SIDS pathogenesis: pathological findings indicate infection and inflammatory responses are involved

Sudden Infant Death Syndrome, Pulmonary Edema, and Sodium Toxicity: A Grounded Theory

Sudden Infant Death Syndrome (SIDS) occurs unexpectedly in an otherwise healthy infant with no identifiable cause of death following a thorough investigation. A general hypervolemic state has been identified in SIDS, and fluid in the lungs suggests the involvement of pulmonary edema and hypoxia as the cause of death. The present perspective paper reviews pathophysiological, epidemiological, and dietary evidence in SIDS. A grounded theory is presented that proposes an association of SIDS with sodium toxicity from excessive sodium chloride intake, mediated by noncardiogenic pulmonary edema, hypoxia, and alveolar damage. The peak of SIDS cases occurs in infants 2-4 months of age, who are less efficient in excreting excessive dietary sodium load. Evidence implicating sodium toxicity in SIDS includes increased levels of sodium associated with fever and with inflammatory/immune responses in the lungs. Conditions in near-miss SIDS cases are linked to dysregulated sodium, and increased sodium dietary intake suggests that sodium toxicity from a high-salt diet potentially mediates the association of seasonality and socioeconomic status with SIDS incidence. In addition, exposure to sodium toxicity meets three main criteria of the triple risk model of SIDS. The proposed pathophysiological effects of pulmonary edema related to sodium toxicity in SIDS merit further investigations.

Increased Platelet-Derived Growth Factor and Cytokine Levels in the Cerebrospinal Fluid of Patients of Sudden Unexpected Death with or without Viral Infection

OBJECTIVE

To clarify the pathogenesis of sudden unexpected natural death (SUD) as well as biomarkers to differentiate the underlying diseases, by performing cytokine analysis in the acute phase of pediatric patients in whom viral infection led to SUD.

METHODS

An acute phase cytokine analysis of pediatric patients in whom viral infection led to SUD was performed, and the data obtained were compared with those from SUD patients not associated with viral infections. Subjects included 4 boys aged 1-16 mo who died of cardiopulmonary arrest associated with viral infections. The causative viruses were identified as enterovirus, parainfluenza virus, respiratory syncytial virus, and rotavirus. The 4 other infants/children (aged 2-12 mo) died of non-infectious episodes, i.e., 1, 2, and 1 died of drowning, falling, and a traffic accident, respectively. Cerebrospinal fluid samples (CSF) of the subjects were collected during cardiopulmonary resuscitation or within 24 h of the events.

RESULTS

The infection-induced sudden death group showed elevated CSF levels of inflammatory cytokines and chemokines. No increase was observed in interleukin-10 levels. Furthermore, in the infection-induced sudden death group, platelet-derived growth factor levels correlated with inflammatory cytokine levels.

CONCLUSIONS

Infection-associated SUD may be differentiated from noninfectious SUD by measuring the levels of acute phase-inflammatory cytokines and chemokines at the onset of SUD.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.

The Sudden Infant Death Syndrome mechanism of death may be a non-septic hyper-dynamic shock

BACKGROUND

Sudden Infant Death Syndrome (SIDS) mechanisms of death remains obscured. SIDS' Triple Risk Model assumed coexistence of individual subtle vulnerability, critical developmental period and stressors. Prone sleeping is a major risk factor but provide no clues regarding the mechanism of death. The leading assumed mechanisms of death are either an acute respiratory crisis or arrhythmias but neither one is supported with evidence, hence both are eventually speculations. Postmortem findings do exist but are inconclusive to identify the mechanism of death. WHAT DOES THE PROPOSED HYPOTHESIS BASED ON?: 1. The stressors (suggested by the triple risk model) share a unified compensatory physiological response of decrease in systemic vascular resistant (SVR) to facilitate a compensatory increase in cardiac output (CO). 2. The cardiovascular/cardiorespiratory control of the vulnerable infant during a critical developmental period may be impaired. 3. A severe decrease in SVR is associated with hyper-dynamic state, high output failure and distributive shock.

THE HYPOTHESIS

Infant who is exposed to one or more stressors responds normally by decrease in SVR which increases CO. In normal circumstances once the needs are met both SVR and CO are stabilized on a new steady state. The incompetent cardiovascular control of the vulnerable infant fails to stabilize SVR which decreases in an uncontrolled manner. Accordingly CO increases above the needs to hyper-dynamic state, high output heart failure and hyper-dynamic shock.

