Microbiology in sudden infant death syndrome (SIDS) and other childhood deaths

The usefulness of post-mortem microbiology in the assessment of sudden unexpected deaths in infants and children has been debated by many pathologists. In our centre, microbiological investigations have been part of the post-mortem protocol for investigation of sudden deaths in infants and children for the past 12 years. The objective of this study was to review the microbiological findings for infants and children examined by our unit during the past 4 years in relation to gross and histological findings of the autopsy and the medical and social histories of the children. We reviewed 57 consecutive sudden deaths in infants and children examined by our Referral Centre between November 1994 and October 1998. These 57 sudden deaths were aged from 1 day to 4 years and 9 months including 40 cases of sudden infant death syndrome (SIDS) and 17 non-SIDS deaths. Results of the microbiological investigations of tissues and body fluids were assessed during the case review with reference to histological shock signs, severe gastric aspiration, and signs of acute thymic involution. Bacteria alone or in association with viruses were identified in 45/57 (79%) cases including 34/40 (85%) SIDS. The most frequent bacterial isolate was Escherichia coli (27), and the virus identified most frequently was enterovirus (8). C-reactive protein was increased in 10 out of the 42 cases tested including 8/32 (25%) SIDS. Significant gastric content aspiration was found in 17/57 (29.8%) including 13/40 (32.5%) SIDS. Histological signs of shock were present in 33/55 (60%) cases including 22/39 SIDS (56.4%). The microbiological findings were positive for 27/33 (81.8%). We conclude that post-mortem microbiology is essential in sudden death investigation. The conclusion that a death is unexplained if no microbiology was done is not valid, even if in some cases it may be difficult to know precisely in what way the pathogen contributed to the death.     

Endpoints in myelin-deficient (MD) rats

Health problems in some animal models remain unexplained, rendering in vivo studies ethically challenging, especially when experimental animals are prone to sudden death. Over the last 3 decades, the myelin-deficient (md) rat, a strain with severe dysmyelination due to mutant proteolipid protein, has been key to important discoveries in mechanisms of myelination and glial cell biology. The usefulness of this mutant rat, however, has been limited by sudden death during the fourth week of life. Timely euthanasia has been difficult because the cause of these mortalities remains unexplained and the endpoint not determined. In this clinicopathologic study, we determined that sudden onset of hindlimb paralysis inevitably leads to paralysis of the urinary bladder and then breathing difficulties because of severe injury to the spinal cord in the midthoracic region with concurrent narrowing of the vertebral canal due to fracture of a vertebral body. Sudden onset of hindlimb paralysis likely is related to seizures and severe muscle spasms that begin to occur at the end of the third week of life. Once seizure activity begins, we recommend frequent monitoring of md rats for hindlimb paralysis and distention of the urinary bladder as indication of endpoints mandating prompt euthanasia.     

Alteration in regulation of inflammatory response to influenza a virus and endotoxin in suckling rat pups: a potential relationship to sudden infant death syndrome

Data increasingly implicate a possible role of immune and inflammatory responses to infection in sudden infant death syndrome (SIDS). We have previously described a dual challenge model that results in pathology, organ damage, vascular collapse and unexplained death similar to that seen in SIDS. In this study, we examined changes in inflammatory cytokine mRNA in the lung and liver and regulation of pathways associated with nitric oxide production. Our data suggest that priming of the immune system by mild viral infection disturbs normal inflammatory response to endotoxin. This results in an increased nitric oxide synthase production, most likely the cause of liver pathology and clotting abnormalities.     

The activity of 2'',3''-cyclic nucleotide 3''-phosphohydrolase in the corpus callosum, subcortical white matter, and spinal cord in infants dying from sudden infant death syndrome

Abstract: The activity of 2',3'-cyclic nucleotide 3'-phos-phohydrolase (CNPase) has been determined in corpus callosum, subcortical white matter, and spinal cord of infants whose death was attributed to the sudden infant death syndrome (SIDS), and compared with enzyme activity in other cases in which the cause of death was not associated with respiratory distress. In nearly half the SIDS cases, CNPase activity and oligodendroglial cell numbers were reduced before the onset of myelination, but only in the corpus callosum. In other SIDS cases, enzyme activity and cell numbers were the same as in non-SIDS cases. If the expression of CNPase activity reflects glioblast differentiation to oligodendrocytes with myelinating potential, then this transformation is abnormal in certain SIDS cases, as also evidenced in cases of prolonged neonatal respiratory insufficiency and gives rise to a subsequent deficit of myelin in the corpus callosum.     

