Surfactant abnormalities in babies dying from sudden infant death syndrome

Tracheomalacia-Lethal Factor in a 17-month-Old Child

A case is reported of a child who died at age 17 months after having repeated respiratory incidents, probably related to mild infections. After a fatal respiratory episode, tracheomalacia was detected at necropsy. No other abnormalities were found. This condition, unless suspected clinically, could easily be overlooked at necropsy.

Surfactant phospholipid changes after antigen challenge: a role for phosphatidylglycerol in dysfunction

In asthma, inflammation-mediated surfactant dysfunction contributes to increased airway resistance, but the mechanisms for dysfunction are not understood. To test mechanisms that alter surfactant function, atopic asthmatics underwent endobronchial antigen challenge and bronchoalveolar lavage (BAL). BAL fluids were sequentially separated into cells, surfactant, and supernatant, and multiple end points were analyzed. Each end point's unique relationship to surfactant dysfunction was determined. Our results demonstrate that minimum surface tension (gamma(min)) of surfactant after antigen challenge was significantly increased with a spectrum of responses that included dysfunction in 6 of 13 asthmatics. Antigen challenge significantly altered the partitioning of surfactant phospholipid measured as a decreased ratio of large surfactant aggregates (LA) to small surfactant aggregates (SA), LA/SA ratio. Phosphatidylglycerol (PG) was significantly reduced in the LA of the dysfunctional asthmatic BALs. There was a corresponding significant increase in the ratio of phosphatidylcholine to PG, which strongly correlated with both increased gamma(min) and decreased LA/SA. Altered surfactant phospholipid properties correlated with surfactant dysfunction as well or better than either increased eosinophils or protein. Secretory phospholipase activity, measured in vitro, increased after antigen challenge and may explain the decrease in surfactant PG. In summary, alteration of phospholipids, particularly depletion of PG, in the LA of surfactant may be an important mechanism in asthma-associated surfactant dysfunction.

Alveolar septal collapse in the transitional infant lung: a possible common mechanism in sudden unexpected death in infancy

Sudden unexpected death in infancy (SUDI) is a category used to represent the largest single group of infant deaths. Although there are several theories, the cause of SUDI remains unknown and the mechanism of co-sleeping associated deaths are also undetermined. We investigate a possible biomechanical mechanism which may be common in SUDI and may provide an explanation for the association of the known risk factors for SUDI such as co-sleeping, prematurity, prone sleeping position, overwrapping, overheating and maternal smoking. The neonatal lung has few, if any, true septa but from about four weeks of age, a period of rapid alveolarisation commences. The developing alveolar walls (septae) have little fibre support against surface tension forces as they grow but are supported by a double layer of capillaries. Until the elastin/collagen supporting network is laid down these nascent septal walls are vulnerable to collapse against sac or duct walls during this transitional period. We hypothesise that such collapse will prevent one side of the septa, and the wall it overlays, from alveolar gas exchange and a functional left-right shunt is formed which may result in hypoxia. Furthermore, lung stretch receptors in bronchi running through or adjacent to collapsed regions will be activated, falsely signalling lung inflation to the brain stem with resultant respiratory inhibition, so precipitating further collapse. The process will continue until lung volume falls below residual capacity, when normal tidal breathing efforts will no longer result in significant air flow, even if stretch receptor signals have not produced complete apnoea. Large inspiratory efforts are then required to break the surface tension seal, which damages capillaries to produce petechial haemorrhages. Many epidemiological risk factors for SUDI could influence such a mechanism, leading to the proposal that Alveolar Septal Collapse in Infancy (ASCI) is a core mechanism via which these factors act.

Inflammation and surfactant

An intact and well-functioning pulmonary surfactant system is critical for normal respiration and protection from lung infection. Surfactant is comprised of phospholipids that reduce surface tension and greatly reduce the work of breathing. The other major component consists of surfactant-associated proteins, which optimise the biophysical function of phospholipids and/or play an important role in host defence by acting as collectins. Alteration of surfactant composition and function occurs with various inflammatory disorders that affect the airways or the lung parenchyma including asthma, infant respiratory distress syndrome/bronchopulmonary dysplasia, cystic fibrosis, acute respiratory distress syndrome and interstitial lung disease. Although surfactant replacement therapy is indicated for infant respiratory distress syndrome, there is no well-proven role for exogenous surfactant in the treatment of inflammatory lung disorders at the present time.

