Infant stress and sleep deprivation as an aetiological basis for the sudden infant death syndrome

Pulmonary immune profiling of SIDS: impaired immune maturation and age-related cytokine imbalance

BACKGROUND

For sudden infant death syndrome (SIDS), an impaired immunocompetence has been discussed for a long time. Cytokines and chemokines are soluble immune mediators (SIM) whose balance is essential for the immune status. We hypothesized that an imbalanced immune response might contribute to the etiology of SIDS.

METHODS

We investigated 27 cytokines, chemokines, and growth factors in protein lysates of lungs derived from 29 SIDS cases and 15 control children deceased for other reasons.

RESULTS

Except for the CCL5, no significant differences were detected in the lungs between SIDS cases with and without mild upper respiratory tract infections. In contrast, IL-1RA, IL-7, IL-13, and G-CSF were decreased in the merged SIDS cases compared to control cases without evidence of infection. Plotting SIM concentrations against infant age resulted in increasing concentrations in control but not in SIDS lungs, indicating a disturbed immune maturation. Moreover, an age-dependent shift towards a Th2-related pattern was observed in SIDS.

CONCLUSIONS

Our findings suggest that an impaired maturation of the immune system, an insufficient response to respiratory pathogens, and an immune response modulated by Th1/Th2 imbalance might play a possible role in triggering SIDS. These findings might in part be explained by chronic stress.

IMPACT

Maturation of the cytokine and chemokine network may be impaired in SIDS. An imbalance between Th1- and Th2-related cytokines, which may reflect a state of chronic stress causing a more Th2 shift. An impaired immune maturation, an insufficient response to respiratory pathogens, and an immune response modulated by Th1/Th2 imbalance might play a possible role in SIDS.

Cardiac ion channelopathies and the sudden infant death syndrome

The sudden infant death syndrome (SIDS) causes the sudden death of an apparently healthy infant, which remains unexplained despite a thorough investigation, including the performance of a complete autopsy. The triple risk model for the pathogenesis of SIDS points to the coincidence of a vulnerable infant, a critical developmental period, and an exogenous stressor. Primary electrical diseases of the heart, which may cause lethal arrhythmias as a result of dysfunctioning cardiac ion channels (“cardiac ion channelopathies”) and are not detectable during a standard postmortem examination, may create the vulnerable infant and thus contribute to SIDS. Evidence comes from clinical correlations between the long QT syndrome and SIDS as well as genetic analyses in cohorts of SIDS victims (“molecular autopsy”), which have revealed a large number of mutations in ion channel-related genes linked to inheritable arrhythmogenic syndromes, in particular the long QT syndrome, the short QT syndrome, the Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Combining data from population-based cohort studies, it can be concluded that at least one out of five SIDS victims carries a mutation in a cardiac ion channel-related gene and that the majority of these mutations are of a known malignant phenotype.

A mechanism for sudden infant death syndrome (SIDS): stress-induced leak via ryanodine receptors

BACKGROUND

Sudden infant death syndrome (SIDS) is the leading cause of postneonatal mortality in the United States. Mutations in the RyR2-encoded cardiac ryanodine receptor cause the highly lethal catecholaminergic polymorphic ventricular tachycardia (CPVT1) in the young.

OBJECTIVE

The purpose of this study was to determine the spectrum and prevalence of RyR2 mutations in a large cohort of SIDS cases.

METHODS

Using polymerase chain reaction, denaturing high performance liquid chromatography, and direct DNA sequencing, a targeted mutational analysis of RyR2 was performed on genomic DNA isolated from frozen necropsy tissue on 134 unrelated cases of SIDS (57 females, 77 males; 83 white, 50 black, 1 Hispanic; average age = 2.7 months). RyR2 mutations were engineered by site-directed mutagenesis, heterologously expressed in HEK293 cells, and functionally characterized using single-channel recordings in planar lipid bilayers.

RESULTS

Overall, two distinct and novel RyR2 mutations were identified in two cases of SIDS. A 6-month-old black female hosted an R2267H missense mutation, and a 4-week-old white female infant harbored a S4565R mutation. Both nonconservative amino acid substitutions were absent in 400 reference alleles, involved conserved residues, and were localized to key functionally significant domains. Under conditions that simulate stress [Protein Kinase A (PKA) phosphorylation] during diastole (low activating [Ca2+]), SIDS-associated RyR2 mutant channels displayed a significant gain-of-function phenotype consistent with the functional effect of previously characterized CPVT-associated RyR2 mutations.

