This study quantitatively compared the radiological findings of patients with COVID-19 pneumonia and those with MDA5-IP. Patients with COVID-19 pneumonia showed fewer faint GGO and consolidation volumes than those with MDA5-IP. Furthermore, we developed a predictive model to distinguish COVID-19 pneumonia from MDA5-IP based on CART analysis. This model contributed to the stratification of risk of COVID-19 pneumonia in 26% of patients.
To the best of our knowledge, this is the first study to compare the radiological findings of COVID-19 pneumonia with those of MDA5-IP. Peripheral and bilateral GGO or multifocal GGO with or without consolidation are typical radiological findings of COVID-19 pneumonia (15). Similar findings were observed in patients with MDA5-IP (11, 16, 17). The radiological findings of COVID-19 pneumonia are similar to those of MDA5-IP, and it is difficult to distinguish between COVID-19 pneumonia and MDA5-IP based on radiological findings (18). We hypothesized that qualitatively comparing radiological findings would make it difficult to distinguish between the two diseases. Therefore, we compared radiological findings quantitatively using AI-based CT image analysis. Using AI-based CT image analysis, the volume of each radiological finding could be measured quantitatively, and faint GGO and GGO could be categorized based on the density of the region. The results showed that faint GGO was observed more extensively in patients with MDA5-IP than in those with COVID-19 pneumonia.
The COVID-19 pneumonia group showed significantly fewer faint GGO than the MDA5-IP group. Additionally, faint GGO < 30% and GGO ≥ 10% were adopted as the thresholds for stratifying the risk for COVID-19 pneumonia in CART analysis. Thus, the difference in the proportion of faint GGO and GGO between the two groups was important in distinguishing COVID-19 pneumonia from MDA5-IP. Intracapillary hyaline thrombi and edema around blood vessels were observed in the early phase of COVID-19 pneumonia (19). Thusen et al. reported that GGO became denser in the hypoperfusion region of the lungs on CT (20). They suggested that the obstruction of small arteries induced edema around the blood vessels, resulting in a denser GGO. According to these previous reports, it is likely that denser GGO was observed in patients with COVID-19 than in those with other interstitial lung diseases. Indeed, the COVID-19 pneumonia group showed a lower ratio of faint GGO/GGO than the MDA5-IP group in the post-hoc analysis. A comparison of the density of GGO may be useful for distinguishing COVID-19 pneumonia from other interstitial lung diseases.
Our study has some limitations. First, this was a case-control study. The rate of COVID-19 pneumonia in patients with MDA5-IP differs from that in clinical practice. However, because MDA5-IP is a rare disease, it is difficult to conduct a cohort study. Secondly, we could not validate a predictive model for the risk of COVID-19 pneumonia. Therefore, it is necessary to confirm whether this also applies to other settings. Third, the radiological findings of COVID-19 pneumonia have temporarily changed. The radiological findings changed according to the duration from onset in patients with COVID-19 pneumonia (21–23). However, in clinical practice, the duration varies from onset to diagnosis. This study included patients who underwent chest CT scans within 2 days of admission. These criteria provided the same population as the patients at the time of diagnosis in clinical practice. Fourth, we did not evaluate differences in radiological findings, including the distribution of opacities, vascular thickening, and tree-in-bud, between COVID-19 pneumonia and MDA5-IP. MDA5-IP is a rare disease, and the evaluation of radiological findings is difficult because of the small number of cases.