Introduction

Otitis media and second-hand smoke (SHS)

Otitis media, infection and inflammation of the middle ear, is one of the most common reasons for children to attend the primary care physician (GP), accounting for $3.8billion in healthcare costs in the U.S.A. alone.¹ The term middle ear disease (MED) encompasses infections of the middle ear comprising of acute otitis media (AOM), serous otitis media with effusion (SOME) or “glue ear”, recurrent acute otitis media, and chronic suppurative otitis media (CSOM). A causal relationship between exposure to secondhand smoke (SHS) and increased incidence of middle ear disease has been proposed by a number of researchers over the past few decades who have shown correlations between parental smoking and incidence of middle ear disease.² However, the issue is by no-means straightforward as there have been conflicting reports from different studies.² Furthermore, in 2006, a systematic review conducted by the US Surgeon General concluded that the evidence linking SHS to otitis media in children was insufficient to establish a causal link. However, since then, an increasing body of research into SHS exposure and otitis media has been produced.³

Does SHS exposure increase the risk of otitis media in children?

The American Academy of Paediatrics (AAP) has reviewed the evidence linking SHS to a variety of diseases in the document Protecting Children and Adolescents From Tobacco and Nicotine, 2023. In the report, the quality of evidence is considered “high” with respect to a causal relationship between SHS and middle ear disease.⁴ One of the difficulties encountered is distinguishing between the risk posed by maternal smoking during pregnancy and the risk from SHS after birth, however the AAP point to a meta-analysis (a type of study that considers the results from multiple studies and draws overall conclusions) from 2012 that demonstrated an increased risk from both mothers who smoked after birth and any smokers within the child’s household.³ Another difficulty is that there are several other causative factors linked to otitis media including family history, presence of siblings at home, whether the child attends pre-school nursery, and social class, which can be difficult to separate from parental smoking. Whilst there are studies that have failed to show the correlation between SHS and otitis media, the most up to date guidelines accept the link between the two. In the UK, NICE (national institute for health  and care excellence), recommends avoidance of SHS in the prevention of otitis media.

Mechanisms linking SHS to otitis media

Colonisation with bacteria causing URTI in children of smokers

In otitis media, infection ascends into the middle ear via the eustachian tube which connects the back of the nose to the ear. There is evidence that the primary bacteria associated with otitis media, namely: streptococcus pneumoniae, haemophilus influenzae and moraxella catarrhalis, colonise the nasopharynx of adult smokers and their children in a study by Greenberg et al.⁵ The proposed mechanisms are:

• Impaired muco-cilliary clearance - smoking affects the small cilia (hair-like protrusions) which along with respiratory mucus secretions trap and clear bacteria and particles from the airways.⁶
• Bacteria appear to be able to attach to the cells lining the respiratory tract of smokers more easily than non-smokers.⁷
• Local inflammatory responses to tobacco smoke; tobacco smoke leads to the production of inflammatory chemicals which may make it easier for bacteria to colonise the area.⁸

It is not clear however, certainly in the case of streptococcus pneumoniae, whether children are the source of the infection, which is then transmitted to the parent or vice versa but the incidence of carriage is certainly higher in smoking households.⁵

Eustachian tube dysfunction: SHS causes inflammation and fluid buildup.

Dysfunction of the connection between the nasopharynx and the middle ear is proposed in otitis media, particularly in recurrent otitis media and serous otitis media with effusion. It is thought that SHS contributes to eustachian tube dysfunction in three ways:

• A direct effect on the clearance of mucus by cilia lining the eustachian tube
• Chemical irritation of the cells lining the eustachian tube
• Allergic inflammatory processes precipitated by chemicals in tobacco smoke.⁹

Studies certainly support this theory. Examination of the respiratory epithelium of those exposed to SHS reveals loss of cilia, changes in the cell type lining respiratory surfaces, and thickening of the lining of the eustachian tube.¹⁰ As a result SHS contributes to negative pressure in the middle ear. In serous otitis media with effusion and recurrent otitis media with effusion, fluid develops in the middle ear in the context of negative pressure. The finding of negative pressure in the middle ear of children from smoking households has shown a direct relationship to the number of smokers in the household and to the blood levels of cotinine, a metabolite of tobacco smoke in several studies.⁹

