Discussion
This report highlights two significant aspects. First, HFNC enabled us to accurately evaluate his oxygenation as a parameter of treatment effectiveness. When his diagnosis was made, it was difficult to predict the treatment responsiveness and prognosis as previous reports.1, 2 Because we would have needed to consider lung transplantation if the treatment had not worked effectively, a precise assessment of the treatment response was crucial for his management. Previous studies have reported that the oxygenation improvement and nocturnal respiratory rate were adequate indicators of treatment responsiveness, although few reports described the details of these parameters.2 The guidelines for idiopathic pulmonary fibrosis in adults recommend pulmonary function tests, 6-min working tests, and improvement of CT findings for treatment evaluation.4 However, these investigations are challenging in infants and young children. Repeated radiological studies involve radiation exposure and the necessity for sedation in young children. In our case, we used oxygen concentration delivered by HFNC adjusted to maintain SpO2 ≥95% to evaluate treatment responsiveness. It has been reported that oxygen concentration delivered to the patient through conventional nasal cannula varies depending on the patient’s respiratory condition.3 HILD patients are supposed to be susceptible to tachypnea and hypoxia due to decreased lung compliance and diffusion impairment, compared to patients with infectious lung diseases. Therefore, an accurate oxygenation evaluation may be difficult, especially in HILD patients receiving conventional oxygen therapy. Because HFNC can provide a specific oxygen concentration, independent of the patient’s respiratory conditions, it was possible to properly assess oxygen saturation even in the presence of severe respiratory distress.3 In our case, this precise assessment stratified whether he needed further therapy such as lung transplantation.
Second, the biochemical parameters of lung injury did not reflect his actual clinical conditions. Although KL-6 elevation has been reported to be helpful in the pediatric interstitial lung disease diagnosis,5 whether the biochemical parameters can reflect respiratory improvements in HILD patients remains unknown. In our case, the decrease in LDH and KL-6 levels was only evident after discharge. We presumed that the complex pathophysiology of surfactant dysfunction might cause this delay. The abnormal pro-surfactant protein C translated from mutant SFTPC was thought to induce endoplasmic reticulum stress and inhibit the function of a normally-matured surfactant protein C.1 Consequently, pulmonary inflammation and pulmonary surfactant dysfunction might cause respiratory insufficiency.1 The decrease in biochemical parameters did not match the improvement of respiratory condition, as these parameters only reflected the resolution of lung inflammation. We speculated that the recovery of surfactant function occurred initially, followed by the resolution of lung inflammation. Although it was not possible to explain this phenomenon precisely, physiological parameters seemed to be more helpful than biochemical parameters to evaluate the treatment responsiveness, at least in our case.
The concrete evidence of HFNC usefulness as a respiratory support for HILD remains to be investigated. Nevertheless, our findings may be useful to evaluate the respiratory condition of HILD with SFTPCvariations.