Environmental tobacco smoke and sudden infant death syndrome: a review

Environmental tobacco smoke (ETS), containing the developmental neurotoxicant, nicotine, is a prevalent component of indoor air pollution. Despite a strong association with active maternal smoking and sudden infant death syndrome (SIDS), information on the risk of SIDS due to prenatal and postnatal ETS exposure is relatively inconsistent. This literature review begins with a discussion and critique of existing epidemiologic data pertaining to ETS and SIDS. It then explores the biologic plausibility of this association, with comparison of the known association between active maternal smoking and SIDS, by examining metabolic and placental transfer issues associated with nicotine, and the biologic responses and mechanisms that may follow exposure to nicotine. Evidence indicates that prenatal and postnatal exposures to nicotine do occur from ETS exposure, but that the level of exposure is often substantially less than levels induced by active maternal smoking. Nicotine also has the capacity to concentrate in the fetus, regardless of exposure source. Experimental animal studies show that various doses of nicotine are capable of affecting a neonate's response to hypoxic conditions, a process thought to be related to SIDS outcomes. Mechanisms contributing to deficient hypoxia response include the ability of nicotine to act as a cholinergic stimulant through nicotinic acetylcholine receptor (nAChR) binding. The need for future research to investigate nicotine exposure and effects from non-maternal tobacco smoke sources in mid to late gestation is emphasized, along with a need to discourage smoking around both pregnant women and infants.     

Sudden infant death syndrome, virus infections and cytokines

Many epidemiological risk factors identified for sudden infant death syndrome (SIDS) suggest a viral aetiology, e.g. exposure to cigarette smoke and winter peak, mild respiratory symptoms. Virus infections and bacterial toxins induce cytokine activity and it has been suggested that uncontrolled inflammatory mediators could be involved in some cases of SIDS. The aim of this review was to assess the evidence for virus infection in SIDS and to examine those findings in relation to individual variations in cytokine responses and various pathophysiological mechanisms proposed for SIDS such as sleep derangement, hypoxia, cardiac arrhythmia, vascular hypotonicity and hypoglycaemia.     

Clostridium perfringens type A cytotoxic-enterotoxin(s) as triggers for death in the sudden infant death syndrome: Development of a toxico-infection hypothesis

In our studies with the pathogenic bacteriumClostridium perfringens type A and its cytotoxic-enterotoxins (CTEs), we have obtained results that imply an involvement of this organism in the sudden infant death syndrome (SIDS). In fecal samples obtained from SIDS infants (n=164) and non-SIDS infants (n=57),C. perfringens type A was present in high numbers in >80% of SIDS and <2% of control non-SIDS cases respectively. Fecal samples from SIDS infants analyzed by ELISA forC. perfringens type A CTEs showed a very strong positive correlation with the presence of the organism. Histopathological examination of ileal tissue from SIDS infants showed remarkable similarity to tissue from animal models affected byC. perfringens type A CTEs, where the patterns of damage were positively correlated with the age of the animal. We propose that systemic distribution of the CTEs acts parasympathomimetically to trigger a biochemical cascade that alters cardiorespiratory control. Death may subsequently ensue in an immunologically vulnerable infant.Florida Agricultural Experiment Station Journal Series No. R-02419.     

