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The major PFT impairments after HCT are airflow obstruction (AFO), restrictive lung disease (RLD), diffusion abnormality (DLCO) or combinations of all three, occurring months to years after HCT. Bronchiolitis obliterans syndrome (BOS) is a cGVHD manifestation with irreversible AFO, and may present insidiously as non‐productive cough, wheeze and dyspnea; patients may be asymptomatic for long periods despite moderate to severe AFO on PFTs. In children aged <6 years, PFT data is often unreliable and NIH consensus criteria for diagnosing BOS may be difficult to satisfy and so parametric response mapping from high‐resolution inspiration/expiration CT scans has been investigated to secure this diagnosis [42].

Restrictive pattern PFTs may be seen with dose‐related pulmonary fibrosis due to pre‐HCT or conditioning exposures to bleomycin, busulfan, carmustine, and lomustine. Other etiologies include cryptogenic organizing pneumonia (COP), or chest wall restriction due to TBI, chest wall irradiation, or cGVHD‐sclerosis. COP may generally be diagnosed after autologous or allogeneic HCT when a child who may have fever, also has solitary or multifocal pulmonary infiltrates on chest CT, and BAL testing has ruled out bacterial, viral, fungal and other opportunistic pathogens. Children transplanted for DC require close follow‐up for pulmonary fibrosis and are also at risk for pulmonary arteriovenous malformations and should be referred to pulmonologists if PFTs or focused radiologic findings are abnormal.

Late idiopathic pneumonia syndrome (IPS) is rare, presents acutely with florid dyspnea and widespread bilateral pulmonary infiltrates. Broad, empiric antimicrobial therapy is often administered while attempts to rule out infection are made, often in an already critically ill patient. IPS is treated with high‐dose steroids, often plus etanercept, but mortality is high.

Because development of BOS after allogeneic HCT can be insidious with a median onset of 1 year, some centers recommend PFTs at 3 and 6 months, then annually for 5 years. Because more than one‐third of patients with cGVHD develop AFO, PFT testing is also advised when cGVHD is initially diagnosed, with repeat spirometry at least every three months times four, then full PFTs at least annually until systemic IST for cGVHD has ended. There is usually a rapid decline in FEV1 in the 6 months preceding a diagnosis of BOS, then stabilization, if a patient survives. Treatment of established BOS with systemic IST is often unsuccessful and worse survival is seen among those with FVC <67% at time of BOS diagnosis. Taking these facts together, early therapy is the goal before there has been extensive irreversible damage of small airways. To this end, a double‐blind randomized controlled study of inhaled corticosteroids (ICS) plus long acting beta agonist (LABA) showed that ICS/LABA can significantly improve FEV1 in moderate to severe BOS without changing IST [43].