SHS impairs immune defences

SHS impairs host defences against pathogens (bacteria, viruses and fungi) causing infection in a number of ways. In addition to disrupting the mechanism by which pathogens are trapped in mucus and cleared by the action of the cilia (mucociliary clearance), SHS has a direct effect on the bodies immune cells and their ability to produce antibodies to pathogens.¹¹ Macrophages - cells which attack bacteria have also been shown to have their responsiveness reduced by SHS.¹¹ Finally, by altering the cells lining the respiratory tract, SHS impairs this important defence against the entry of pathogens.

Evidence of genetic changes caused in middle ear cells exposed to tobacco smoke

Laboratory studies have demonstrated that exposure to condensed tobacco smoke can induce changes in the mucin gene in cells lining the middle ear. Whilst this has only been demonstrated in a laboratory setting with cells isolated from the middle ear of mice, it suggests a proposed mechanism by which SHS may affect the middle ear.¹²

Evidence of increased risk

Studies showing correlation between SHS and otitis media.

Because it would be obviously unethical to intentionally expose children to SHS to observe the effects on middle ear disease, the highest quality of scientific evidence, a randomised controlled trial is impossible to perform. However the relationship between SHS exposure and middle ear disease can be observed in studies such as prospective cohort studies in which two groups of children, one from a smoking household and one from a non-smoking household are observed over time to look for differences in the incidence of otitis media. Alternatively, a retrospective cohort study can be performed, starting with children who have otitis media and enquiring about parental smoking habits. Obviously, the quality of the evidence increases with the number of children studied. Large cohort studies were undertaken in Norway (2010) and Japan (2024) subsequent to the US Surgeon general’s conclusion that there was a lack of sufficient evidence linking SHS and middle ear disease.^13,15 The best way to collate the evidence from multiple studies is to perform a meta-analysis in which the results of several previous studies are considered together to derive overall conclusions. Several comprehensive meta-analyses were undertaken between 2012 and 2024.^3,15 All of these studies have demonstrated an established link between parental smoking and middle ear disease, in line with current guidelines from healthcare bodies such as NICE and the AAP.

Health burden of otitis media

The health burden of otitis media is considerable. A comprehensive review of cases worldwide from 2005 concluded the incidence to be 10.5%, which equates to 709 million cases per year, half of which are in children less than 5 years old.¹⁶ Whilst this is uncommon, otitis media can lead to complications such as meningitis and brain abscess, causing approximately 21,000 deaths per year worldwide. Hearing loss due to otitis media has a prevalence of 30.82 per 100,000 children.¹⁶ Whilst a 2021 survey of ENT (ear nose and throat) departments in the UK showed wide variations between each department, a substantial number of children with recurrent otitis media require treatment either with long term antibiotics or surgery to insert a ventilation tube (“grommet”) into the eardrum.¹⁷ According to statistics from the House of Commons Library, 12.9% of adults smoked as of 2022, which despite reduction in recent years remains a preventable cause of otitis media in children. Reduction of exposure of children to SHS can be promoted by increasing parental awareness and education, and promoting smoke-free environments for children. Other preventative factors include uptake of both Streptococcus pneumoniae and HiB (Haemophilus Influenzae B) vaccines, breastfeeding, and reduction in exposure to air pollution.

Summary

Second hand smoke (SHS) has a demonstrable link to otitis media in children. Whilst older analysis of the evidence commissioned by the US Surgeon General in 2006 had not established a definite link, more recent studies support a causal link. The American Academy of Paediatrics and healthcare bodies such as NICE in the UK consider the evidence of a link between SHS and development of middle ear disease to be of high quality. Several mechanisms are proposed in the development of otitis media related to SHS. These include colonisation of the nasopharynx by harmful bacteria, dysfunction of the eustachian tube, and impaired immune response to common pathogens causing otitis media. Otitis media presents a considerable health burden in the UK and worldwide. Reduction in exposure to SHS from parental smoking remains a modifiable cause.