CONCLUSIONS

The proposed hypothesis provides an appropriate alternative to either respiratory crises or arrhythmia though both speculations cannot be entirely excluded.

Clinical and experimental aspects of breathing modulation by inflammation

Neuroinflammation is produced by local or systemic alterations and mediated mainly by glia, affecting the activity of various neural circuits including those involved in breathing rhythm generation and control. Several pathological conditions, such as sudden infant death syndrome, obstructive sleep apnea and asthma exert an inflammatory influence on breathing-related circuits. Consequently breathing (both resting and ventilatory responses to physiological challenges), is affected; e.g., responses to hypoxia and hypercapnia are compromised. Moreover, inflammation can induce long-lasting changes in breathing and affect adaptive plasticity; e.g., hypoxic acclimatization or long-term facilitation. Mediators of the influences of inflammation on breathing are most likely proinflammatory molecules such as cytokines and prostaglandins. The focus of this review is to summarize the available information concerning the modulation of the breathing function by inflammation and the cellular and molecular aspects of this process. I will consider: 1) some clinical and experimental conditions in which inflammation influences breathing; 2) the variety of experimental approaches used to understand this inflammatory modulation; 3) the likely cellular and molecular mechanisms.

A systematic review exploring the relationship between infection and sudden unexpected death between 2000 and 2016: A forensic perspective

Death due to infectious diseases is a major health concern worldwide. This is of particular concern in developing countries where poor-socio economic status and a lack of healthcare resources contribute to the high burden of disease. In some cases death due to infection can be acute and aggressive, and death may occur without a diagnosis whilst the person is still alive. These deaths may ultimately lead to a medico-legal autopsy being performed. There are various mechanisms by which sudden death due to infection may occur. In addition, there are many risk factors associated with sudden death due to infection, which differ between infants and older individuals. However, it is unclear which pathogens and risk factors are most frequently associated with sudden death due to infection. Therefore a systematic review of articles and case reports published between 1 January 2000 and 30 June 2016 was undertaken in order to (1) explore the relationship between pathogens and their causative role and (2) identify the relationship between predisposing and/or risk factors associated with sudden death due to infection. Major databases were searched and after critical appraisal 143 articles were identified. It was found that respiratory infections and deaths involving bacterial pathogens were most commonly associated with these deaths. In addition the most common risk factors in infants were exposure to tobacco smoke and co-sleeping. In adults the most common risk factors were co-morbid conditions and illnesses. This information aids in a better understanding of these deaths and highlights the need for more research in this field, particularly in developing countries.

Virus Infections and Sudden Death in Infancy: The Role of Interferon-γ

Respiratory infections have been implicated in sudden infant death syndrome (SIDS). As interferon-γ (IFN-γ) is a major response to virus infection, we examined (1) the frequency of single nucleotide polymorphism (SNP), IFNG T + 874A, in SIDS infants, their parents, and ethnic groups with different incidences of SIDS; (2) model systems with a monocytic cell line (THP-1) and human peripheral blood monocytes (PBMC) for effects of levels of IFN-γ on inflammatory responses to bacterial antigens identified in SIDS; (3) interactions between genetic and environmental factors on IFN-γ responses. IFNG T + 874A genotypes were determined for SIDS infants from three countries; families who had a SIDS death; populations with high (Indigenous Australian), medium (Caucasian), and low (Bangladeshi) SIDS incidences. The effect of IFN-γ on cytokine responses to endotoxin was examined in model systems with THP-1 cells and human PBMC. The IFN-γ responses to endotoxin and toxic shock syndrome toxin (TSST-1) were assessed in relation to genotype, gender, and reported smoking. There was a marginal association with IFNG T + 874A genotype and SIDS (p = 0.06). Indigenous Australians had significantly higher proportions of the IFNG T + 874A SNP (TT) associated with high responses of IFN-γ. THP-1 cells showed a dose dependent effect of IFN-γ on cytokine responses to endotoxin. For PBMC, IFN-γ enhanced interleukin (IL)-1β, IL-6, and tumor necrosis factor-α responses but reduced IL-8 and IL-10 responses. Active smoking had a suppressive effect on baseline levels of IFN-γ. There was no effect of gender or genotype on IFN-γ responses to bacterial antigens tested; however, significant differences were observed between genotypes in relation to smoking. The results indicate virus infections contribute to dysregulation of cytokine responses to bacterial antigens and studies on physiological effects of genetic factors must include controls for recent or concurrent infection and exposure to cigarette smoke.

