1 INTRODUCTION

Asthma is usually characterized by chronic airway inflammation, and defined by the history of respiratory symptoms such as wheezing, shortness of breath, chest tightness, and cough that vary over time and in intensity, together with variable expiratory airflow limitation (Reddel et al., 2015). It is a serious global health problem affecting all age groups, with global prevalence of up to 20% of children aged 6–7 years experiencing severe wheezing episodes within a year (Lai et al., 2009). The treatment of asthma starts with avoidance to stimuli, but controlling the symptoms with anti-asthmatic medicines is the main component of most asthma treatments. Pharmacological management of chronic childhood asthma involves two main categories of drugs: bronchodilators and anti-inflammatory agents. Children with mild asthma often are managed only with inhaled β2-agonist bronchodilators. Inhaled glucocorticoids are effective anti-inflammatory agents recommended for use in children with moderate to severe asthma. If child's asthma is not well controlled by the above medications, alternative treatments include leukotriene receptor antagonist (Castillo et al., 2017).

Dental caries is the most prevalent and ubiquitous noncommunicable disease (Meyer & Enax, 2018). It develops through a complex interaction over time between acid-producing bacteria and fermentable carbohydrate, and many host factors including teeth and saliva (Selwitz et al., 2007). The caries process is a continuum resulting from many cycles of demineralization and remineralization, which is the body's natural attempt of repairing caries lesions. Initial demineralization is often subclinical but can lead to the development of caries lesions, which range from incipient areas of increased enamel opacity and porosity to frank cavitation. Risk of dental caries development includes physical, biological, environmental, behavioral, and lifestyle-related factors, such as presence of dental plaque, high numbers of cariogenic bacteria, inadequate salivary flow, insufficient fluoride exposure, poor oral hygiene, inappropriate sugar consumption, and usage of various medicines (Selwitz et al., 2007).

A link between asthma and dental caries has long been discussed (Alavaikko et al., 2011) but the results seem to be conflicting. A systematic review with meta-analysis reported that asthma doubles the risk of dental caries in both the primary and permanent dentitions (Alavaikko et al., 2011). Although the authors were unable to determine a mechanism to explain the link, they suggested use of asthma medication or a factor related to asthma per se, such as inflammation. An earlier review by Maupomé et al. (2010) claimed there is no strong evidence for a causal relation between caries and asthma. In 2019, another systematic review provided evidence that asthmatic subjects had nearly 1.5 times higher odds of the occurrence of dental caries in both dentitions (Agostini et al., 2019). The inconclusive results could be attributed to several limitations associated to methodological aspects and definitions of asthma and dental caries (Agostini et al., 2019). A need for longitudinal cohort studies is evident (Agostini et al., 2019; Alavaikko et al., 2011; Maupomé et al., 2010), to provide further knowledge to establish a causal relation between asthma and dental caries.

Potential mechanisms by which asthma could increase caries development include decreased salivary flow rate (Paganini et al., 2011; Ryberg et al., 1991), changes in saliva composition and its pH, lower plaque pH values (Stensson, Wendt, Koch, Oldaeus, et al., 2010), increased salivary levels of Streptococcus mutans (Botelho et al., 2011), and more frequent consumption of sweet drinks (Samec et al., 2013; Stensson, Wendt, Koch, Oldaeus, et al., 2010), which could be explained by frequent mouth breathing by asthmatic children leading to dry mouth and quest for drink (Samec et al., 2013; Stensson et al., 2008; Stensson, Wendt, Koch, Oldaeus, et al., 2010). The role of asthma medication is likely to be a contributing factor.

In our previous study, we showed asthmatic children had significantly higher prevalence of caries on primary and permanent teeth (Samec et al., 2013). In multivariate regression analysis, various risk factors were associated with caries experience of asthmatic children, indicating a need for further studies. The objective of the present study was therefore to examine the influence of childhood asthma on dental caries development and caries risk factors among asthmatic children in Slovenia. To assess the influence of asthma on the progression of dental caries, and as such the threat to the health of the dentition, a longitudinal design was utilized. To have a more homogenous group by disease severity, the asthmatic children must have used anti-asthmatic medicines daily. We investigated children with asthma and healthy control siblings to infer genetic and environmental contributions. International Caries Detection and Assessment System-II (ICDAS II) scoring criteria, which measures the disease process at its different stages, was used.