Intestinal epithelial damage in sids babies and its similarity to that caused by bacterial toxins in the rabbit

Sections of the duodenum, jejunum, ileum, caecum and large intestine from 14 sudden infant death syndrome (SIDS) babies were examined by scanning (SEM) and transmission electron microscopy (TEM). The type and amount of damage was characterised and quantitated and compared with the presence of Clostridium perfringens, Clostridium difficile, Escherichia coli and Staphylococcus aureus in faecal samples from the babies and toxins from the bacteria in faecal samples and serum from the babies. The data were compared with the damage that these toxins cause to the rabbit intestinal epithelium (see the previous paper in this issue). Damage was present in most of the SIDS samples, varying from 0 to 96%, and most damage occurred when the faecal samples contained the above bacteria and their toxins. Damage varied from removal of microvilli, damage to villus tips, separation of and removal of epithelial cells from the lamina propria, and removal of enterocytes leaving goblet and tuft cells, to damage and breakdown of the lamina propria. The results support the hypothesis that the cause of death in a significant proportion of SIDS babies may result from the absorption of toxins from the intestinal tract initiating a toxic shock reaction.

Association of primary Pneumocystis carinii infection and sudden infant death syndrome

To delineate clinical and histological features of the first Pneumocystis carinii infection affecting the immunocompetent host, P. carinii-specific histological stains were performed on autopsy lung specimens from 534 consecutive pediatric patients (those with AIDS and malignancies were excluded) in Santiago, Chile. P. carinii clusters were found in 4 (25%) of 16 infants who died of no apparent cause at arrival to the emergency department, and in 10 (2.9%) of 342 infants who died of multiple conditions at the hospital (P=.002, Fisher's exact test). This prompted us to analyze additional series of infants with sudden infant death syndrome (SIDS). In 161 additional SIDS cases, 47 (35.1%) of 134 infants from Chile and 4 (14.8%) of 27 infants from Oxford, United Kingdom, were found to have P. carinii clusters in the lungs. The quantity of P. carinii cysts was small compared with the numbers seen in immunocompromised hosts with P. carinii pneumonitis. This study provides histological evidence that primary P. carinii infection is associated with SIDS.

Raised bile acid concentrations in SIDS lungs at necropsy

Previous studies of the abnormal physical properties of lung surfactant derived from infants experiencing prolonged expiratory apnoea, or who have died of sudden infant death syndrome (SIDS), have led to a search for the agent responsible. Bronchoalveolar lavage (BAL) has been performed upon 12 infants under 12 months at necropsy and the rinsings analysed for up to 26 bile acids using high performance liquid chromatography, which requires nanomolar quantities. They were also analysed for simultaneously retrieved phospholipid and proteolipid-a minor component of lung surfactant-as markers of lavage efficiency. Total bile acid (TBA) was found to be higher in six SIDS cases, reaching a mean (SE) 8.54 (2.24) mumol/l in the BAL fluid compared with 4.66 (1.47) mumol/l in the six controls of similar age. When related to the concomitant surfactant yield, the TBA/ proteolipid and TBA/phospholipid ratios both showed highly significant differences between index lungs and controls, providing another postmortem marker of SIDS with potential for development as a test of risk. Since the bile:phospholipid ratio determines whether phospholipase A2 synthesises or hydrolyses phospholipid, the raised TBA/phospholipid ratio could be highly significant, causing this enzyme to function more like its role in the gut than in the lung.

Abnormalities in surfactant in sudden infant death syndrome as a postmortem marker and possible test of risk

OBJECTIVE

To determine whether physical abnormalities in lung surfactant, particularly inversion of the hysteresis between surface tension and surface area, are an effective postmortem marker for sudden infant death syndrome (SIDS).

METHODOLOGY

Bronchoalveolar lavage (BAL) was employed to obtain lung rinsings from 55 infants under 24 months of age at autopsy, comprising 34 index cases and 21 controls. Folch reagent was used to extract the lipoid content which was then applied at two surface concentrations, fixed and BAL-determined (BALD), to the pool of a Langmuir trough in which surface tension (gamma) was measured continuously by the Wilhelmy method as surface area (A) was cycled.

