Being hospitalized for a respiratory infection before the age of two years, having had recurrent otitis in childhood, having been exposed to a dog as a newborn, having grown up in a large family were associated with habitual snoring later in life. These findings were independent of other childhood exposures and adult risk factors for snoring. When considering habitual snoring with daytime sleepiness combined, the same childhood factors were associated with increased adult risk. Our observations were demonstrated in a large population study in Northern Europe and were consistent across the seven centers. The findings are new and indicate that a predisposition for adult snoring and possibly also for obstructive sleep apnea could be established early in life.
Obesity is a major cause of snoring and sleep apnea. It is, however, important to increase knowledge about other preventable causes of habitual snoring, since a large number of snorers suffer from daytime sleepiness and an increased risk of cardiovascular disease and even early death [1
]. The present study showed that early life environment may be of importance for snoring later in life. Further knowledge of this subject could contribute to primary prevention of adult snoring.
Our results indicate an association between early life environment and snoring later in life. It is, however, only possible to speculate about causal relationships and mechanisms based on the present data. Previous studies have shown that children with large tonsils develop retrognathia and posteriorly inclined mandibles as a result of changes in tongue posture and mouth breathing [30
]. Studies on growing monkeys has also shown that induced oral respiration leads to a lowering of the chin, a steeper mandibular plane angle, and an increase in gonial angle as compared with control animals [32
]. It is possible that subjects reporting otitis, severe respiratory infections or living in a large family in childhood more frequently had infections in the upper airways with hypertrophy of the tonsils and subsequent narrowing of the adult upper airways. Further, endotoxins are proinflammatory cell wall components from gram-negative bacteria and airborne endotoxins that are prevalent especially in homes with dogs [33
]. We hypothesize that infections in childhood and exposure to airborne endotoxins in infancy stimulate the lymphatic system with subsequent enlargement of the tonsils. Remaining large tonsils or retrognathia due to large tonsils in childhood may compromise the upper airways, and could explain the associations between early life factors and snoring in adulthood as observed in this study. Unfortunately, we do not have information about history of tonsillectomy and/or adenoidectomy which might have been valuable for further understanding of these mechanisms.
A severe infection in childhood was only related to snoring later in life among overweight subjects, indicating that subjects who suffered from severe infections in childhood run a higher risk of habitual snoring if they become obese later in life. It is difficult to speculate on this relationship, but it seems reasonable that obesity, which is a common cause of snoring, increase potential negative consequences related to severe airways infections early in life.
The strengths of the present study are the large number of subjects, the multi-centre structure, the detailed analysis of childhood environmental factors and the high response rate to the questionnaire. The response rate analysis showed that men and younger subjects were slightly underrepresented. As the absolute differences between non-responders and responders were relatively small and we do not think that this has affected our results substantially.
The present analysis is limited by recall bias in assessments of childhood environment based on information in adulthood. A recent analysis, based on a multi-cultural study of childhood pet keeping, indicated that adults report important childhood events like having a dog or cat very consistently [34
]. We therefore assume that the reporting of pets and household size in this study is fairly reliable. Reports of childhood hospitalization could possibly be biased with regard to childhood social class and subsequent respiratory infection; however, the analyses were adjusted for parental education. It is unlikely that the misclassification of any of these childhood factors would be differential with regard to adult snoring and we believe that the misclassification in this study is non-differential and may have attenuated the effects.
Other limitations include residual confounding from variables not included in the present study, such as current pet keeping, current household size, seasonal allergies, mouth breathing in sleep and childhood snoring. It is for example possible that persons exposed to pets during childhood are more likely to keep pets as adults, and that the association with current snoring and dogs is explained by current exposure rather than by previous exposure.
Snoring was based on subjective reports, which is a common limitation in epidemiological studies. Subjective reports are, however, the most commonly used instrument for measuring snoring, in part because of the technical problems involved with microphone recordings as well as the ability of subjective reports to give an average of the subject's degree of snoring, whereas the result of a single night's recording may be misleading. Objective recordings using microphones correlate well with subjective snoring in young adults [35