Animal models used to test the interactions between infectious agents and products of cigarette smoked implicated in sudden infant death syndrome

Animal test systems are reviewed that have relevance to sudden infant death syndrome (SIDS) are reviewed. These test interactions between infectious agents (or their toxins) and products of cigarette smoke. Infectious agents implicated in SIDS include members of the enterobacteria and clostridia, Staphylococcus aureus and Streptococcus pyogenes. Smoking is thought to be the single most preventable cause of SIDS. Tobacco smoke contains many extremely toxic products including cyanide and nicotine. Many animal test systems are available to examine the potency of bacterial toxins and smoke-derived components. These include mice, hamsters, rats and chick embryos. Such systems reveal synergy between bacterial toxins, especially endotoxin and superantigens. They have also demonstrated potentiation of low levels of bacterial toxin by low levels of both nicotine and its primary metabolite, cotinine. These findings suggest a possible causal explanation for the fact that passive exposure to cigarette smoke is a risk factor in sudden infant death syndrome.     

The role of infection in sudden infant death syndrome

Studies on the potential role of infectious agents in sudden infant death syndrome (SIDS) have been published over the years in a variety of journals. The aim of this special issue of FEMS Immunology and Medical Microbiology is to bring together a group of the most recent studies from Europe, Australia and Canada which cover epidemiology and laboratory studies examining hypotheses relating to infection and inflammation in SIDS. The articles in this issue examine evidence for the involvement of specific micro-organisms in SIDS and the problems relating to experimental studies on infection in relation to the underlying pathology of these deaths. There is an update on the evidence for the common bacterial hypothesis proposed in 1987 examining risk factors identified in epidemiological studies, particularly how the prone sleeping position could affect bacterial colonisation or induction of toxins. Evidence for induction of inflammatory responses in SIDS infants is reviewed and the relation of these responses to mechanisms proposed as causes of death assessed. Factors found to be associated with reduction of the risk of SIDS (breast feeding and immunisation) are examined in relation to some of the toxigenic bacteria implicated in these deaths. Finally, the high incidence of SIDS in some ethnic groups is examined as a potential model to investigate the contributions of genetic, environmental and cultural differences to susceptibility of infants not only to SIDS but to serious respiratory tract infections.     

Some aspects of clinical relevance in the maturation of respiratory control in infants.

Two reflex mechanisms important for survival are discussed. Brain stem and cardiovascular mechanisms that are responsible for recovery from severe hypoxia (autoresuscitation) are important for survival in acutely hypoxic infants and adults. Failure of this mechanism may be important in sudden infant death syndrome (SIDS), because brain stem-mediated hypoxic gasping is essential for successful autoresuscitation and because SIDS infants appear to attempt to autoresuscitate just before death. A major function of another mechanism is to protect the airway from fluid aspiration. The various components of the laryngeal chemoreflex (LCR) change during maturation. The LCR is an important cause of prolonged apneic spells in infants. Consequently, it also may have a role in causing SIDS. Maturational changes and/or inadequacy of this reflex may be responsible for pulmonary aspiration and infectious pneumonia in both children and adults.     

Endotoxin in blood and tissue in the sudden infant death syndrome

Although the explanation for sudden infant death syndrome (SIDS) remains unknown, an increasing body of evidence now exists to suggest a possible role for bacterial toxins in the aetiology, and a number of investigators have considered that endotoxaemia could explain some of the associated features. Following the development of an animal model which confirmed that endotoxaemia could be detected after death, we studied endotoxin levels in blood and tissue samples taken at autopsy from SIDS infants, child controls and adult controls. There were significant correlations between endotoxin levels in blood and the various organs sampled particularly in SIDS cases and child controls, and blood endotoxin levels in SIDS cases were higher in those infants where there was histological evidence of mild to moderate inflammation. However, overall no significant differences were found between endotoxin levels in blood or tissue in the three study groups. Further studies into possible actions or interactions of endotoxin in SIDS are required.     

Prenatal nicotine alters vigilance states and AchR gene expression in the neonatal rat: implications for SIDS

Maternal smoking is a major risk factor for sudden infant death syndrome (SIDS). The mechanisms by which cigarette smoke predisposes infants to SIDS are not known. We examined the effects of prenatal nicotine exposure on sleep/wake ontogenesis and central cholinergic receptor gene expression in the neonatal rat. Prenatal nicotine exposure transiently increased sleep continuity and accelerated sleep/wake ontogeny in the neonatal rat. Prenatal nicotine also upregulated nicotinic and muscarinic cholinergic receptor mRNAs in brain regions involved in regulating vigilance states. These findings suggest that the nicotine contained in cigarette smoke may predispose human infants to SIDS by interfering with the normal maturation of sleep and wake.     

