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.