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.