Prevalence of aspiration pneumonitis not requiring antibiotics among patients with aspiration pneumonia

Antimicrobial resistance is a global health threat. Given the positive correlation between the prevalence of antibiotic-resistant bacteria and increasing antibiotic treatment,1 indications for antibiotic use should be carefully determined. The percentage of older persons among the global population continues to increase owing to increasing life expectancy.2 Aspiration pneumonia is a common mechanism of pneumonia development in older patients with community-acquired pneumonia and is characterized by patients having various swallowing disorders, including mis-swallowing and neuromuscular disorders, aging deterioration, and dementia.3 It is classified as chemical pneumonitis (aspiration pneumonitis), an inflammatory reaction to irritative gastric contents, and bacterial pneumonia (aspiration pneumonia with a bacterial source).4 Aspiration pneumonitis can be categorized into the Mendelson syndrome, characterized by massive gastric juice aspiration, and mild pneumonitis, characterized by oral secretion aspiration. The latter case appears to be mainly caused by aspiration pneumonitis in older people because cases presenting the diffuse lung infiltration observed in the Mendelson syndrome are rare in clinical practice.5 Most cases of aspiration pneumonitis improve spontaneously without antibiotic treatment.6 However, no specific diagnostic criteria for aspiration pneumonitis have been established, and its incidence among patients with aspiration risks and pneumonia remains unknown. A prospective study to clarify its prevalence would be ethically challenging because antibiotics should not be administered when aspiration pneumonitis is suspected. Occasionally, respiratory physicians in our hospital do not use antibiotics to treat some patients with pneumonia believed to have aspiration pneumonitis. Given this current standard of care in our hospital, we retrospectively assessed the incidence of aspiration pneumonitis and baseline characteristics of patients whose pneumonia improved and persisted without antibiotics for aspiration pneumonia. Overall, 52 patients (≥75 years old) who completed treatment for community-onset aspiration pneumonia, remained hospitalized for rehabilitation, and subsequently developed recurrent pneumonia from January 2018 to December 2021 were included. Pneumonia was diagnosed based on clinical symptoms and chest radiographs. Patients were classified into two groups: those who were treated with antibiotics within 7 days of recurrent pneumonia diagnosis and those who were not. Among patients who did not receive antibiotics, pneumonia that spontaneously improved in 7 days after its diagnosis was defined as “aspiration pneumonitis” not requiring antibiotics. The proportion of aspiration pneumonitis was calculated, and patients' baseline characteristics were compared between patients whose pneumonia improved and those whose pneumonia persisted without antibiotics. The study protocol was approved by the institutional ethics committee of our institution (approval number 2463; approval date 26 January, 2023). Informed consent was waived by the committee because of the retrospective design of the study. Of the 52 patients (median age: 88 years, interquartile range: 86–91 years), 13 were not treated with antibiotics within 1 day of pneumonia diagnosis, whereas 39 received antibiotics immediately. Of the 13 patients who did not receive antibiotics, pneumonia improved in 4 (31%) without the use of antibiotics (Figure 1). These cases likely fit the diagnosis of aspiration pneumonitis. Baseline characteristics did not differ significantly between patients whose pneumonia improved and patients whose pneumonia persisted without antibiotics. This analysis indicates that aspiration pneumonitis may occur in approximately 30% of the patients with a risk of aspiration and pneumonia. Possibly, a greater number of patients had aspiration pneumonitis because some patients receiving antibiotic therapy had the potential to improve without antibiotics. However, the analysis did not reveal which patient characteristics would be useful for distinguishing aspiration pneumonitis from aspiration pneumonia with a bacterial source among those with aspiration pneumonia. It was confirmed that a certain amount of aspiration pneumonitis exists among aspiration pneumonia, and the indication of antibiotics should be carefully determined. Furthermore, the presence of pneumonia with spontaneous regression is assumed to include not only typical aspiration pneumonitis caused by sterile materials but also diffuse aspiration bronchiolitis or mild aspiration pneumonia. Thus, diffuse aspiration bronchiolitis or mild aspiration pneumonia not requiring antibiotics needs to be further studied. A large-scale study is required to validate our results and determine the predictive factors for aspiration pneumonitis not requiring antibiotics; this will help reduce unnecessary antibiotic prescriptions. This study was supported by Daiwa Securities Foundation Research in 2022. All authors have stated that there are no conflicts of interest in connection with this article. The study protocol was approved by the institutional ethics committee of our institution (approval number 2463; approval date 26 January, 2023). Informed consent was waived by the committee because of the retrospective design of the study. The data that support the findings of this study are available from the corresponding author, upon reasonable request