The male excess in sudden infant deaths

The peak age at which sudden infant death syndrome (SIDS) occurs corresponds to the developmental period in which infants are dependent on their innate responses to infection. There is a growing body of evidence indicating that dysregulation of inflammatory responses might contribute to the physiological changes leading to these sudden deaths. This study examined the effects of three important risk factors for SIDS on inflammatory responses: cigarette smoke, virus infection and male sex. Cytokine responses of peripheral monocytic blood cells of healthy, non-smoking males and females to endotoxin were measured. Surrogates for virus infection or cigarette smoke were assessed using IFN-γ or water-soluble cigarette smoke extract (CSE). For most conditions, cells from males had lower pro-inflammatory cytokine responses than those of females. An opposite trend was observed for IL-10. Significantly lower levels of some cytokines were noted for cells from male donors exposed to CSE. In females, there were significant correlations between testosterone levels and levels of pro-inflammatory cytokines, but none for males. Testosterone levels in females correspond to those among male infants in the age range at greatest risk of SIDS. The effects of the testosterone surge in male infants need to be examined in relation to changes in cortisol levels that occur during the same period of infant development.

Is there any correlation between HLA-DR expression in laryngeal mucosa and interleukin gene variation in sudden infant death syndrome?

AIM

The mucosal immune system and cytokines are activated in a large proportion of cases of sudden infant death syndrome (SIDS). Our aim was to search for a possible association between cytokine polymorphisms and immune stimulation of the laryngeal mucosal in SIDS.

METHODS

HLA-DR expression in laryngeal mucosal glands and surface epithelium in 97 SIDS victims was evaluated applying a semi-quantitative scoring system. The findings were related to cytokine gene polymorphisms as well as to the level of various cytokines in the cerebrospinal fluid (CSF). A risk score was established: a score of 0 prepresenting negative HLA-DR, supine position and no fever prior to death.

RESULTS

The IL-6 -176CG/CC genotype was found in 92.3% of the SIDS cases with positive score for all risk factors (p = 0.01). Infants with high HLA-DR score had high levels of IL-6 in the cerebrospinal fluid (>30 μg/L) (p = 0.005). Furthermore, the IL-8 SNPs -781 CT/TT genotypes and -251 AA/AT genotypes were observed in 93% of the SIDS cases with one or more of the risk factors present compared with SIDS cases no risk factors reported (p = 0.003 and p = 0.016, respectively).

CONCLUSION

This study adds further evidence to the hypothesis that there are genetically associated disturbances of immunological homoeostasis in SIDS.

Cytokines and sudden infant death

BACKGROUND

It has been hypothesised that inflammatory reactions could play an important role in the pathway(s) leading to sudden and unexpected death in infancy. On a molecular level, these reactions are regulated by various cytokines.

METHODS

To characterise the role of IL-1ß, IL-6 and TNFα more precisely, the concentrations of these cytokines were determined quantitatively using specific ELISA techniques in serum and cerebrospinal fluid (CSF) in 119 cases of sudden infant death. The infants were grouped into four categories (SIDS, SIDS with infection, natural death due to infection and unnatural death).

RESULTS

A good correlation was found between CSF and serum for IL-6 (Spearman correlation coefficients (SCC), 0.73) and also for TNFα (SCC, 0.57), although the CSF concentrations were lower than that from the serum. There were no significant differences between the categories of death for any of the serum or CSF cytokines. Compared with normal values, increased serum concentrations of IL-1ß, IL-6 and TNFα were found in 70%, 69% and 38% of the cases respectively, indicating possible agonal or post-mortem changes of cytokine concentrations. In three cases very high cytokine concentrations were found (mainly for IL-6). This may have contributed to the mechanism of death (cytokine storm) in two of the cases.

CONCLUSIONS

In a small group of patients, very high cytokine concentrations are a possible explanation for the cause of death ("cytokine storm").