CONCLUSIONS

Here we report a novel pathogenic mechanism for SIDS, whereby SIDS-linked RyR2 mutations alter the response of the channels to sympathetic nervous system stimulation such that during stress the channels become "leaky" and thus potentially trigger fatal cardiac arrhythmias.

Co-sleeping, an ancient practice: issues of the past and present, and possibilities for the future

Co-sleeping-infants sharing the mother's sleep space-has prevailed throughout human evolution, and continued over the centuries of western civilization despite controversy and blame of co-sleeping mothers for the deaths of their infants. By the past century, "crib death" was recognized, later identified as Sudden Infant Death Syndrome (SIDS), and generally found to occur more frequently during bed sharing. Pediatricians warned parents of the dangers of SIDS and other risks of bed sharing, and the frequency of bed sharing decreased markedly over the years. However, during recent decades, bed sharing began to increase, though major issues were raised, including: whether bed sharing actually exacerbates or is protective against the occurrence of SIDS, whether the practice facilitates breast feeding, whether bed sharing is beneficial for an infant's development, and other concerns. Dissention may soon be diminished by use of a crib which opens at the mother's bed-side and is becoming a popular approach to mother-and-infant closeness through the night.

Inhibition of serotonergic neurons in the nucleus paragigantocellularis lateralis fragments sleep and decreases rapid eye movement sleep in the piglet: implications for sudden infant death syndrome

Serotonergic receptor binding is altered in the medullary serotonergic nuclei, including the paragigantocellularis lateralis (PGCL), in many infants who die of sudden infant death syndrome (SIDS). The PGCL receives inputs from many sites in the caudal brainstem and projects to the spinal cord and to more rostral areas important for arousal and vigilance. We have shown previously that local unilateral nonspecific neuronal inhibition in this region with GABA(A) agonists disrupts sleep architecture. We hypothesized that specifically inhibiting serotonergic activity in the PGCL would result in less sleep and heightened vigilance. We analyzed sleep before and after unilaterally dialyzing the 5-HT1A agonist (+/-)-8-hydroxy-2-(dipropylamino)-tetralin (8-OH-DPAT) into the juxtafacial PGCL in conscious newborn piglets. 8-OH-DPAT dialysis resulted in fragmented sleep with an increase in the number and a decrease in the duration of bouts of nonrapid eye movement (NREM) sleep and a marked decrease in amount of rapid eye movement (REM) sleep. After 8-OH-DPAT dialysis, there were decreases in body movements, including shivering, during NREM sleep; body temperature and heart rate also decreased. The effects of 8-OH-DPAT were blocked by local pretreatment with N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane-carboxamide, a selective 5-HT1A antagonist. Destruction of serotonergic neurons with 5,7-DHT resulted in fragmented sleep and eliminated the effects of subsequent 8-OH-DPAT dialysis on REM but not the effects on body temperature or heart rate. We conclude that neurons expressing 5-HT1A autoreceptors in the juxtafacial PGCL are involved in regulating or modulating sleep. Abnormalities in the function of these neurons may alter sleep homeostasis and contribute to the etiology of SIDS.

Stable prevalence but changing risk factors for sudden infant death syndrome in child care settings in 2001

OBJECTIVE

A total of 20% of sudden infant death syndrome (SIDS) cases in the 1990s occurred in child care settings. This is much higher than the 8% expected from Census Bureau data. Factors that were associated with child care SIDS included older age; white race; older, more educated mothers; and unaccustomed prone position. Since these findings, much emphasis has been placed on promoting a safe sleep environment in child care. The objectives of this study were to determine the proportion of SIDS occurring in child care in 2001 and to assess risk factors for SIDS in child care.

METHODS

We conducted a retrospective review of all SIDS deaths that occurred in 2001 in 13 US states. Information regarding demographics, SIDS risk factors, and child care arrangements were collected and analyzed. Deaths that occurred in child care were compared with deaths that occurred during parental care.

