Sudden infant death syndrome — a ‘diagnosis’ in search of a disease

Investigating genetic variants in microRNA regulators of Neurokinin-1 receptor in sudden infant death syndrome

Sudden infant death syndrome (SIDS) occurs more often in male than in female infants, suggesting involvement of the X-chromosome. Histopathological studies have suggested that altered expression of the Neurokinin-1 receptor may also play a role in the pathogenesis of SIDS. It was hypothesised that genetic variants in three X-chromosome-encoded microRNA (miRNA/miR), known to down-regulate expression of the Neurokinin-1 receptor, may contribute to SIDS.

AIM

To identify sequence variants in the miRNAs within a study cohort (27 cases of SIDS and 28 controls) and determine if there was a difference in the frequencies in male and female SIDS infants.

METHODS

Genomic DNA prepared from stored blood spots was amplified and sequenced to identify genetic variants in miR500A, miR500B and miR320D2.

RESULTS

No novel variants in the miRNAs were identified in our study cohort. We identified one known single-nucleotide polymorphism (SNP) in miR320D2: rs5907732 G/T, in both cases and controls. No significant difference in the SNP frequency was observed between male and female SIDS cases.

CONCLUSION

This pilot study suggests that sequence variants in three miRNAs do not contribute to the reported higher prevalence of SIDS in male infants and do not contribute to the pathogenesis of SIDS in our cohort.

Neuropathological Developments in Sudden Infant Death Syndrome

A wide variety of neuropathological abnormalities have been investigated in infants who have died of sudden infant death syndrome (SIDS). Issues which detracted from early studies included failure to use uniform definitions of SIDS and lack of appropriately matched control populations. Development of the triple risk model focused attention on the concept of an inherent susceptibility to unexpected death in certain infants, with research demonstrating a role for the neurotransmitter serotonin within the brainstem. However, it now appears that neuropathological abnormalities in SIDS infants are more complex than a simple serotonergic deficiency in certain medullary nuclei but instead could involve failure of an integrated network of neurochemical transmitters in a variety of subcortical locations. The following overview examines recent research developments looking particularly at the potential role of the peptide neurotransmitter substance P and its neurokinin-1 receptor in multiple nuclei within the brainstem, asymmetry and microdysgenesis of the hippocampus, and decreased orexin levels within dorsomedial, perifornical, and lateral levels in the hypothalamus. Whether such research will lead to identifiable biomarker for infants at risk of SIDS is yet to be established. Use of standardized and consistent methods of classifying and categorizing infant deaths will be pivotal in generating reproducible research results.

Evolution and significance of the triple risk model in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is a leading cause of death in infants, although the mechanisms leading to death remain unclear. Multiple theories have emerged over time, with one of the most influential hypotheses being the triple risk model. This model, first devised in 1972 and later revised in 1994 by Filiano and Kinney, is still widely used in assisting with conceptualising and understanding sudden death in infancy. This model has evolved over time, with each version stressing that SIDS is likely to occur when certain risk factors coincide, suggesting that the lethal mechanisms in SIDS are likely to be multifactorial. All versions of the triple risk model from 1972 to the present have emphasised the complexity of SIDS and serve as useful guides for current and future research into the enigma of sudden and unexpected death in infancy.

A re-audit of the use of definitions of sudden infant death syndrome (SIDS) in peer-reviewed literature

The use of different definitions of sudden infant death syndrome (SIDS) may make comparison of data among studies difficult. Fifty randomly selected papers dealing with SIDS that were published between 2010 and 2011 in peer-reviewed journals were reviewed to determine whether one of three internationally accepted definitions of SIDS had been either written in the text or referenced. A significant improvement in the use of definitions has occurred since 2005, with the percentage of papers either quoting or referencing a standard definition increasing by 26%, from 42 to 68%. The 1989 NICHD definition remained the most commonly used definition (35.1%) followed by the 2004 San Diego definition (26.3%). Although the percentage of papers where either no definition was provided or where an idiosyncratic or mis-cited definition was used fell 26%, from 58 to 32%, nearly one in three papers published on SIDS in peer-reviewed journals that were included in this study still did not cite a standard definition.