RESULTS

The gamma:A loops from SIDS samples were inverted relative to controls (i.e. they were hysteresis reversed, this inversion being quantified by an empirical surface tension: area reversal (STAR) score). There was a wide scatter of STAR scores, but a critical value was found which offered a significant (P = 0.017) separation of SIDS cases from controls for a fixed surface concentration and a highly significant separation (P = 1.0 x 10(-4)) for BALD surface concentrations. Differences in the yields of phospholipid and proteolipid, or their correlation to STAR scores, did not reach statistical significance.

CONCLUSIONS

Inversion of the gamma :A loops (i.e. hysteresis inversion) would appear to offer a better postmortem marker of SIDS than any reported previously, the procedure having potential for development as a prospective test indicating the risk of this disease.

Pathophysiology of overheating in a piglet model: findings compared with sudden infant death syndrome

OBJECTIVE

To examine the nature of hyperthermia-induced pathophysiological changes in an animal model including effects on lung compliance.

METHODOLOGY

Piglets were randomly assigned to heated or non-heated groups. Heated animals were warmed to 4 degrees C above normal body temperature while sedated and breathing spontaneously. Cardiorespiratory variables were recorded serially and haematological assessments and blood cultures taken at 0 and 6 h. After 6 h the animals were killed and a limited postmortem was performed. Control animals had all procedures without heating.

RESULTS

Heated piglets developed tachycardia, hypotension and a metabolic acidosis in addition to tachypnoea, hypocapnic alkalosis and a neutrophil leucocytosis. Rectal temperature after death fell at the same rate in both groups. Lung histology revealed an excess of lung haemorrhage and alveolar oedema in the heated group. No significant group differences in dynamic lung compliance were demonstrated.

CONCLUSIONS

The pathological changes that occur during hyperthermia are non-specific but not incompatible with those found in sudden infant death syndrome. There was no confirmation of the thesis that hyperthermia causes death by altering lung compliance.

A novel role for surfactant in the lung with implications for the sudden infant death syndrome

Feedback from mechanoreceptors in the lungs to the brainstem has long been known to be vital for control of rhythmogenesis during normal breathing. Action potentials transmitted via the vagi are shown to display an irreversible relationship with respect to lung area (A), i.e. hysteresis, which closely resembles the hysteresis in surface tension (gamma) versus A for normal surfactant which coats that surface. Since lung recoil is largely determined by surface tension for resting tidal volumes, this will dominate the stretch of mechanoreceptors. Hence, it is postulated that, through the Hering-Breuer reflex, surfactant largely determines afferent neural feedback, explaining the above similarity in clockwise hysteresis loops. Thus the ability of normal surfactant to impart normal clockwise gamma: A hysteresis is seen as a desirable property enabling the brainstem to differentiate between the inspiratory and expiratory phases of the breathing cycle at the same lung volume. It is further hypothesized that the very abnormal surfactant found recently in some infants with prolonged expiratory apnoea by displaying anti-clockwise gamma: A loops would render afferent neural feedback to the brainstem highly confusing and could cause prolonged expiratory apnoea. This concept is discussed as a possible cause of sleep apnoea, recurrent cyanotic episodes and the Sudden Infant Death Syndrome (SIDS).

Surfactant abnormalities in ALTE and SIDS

Abnormalities in the relative concentrations of the components of surfactant have been implicated in prolonged expiratory apnoea (PEA) and sudden infant death syndrome (SIDS). Controversy has, however, surrounded these findings, as they may be secondary to terminal life events. In this study the physical properties of surfactant were measured in children with recurrent apparent life threatening events (ALTEs), PEA, and SIDS. Bronchial lavage samples were obtained from 21 children with recurrent ALTEs, two SIDS victims, and 26 control patients. Lipid components were immediately elutriated from these samples with liquid chloroform. The physical properties of the extracted surfactant were studied on a Langmuir trough in which the area (A) of the monolayer was cycled continuously as the surface tension (gamma) was measured by the Wilhelmy method using a platinum 'flag'. The investigators performing these tests were unaware of the clinical diagnosis. Twenty one of 23 patients displayed abnormal physical properties while seven of 26 controls displayed similar abnormalities. These abnormalities were partially inverted hysteresis (figure of eight) loops and inverted (anticlockwise) loops that also generally exhibited less hysteresis. Of the 26 controls 20 exhibited a wide hysteresis pattern that cycled in a normal (clockwise) direction. These differences were significantly different. It is concluded that children with recurrent ALTEs have definable abnormalities in the physical properties of surfactant and that these findings may provide a sensitive means of identifying those at risk of recurrent ALTEs and SIDS.