RESULTS

Of 480 deaths, 79 (16.5%) occurred in child care settings. Of these child care deaths, 36.7% occurred in family child care homes, 17.7% occurred in child care centers, 21.3% occurred in relative care, and 17.7% occurred with a nanny/babysitter at home. Infants in child care were more likely to be older and to die between the hours of 8 am and 4 pm and less likely to be exposed to secondhand smoke. There was no difference in usual, found, or placed sleep position between child care and home deaths. Approximately one half of the infants who died of SIDS in both settings were found prone, and 20% of deaths in both settings were among infants who were unaccustomed to prone sleep.

CONCLUSIONS

The proportion of SIDS deaths in child care has declined slightly but still remains high at 16.5%. Infants in child care are no more likely to be placed or found prone and no more likely to be on an unsafe sleep surface. Educational efforts with child care providers have been effective and should be expanded to unregulated child care providers. In addition, there may be other, yet-unidentified factors in child care that place infants in those settings at higher risk for SIDS.

Possible pathomechanisms of sudden infant death syndrome: key role of chronic hypoxia, infection/inflammation states, cytokine irregularities, and metabolic trauma in genetically predisposed infants

Chronic hypoxia, viral infections/bacterial toxins, inflammation states, biochemical disorders, and genetic abnormalities are the most likely trigger of sudden infant death syndrome (SIDS). Autopsy studies have shown increased pulmonary density of macrophages and markedly more eosinophils in the lungs accompanied by increased T and B lymphocytes. The elevated levels of immunoglobulins, about 20% more muscle in the pulmonary arteries, increased airway smooth muscle cells, and increased fetal hemoglobin and erythropoietin are evidence of chronic hypoxia before death. Other abnormal findings included mucosal immune stimulation of the tracheal wall, duodenal mucosa, and palatine tonsils, and circulating interferon. Low normal or higher blood levels of cortisol often with petechiae on intrathoracic organs, depleted maternal IgG antibodies to endotoxin core (EndoCAb) and early IgM EndoCAb triggered, partial deletions of the C4 gene, and frequent IL-10-592*A polymorphism in SIDS victims as well as possible hypoxia-induced decreased production of antiinflammatory, antiimmune, and antifibrotic cytokine IL-10, may be responsible for the excessive reactions to otherwise harmless infections. In SIDS infants, during chronic hypoxia and times of infection/inflammation, several proinflammatory cytokines are released in large quantities, sometimes also representing a potential source of tissue damage if their production is not sufficiently well controlled, eg, by pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP). These proinflammatory cytokines down-regulate gene expression of major cytochrome P-450 and/or other enzymes with the specific effects on mRNA levels, protein expression, and enzyme activity, thus affecting metabolism of several endogenous lipophilic substances, such as steroids, lipid-soluble vitamins, prostaglandins, leukotrienes, thromboxanes, and exogenous substances. In SIDS victims, chronic hypoxia, TNF-alpha and other inflammatory cytokines, and arachidonic acid (AA) as well as n-3 polyunsaturated fatty acids (FA), stimulated and/or augmented superoxide generation by polymorphonuclear leukocytes, which contributed to tissue damage. Chronic hypoxia, increased amounts of nonheme iron in the liver and adrenals of these infants, enhanced activity of CYP2C9 regarded as the functional source of reactive oxygen species (ROS) in some endothelial cells, and nicotine accumulation in tissues also intensified production of ROS. These increased quantities of proinflammatory cytokines, ROS, AA, and nitric oxide (NO) also resulted in suppression of many CYP450 and other enzymes, eg, phosphoenolpyruvate carboxykinase (PEPCK), an enzyme important in the metabolism of FA during gluconeogenesis and glyceroneogenesis. PEPCK deficit found in SIDS infants (caused also by vitamin A deficiency) and eventually enhanced by PACAP lipolysis of adipocyte triglycerides resulted in an increased FA level in blood because of their impaired reesterification to triacylglycerol in adipocytes. In turn, the overproduction and release of FA into the blood of SIDS victims could lead to the metabolic syndrome and an early phase of type 2 diabetes. This is probably the reason for the secondary overexpression of the hepatic CYP2C8/9 content and activity reported in SIDS infants, which intensified AA metabolism. Pulmonary edema and petechial hemorrhages often present in SIDS victims may be the result of the vascular leak syndrome caused by IL-2 and IFN-alpha. Chronic hypoxia with the release of proinflammatory mediators IL-1alpha, IL-1beta and IL-6, and overloading of the cardiovascular and respiratory systems due to the narrowing airways and small pulmonary arteries of these children could also contribute to the development of these abnormalities. Moreover, chronic hypoxia of SIDS infants induced also production of hypoxia-inducible factor 1alpha (HIF-1alpha), which stimulated synthesis and release of different growth factors by vascular endothelial cells and intensified subclinical inflammatory reactions in the central nervous system, perhaps potentiated also by PACAP and VIP gene mutations. These processes could lead to the development of brainstem gliosis and disorders in the release of neuromediators important for physiologic sleep regulation. All these changes as well as eventual PACAP abnormalities could result in disturbed homeostatic control of the cardiovascular and respiratory responses of SIDS victims, which, combined with the nicotine effects and metabolic trauma, finally lead to death in these often genetically predisposed children.

