Increased Airway Smooth Muscle Shortening Is Associated With an Earlier Age of Sudden Unexpected Death in Infancy

Sudden infant death syndrome (SIDS) is a leading cause of infant deaths with a mortality rate of 20.98 per 100 000 infants worldwide [1]. The definition of SIDS is the sudden and unexplained death of a baby younger than 1 year of age after a thorough case investigation [2], no longer extending to ‘young child’ as outlined in the original classification. Sudden unexpected death in infancy (SUDI) is now the term used to capture all sudden, unexpected infant deaths, including SIDS [3]. Cases where children over 1 year of age die without a clear cause are classified as sudden unexplained death in childhood (SUDC) [2]. The clinical manifestation of SUDI/SUDC is heterogenous. Of interest to the Journal of Paediatric and Child Health community, there is evidence for an effect of airway pathology that may enhance airway smooth muscle (ASM) shortening and lead to airflow limitation on SUDI/SUDC. For example, we have previously shown the layer of ASM in SIDS infants is thicker compared with controls [4]. Other studies report an increase in eosinophil numbers, T lymphocytes, B lymphocytes and peri-bronchial mast cells [5] in SIDS infants compared with control. An adverse in utero environment also affects the airway wall and is a known risk factor for SIDS. Infants who died from SIDS exposed to maternal smoking during and after pregnancy had increased inner airway wall thickness compared with SIDS cases who were not exposed to maternal smoking [6]. Prematurity is associated with a greater incidence of SUDI [2], although whether the effects of prematurity arise through alterations to airway wall structure is not known. The aim of the present study was therefore to examine the effect of prematurity on airway wall structure and ASM shortening in cases of SUDI or SUDC. All infants involved in this study had post-mortem examinations performed by an experienced paediatric pathologist at the Victorian Institute of Forensic Medicine. Permission was obtained from the Institute's ethics committee to access the stored lung tissue from those post-mortems performed between 1991 and 1993 [6]. Approval for this study was obtained from the Royal Children's Hospital Ethics committee (94064C), Victorian Institute of Forensic Medicine Ethics Committee (Access Number 36/94, Human Ethics number 119/94) and Sir Charles Gairdner Hospital Human Research Ethics Committee (HREC 2015–053). Airways were available from 68 infants (post-mortem) who died between 1 and 18 months of age and were classified as SUDI or SUDC. The sample size in this study does not represent the total number of sudden infant deaths in Victoria between 1991 and 1993. Characteristics of the case series: 56% male (n = 38), average birth weight 3.23 ± 0.085 kg (±SEM) and 13% premature births (n = 9; < 37 gestational weeks). Cases examined in this study had similar characteristics to other cases within the Victorian epidemiological study of SIDS. Quantitative assessment of airway dimensions using histological sections has been previously described [4, 6]. In 5 μm airway cross-sections stained with haematoxylin and eosin, planimetry was used to quantify epithelial height and areas of the ASM, inner, outer and total wall, normalised to the perimeter of the basement membrane (Pbm), a marker of airway size [4]. The percentage of ASM shortening was estimated morphometrically from the outer perimeter of the measured ASM layer (Pmo) and the calculated relaxed perimeter (Pmor), using (Pmor − Pmo)/Pmor × 100 [6, 7]. Analyses were grouped by airway size: small (Pbm, < 1 mm) or large (Pbm, 1–4 mm). t Test was used to determine the effect of prematurity (26–36 gestational weeks) compared with term (37–43 gestational weeks; n = 59) infants on the measured airway parameters. Linear regressions were performed to assess the relationship between all collected variables. Perinatal cigarette smoke exposure was previously shown to not affect the thickness of the ASM layer and ASM shortening; smoking was therefore ignored in the analysis [6, 8]. Data were analysed using SigmaPlot (version 14.5, Chicago, IL, USA), with statistical significance defined as p < 0.05. Prematurity did not alter the thickness of the ASM layer in small (p = 0.58) and large (p = 0.94) airways of SUDI or SUDC cases. There was no effect of prematurity on epithelial height (small airway, p = 0.42; large airway, p = 0.58), inner (small airway, p = 0.44; large airway, p = 0.73), outer (small airway, p = 0.31; large airway, p = 0.36) or total wall thickness (small airway, p = 0.68; large airway, p = 0.27) of SUDI or SUDC cases. There was also no impact of prematurity on ASM shortening in small (p = 0.10) and large (p = 0.90) airways of SUDI or SUDC cases. Separate from the factorial effects of prematurity on airway structure, age at death was related to the ‘contractile state’ of the ASM. In large airways, ASM shortening was negatively correlated with age at death (p = 0.04, r = −0.25, Figure 1), suggesting that greater ASM shortening (and therefore lumen narrowing) contributes to earlier mortality. The effect does not appear to be related to ASM thickness, which was not correlated with age at death in large airways (p = 0.58) and positively correlated in small airways (p = 0.01, r = 0.33), that is, ASM thickness was greater in subjects who were older at the time of death. Greater ASM thickness with advancing age is expected as a result of maturation [9]. Associations remained the same after controlling for maturational effects by correcting age at death to 40 gestational weeks. Other explanations for an increase in ASM shortening at early time points include enhanced contractility of the immature ASM [10] and greater compliance and therefore collapsibility to the same mechanical stimulus [11]. Changes in airway wall compliance due to increased thickness of the airway wall with normal airway development may influence the loads limiting ASM shortening. However, we found no correlation between inner, outer, or total wall thickness, epithelial height and age at death in both small and large airways (p > 0.05). There were also no relationships between ASM thickness or ASM shortening of small or large airways and gestational age at birth or birth weight (p > 0.05). Methodological limitations are acknowledged. There was a statistical inequality in terms of the low sample size for preterm infants compared with control. These factors were, however, independent and appropriate non-parametric tests were applied for non-normally distributed data. Low birth weight is commonly associated with preterm birth and also a risk factor for SUDI/SUDC [12]. Given that low birth weight infants exhibit ASM hyperplasia and a greater proportion of extracellular matrix compared with infants with a healthy birth weight [13], future analysis could assess the effect of low birth weight on airway wall structure and ASM shortening in cases of SUDI or SUDC. The present study was also limited to examining changes in gross wall thickness, which provides no information on compositional changes (e.g., extracellular matrix) that may affect functional and mechanical properties such as compliance. In closing, our short communication reports that prematurity does not affect airway wall thickness or ASM thickness and shortening in SUDI and SUDC cases; however, excessive bronchoconstriction appears to be associated with an earlier age of infant deaths. Conceptualization: K.C.W.W. and J.G.E. Data curation: K.C.W.W., P.J.R. and J.G.E. Formal analysis: K.C.W.W., P.B.N. and J.G.E. Funding acquisition: K.C.W.W., P.J.R. and J.G.E. Investigation: K.C.W.W. and J.G.E. Methodology: K.C.W.W., A.L.J. and J.G.E. Project administration: K.C.W.W. and J.G.E. Resources: K.C.W.W. and J.G.E. Supervision: K.C.W.W. Validation: K.C.W.W. and P.B.N. Visualisation: K.C.W.W. and P.B.N. Writing – original draft: K.C.W.W., P.B.N. and J.G.E. Preparation writing – review and editing: K.C.W.W., P.J.R., A.L.J., P.B.N. and J.G.E. The authors thank the Victorian Institute of Forensic Medicine for assistance in histologic preparations. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.