DISCUSSION
With this study, we demonstrated a similar level of physical activity in children born extremely preterm, with and without BPD, both from objective pedometer data and self-reported activity level. Notably, after adjusting for both MRI total proton density and BPD status, a 5%-predicted increase in FEV1 was associated with approximately 750 more steps taken per day, suggesting that airflow limitation affects physical activity. Further, both higher daily step count and greater daily MVPA were moderately correlated with greater proton density at full expiration, suggesting less gas trapping.
Although we expected that there may be a difference between groups, levels of physical activity did not differ between children with and without BPD. Previous research has shown that children with BPD and lower FEV1 have reduced exercise capacity5,10, and that children with BPD have pulmonary function abnormalities compared to their peers without BPD7-9,11, which may correlate with physical activity limitations. Our study did find that lower physical activity levels were associated with pulmonary function abnormalities consistent with gas trapping, across both groups. This suggests that there may be factors related to extreme prematurity and lung function that affect physical activity, independent of the diagnosis of BPD, some of which may not be captured in this study, such as leg discomfort or being shorter than children without BPD, which may result in a greater number of steps needed to travel the same distance27. Diagnosing BPD relies on clinical characteristics and treatment, rather than physiologic or histologic changes, which may lead to misdiagnosis or misclassification of disease28-30. Tests which evaluate the lung function or structure, such as pulmonary MRI, may provide us with new information for diagnosis and classification of prematurity-associated lung disease, and may correlate better with functional measures such as physical activity20,22,31-33.
Our study found that both lower daily step count and lower MVPA were correlated with diminished proton density at full expiration on MRI, a measure of gas trapping. In our previous PICTURE study, diminished MRI proton density at full expiration was correlated with PFT measures suggestive of gas trapping (higher RV/TLC and lower FEV1)13, which have been shown to be associated with impaired exercise capacity in a previous study34. As pulmonary MRI measures may provide different and useful information compared to a clinical BPD diagnosis, further research is needed to understand factors that determine physical activity in this population13.
Nonetheless, it was encouraging that the average number of steps taken by this cohort was significantly higher than expected when compared to CanPlay normative data for average step counts (11500-12200, depending on gender) in this age group35. Overall, our data suggest that children born extremely prematurely are more active than their peers, regardless of whether they were given a diagnosis of BPD in infancy. This is in contrast to a previous study suggesting that children born extremely preterm are less active than their term-born peers5.
Given the high prevalence of ADHD and hyperactivity in children with history of extreme prematurity36, we conducted analyses to determine if ADHD diagnosis explained differences in physical activity. While we observed a lower step count in children with BPD who were treated with ADHD medications compared to their peers without ADHD medications, treated individuals still had a daily step count well above the average based on normative data. Ultimately, we found that there does not seem to be an effect of a diagnosis of or treatment for ADHD on overall activity level, although the prevalence of ADHD in the study population may be underreported or underdiagnosed37. This may account for the difference between our cohort and normative range data.
Our study was limited by small sample size. Although we increased the rigour of our study data by only considering children with valid pedometer data (i.e. at least 4 days of recording, including 1 weekend day), this further limited our sample. As such, our study was not powered to detect small differences between variables. Furthermore, our sub-analysis examining the impact of ADHD medications was unable to assess whether children with ADHD who are not treated with medication were present, limiting the reliability of this analysis. Children with untreated ADHD may be more active than their peers in all cohorts, which may account for the variation seen, however this is purely speculative. Lastly, due to the cross-sectional nature of this study, we could not evaluate the causal nature of the relationship between physical activity, BPD, PFT measures, and MRI indices.
Larger studies are needed to further evaluate the associations between pulmonary structure on MRI, pulmonary function and physical activity in children born extremely pre-term, with and without BPD. It would also be beneficial to reassess the physical activity level of this and similar cohorts of children as they approach adolescence and adulthood to determine if the participation and perception of physical activity level remains comparable to their peers, both between groups of BPD/non-BPD, and compared to non-preterm peers. These findings also support further research into novel imaging strategies which may provide alternate means to identify those at risk of impaired pulmonary function and lower physical activity, as the current clinical diagnostic classifications are limited in their ability to identify these children31,32.
CONCLUSIONS
This study suggests that school-aged children born extremely prematurely have similar physical activity to their peers, regardless of a diagnosis of BPD in infancy. Nonetheless, pulmonary function measures suggestive of gas trapping and diminished proton density on MRI were associated with lower physical activity. These findings support ongoing encouragement of participation in physical activity programs in children born prematurely, particularly for children with more pulmonary abnormalities, who may become less physically active as they age.