700 deaths per year. Although formally SIDS dates back at least 2,000 years and was even mentioned in the Hebrew Bible (Kings 3:19), its etiology remains unexplained prompting the CDC to initiate a sudden unexpected infant death case registry in 2010. Due to their total dependence, the ability of the infant to allostatically regulate stressors and stress responses shaped by genetic and environmental factors is severely constrained. We propose that SIDS is the result of cumulative painful, stressful, or traumatic exposures that begin in utero and tax neonatal regulatory systems incompatible with allostasis. We also identify several putative biochemical mechanisms involved in SIDS. We argue that the important characteristics of SIDS, namely male predominance (60:40), the significantly different SIDS rate among USA Hispanics (80% lower) compared to whites, 50% of cases occurring between 7.6 and 17.6 weeks after birth with only 10% after 24.7 weeks, and seasonal variation with most cases occurring during winter, are all associated with common environmental stressors, such as neonatal circumcision and seasonal illnesses. We predict that neonatal circumcision is associated with hypersensitivity to pain and decreased heart rate variability, which increase the risk for SIDS. We also predict that neonatal male circumcision will account for the SIDS gender bias and that groups that practice high male circumcision rates, such as USA whites, will have higher SIDS rates compared to groups with lower circumcision rates. SIDS rates will also be higher in USA states where Medicaid covers circumcision and lower among people that do not practice neonatal circumcision and/or cannot afford to pay for circumcision. We last predict that winter-born premature infants who are circumcised will be at higher risk of SIDS compared to infants who experienced fewer nociceptive exposures. All these predictions are testable experimentally using animal models or cohort studies in humans. Our hypothesis provides new insights into novel risk factors for SIDS that can reduce its risk by modifying current infant care practices to reduce nociceptive exposures.