Cerebral hemodynamics during arousals in preterm infants

The aim of the study was to evaluate potential changes of cerebral blood volume (CBV) related to arousals in preterm infants. As arousals are known to change different physiological parameters, it was postulated that this could also hold true for CBV. Polygraphic recordings were performed in 38 preterm infants (18 female, 20 male). The infants' gestational age at birth was 32.0+/-2.3 weeks, postconceptional age was 35.1+/-1.2 weeks and postnatal age at study entry was 24.3+/-2.9 days, birth weight was 1793+/-527 g and actual weight at study entry was 2011+/-324 g [mean (+/- standard deviation)]. CBV was measured using near infrared spectroscopy. Arousals were scored due to the guidelines of the "International Paediatric Work Group on Arousals" and categorized as either cortical (CA) or subcortical arousals (SCA). Altogether, 122 arousals (66 CA, 56 SCA) were scored. According to sleep stage, 77 arousals were analyzed in active sleep, 23 in quiet sleep and 22 in intermediate sleep. Mean duration of arousals was 8.8+/-0.3 s. CBV, cerebral vascular oxygenation and the balance between oxygen delivery and oxygen consumption remained constant during arousals in preterm infants. This was demonstrated for both CA and SCA and was independent of sleep stage, suggesting that the impact of arousals in stable preterm infants is too small to alter cerebral vascular autoregulation.

Hazardous infant and early childhood sleeping environments and death scene examination

Infants and young children are at risk of accidental asphyxia from unsafe sleeping environments Specific dangers include wedging/entrapment and positional asphyxia when infants slide into gaps between mattresses and cot sides or between beds and walls. Faulty cot construction may result in defects through which infants may fall and become suspended, or in spaces which entrap the head. Projections on the inside of cots may ensnare clothing causing hanging, as may nearby curtain cords and restraining harnesses. The mouth and nose may be covered by plastic sheeting or by the mattress in partially filled waterbeds. Rocking cradles with excessive angles of tilt may also place infants' heads into positions where breathing is compromised. Finally, external pressure by adults in shared sleeping situations may result in fatal chest compression or airway occlusion. In many of these situations the autopsy findings are entirely nonspecific and do not help in establishing a cause of death. Thus, careful death scene examination with later formal review of findings may be the only way to accurately determine the manner of death.

Effects of sleep position, sleep state and age on heart rate responses following provoked arousal in term infants

Previous studies have suggested that autonomic dysfunction may be involved in Sudden Infant Death Syndrome (SIDS). The major risk factors for SIDS are the prone sleeping position and maternal smoking. Our aim was to examine the effects of sleeping position and maternal smoking on the postnatal maturation of autonomic function by examining heart rate responses following arousal in healthy term infants. Twenty-four infants (11 born to mothers who smoked during pregnancy and 13 to mother who did not smoke) were studied using daytime polysomnography and multiple measurements of arousal threshold (cm H(2)O) in response to air-jet stimulation applied alternately to the nares were made in both active sleep (AS) and quiet sleep (QS). We demonstrated no difference between smoking and non-smoking groups of infants in any of our measurements, and thus combined data from the groups. Baseline (BHR) was elevated in the prone compared to the supine position in quiet sleep (QS) at 2-3 weeks (p<0.001) and 5-6 months (p<0.001), and in active sleep (AS) at 2-3 and 5-6 months (p<0.05). BHR was significantly elevated in AS compared to QS in the supine position at all ages (p<0.01) and in the prone position at 2-3 (p<0.001) and 5-6 months (p<0.05). Increases in heart rate (deltaHR%) following arousal were significantly greater in the supine compared to the prone position in QS at 2-3 weeks (p<0.05) and in AS at both 2-3 (p<0.01) and 5-6 months (p<0.05). DeltaHR% was significantly greater in AS compared to QS in both supine (p<0.05) and prone (p<0.001) positions at 2-3 weeks and in the supine position at 2-3 months (p<0.001). We conclude that sleep state, sleep position and postnatal age affect the cardiac responses following arousal from sleep in healthy term infants. Impairment of heart rate control in the prone position may be important in understanding the increased risk for SIDS in this position.

Sudden infant death syndrome: overview and update

The past decade and a half has seen marked changes in the epidemiology of sudden infant death syndrome (SIDS). The avoidance of certain risk factors such as sleeping prone and cigarette smoke exposure has resulted in the death rate falling dramatically. Careful evaluation of environmental factors and endogenous characteristics has led to a greater understanding of the complexities of the syndrome. The development and implementation of death scene and autopsy protocols has led to standardization in approaches to unexpected infant deaths with increasing diagnoses of accidental asphyxia. Despite these advances, there is still confusion surrounding the diagnosis, with deaths being attributed to SIDS in many communities and countries where death scene investigations and autopsies have not been conducted. The following review provides a brief overview of the historical background, epidemiology, pathology, and pathogenesis of SIDS. Contentious issues concerning the diagnosis and current problems are discussed. Despite calls to abandon the designation, SIDS remains a viable term for infants who die in their sleep with no evidence of accident, inflicted injury, or organic disease after a full investigation has been conducted according to standard guidelines.