Sudden infant death syndrome, infection and inflammatory responses

Sudden infant death syndrome (SIDS) is sudden unexpected death in infancy for which there is no explanation after review of the history, a death scene investigation and a thorough autopsy. The use of common diagnostic criteria is a prerequisite for discussing the importance of infection, inflammatory responses and trigger mechanism in SIDS. Several observations of immune stimulation in the periphery and of interleukin-6 elevation in the cerebrospinal fluid of SIDS victims explain how infections can play a role in precipitating these deaths. Finally, these findings and important risk factors for SIDS are integrated in the concept of a vicious circle for understanding the death mechanism. The vicious circle is a concept to elucidate the interactions between unfavourable factors, including deficient auto-resuscitation, and how this could result in death.     

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

This article explores the pathological evidence that supports the hypothesis that infection and inflammation are underlying mechanisms in SIDS. It reviews the pathological findings in relation to the risk factors reported for SIDS and compares these findings with other hypotheses suggested as causes of these unexplained deaths in infants. The roles of environmental factors and bacterial products such as soluble curlin detectable in SIDS sera in triggering cytokine cascades and aberrant inflammatory responses resulting in a toxic shock-like event are also explored. Areas for future research are outlined.     

A Possible Murine Model for Investigation of Pathogenesis of Sudden Infant Death Syndrome

Several studies have indicated a possible causative role of toxigenic bacteria in sudden infant death syndrome (SIDS). This study examined the effect of toxigenic E. coli on pregnant and infant mice to determine if these animals could be used as a model for SIDS pathogenesis. Strains of E. coli from the intestinal contents of infants who have died of SIDS or other causes and from the faeces of healthy infants were collected over a broad time scale. The isolates were tested for their ability to produce then known toxins of E. coli and were serotyped (O and H antigens). Certain serotypes (e.g. O1:H- and O25:H1) emerged significantly more frequently from cases of SIDS than from healthy infants and isolates of these types were generally toxigenic in Vero-cell cultures but whose verotoxicity was not related to classical Shiga or other known toxins. This mouse model was developed to test the effects of these toxigenic and also non-toxigenic strains. Four apparently healthy pups aged between 17 and 21 days died unobserved overnight but no pups of the 54 control mice died suddenly (P = 0.0247, Fisher’s exact test). These were considered to represent sudden unexpected deaths. Pathological effects compatible with those in SIDS were observed in mouse pups exposed to toxigenic strains indicating this model may be suitable for further study into the pathogenesis of unexpected deaths in infancy. Providing an animal model of SIDS would promote a much better avenue for studying the pathogenesis of this enigmatic condition.     

Alterations of biological features of the cerebellum in sudden perinatal and infant death

This article intends to show how the cerebellum, a structure ordinarily not considered in mediating breathing or cardiovascular control, may play a critical role in compensatory responses particularly to hypoxic insults occurring pre and/or postnatally and thus may be involved in the sudden unexplained perinatal and infant death. Besides the ontogenesis of the cerebellar cortex in man, we reported alterations of biopathological features (neuronal immaturity, altered apoptotic programs, negative expression of somatostatin and EN2 gene, intense c-fos expression positivity, astrogliosis) in the cortex and in the dentate nucleus of the 63% of sudden deaths, and only in 10% of the controls. The correlation of these results with the mother's smoking habit was highly significant. Therefore, we support the hypothesis, already expressed in previous studies on brainstem, of a close relation between maternal cigarette smoking and a wide range of morpho-physiological defects of the brain, leading to unexplained sudden death in stillbirths, newborns, and Sudden Infant Death Syndrome (SIDS) victims.     

