Pulmonary MRI of Ex-preterm Children With and Without BPD To Understand Risk of Emphysematous Changes

Knowledge to Date: Bronchopulmonary dysplasia (BPD), the most common pulmonary complication of prematurity, occurs in 41% of infants born before 28 weeks' gestation.

Defined as a need for oxygen at 36 weeks' postmenstrual age,2 BPD results in longterm morbidity in children and adults, including reduced lung function, early cardiovascular disease and premature death The reported severity of such impairments varies, as studies have been conducted on small numbers of children with differing degrees of prematurity. Furthermore, it is unclear what markers are most sensitive to predict long-term respiratory compromise. While functional measures have been studied, micro-structural differences, that may be quantified using pulmonary magnetic resonance imaging (MRI), have not been evaluated in this population, and may provide earlier quantitative markers of future respiratory disease, including chronic obstructive pulmonary disease (COPD). Recent advances in the clinical care of preterm infants have permitted survival of ever more premature infants, in whom alveolar development is incomplete, representing a new, relatively unstudied and growing cohort with a new form of BPD.

This BPD is associated with arrested alveolar growth and development, with reduction in alveolar surface area and pulmonary capillary blood volume. Yet, little is known about the trajectory of associated lung changes through childhood. Traditional measures of pulmonary function are lower in school aged children with history of BPD than in those preterm-born without BPD, but they may still be within normal reference ranges. There is, however, evidence that pulmonary function declines more steeply between age 8-18 years in those with BPD and that lower infant lung function predicts lower adult lung function.

This is particularly concerning since there is increasing recognition that preterm born adults have a significant burden of early lung disease. Emphysema has been reported in young adults with a history of prematurity less extreme than the prematurity seen today. X-ray computed tomography (CT) imaging detects pulmonary abnormalities in 98%, with emphysema in 47% of this less premature cohort as young adults. Not all of those with CT abnormalities, however, have impaired pulmonary function tests (PFTs), thus highlighting the limitations of conventional PFTs to detect disease. In childhood, PFTs, while very powerful indicators of global lung health, may be relatively insensitive to detect early pulmonary structural changes and regional lung abnormalities. Pulmonary dysfunction may not cause symptoms at rest or correlate with PFTs in childhood, but may become apparent with exercise, infection, or aging. Structural changes in the lung parenchyma may already be present in childhood and may be earlier markers of current and future respiratory disease than conventional PFT. Such changes may be detected with pulmonary MRI of the lung, as has been shown in COPD and in pilot work in BPD MRI has significant advantages over CT, including lack of exposure to ionizing radiation and high resolution of fine tissue structure. Recent advances in pulmonary MRI, including ultra-short echo time pulse sequences, generate pulmonary images with enhanced parenchymal signal intensity which rival CT images for visualization of lung parenchyma and vasculature. MRI is emerging as a research tool and may ultimately replace CT for pediatric thoracic imaging tasks and serial evaluations.

To the Investigators knowledge, conventional and ultra-short echo time 1H MRI studies of children with histories of prematurity and BPD have not been performed to quantify microstructural pulmonary abnormalities. The Investigators therefore propose a study to compare pulmonary micro-structural (MRI) and functional (PFT) abnormalities in premature children with and without history of BPD. The investigators hypothesize that pulmonary tissue destruction and/or emphysematous changes will be evident on MRI and will be worse in ex-premature infants with a history of BPD than in those without history of BPD. Further, the investigators hypothesize that MRI will be more sensitive to detect lung changes than PFT, but that MRI changes will correlate with PFT abnormalities. Ultimately, this information will form the basis of future longitudinal studies to evaluate progression of pulmonary disease and assess effects of new treatment strategies in this population.

Research Questions Amongst 7-9 year old children with a history of prematurity (< 28 weeks' gestation), comparing those with and without a history of BPD

1. Are there measureable differences in pulmonary micro-structure, quantified using MRI signal intensity (primary question)?
2. What is the association between pulmonary micro-structural measurements quantified using pulmonary MRI and pulmonary function outcomes (secondary questions):
3. airflow limitation, as measured by forced expiratory volume in one second (FEV1), FEV1/FVC, mid-expiratory flows (FEF25-75)
4. lung volumes and gas trapping , as measured by total lung capacity (TLC), residual volume (RV) and RV/TLC ratio
5. diffusion capacity of carbon monoxide (DLCO), a marker of pulmonary alveolar volume and pulmonary diffusing capacity
6. respiratory symptoms, social and environmental exposure history and healthcare utilization, assessed by validated questionnaire (ATS-DLD-78-a) Methods Study Setting: This cross-sectional study will be carried out at three Canadian tertiary care pediatric centres (Children's Hospital of Eastern Ontario, Ottawa, Ontario, The Hospital for Sick Children, Toronto, Ontario and Centre Hospitalier Universitaire (CHU) Sainte-Justine, Montreal, Quebec). Ethics approval will be obtained at all participating sites. Analysis: Descriptive statistics will be used to describe the two groups. The primary analysis will be a comparison of signal intensity between children with a history of BPD and those without, adjusting for exposure to earlier (24-26 weeks) or later (27-28 weeks) gestational age, using a two-way analysis of variance. Secondary analyses will examine the association between MRI signal intensity and PFT, as well as signal intensity and respiratory symptoms from the ATS-DLD-78-c questionnaire, using Spearman correlations. Exploratory analyses will also examine the association between PFT and respiratory symptoms, as well as parental smoking exposure, the relationship of obstetrical and neonatal historical factors to signal intensity, PFT, and respiratory symptoms.