Effects of body position on sleep and arousal characteristics in infants

The prone sleeping position has been identified in world-wide epidemiological studies as a major risk factor for sudden infant death syndrome (SIDS). Public awareness campaigns throughout the western world have led to an over 50% reduction in postneonatal mortality and frequency of SIDS. This reduction in mortality has been mainly attributed to the avoidance of the prone sleep position. Various mechanisms have been postulated to explain the increased risk of SIDS associated with prone sleeping, among these, impairment of arousal from sleep. This paper reviews the effects of prone sleeping on infant sleep architecture, arousability from sleep and cardiorespiratory controls. Sleeping in the prone position has been shown to increase the amount of time spent sleeping, particularly time spent in quiet sleep (QS). Sleeping prone has also been demonstrated to be associated with a reduced responsiveness to a variety of arousal stimuli. Such impairment of arousal has been demonstrated to be associated with changes in control of autonomic cardiac function. During arousal, heart rate, blood pressure and breathing movements increase, while gross body movements occur to avoid the stimulus. Any impairment in arousability from sleep such as could occur when infants sleep in the prone position, could possibly contribute to the final pathway to SIDS.

Effects of maternal tobacco smoking, sleeping position, and sleep state on arousal in healthy term infants

OBJECTIVES

To investigate whether a history of maternal tobacco smoking affected the maturation of arousal responses and whether sleeping position and infant age alters these relations.

DESIGN

Healthy term infants (13 born to mothers who did not smoke and 11 to mothers who smoked during pregnancy) were studied using daytime polysomnography on three occasions: (a) two to three weeks after birth, (b) two to three months after birth, and (c) five to six months after birth. Multiple measurements of arousal threshold in response to air jet stimulation were made in both active sleep (AS) and quiet sleep (QS) when infants slept both prone and supine.

RESULTS

Maternal smoking significantly elevated arousal threshold in QS when infants slept supine at 2-3 months of age (p<0.05). Infants of smoking mothers also had fewer spontaneous arousals from QS at 2-3 months in both prone (p<0.05) and supine (p<0.001) sleeping positions. In infants of non-smoking mothers, arousal thresholds were elevated in the prone position in AS at 2-3 months (p<0.01) and QS at 2-3 weeks (p<0.05) and 2-3 months (p<0.001).

CONCLUSIONS

Maternal tobacco smoking significantly impairs both stimulus induced and spontaneous arousal from QS when infants sleep in the supine position, at the age when the incidence of sudden infant death syndrome is highest.

Pathological findings in overlaying

A 2-month-old girl is reported who was found in cardiorespiratory arrest beneath her unconscious mother. Full autopsy examination failed to reveal any features which would give an indication of the nature of the terminal event. This case confirms the non-specificity of autopsy findings in an infant who suffocated underneath an adult--a situation mimicking classical 'overlaying'. Given that there may be no specific histopathological findings when an infant suffocates underneath an adult, a diagnosis should not be formulated if a full death scene description is not available and no specific lesions or diseases are found. The manner of death should be designated as 'undetermined'.

The prone sleeping position impairs arousability in term infants

OBJECTIVE

To investigate whether the prone sleeping position impaired arousal from sleep in healthy infants and whether this impairment was related to cardiorespiratory variables, temperature, or age.

STUDY DESIGN

Healthy term infants (n = 24) were studied with daytime polysomnography on 3 occasions: 2 to 3 weeks after birth, 2 to 3 months after birth, and 5 to 6 months after birth. Multiple measurements of arousal threshold (cm H(2)O) in response to air-jet stimulation applied alternately to the nares were made in both active sleep and quiet sleep when infants slept both prone and supine.

RESULTS

Arousal thresholds were significantly higher in both active sleep and quiet sleep when infants slept prone at 2 to 3 weeks and 2 to 3 months, but not at 5 to 6 months. These increases were independent of any sleep position-related change in either rectal or abdominal skin temperature, respiratory rate, oxygen saturation, or heart rate.

CONCLUSIONS

The prone position significantly impairs arousal from both active sleep and quiet sleep in healthy term infants. This impairment in arousability occurred with no clinically significant changes in cardiorespiratory variables or body temperature. Decreased arousability from sleep in the prone position provides an important insight into its role as a risk factor for sudden infant death syndrome.