Sudden infant death syndrome in child care settings in the Netherlands

BACKGROUND

In the Netherlands, there is a very low incidence of sudden infant death syndrome (SIDS) due to effective preventive campaigns.

METHODS

During the period September 1996 to August 2002, nationwide 161 deaths from SIDS (about 85% of all cases of SIDS during that time) were investigated by the Cot Death Committee of the Dutch Paediatric Association.

RESULTS AND DISCUSSION

Over 10% of cases of SIDS took place during some type of child care. From a national survey carried out in 2000/01 information was available on the child care attendance of 2000 Dutch infants aged 3-6 months. Based on the hours usually spent in child care by these infants, the number of similarly aged infants that died from SIDS while attending child care was 4.2 times higher than expected. Remarkably, the prevalence of known risk factors for SIDS, such as sleeping position and parental smoking, was favourable in the SIDS cases in child care settings. The adherence of child care facilities to the safe sleeping recommendations is high in the Netherlands, and no explanation as to why child care settings may be associated with an increased risk of SIDS is apparent. The possibility of other explanations, such as stress and change in routine care, is hypothesised.

Sudden infant death syndrome: a cybernetic etiology

The brain's processes, by hypothesis, involve information processing by an extraordinarily complex, highly sophisticated, self-organizing cybernetic system embedded in the central nervous system. This cybernetic system generates itself in successive stages. Breathing is, by default, an autonomous function, but breath control is learned. If there is not a smooth transfer of function at the time when a successor system (one that enables autonomous breathing to be overridden by voluntary control) takes over, breathing may cease, without any overt cause being detectable, even with a thorough postmortem examination. If conditions are such that, at that point, the infant's body lacks the strength to resume breathing again under autonomic control, Sudden Infant Death Syndrome may result. The theory explains why infants are at greater risk if they sleep face down.

Effect of pacifier use on oral breathing in healthy newborn infants

We tested the hypothesis that the use of a pacifier may affect the ability of some term infants to maintain effective oral breathing during prolonged nasal occlusion. Three nasal occlusion tests without a pacifier and 3 with a pacifier were alternately carried out in 20 healthy term infants (age 2-5 days). Once the infant commenced oral breathing, nasal occlusion was continued for up to 90 sec (prolonged nasal occlusion), provided the infant did not start crying and that arterial oxygen saturation (SaO(2)) did not drop to < or = 80%. The response to nasal occlusion was considered maladaptive if oral breathing was accomplished with signs of upper airway obstruction. After nasal occlusion, the infants succeeded in starting oral breathing in all instances after a delay which was strongly correlated to the drop in SaO(2) (P < 0.001). Once the infants commenced oral breathing, 17/20 infants presented a maladaptive response to 62% of all tests without pacifier, whereas 10/20 infants presented a maladaptive response to 30% of all tests with a pacifier in place (P < 0.001). Following prolonged nasal occlusion, 18 of 19 infants presented a maladaptive response to 84% of all tests without pacifier, whereas 12 of 19 infants presented a maladaptive response to 41% of all tests with a pacifier in place (P < 0.001). Thus, after prolonged nasal occlusion with or without pacifier, the drop in mean SaO(2) from baseline values changed in accordance with an appropriate and maladaptive response (-4 +/- 1 vs. -7 +/- 1; P < 0.001). We conclude that normal term infants often present with a maladaptive response to prolonged nasal occlusion. The use of a pacifier enhances the infant's ability to maintain a more adequate oral air flow.