Biochemical, cytological and histopathological examination of sudden unexpected death in infancy

To evaluate the pathophysiological mechanisms underlying sudden infant death syndrome (SIDS), four sudden unexpected death in infancy (SUDI) and four sudden deaths in children over 1 year of age were examined. In the SUDI cases, increased numbers of scavenger receptor A positive (SRA(+) ) cells (4/4), numerous platelet aggregates (3/4), and tumor necrosis factor (TNF)-α(+) cells (4/4) were observed in the peripheral blood (PB) smear preparations. Macrophage colony stimulating factor, interleukin (IL)-6, IL-8, TNF-α and IL-1β all exceeded the normal levels. Minute foci of inflammatory lung injury (4/4), numerous platelet emboli in lungs and among cardiac myocytes (3/4) and appreciable contraction band necrosis (1/4) were observed. And neutrophils accumulated in the capillaries of injured organs and endothelial cells were extensively injured. From these findings, cytokine abnormality induced by SRA(+) cells in PB was considered to play an important role in the development of tissue injury, and platelet emboli or contraction band necrosis might have been the leading cause of death in our SUDI cases. Patients with characteristics thought to be similar to our SUDI cases were included in the SIDS group; cytokine abnormality was considered to be one of the underlying mechanisms in SIDS.

A polymorphism in a staphylococcal enterotoxin receptor gene (T cell receptor BV3 recombination signal sequence) is not associated with unexplained sudden unexpected death in infancy in an Australian cohort

Polymorphisms in genes that influence the expression of toxin receptors could contribute to Sudden Infant Death Syndrome (SIDS) and unexplained Sudden Unexpected Death in Infancy (uSUDI) for which there is evidence of toxin involvement. We aimed to determine whether TCRBV3S1 allele 2 could be involved in a staphylococcal toxic shock hypothesis for uSUDI. Observed frequencies of the TCRBV3S1*2 allele and genotype in 48 Australian uSUDI cases and 96 live comparison infants did not differ. In future the role of other toxin receptor gene polymorphisms deserves investigation.

Distribution of interleukin-1 receptor antagonist genotypes in sudden unexpected death in infancy (SUDI); unexplained SUDI have a higher frequency of allele 2

AIMS

This investigation was designed to explore the role of IL-1RN genotype in unexplained infant deaths (including sudden infant death syndrome (SIDS)), non-infectious infant deaths, and infectious infant deaths, and to investigate whether IL-1RN genotype is related to the finding of organisms in normally sterile sites in infant deaths.

METHODS

IL-1RN 89bp variable number of tandem repeat polymorphism genotype was determined using polymerase chain reaction for 49 cases of unexplained sudden unexpected death in infancy (uSUDI), 13 cases of infectious sudden unexpected death in infancy, 10 cases of non-infectious sudden unexpected death in infancy, and 103 live control infants. IL-1RN genotype was then compared with the presence of bacteria in normally sterile sites in infant deaths.

RESULTS

An association was found between the homozygous A2 allele and uSUDI (P = 0.007; 95% confidence interval 1.41-17.67) where carriage of the 2/2 genotype was 4.85 times more likely to increase risk of uSUDI compared with the predominant 1/1 genotype.

CONCLUSIONS

The role of infection in uSUDI and SIDS may be via an immune response pathway where IL-1RN A2 affects interleukin (IL)-1 regulation. These results are consistent with previous research where polymorphic genotypes conferring more severe proinflammatory responses are found more frequently in uSUDI/SIDS infants than in controls.

Sudden infant death syndrome and cardiac arrhythmias

There is a considerable body of evidence that common bacterial toxins, absorbed from the mucosal surface or delivered as part of a transient bacteremia, have a pathogenic role in sudden infant death syndrome (SIDS). The candidate organisms are Staphylococcus aureus and Escherichia coli. Death in SIDS is rapid, with infants progressing from well, or only mildly unwell, to death in less than 20 min. This mode of death is not typical of infection but it is consistent with toxin action on cardiovascular or respiratory control. Both S. aureus and E. coli secrete toxins (cytolysins and colicins) that create channels in cell membranes and disturb ion currents. Recent evidence indicates that between 5 and 15% of SIDS cases carry potentially lethal loss-of-function mutations in cardiac channelopathy genes. However, only a minority of individuals with these mutations die of SIDS and the hypothesis proposed is that toxin-gene interaction could explain the deaths. Furthermore, channelopathy mutations predispose to sudden death at all ages and since episodes of transient bacteremia occur throughout life the idea of toxin-gene interaction could have wider applicability. These ideas can be investigated and answered in the near future using the new science of proteomics.