2 MATERIALS AND METHODS 2.1 Study population

The study population consisted of 2–17-year-old children under treatment for chronic bronchial asthma at the University Children's Hospital, Ljubljana, Slovenia. To be eligible for the study, the asthmatic children must have used anti-asthmatic medicines daily for at least 1 year and had physician-diagnosed asthma. Children suffering from additional diseases such as heart diseases, gastro-esophageal reflux, chromosomal abnormalities, infectious diseases, eating disorders, and frequent vomiting were excluded from the study. Controls were non-asthmatic siblings of the asthmatic study patients. In 2008, we examined 278 children (138 asthmatic children and 140 siblings). Baseline participants were followed up in 2011, when we reexamined 106 children (53 asthmatic children and 53 siblings). We examined whether there were systematic differences between parents and children who participated in follow-up and those who dropped out. There was no significant difference between the two groups in terms of dental caries and demographic characteristics. For the purpose of the study, the population was divided into three age groups based on the stage of development of the dentition. The first age group was composed of 2- to 6-year-old children (n = 72), the second age group of 7- to 12-year-old children (n = 153), and the third age group of 13- to 17-year-old children (n = 53). The study protocol was approved by ethics committee at the Ministry of Health in Slovenia (No. 165/07/09). All the parents gave written informed consent for inclusion in this study.

In the second part of the study, we assessed the influence of various confounding factors on caries experience in the group of asthmatic children. Additional 308 asthmatic children were examined. Thus, this cohort group was composed of 446 2–17 years old asthmatic children.

Dental caries assessment was carried out at the University Dental Clinic in a dental chair under artificial light by two calibrated dentists using a standard dental mirror and rounded dental probe. The dental examiners were blinded to children with and without asthma. Children had their teeth cleaned before the examination. Caries status was determined by the number of decayed [non-cavitated primary (d1) and permanent (D1) and cavitated primary (d2) and permanent (D2)], missing (M), and filled (f/F) surfaces in primary (dfs) and permanent (DMFS) teeth through clinical examination using the subtle ICDAS II scoring criteria (Ismail et al., 2007). The basic codes range from measurement of the first visible carious change in enamel caused by carious demineralization (code 1) to extensive cavitation with visible dentin (code 6). Radiograph was not included in the examination. Data on caries experience were presented in primary teeth as d1fs (non-cavitated decayed and filled surfaces), d2fs (cavitated decayed and filled surfaces), d12fs (non-cavitated, cavitated, and filled surfaces), and in permanent teeth as D1MFS (non-cavitated decayed, missing, and filled surfaces), D2MFS (cavitated decayed, missing, and filled surfaces), D12MFS (non-cavitated, cavitated, missing, and filled surfaces).

To measure the transition of caries in the longitudinal study, a scoring system from Ismail et al. (2011) was used. It incorporates transition of non-cavitated lesions and reversals at the tooth surface level. Data in primary and permanent teeth were presented as s/S (sound tooth surfaces), dnc/DNC (decayed non-cavitated lesions), dc/DC (decayed cavitated lesions), f/F (filled lesions), fnc/FNC (filled non-cavitated lesions), fc/FC (filled cavitated lesions), m/M (missing tooth surfaces because of caries), m0/M0 (missing tooth surfaces because of other reasons than caries), x/X (not-examined or excluded tooth surfaces).

To ensure accurate assessment for dental caries, the examiners were calibrated prior to the study by a cariologist, who is experienced in caries diagnosis. Intra- and inter-examiner reproducibility was tested on 10% of the children. Weighted Cohen's kappa values for intra-examiner reproducibility were 0.87 and 0.91 and for inter-examiner reproducibility 0.81. These values met the 0.80 pre-established value for qualification. Agreement of clinical assessments was therefore established to be good which validated the examination procedure.

Questionnaires completed by parents and data from the patients' medical records provided information on demographics, medical history, medication usage, dietary history, oral hygiene habits, fluoride exposure, and for asthmatic children also on type, dose, frequency, length, and mode of medicine application. For the glucocorticoid dose, we used the dose the children had been using for the previous 6 months.