Feasibility: Imaging Feasibility: The MRI pulse sequence that is the foundation for this study was previously developed and validated at 3T on a GE scanner by Dr. Parraga, who serves as Co-Principal Investigator of this proposed research.42 The MR acquisitions will be enabled using MR infrastructure at all 3 sites and 32 channel cardiac coils. Proof-of-concept for the presence of extensive emphysema is provided for a 25 year old with a history of prematurity and BPD, with significant MRI-evidence of widespread alveolar and acinar duct abnormalities that are consistent with emphysema Recruitment Feasibility: Each year, approximately 400 infants born < 28 weeks' gestation attend neonatal clinic follow-up visits at 18 months of age, at the three participating centres. As there are already established databases of these children at each site, identification of those eligible for study participation will be easily achieved.

At CHU Sainte-Justine, approximately 80 children aged 5-7 years are seen annually and from a previous study recruiting at 5 years, participation rate was 60%, further supporting feasibility for this study. Anticipated Results and Conclusions The Investigators anticipate that there will be significantly more severe pulmonary micro-structural abnormalities, as evidenced by lower pulmonary mean signal intensity, in 7-9 year olds born at less than 28 weeks' gestation, with a history of BPD, as compared to children born at less than 28 weeks' gestation without BPD. The investigators expect that mean MRI-derived signal intensity will correlate significantly with PFT measurements, as well as with the presence of chronic respiratory symptoms and increased healthcare utilization. This is the first study using innovative MRI techniques in relation to functional testing that will help characterize regional pulmonary tissue destruction and/or emphysematous changes present in children with a history of extreme prematurity, with and without BPD. In particular, MRI measurements may be more sensitive markers of early lung disease than PFT or respiratory symptoms. Since these children may only manifest signs of respiratory impairment in the presence of stressors, knowledge of these MRI changes may prompt earlier or more aggressive respiratory support and treatment to prevent respiratory compromise. Ultimately, there may be opportunity in future to intervene with additional treatments that may halt progression to adult lung disease. This is particularly important, given the increasing body of evidence for early COPD-like disease in this group. This work may identify a BPD-COPD overlap syndrome, with unique pathophysiology and potentially differential response to treatments from traditional COPD.

Potential Challenges: Children will be recruited for this study from neonatal follow-up clinic databases that capture all of those graduating from the neonatal nursery and surviving to 18 months. It is possible that some families may have since relocated or have been lost to follow-up and/or that those willing to participate in a research study at age 7-9 years will differ from those not willing/able to participate. Further, by studying children able to perform PFT and to cooperate for MRI, the investigators will exclude those with significant neurodevelopmental delay, who may also have more severe lung disease. As these are the main study outcomes, however, this will be necessary. Inclusion of children with a history of BPD ensures that those at the more severe end of the spectrum of early pulmonary complications, who are expected to have more severe lung disease, are studied. Exercise testing is another methodology to assess pulmonary and cardiovascular function. It is beyond the feasibility and budget ($200 per test) for this study and also requires coordination tasks that may be challenging for some participants. Nonetheless, it is an important consideration for future work. There are few precedents for pulmonary MRI in children, particularly in those with prematurity and BPD. Traditionally, MRI has been considered suboptimal in comparison to CT for lung parenchyma imaging, due to the low tissue density and loss of signal from magnetic field inhomogeneity at the air lung interfaces. Novel ultra-short echo time (UTE) pulse sequences, however, permit greater signal from the pulmonary parenchyma, which makes MRI a viable strategy to assess emphysematous changes in this population, without exposure to ionizing radiation. In the neonatal intensive care unit, MRI has shown signal decreases suggestive of alveolar simplification (emphysema) in those with severe BPD. It is possible that children with a history of extreme prematurity without BPD will also have some degree of pulmonary parenchymal change on MRI, although it is still expected that abnormalities will be more severe in those with a history of BPD. This study is, however, powered to detect smaller differences in signal intensity between groups than observed in adult COPD studies. The investigators are not using a healthy term control group because the purpose of this research is to assess radiological and functional biomarkers that will help clinicians and researchers identify among preterm born children those at greater risk of long-lasting pulmonary problems who would benefit the most from treatment.

Relevance: The ever growing population of extremely preterm born individuals, representing approximately 27,000 Canadians less than 18 years old, of whom approximately 40% have BPD, is at risk of development of early COPD-like emphysematous changes. Thus, neonatal conditions may be at the origin of a substantial and disproportionately high burden of adult lung disease, associated with significant morbidity and mortality. Prematurity and BPD, in addition to smoking, is one of the strongest predictors of obstructive airways disease in later life. In order to promote lung health and reduce the global burden of chronic lung disease, it is necessary to understand the trajectory of lung growth, development and decline across the lifespan in this at-risk population. Use of novel imaging strategies, more sensitive than PFT or respiratory symptoms to quantify early changes in the lung, provides new knowledge about the natural history of BPD in this population. Furthermore, MRI changes can quantify improvement associated with therapeutic interventions and can be safely serially evaluated across the lifespan. This is particularly important and exciting, given new therapies on the horizon to treat BPD, including stem cell treatments.