Prolonged QT interval: a marker of sudden infant death syndrome?

A prospective electrocardiographic study was performed in 1830 newborns to evaluate the predictive value of QT interval for sudden infant death syndrome (SIDS). Standard ECG, with babies asleep, was recorded at the ages of 4 days, 2, 4, 6 months, and 1 year. The QTc (+/- SD) was 392 +/- 22 at 4 days, 410 +/- 16 (p less than 0.0001) at 2 months (n = 1256), 404 +/- 16 at 4 months (n = 1015), 400 +/- 16 at 6 months (n = 895), and 398 +/- 15 at 1 year (n = 890). QTc values longer than the mean plus 3 standard deviations were considered prolonged. Heart rate values (beats/min) were 138 +/- 19 at 4 days, 141 +/- 13 at 2 months, 134 +/- 13 at 4 months, 133 +/- 13 at 6 months, and 128 +/- 14 at 1 year. In 34 babies the QT interval was prolonged (mean + 3SD) and 3 of these died suddenly: the first, at 3 months (QTc = 470 and HR = 147 at 4 days); the second after 7 weeks (QTc = 514, HR = 115); the third at 3 months (QTc = 464 and HR = 140 at 4 days).

Surfactant abnormality and the sudden infant death syndrome--a primary or secondary phenomenon?

A prospective study of 46 infant deaths occurring between 3 and 100 weeks of age was performed and comprised a structured necropsy followed by collection of lung washings for surfactant phospholipid analysis and samples for microbiological examination. Of the 46 infants studied, 23 died from sudden infant death syndrome (SIDS) alone; SIDS was the cause of death in a further 12 but there were additional clinical or pathological findings insufficient in themselves to account for the death ('SIDS-plus'). In 11 there were other causes of death ('non-SIDS'). The lung washings from infants dying from SIDS had significantly lower concentrations of phosphatidylcholine and a significantly lower palmitate content in the phosphatidylcholine. There was no association between surfactant phospholipid abnormality and the presence of recognised pathogens, histological evidence of pulmonary inflammation, aspiration of stomach contents, age at death, sex, and death-postmortem interval. There were, however, lower concentrations of phosphatidylcholine and phosphatidylcholine palmitate content in infants colonised by organisms with reported phospholipase A2 activity.

Sudden infant deaths in Tasmania, 1980-1986: a seven year prospective study

This investigation was carried out by questionnaire to parents of SIDS cases and two controls for each case and aimed to follow up the fact that Tasmania has been shown to have an unusually high rate of this cause of death. Both chi 2 and relative risk calculations were employed. The high rate of SIDS of the 1970s has been confirmed in this series in the 1980s. Many of the epidemiological findings are closely in line with those found internationally. Of particular usefulness are new indications which lie within parental choice; cigarette smoking by parents leading to passive smoking by the baby carries a high relative risk (RR = 3.0, P less than 0.001) as does sleeping in the prone position (RR = 1.9, P less than 0.01) as against sleeping in a lateral position. Tenants of Housing Department homes are at raised risk of SIDS (RR = 2.6, P = 0.001). Density of persons within the home also raises the risks to babies (P less than 0.001) and a well ventilated bedroom lowers the risk (P less than 0.001).

Pulmonary surfactant. I. In immature and mature babies

Lung surfactant was obtained by postmortem lavage from: (A) premature babies: 34 dying acutely within 2 days of birth from Hyaline Membrane Disease (HMD), 20 dying several days after birth with HMD and its consequences, 8 dying from causes other than HMD; (B) mature babies: 24 dying stillborn, 15 dying soon after birth and 16 dying between 2 weeks and 1 year of age with minimal lung pathology. The phospholipid composition of the surfactant was analysed. Compared to the surfactant of babies dying acutely from HMD, that of the babies dying later from HMD contained significantly higher proportions of phosphatidylcholine (PC) and significantly lower proportions of sphingomyelin while that of the mature babies contained significantly higher proportions of PC and phosphatidylglycerol but significantly lower proportions of sphingomyelin and combined phosphatidylinositol and phosphatidylserine. The surfactant of premature babies dying of causes other than HMD was similar and intermediate to that of both groups of babies dying from HMD. The PC fraction composition of the surfactant of the babies dying acutely from HMD contained significantly lower proportions of the disaturated fraction than those of the babies dying later from HMD, stillborn babies or mature babies.