Changing trends in the diagnosis of sudden infant death

A study of 114 consecutive cases of unexpected infant death that occurred in South Australia over a 5-year period from January 1994 to December 1998 was undertaken. There were 45 deaths attributed to sudden infant death syndrome (SIDS), 19 to natural causes, 21 to accidents. and 5 to homicides; 24 cases were listed as "undetermined." Although there has been a genuine and continued decline in SIDS numbers in this population, there has also been an increase in the diagnosis of cases of accidental asphyxia due to unsafe sleeping environments and of cases in which the family background and autopsy findings suggested more complex mechanisms. The change in diagnostic profile has followed the introduction of more rigorous clinical history review, death scene examination, and autopsy testing. Thus, although diagnostic outcomes have altered in this population, it is more likely the result of more careful interpretation of the extensive investigations that are now undertaken rather than arbitrary reclassification.

Accidental childhood death and the role of the pathologist

The following study provides an overview of accidental childhood death. This study is based on a review of 369 cases of fatal childhood accidents taken from the records of the Department of Histopathology, Women's and Children's Hospital, Adelaide, Australia, over a 34-year period from 1963 to 1996. Data provide information on deaths due to motor vehicle accidents, drownings, accidental asphyxia, burns, poisonings, electrocution, and miscellaneous trauma. In addition, certain categories have undergone further examination, including asphyxial deaths due to unsafe sleeping environments and unsafe eating practices, drowning deaths, and deaths on farms, following identification of significant child safety problems in these areas as part of the "Keeping Your Baby and Child Safe" program. Previously unrecognized dangers to children detected through this program include mesh-sided cots, V-shaped pillows, and certain types of stroller-prams. The production of information pamphlets and packages for parents and the recall of certain dangerous products following recommendations made by pathologists demonstrate that pediatric and forensic pathologists have an important role to play in preventive medicine issues and in formulating public health strategies.

Gastric aspiration and sleeping position in infancy and early childhood

Concern has been expressed that the recommendation of supine sleeping position for infants would result in an increase in deaths due to gastric aspiration. A review of 196 cases of infant and early childhood death in children under 3 years of age, occurring over a 9-year period (September 1989 to August 1998) was undertaken to ascertain how many cases of significant gastric aspiration had occurred. Extensive and widespread filling of the airways/alveoli with gastric contents was found in three infants/young children aged 5, 6 and 30 months, respectively. In each instance the body had been found lying face down (prone), with the face in a pool of vomitus in at least one case. No cases of significant gastric aspiration were found in infants who had been found lying on their sides or backs (supine). In addition, no significant increase in numbers of infant and early childhood deaths in South Australia due to gastric aspiration over this time could be demonstrated. Concerns that the supine rather than the prone position is more likely to result in significant gastric aspiration are not supported by this study.

Effects of prematurity on arousal from sleep in the newborn infant

The incidence of sudden infant death syndrome has been found to be consistently higher in preterm and low birth weight infants than in infants born at term. Failure to arouse from sleep is one possible mechanism for sudden infant death syndrome. This study compared the arousal responses to nasal air-jet stimulation in a longitudinal study between groups of healthy preterm and term infants. Preterm infants (n = 9) were born at 31-35 wk gestation with normal birth weights for gestational age and studied on three occasions: a preterm study at 36 wk, at 2-3 wk post-term, and at 2-3 mo post-term. Term infants (n = 22) were born at 37-42 wk and were studied at 2-3 wk and 2-3 mo post-term. Arousal thresholds were determined in both active sleep (AS) and quiet sleep (QS). In preterm infants, there was no state-related difference in arousal thresholds at either the 36 wk or 2-3 wk study; however, at 2-3 mo, arousal threshold was significantly greater in QS than AS (p < 0.05). In contrast, in term infants, arousal thresholds were significantly elevated in QS compared with AS at both 2-3 wk and 2-3 mo (p < 0.001). Arousal thresholds in AS were not different between the two groups of infants, with both groups of infants remaining readily arousable. However, in QS at 2-3 mo, arousal thresholds were significantly lower in the preterm infants (p < 0.05). This study has demonstrated that arousability is altered by gestational and postnatal age. The lower arousability that characterizes QS in term infants regardless of age is not evident in preterm infants until 2-3 mo post-term age.

Petechial hemorrhages and unexpected infant death

The significance of petechial hemorrhages in cases of unexpected infant death remains uncertain. While intrathoracic petechiae occur in the majority of cases of sudden infant death syndrome (SIDS), their relationship to terminal mechanisms has been debated. Facial, conjunctival and external upper chest petechiae are not a feature of SIDS and raise the possibility of underlying illness such as sepsis, or of forceful coughing or vomiting. Alternatively the presence of cutaneous or conjunctival petchiae may suggest trauma or asphyxia due to chest or neck compression. Given the possibility of alternative mechanisms of death it is preferable to designate the cause and manner of death as 'undetermined', rather than 'SIDS', when petechiae are found in unusual locations.

