At baseline, CPAP-treated and control patients did not differ in terms of age, gender, or AHI. However, differences in comorbidities, BMI, and smoking habits were observed (Table 1) . In the CPAP-treated patients, the median CPAP use across the 9-year treatment period was 6.4 (IQR 2.3) h/day, and 913 (88.6%) of the patients were compliant to CPAP (≥ 4 h/day). Of the CPAP-treated and control patients, 1.2% and 0.9%, respectively, had engaged in bariatric surgery (p = 0.5), and 6.3% of the controls had used a mandibular advancement device (MAD) treatment after CPAP. Data on MAD compliance was not available.
Of the 1030 CPAP-treated patients, MVAs were registered in total of 30 patients before and 39 patients after CPAP. Of the latter, 3 MVAs occurred after a several months’ break from CPAP and were thus excluded from the analyses. Of the 1030 controls, MVAs were registered in 17 patients. The incidence of MVAs per 1000 person years was 3.2 (95% CI 2.3–4.6) before, 3.9 (95% CI 2.8–5.3) after CPAP, and 2.6 (95% CI 1.6–4.1) in controls (Fig. 2). Risk for having a MVA did not differ significantly between after and before treatment groups (risk ratio (RR) 1.19, 95% CI 0.73–1.94, p = 0.5), or between CPAP-treated and control patients (RR 1.5, 95% CI 0.85–2.69, p = 0.2). One patient had two MVAs after CPAP, and one patient had a MVA before and after CPAP. None of the controls had multiple MVAs. Excluding those controls who had used MAD after CPAP did not change the results. Furthermore, the MVA incidence did not differ between the compliant and noncompliant CPAP-treated patients. Among the latter, the incidence tended to increase after CPAP, but the difference was not significant (Fig. 2).
Among the patients with MVA, the only baseline difference was higher BMI in CPAP-treated than in control patients (median 34.2 vs. 31.3 kg/m2, p = 0.03) (Table 2). The prevalence of cardiovascular disease and the number of professional drivers were low and did not differ between the groups. No differences were observed in sleep study data (Table 2) or accident conditions including the hour of the accident (Table 3). Time to the first MVA, however, was longer among CPAP-treated than that of among controls (median 42.5 vs. 15.0 months, p = 0.02). In the CPAP-treated patients with MVA, a mean increase of 1.2 ± 3.7 kg/m2 units and a mean decrease of 3.7 ± 3.6 scores were observed during treatment in BMI and Epworth sleepiness scale (ESS) score, respectively. The median use of CPAP did not differ between CPAP-treated patients with or without MVA (median 6.4, IQR 2.2 vs. 6.4, IQR 2.3 h/day, p = 0.8).