Pulmonary surfactant. II. In sudden infant death syndrome

The lung surfactant phospholipid composition of lavage samples from 102 babies dying from Sudden Infant Death Syndrome (SIDS) (one-third with minor signs of inflammation) was compared with that of: 34 babies dying from Hyaline Membrane Disease (HMD), 15 mature babies dying soon after birth, 16 mature babies dying in the same age range as the sudden infant death syndrome cases, 13 babies dying from pneumonia and 6 from septicaemia. The surfactant of the two groups of babies dying from SIDS was identical and approximated that obtained from babies dying from HMD, pneumonia or septicaemia. Compared to that obtained from mature babies, the surfactant of babies dying from SIDS contained significantly lower proportions of phosphatidylcholine (PC) and significantly higher proportions of lyso-PC and sphingomyelin. The proportion of disaturated PC was similar to that of the surfactant of the age-matched mature babies. The surfactant composition of the babies dying from SIDS did not change appreciably after death nor vary with age at death. The surfactant phospholipid composition of postmortem samples from mature babies was similar to that of aspirates from living babies and infants and to that of bronchoalveolar lavage samples from living adults.

Fatty acid composition of hepatic and cardiac tissue from chickens dying of sudden death syndrome

Gas chromatographic analysis of tissue lipids from broiler chickens dying from sudden death syndrome (SDS) were carried out to determine if anomalies in fatty acid composition could be linked to the disease. Crude extracts of lipids from hearts and livers of chickens dying from SDS and their matched controls (matched for pen number, sex and time of death) were fractionated by thin layer chromatography and the phospholipid, triacylglycerol (TG) and nonesterified aliphatic carboxylic acid [C14-C22] (FA) fractions were transmethylated and analysed by capillary gas chromatography. A number of significant (P less than .05) differences in fatty acid composition were found to exist between males dying of SDS and their controls, the most notable being elevated levels of arachidonic acid in the hepatic TG and cardiac FA fractions of SDS males. Few significant differences were found between SDS females and their controls. There was a trend toward increased desaturation of cardiac and hepatic tissue lipids of male SDS chickens, although this was significant only for the hepatic TG fraction. The results did not support the hypothesis that a lack of arachidonic acid as a precursor of prostaglandin synthesis was a causative factor in the disease.

Hypoplastic lungs and abnormal phospholipids in asphyxiating thoracic dystrophy

The respiratory impairment of asphyxiating thoracic dystrophy previously has been attributed to slower growth of the ribs which reduces chest size and limits chest expansion during breathing. Two siblings with this condition are described. One was found to have an abnormally low crying vital capacity; in the other the peak flow rate was reduced markedly. Chest X-rays and ventilation-perfusion nuclear scans were suggestive of hypoplastic lungs. Nasopharyngeal aspirates of airway secretions were found to contain significantly less total phospholipids and differences in phospholipid composition in comparison with a normal control group. These findings raise the possibility that the lungs are hypoplastic and have an abnormal phospholipid content in asphyxiating thoracic dystrophy.

High-performance liquid chromatography of adult human bronchoalveolar lavage: assay for phospholipid lung profile

High-performance liquid chromatography has been used to separate pulmonary phospholipids from adult human bronchoalveolar lavage. A solvent system consisting of acetonitrile-water (80:20) as solvent A and pure acetonitrile as solvent B was used with a silica column (Bio-Sil HP 10) coupled to an Si-100 Polyol precolumn. A linear gradient from 87.5 to 25% of solvent B was found to separate all biologically relevant surfactant phospholipids in the following sequence and composition: phosphatidic acid (1.1%), phosphatidylglycerol (10.6%), phosphatidylinositol (9.9%), phosphatidylethanolamine (3.6%), phosphatidylserine (4.5%), phosphatidylcholine (60.8%), sphingomyelin (8.1%) and lysophosphatidylcholine (1.6%). These results were very similar to the phospholipid pattern obtained by two-dimensional thin-layer chromatography. It is concluded that high-performance liquid chromatography is a useful and rapid method for the separation of phospholipids in biological fluids containing pulmonary surfactant.