Is breast feeding in bed always a safe practice?

OBJECTIVES

To examine whether infants who are being breast fed in their mother's bed are at increased risk of accidental suffocation.

MATERIALS AND METHODS

Review of all cases of unexpected infant death occurring in South Australia, Australia during 1996 was undertaken to ascertain whether any cases of sudden infant death had occurred in association with breast feeding in the parental bed. All infants had death scene examinations, history reviews and full autopsies performed.

RESULTS

Three of a total of 28 cases of unexpected infant death were identified where accidental asphyxia associated with breast feeding-related cosleeping was considered a likely cause of death. Maternal fatigue was a factor in each of the three cases. There were nine SIDS cases and 16 other cases which included non-accidental injury, accidental asphyxia, congenital cardiovascular disease and sepsis.

CONCLUSIONS

Accidental asphyxia is a rare but possible outcome if mothers fall asleep in bed with their infants while breast feeding. Nursing mothers should be made aware of the potential dangers of fatigue and sedation in such circumstances. Breast feeding out of bed, or in the company of a second person who can ensure the safety of the infant if breast feeding is occurring in bed may prevent these unfortunate fatalities.

V-shaped pillows and unsafe infant sleeping

OBJECTIVE

To investigate the possibility that adult size V-shaped pillows may be associated with accidental asphyxial deaths in infants.

METHODOLOGY

Review was undertaken of all cases of sudden infant death presenting to the Women's and Children's Hospital in Adelaide involving infants who had been found on adult size V-shaped pillows.

RESULTS

Four cases of sudden infant death were found in which infants were found sleeping on V-shaped pillows. All of the deaths had occurred in 1995. In two of the cases the pillows were considered to be implicated in the cause of death as the potential for obstruction of the infants' airways could be demonstrated on death scene reconstruction.

CONCLUSION

Adult size V-shaped pillows may be unsafe for use in small infants who may accidentally suffocate if trapped between the two arms, or under, the pillow. The use of such pillows to maintain the body position of sleeping infants should be discouraged.

Mesh-sided cots--yet another potentially dangerous infant sleeping environment

Two cases of accidental asphyxia involving an 11.5 month old boy and a 3.5 month old boy who each died after being trapped between the elastic mesh side of their cots and the cot mattress are reported. In both cases the original cot mattress has either been replaced or augmented by a less well fitting, thicker mattress. Particular problems that exist with these type of mesh sided cots are the potential for considerable stretching of the side of the cot admitting the relatively larger, poorly supported infant head, with elastic recoil of the mesh holding the head in potentially dangerous positions. To help determine whether accidental asphyxia has occurred, death scene examination in cases of sudden infant death during sleep should include reconstruction of the position of the body in the cot or bed, with careful examination of the structure of the cot/bed.

Routine microbiological testing in sudden and unexpected infant death

OBJECTIVE

To evaluate the significance of microbiological test results in a series of infants who had died suddenly and unexpectedly.

METHODOLOGY

Following a review of all cases of sudden natural death in infants presenting to the Adelaide Children's Hospital (ACH) division of the Women's and Children's Hospital (WCH) over the 10 year period between 1983 and 1992, specific evaluation of microbiological test results was undertaken.

RESULTS

There were 329 cases of sudden infant death syndrome (SIDS) and 23 cases in which sudden infant death was either attributed to other conditions or was unclassifiable. Positive microbiological results were recorded in the majority of cases, most being considered to be due to postmortem overgrowth or to contamination at autopsy. Of the remaining cases, microbiological results were essential to the establishment of the diagnosis in three cases, and were a useful adjunct to the diagnosis in a further six cases.

CONCLUSIONS

Routine microbiological testing in cases presenting as SIDS did not reveal occult sepsis in most instances. Such testing did, however, add support to the diagnosis of SIDS where no pathogens were isolated and, if not undertaken, would have resulted in a small percentage of cases of sudden infant death due to infections remaining undiagnosed.

Accidental infant death and stroller-prams

A three-month-old boy and an eight-month-old boy died from accidental positional asphyxia and hanging, respectively, after being placed to sleep unsupervised in stroller-prams. Both infants had moved down towards the fronts of the stroller-prams. The younger infant fell out when the footplate collapsed and he was found hanging from a metal bar on the side. The older infant had partly slipped through the front and was suspended with his head and arms within the stroller-pram and with his face pushed firmly into the mattress by a horizontal metal bar. Stroller-prams are a potentially dangerous sleeping environment unless infants are closely supervised, gaps in the front of stroller-prams closed and upright footplates stabilised.