Differential diagnosis
The CT appearance of COVID-19 shares some similarities with other diseases that cause viral pneumonia, including influenza viruses, parainfluenza virus, adenovirus, respiratory syncytial virus, rhinovirus, human metapneumovirus, etc. (Table 4). In particular, those within the same viridae (SARS and MERS) have great similarities in imaging findings because they belong to the same coronaviridae family, and thus need to be excluded through clinical manifestations and laboratory pathogen detection. Imaging in MERS pneumonia can also show ground glass lesions in the subpleural and basal parts with consolidation, and fibrosis changes can be left after healing [34]. In the pneumonia patients infected by respiratory syncytial virus, chest CT mainly manifested as small centrilobular nodules and areas of consolidation which are often asymmetrically distributed in the lungs. Adenovirus pneumonia shows bilateral multifocal ground glass opacities with patchy consolidations on CT images and may show lobar or segmental distribution. Human parainfluenza virus pneumonia may show centrilobular nodules with bronchial wall thickening that differentiates it from the imaging appearances of the other viruses. Radiographs in patients with influenza pneumonia show bilateral patchy areas of ground glass opacity with or without focal areas of consolidation, usually in the lower lobes [34]. In a report on H1N1 influenza infection, in addition to the most common findings of ground glass opacity, interlobular septal thickening, and centrilobular nodules were the second most frequent findings [35]. Rapidly progressive ground glass opacities and consolidations with air bronchograms and interlobular septal thickening, with right lower lobe predominance, are the main imaging findings in H7N9 pneumonia [36]. Viruses are a common cause of respiratory infection and recognition of viral pneumonia patterns may help in the differentiation among viral pathogens, while the definite diagnosis is achieved by laboratory detection of virus.
Table 4 Differential diagnosis of different viral pneumonia
Virus Imaging characteristics
MERS-COV Ground glass lesions in the subpleural and basal portions of the lung parenchyma with areas of consolidation; fibrotic changes can be present after healing
H1N1 Ground glass opacity, interlobular septal thickening, and centrilobular nodules
H7N9 Ground glass opacity and consolidation with air bronchograms and interlobular septal thickening
Human parainfluenza virus Centrilobular nodules with bronchial wall thickening, findings which differentiate it from other viral infections
Respiratory syncytial virus Small centrilobular nodules and areas of parenchymal consolidation; asymmetrically distributed in the lungs
Adenovirus pneumonia Bilateral multifocal ground glass opacities, patchy consolidation in a lobar and/or segmental distribution
In addition, COVID-19 also needs to be distinguished from mycoplasma pneumonia, chlamydia pneumonia, and bacterial pneumonia. Other diseases that need to be identified are vasculitis, acute interstitial pneumonia, connective tissue-related lung disease, and cryptogenic organizing pneumonia [15].