Prolonged expiratory apnoea: a disorder resulting in episodes of severe arterial hypoxaemia in infants and young children

Ten infants with rapidly developing and severe episodes of hypoxaemia (15-120 s duration) were studied. In infants over 2 months old most episodes occurred when awake, after a sudden noxious stimulus. In younger infants frequent yet undetected episodes occurred during sleep and feeding. Arterial PO2 fell below 20 mm Hg within 20 s, and loss of consciousness, sometimes with convulsions, occurred after 30 s. Clinical observations, measurements of respiratory movements, air flow, oesophageal pressure, external oblique surface electromyogram, and, in two cases, chest fluoroscopy and microlaryngoscopy documented episodes of no inspiratory flow but continued expiratory activity at low lung volume with partial or complete glottic closure. In five infants, episodes continued despite tracheostomy or an indwelling nasotracheal tube. No intracardiac shunt could be demonstrated and the rapid fall in arterial PO2 was attributed to lack of ventilation at a maximum expiratory position in the presence of a rapid recirculation time. In five infants tested there was a low proportion of phosphatidylcholine in the tracheal aspirate. In one infant audible expiratory braking (grunting) was present for most of the awake time. This previously unrecognised mechanism for severe hypoxaemia may be one cause of neurodevelopmental damage and sudden death in infants and young children.

Pressure volume characteristics of the lungs in sudden infant death syndrome

Data on the pressure volume characteristics of left lungs obtained from 23 babies dying from sudden infant death syndrome were compared with results from 18 length-matched babies dying from established but primarily non-pulmonary causes. Volume distension at 30 cm of water and deflation flow volume characteristics were very similar in the two groups. These findings do not suggest that babies dying from sudden infant death syndrome have abnormally stiff lungs.

A bimodal form of alveolar behaviour induced by a defect in lung surfactant--a possible mechanism for sudden infant death syndrome

It is proposed that in the presence of a defective surfactant at a critical period in lung development, large areas of the lungs may collapse suddenly, greatly reducing oxygen stores, producing a right-to-left shunt, and deranging the breathing control system. Yet the situation may be reversed just as rapidly to an apparently normal one if the child responds appropriately. Such events may be repeated, unnoticed until either the condition improves or the child fails to respond in time. The latter situation may be one mechanism for a proportion of sudden and unexplained infant deaths.

The effect of components of rabbit pulmonary surfactant on the activity of phospholipases

This study investigates the ability of two components of pulmonary surfactant, protein and phosphatidylglycerol, to inhibit the action of phospholipases against phosphatidylcholine. Broncho-alveolar protein, prepared by de-lipidation of rabbit lung lavage material had an inhibitory effect on phospholipases A1 and A2 from rabbit lung lysosomes, comparable to the effect of bovine serum albumin. The degree of inhibition was found to increase with increasing enzyme activity. De-lipidated broncho-alveolar protein was separated into two fractions by gel chromatography. Inhibitory activity was associated only with the second peak, corresponding to rabbit albumin. The effect of phosphatidylglycerol (PG) on the activity of phospholipases A against dipalmitoyl phosphatidylcholine and unsaturated phosphatidylcholine (USPC) was investigated, and compared with the effects of two substrate analogues on the hydrolysis of USPC. There was no indication of true inhibition by PG, but some stimulation of USPC hydrolysis by 10 mol % PG. The relevance of these findings to the fate of surfactant in vivo is discussed.

Biochemical and histological assessment of hepatic lipid in sudden infant death syndrome

A biochemical and histological study of hepatic lipid in children dying from the sudden infant death syndrome (SIDS) and children of a similar age dying explicably are reported. Contrary to a previous report based on histological assessment of hepatic lipid, no significant increase of total lipid content in livers of children dying from SIDS was found. Analysis of hepatic phospholipid fatty acid esters, however, revealed a significant difference between SIDS and children of similar age dying acutely and explicably. The phospholipid abnormality found in SIDS was similar to that found in children dying subacutely with hypoxia and would be consistent with increased cell membrane fluidity. The implications of these findings in the pathogenesis of SIDS are discussed.