2.2 Statistical analysis

A chi-square test was used to test the distribution of subjects between groups for categorical independent variables. Mann–Whitney U-tests were used to test the association between dental caries as dependent variable and independent variables. Children were categorized according to frequency of medicine application (1, 2 times/day), spacer use (no, yes), mouth rinsing after medicine application (no, yes), sugar content in medications (no, yes), length of medicine applications (1–3, ≥4 years), consumption of food and drinks (≤5, >5 times/day), frequency of toothbrushing (1, 2, 3 times/day), glucocorticoid dose (50, 125, 250 μg), leucotriene antagonist use (no, yes), and parents' education (elementary school, vocational school, secondary school, high school, university, postgraduate studies). The data were analyzed using the SPSS 26.0 statistical software package for Windows (SPSS Inc., Chicago, IL, USA). The level of statistical significance was set at p ≤ 0.05.

3 RESULTS

At baseline in year 2008, 278 children (average age 9.14 ± 3.56 years) were included. There was no statistically significant difference between the asthmatic children and non-asthmatic siblings with respect to age, fluoride intake, dietary habits, oral hygiene, last dental visits, and parents' education.

The mean length of anti-asthmatic medication use in 138 asthmatic children was 5.46 ± 3.32 years. 31.9% asthmatic children used medicines from one up to 3 years, 68.1% used them for more than 4 years. All asthmatic children used glucocorticoid daily and bronchodilatator as circumstances required, 18.8% used additional leucotriene antagonists, and 5.1% used antihistamines. Medicines in metered-dose inhaled form were used by 75.2% asthmatic children, 24.8% used them in dry powder inhaled form. Inhalers with spacer were used by 65.9% children. After medicine application, 76.8% asthmatic children rinsed their mouths with water. Non-sugar-containing medications were used by 63.8% children.

Asthmatic children had significantly (p < 0.05) lower prevalence of sound tooth surfaces in the first and third age groups than their non-asthmatic siblings. Significantly (p < 0.01) a greater number of their non-asthmatic siblings were caries-free in the first and third age groups when compared with the asthmatic children (Table 1).

TABLE 1. Prevalence of sound tooth surfaces and caries-free children in primary and permanent teeth in different age groups in year 2008 in asthmatic children and non-asthmatic siblings Age group N Sound tooth surfaces (mean ± SD) Caries-free childrenn (%) Asthmatic children 2–6 72 82.17 ± 18.46** 4 (13.8%)*** 7–12 153 88.69 ± 31.29 8 (9.9.%) 13–17 53 134.86 ± 13.67* 0 (0%)*** Non-asthmatic siblings 2–6 72 91.79 ± 12.41** 23 (53.5%)*** 7–12 153 87.96 ± 37.76 15 (20.8%) 13–17 53 142.60 ± 11.46* 8 (32.0%)*** * p < 0.05, Mann–Whitney U-test. ** p < 0.01, Mann–Whitney U-test. *** p < 0.05, test χ2.

We focused on 106 children whose tooth surfaces data were collected in both 2008 and 2011. Asthmatic children had significantly (p < 0.05) higher mean d12fs in year 2008 and in 2011 compared to non-asthmatic siblings on their primary teeth, as well as significantly higher mean D12MFS on their permanent teeth (Table 2). The 3-year mean increment of D12MFS was higher among asthmatic children in the second and third age group in permanent teeth than that from non-asthmatic siblings (Figure 1).

TABLE 2. Mean ± SD d12fs and mean ± SD D12MFS in year 2008 and 2011 in asthmatic children and non-asthmatic siblings Age group N d12fs 2008 (mean ± SD) d12fs 2011 (mean ± SD) D12MFS 2008 (mean ± SD) D12MFS 2011 (mean ± SD) Asthmatic children 2–6 39 10.94 ± 9.71* 8.88 ± 7.71** – – 7–12 51 – – 4.24 ± 4.60 12.86 ± 10.10** 13–17 16 – – 12.29 ± 7.65* 19.14 ± 13.86** Non-asthmatic siblings 2–6 39 4.09 ± 5.46* 2.91 ± 3.88** – – 7–12 51 – – 2.32 ± 3.31 2.86 ± 3.44** 13–17 16 – – 4.11 ± 5.11* 5.00 ± 4.95** * p < 0.05, Mann–Whitney U-test. ** p < 0.01, Mann–Whitney U-test.