Sudden infant death syndrome (SIDS) in African Americans: polymorphisms in the gene encoding the stress peptide pituitary adenylate cyclase-activating polypeptide (PACAP)

AIMS

Mice lacking pituitary adenylate cyclase-activating polypeptide (PACAP) are prone to sudden death in the second post-natal week, having respiratory and metabolic disturbances reminiscent of the human Sudden Infant Death Syndrome (SIDS). Here we test the hypothesis that the human PACAP gene is a site of genetic variance associated with SIDS in a cohort of 92 victims and 92 matched controls.

METHODS

Using polymerase chain reaction and sequencing, we examined the PACAP gene in 92 SIDS cases (46 Caucasians and 46 African Americans) and 92 race- and gender-matched controls.

RESULTS

We found no significant associations between PACAP and SIDS in Caucasians. However, in the African Americans, a non-synonymous single nucleotide polymorphism (i.e. an aspartic acid/glycine coding variant, rs2856966) within exon 2 of PACAP was significantly associated with SIDS (p = 0.004), as were haplotypes containing this polymorphism (p < 0.0001). Glycine was three times more likely at this location in the African-American SIDS victims (17 cases) than African-American controls (5 cases).

CONCLUSION

These data are the first to suggest an association between a variant within the coding region of the PACAP gene and SIDS. Based on these findings, further investigations are warranted into the functional importance of PACAP signaling in neonatal survival and the role of PACAP-signaling abnormalities in SIDS.

Sudden Infant Death Syndrome: review of implicated genetic factors

Genetic studies in Sudden Infant Death Syndrome (SIDS) have been motivated by clinical, epidemiological, and/or neuropathological observations in SIDS victims, with subsequent pursuit of candidate genes in five categories: (1) genes for ion channel proteins based on electrocardiographic evidence of prolonged QT intervals in SIDS victims, (2) gene for serotonin transporter based on decreased serotonergic receptor binding in brainstems of SIDS victims, (3) genes pertinent to the early embryology of the autonomic nervous system (ANS) (and with a link to the 5-HT system) based on reports of ANS dysregulation in SIDS victims, (4) genes for nicotine metabolizing enzymes based on evidence of cigarette smoking as a modifiable risk factor for SIDS, and (5) genes regulating inflammation, energy production, hypoglycemia, and thermal regulation based on reports of postnatal infection, low birth weight, and/or overheating in SIDS victims. Evidence for each of these classes of candidate genes is reviewed in detail. As this review indicates, a number of genetically controlled pathways appear to be involved in at least some cases of SIDS. Given the diversity of results to date, genetic studies support the clinical impression that SIDS is heterogeneous with more than one entity and with more than one possible genetic etiology. Future studies should consider expanded phenotypic features that might help clarify the heterogeneity and improve the predictive value of the identified genetic factors. Such features should be evaluated to the extent possible in both SIDS victims and their family members. With 2,162 infants dying from SIDS in 2003 in the U.S. alone, and improved but still imperfect parent and caretaker compliance with known modifiable risk factors for SIDS, it behooves clinicians, researchers, and parents to combine efforts to reach a common goal. The message of the "Back to Sleep" campaign needs to be re-introduced/re-engineered to reach families and caretakers of all ethnic groups. Clinicians and researchers need to gently inform new SIDS parents about the opportunity to contribute tissue to the NICHD-funded University of Maryland Brain and Tissue Bank. By expanding the network of clinicians, scientists, and families working together, and by combined efforts in a collaborative multi-center study of candidate genes and/or genomics, the discovery of the genetic profile of the infant at risk for SIDS can ultimately be determined.

Curli biogenesis and function

Curli are the major proteinaceous component of a complex extracellular matrix produced by many Enterobacteriaceae. Curli were first discovered in the late 1980s on Escherichia coli strains that caused bovine mastitis, and have since been implicated in many physiological and pathogenic processes of E. coli and Salmonella spp. Curli fibers are involved in adhesion to surfaces, cell aggregation, and biofilm formation. Curli also mediate host cell adhesion and invasion, and they are potent inducers of the host inflammatory response. The structure and biogenesis of curli are unique among bacterial fibers that have been described to date. Structurally and biochemically, curli belong to a growing class of fibers known as amyloids. Amyloid fiber formation is responsible for several human diseases including Alzheimer's, Huntington's, and prion diseases, although the process of in vivo amyloid formation is not well understood. Curli provide a unique system to study macromolecular assembly in bacteria and in vivo amyloid fiber formation. Here, we review curli biogenesis, regulation, role in biofilm formation, and role in pathogenesis.