Affiliations: Kwong Wah Hospital, Kowloon, Hong Kong, SAR,
University Hospital of Tuebingen, Tuebingen, Germany
Correspondence to: Daniel K. Ng, M Med Sc, Department of Paediatrics, Kwong Wah Hospital, 25 Waterloo Rd, Kowloon, Hong Kong, SAR; e-mail: firstname.lastname@example.org
We read with interest the article “Risk Factors and Natural History of Habitual Snoring” by Urschitz et al (September 2004).1Urschitz et al claimed that their study was the first one to show differences in risk for habitual snoring between boys and girls. This is actually not true, as two studies conducted in our department2–3 and a recent study by Ersu et al4demonstrated a male predominance of habitual snoring. In the adult population, the male predominance of habitual snoring was attributed to the influence of male sex hormones. In prepubertal children, this would not apply, and this male dominance in children is likely due to the presence of allergic rhinitis. Allergic rhinitis has long been recognized as a risk factor of sleep-disordered breathing in children,5and the prevalence of allergic rhinitis was higher in boys in Hong Kong.6
Interestingly “respiratory allergies” and “frequent daytime mouth breathing” were assessed in the study by Urschitz et al, and both were found to be more common in boys. Both parameters are closely related to allergic rhinitis and corroborate previous findings of allergic rhinitis as a risk factor for snoring.
Even though respiratory allergies was not found to be a significant risk factor of habitual snoring in a univariate analysis (Table 3 of the study by Urschitz et al), respiratory allergies should be an important confounding factor that should be adjusted by logistic model. It is unclear why this important confounding factor was not adjusted in either model A or model B in the study by Urschitz et al (see Table 4 in the article) Nonetheless, “frequently daytime mouth breathing” emerged as an independent risk factor for childhood habitual snoring in all models (Table 4). The collinearity between mouth breathing and allergic rhinitis was not addressed. Hence, it is unfortunate that the possibility of allergic rhinitis as a risk factor was ignored by Urschitz et al as allergic rhinitis is readily treatable.
We thank Ng et al for their interest in our study. We assessed associations between known and suggested risk factors and habitual snoring in primary school children.1We did not find a gender difference in the prevalence of habitual snoring. Despite this, we found a higher prevalence of respiratory allergies in boys compared to girls, but were unable to identify a significant relationship between parentally reported respiratory allergies (which included allergic rhinitis) and habitual snoring. Thus, our data do not support the hypothesis of Ng et al that allergic rhinitis may be the underlying cause for a higher prevalence of snoring in male school children.2
In fact, another large European study2on snoring in children was also unable to find a gender difference in the prevalence of habitual snoring before the age of 15 years. Moreover, the Turkish study referenced by Ng et al also found significant gender differences only in children > 11 years old.3In the light of all these population-based studies, we speculate that the studies of Ng et al4–5 may have been subject to referral bias as they included only children referred to a hospital. Thus, male sex may be a predictor for referral but not for habitual snoring in primary school children < 10 years of age.
One major limitation of our study is the fact that participating children were not objectively examined for the presence of respiratory allergies (including allergic rhinitis). Parental observations were used instead. This may have led to some misclassification and lowered associated risks. This limitation is explicitly stated.1 In addition, some of the allergic children in our study were possibly receiving treatment for their allergy and were thus nonsymptomatic regarding their nocturnal breathing. As we did not obtain data on medication, this potential explanation cannot be fully ruled out.
However, we agree with Ng et al that some children presenting with daytime mouth breathing may have allergic rhinitis unrecognized by parents. As daytime mouth breathing was a significant and independent predictor for habitual snoring in our study, it cannot be ruled out that allergic rhinitis was in fact the underlying cause for snoring in some of these children. Our results, however, underscore the importance of nasal obstruction in children. We encourage physicians to search for the underlying clinical problem in snoring children.
We do not agree with Ng et al, however, that the variable “respiratory allergies” should have been introduced as a confounder into our logistic regression analysis. A confounder is strongly and significantly related to both exposure and outcome and accounts in some extent for the effect of exposure on outcome. In our study, the respiratory allergies variable was not significantly related to habitual snoring in univariate analysis and thus did not meet criteria for confounding.
In conclusion, Ng et al rightly point out that allergic rhinitis is most likely related to daytime mouth breathing and may lead to nighttime snoring. In our study, there was a steady and significant increase in the prevalence of respiratory allergies with increasing frequency of mouth breathing (ranging from 7.3% in children who “never” had mouth breathing to 20.3% in those who were reported to have this “always”; χ2 test for trend, p < 0.001). Also, allergic rhinitis may be more prevalent in boys than girls, possibly leading to a higher prevalence of snoring in school children. However, we were unable to find a significantly higher prevalence of snoring in boys and/or in children with allergies. Thus, the hypotheses put forward by Ng et al are not supported by our data.
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