Obstructive sleep apnoea and bone health

From the authors:

We thank J-Y. Zhong and L-Q. Yuan for their comments on our paper investigating bone mineral density (BMD) in male obstructive sleep apnoea (OSA) patients [1]. In our report, published in the European Respiratory Journal [1], we documented that male patients with severe OSA (apnoea–hypopnoea index >30 events·h⁻¹) show a severely decreased BMD in the lumbar spine and femur compared with male controls matched for age, body mass index and physical activity. We also documented that OSA patients present an increased risk for osteopenia and osteoporosis. In keeping with our findings, Chi-squared analysis corrected for continuity showed that T-scores consistent with osteopenia/osteoporosis were more frequent in OSA patients compared with controls at all regions of the lumbar spine and at the femur neck, upper femur neck and Ward triangle [1].

In agreement with the recent consensus from the International Society for Clinical Densitometry (ISCD) [2], in their correspondence Zhong and Yuan suggested that Z-score should be used when research about BMD is performed in males younger than 50 years of age. Although we agree with their suggestions, we doubt that there is actually an agreement about the definition of osteopenia/osteoporosis in heterogeneous populations of patients ranging from young-adults to the elderly. Accordingly, since we included subjects ranging from age 30 to 78 years in our study in order to consecutively evaluate patients affected by severe OSA referred to our Sleep Medicine Centre, we used T-scores for defining osteopenia and osteoporosis taking our cue from previous studies evaluating dual-energy X-ray absorptiometry measurement of BMD in the lumbar spine and femur in groups of men patients ranging from young to older ages [3, 4]. Although the official positions of the ISCD highlighted that the use of Z-scores, not T-scores, are preferred in males younger than 50 years of age, especially in children, these criteria are clinically useful at an individual level, but do not define the direction for researchers investigating large groups of patients of all ages. However, in keeping with the proposition by Zhong and Yuan, we are performing a further investigation of BMD in young-adult male OSA patients, where we will use Z-score to define if BMD is “below the expected range for age” [2]. In fact, the ISCD proposed not to use the terms osteopenia and osteoporosis in this young population, although BMD could be severely decreased in young OSA patients, as experienced in our previous study [1].

Moreover, Zhong and Yuan also suggest quantifying the fracture risk in OSA patients, since fractures are the most serious consequence of osteoporosis. Although they propose a clinic study evaluating this issue, we suppose that this study could be troublesome to complete since OSA is treatable using continuous positive airway pressure (CPAP) therapy, which could totally restore sleep bringing it back to physiological conditions.

Therefore, although the suggestions from Zhong and Yuan are interesting, they are not applicable to our study since there is no agreement about using Z-score or T-score in large group of patients ranging from young to elderly ages. Furthermore, considering that OSA is easily treated using CPAP, fracture risk could be challenging to assess in this population.

In conclusion, taking into account that bone health could be considerably affected by sleep apnoea, future studies are needed to better score the risk for osteopenia and osteoporosis, low BMD and fracture risk at all ages in patients affected by OSA.