Mean ± SD increment of D12MFS (non-cavitated, cavitated, missing, and filled surfaces) in asthmatic children and non-asthmatic siblings. Statistically significant differences (Mann–Whitney U-test) between the asthmatic children group and non-asthmatic siblings group are marked with (*p < 0.05) and (**p < 0.01)

Caries increment over 3 years is presented in Tables 3 and 4. Asthmatic children had significantly (p < 0.05) more dc on their primary teeth in year 2008 and 2011, and in year 2011 significantly more f, fc, and m (Table 3). There was a significantly higher increment of m on primary teeth among asthmatic children.

TABLE 3. Number and percent of s (sound tooth surfaces), dnc (decayed non-cavitated lesions), dc (decayed cavitated lesions), f (filled lesions), fnc (filled non-cavitated lesions), fc (filled cavitated lesions), m (missing tooth surfaces, because of caries), m0 (missing tooth surfaces, because of other reasons than caries), x (not examined or excluded tooth surfaces) in year 2008, 2011, and caries increment between year 2008 and 2011 in asthmatic children and non-asthmatic siblings (n = 106) in primary teeth Year 2008 Year 2011 Caries increment over 3 years Asthmatic children Non-asthmatic siblings Asthmatic children Non-asthmatic siblings Asthmatic children Non-asthmatic siblings Primary teeth Surfaces % Surfaces % Surfaces % Surfaces % Surfaces % Surfaces % s 2148 40.53% 2615 49.34% 994 18.75% 1441 27.19% −1154 −21.78% −1174 −22.15% dnc 48 0.91% 26 0.49% 6 0.11% 15 0.28% −42 −0.80% −11 −0.21% dc 192* 3.62%* 68* 1.28%* 51* 0.96%* 5* 0.09%* −141 −2.66% −63 −1.19% f 1 0.02% 3 0.06% 131* 2.47%* 43* 0.81%* 130 2.45% 40 0.75% fnc 106 2.00% 61 1.15% 7 0.13% 1 0.02% −99 −1.87% −60 −1.13% fc 5 0.09% 2 0.04% 5* 0.09%* 0* 0%* 0 0% −2 −0.04% m 30 0.57% 0 0% 125* 2.36%* 21* 0.40%* 95* 1.79%* 21* 0.40%* m0 5 0.09% 0 0% 0 0% 0 0% −5 −0.09% 0 0% X 2765 52.17% 2525 47.64% 3981 75.11% 3774 71.21% 1216 22.94% 1249 23.57% * p < 0.05, Mann–Whitney U-test. TABLE 4. Number and percent of S (sound tooth surfaces), DNC (decayed non-cavitated lesions), DC (decayed cavitated lesions), F (filled lesions), FNC (filled non-cavitated lesions), FC (filled cavitated lesions), M (missing tooth surfaces, because of caries), M0 (missing tooth surfaces, because of other reasons than caries), X (not examined or excluded tooth surfaces) in year 2008, 2011, and caries increment between year 2008 and 2011 in asthmatic children and non-asthmatic siblings (n = 106) in permanent teeth Year 2008 Year 2011 Caries increment over 3 years Asthmatic children Non-asthmatic siblings Asthmatic children Non-asthmatic siblings Asthmatic children Non-asthmatic siblings Permanent teeth Surfaces % Surfaces % Surfaces % Surfaces % Surfaces % Surfaces % S 3763 39.01% 3598 37.30% 5562 57.66% 5679 58.87% 1799 18.65% 2081 21.57% DNC 22 0.23% 21 0.22% 97 1.01% 29 0.30% 75 0.78% 8 0.08% DC 54* 0.56%* 11* 0.11%* 149* 1.54%* 12* 0.12%* 95* 0.98%* 1* 0.01%* F 154 1.60% 57 0.59% 245* 2.54%* 74* 0.77%*