Common bacterial toxins and physiological vulnerability to sudden infant death: the role of deleterious genetic mutations

The common bacterial toxin hypothesis of sudden infant death syndrome (SIDS) is consistent with the epidemiological features of the condition including the age distribution, seasonal incidence, association with prone sleeping and with exposure to tobacco smoke. The hypothesis is supported by experimental evidence but there are two barriers to its acceptance: the speed of onset does not fit with conventional concepts of an infective process; furthermore, the hypothesis appears to offer a single explanation for what is regarded as a multifactorial disease. Concepts from information theory are used to explore these objections. Complex physiological systems process information and need a high level of redundancy to minimise error. Models show that deleterious mutations in such a system will interact synergistically. Environmental perturbations are most likely to cause failure (sudden death) in systems with several mutations. Models also indicate that mutation rates will pose a limit to the size of the functioning genome and, therefore, increased complexity in evolution depends on using old genes in new combinations rather than the chance appearance of new genes. The idea that we share our genes with the rest of creation (same genes but different combinations) leads to the following conjecture: for every receptor controlling the flow of information across a cell membrane there will be a bacterially coded molecule that can switch it off or on. Based on this premise, bacterial toxaemia could cause sudden death, merely the time it takes for a molecule to associate with or dissociate from its receptor. Regardless of the number of physiological systems involved in SIDS, the age distribution will have a unimodal peak corresponding to the age range during which infant serum IgG reaches its nadir. In this way, the two barriers to the common bacterial toxin hypothesis can be overcome: one explanation but multiple bacteria and toxins acting with variable speed on multiple target systems.     

Evidence for infection, inflammation and shock in sudden infant death: parallels between a neonatal rat model of sudden death and infants who died of sudden infant death syndrome

This study compared pathological findings from a neonatal rat model of sudden death with those from 40 sudden infant death syndrome (SIDS) infants collected at autopsy. In the rat model, influenza A virus was administered intranasally on postnatal day 10, and on day 12 a sublethal, intraperitoneal dose of Escherichia coli endotoxin; mortality was 80%. Tissue samples from the animals and infants were fixed in formaldehyde, embedded in paraffin, and sections stained with hematoxylin and eosin. Tissues from the SIDS specimens were additionally cultured for bacteria and viruses; post-mortem blood samples were evaluated for signs of inflammation. All sections were examined by a pediatric forensic pathologist familiar with SIDS pathology. Comparisons between the rat model and the human SIDS cases revealed that both exhibited gross and microscopic pathology related to organ shock, possibly associated with the presence of endotoxin. Uncompensated shock appeared to be a likely factor that caused death in both infants and rat pups. Response to a shock-inducing event might have played an important role in the events leading to death. The similarities between the neonatal rats and the human cases indicate that further research with the model might elucidate additional aspects of SIDS pathology.     

Preliminary investigation of lethally toxic sera of sudden infant death syndrome victims and neutralisation by commercially available immunoglobulins and adult sera

The aim of the study was to test the hypotheses (i) that sudden infant death syndrome sera are toxic to 11-day old chick embryos and (ii) that such a toxicity can be counteracted by immunoglobulin or adult sera. Serum samples from 11 SIDS victims and five controls were tested for lethal toxicity in the chick embryo bioassay. Five serum samples were used to challenge chick embryos injected with the following: sudden infant death syndrome serum plus Hank's balanced salt solution; Hank's balanced salt solution alone; sudden infant death syndrome serum plus 3% w/v commercial immunoglobulin; sudden infant death syndrome serum plus 6% w/v immunoglobulin; sudden infant death syndrome serum plus pooled sera of 40 healthy adults. Results obtained revealed that Hank's balanced salt solution, the pooled adult serum and the commercial immunoglobulin were all non-lethal, in the chick embryo test system. By contrast. 10 sudden infant death syndrome victims yielded sera containing lethal levels of toxin(s) compared to 2/5 controls which was statistically significant (P < 0.05, Fischer's exact test). In the tests of sudden infant death syndrome serum plus immunoglobulin or pooled adult serum, the lethality of sudden infant death syndrome serum was abolished in all cases. The reduction in toxicity of individual sudden infant death syndrome serum plus immunoglobulin or pooled adult serum was often statistically significant (P<0.05-P<0.00005, Fischer's exact test). We conclude that lethal levels of toxin are present in sudden infant death syndrome sera and that they can be neutralised by normal immune serum. These results indicate that passive immunisation is a potential treatment to protect babies considered at risk from sudden infant death syndrome.