At present, retrospective studies on various aspects of COVID-19 are being further developed. Due to the strong infectivity of COVID-19, the source of the pathogen is unknown, no specific treatment method, and the mortality rate is high, so the early diagnosis of COVID-19 a challenge for us. Correct and timely diagnosis is of great significance for the prognosis and prevention of dissemination. Due to the delayed laboratory etiology results and limited sensitivity, there is still controversy for early clinical diagnosis . Thoracic imaging plays an important role in early diagnosis. This study believes that in the case of an epidemic area, the key to early imaging diagnosis is to distinguish it from other CAP.
The data were similar to most clinical observations.[14-16] COVID-19 group age was mostly young adults, 46 cases (71.9%) were <60 years old, and only 7 cases (15.2%) in the CAP group, It does not comply with the general law of spread of infectious diseases, that is, children and the elderly are often susceptible groups, Young adults had the strongest resistance to infectious diseases and the incidence rate should be low. 72314 cases of Chinese Center for Disease Control and Prevention showed that the incidence rate of China under 10 years old was less than 1%.The incidence rate of children is low, clinical manifestation is light, and infectious is slight. This may be related to cross immunity of antibodies produced by children's vaccination and coronavirus. The high incidence rate of young adults can’t exclude the possibility of more virus contact and infection opportunities due to more social interactions. Of course, further studies on epidemiology and pathogenesis are needed. Fever was the prominent clinical manifestation in both groups, but the degree of temperature rise was more obvious in COVID-19 group than that in cap group. In the early stage of the disease (within 7 days), the total number of leukocytes in the observed cases of COVID-19 was normal or low (< 4 × 109 / L), or showed a decreasing trend, similar to that reported in the literature , which may be caused by the decrease of lymphocytes in the first few days after the disease. The symptoms of muscle soreness and fatigue in COVID-19 group were significantly higher than those in cap group, while the symptoms of expectoration in the latter group were significantly higher than those in the former group, 62 cases (52.5%) and 39 cases (23.6%) in cap group and COVID-19 group, respectively, which may be related to the fact that cap is prone to produce more purulent secretion due to bacterial infection. The results showed that the incidence of heart disease, hypertension, chronic lung disease and diabetes in cap group was significantly higher than that in COVID-19. The average age of patients with comorbidities in COVID-19 group and cap group was 68.8 years (range 39-93 years) and 63.25 years (range 49-76 years), respectively, suggesting that the majority of patients with comorbidities in the two groups were older than 60 years old.
The chest CT changes of COVID-19 usually showed ground glass opacities shadow of different size 3-4 days after clinical onset. The incidence of this group was 78.2% (129 cases). The size and shape of COVID-19 ground glass opacities shadow are various, among which the most typical is round or quasi round ground glass shadow, accounting for 32.1% (53 cases) in this group, most of which are the early changes of young people. The statistics of this group showed that the single lesion of COVID-19 was significantly higher than that of cap group, which may be related to the early stage of COVID-19. With the progress of the disease, the area of ground glass opacities shadow in some patients has expanded, and gradually developed into ground glass shadow with multiple patches, large fusion or large and small pieces coexisting, but the density change is relatively small, which is in contrast with the rapid consolidation in the progress of CAP pneumonia, while the area expansion is relatively not obvious. Of course, there are also some light patients with lung lesions area no longer expanded until absorption. The pathological basis of ground glass changes is mainly related to the local severe pulmonary edema, hyaline membrane formation and fluid exudation in the alveolus cavity in the early lesions, but little to the infiltration of polynuclear and monocyte. Another 32.7% (54 cases) of COVID-19 showed fine reticular shadow overlapped in ground glass shadow, which may be related to the alveolar edema in the lung lesion area of COVID-19 and the slight thickening of alveolar septum with infiltration of monocyte, lymph and plasma cells. Because some CAP patients also showed ground glass opacities shadow (47 cases, 39.8%), which overlapped with the change of COVID-19 lung disease, the imaging manifestations must be combined with clinical and other lung changes.
In CAP group, the new exudative inflammatory lesions were often accompanied by obvious fibrous components or texture aggregation and cord adhesion in other parts of the lung. In this group, the incidence was 57.6% (68 cases), and 31.4% (37 cases) with thickening of bronchitis or peribronchitis and blurring of the outer edge, suggesting that these patients had experienced the process of lung inflammation in the past, and left some chronic inflammation or later changes, This is quite different from the acute onset, rapid progress and multiple onset of lung inflammation of COVID-19. Therefore, this study suggests that the lung with fibrous cord and bronchial wall thickening can be used as a counter indication to exclude COVID-19. In addition, a certain proportion of emphysema, pulmonary bullae, reticular or honeycomb changes under the pleura and "mosaic" sign and/or small cavity shadow in the consolidation area were observed in cap group. In conclusion, the diversity of lung lesions and the coexistence of new and old lesions in cap group are helpful to distinguish from COVID-19.
In this group, 6 patients with COVID-19 underwent chest CT Reexamination about one week after the onset of the disease, the lesions showed "wandering" characteristics, which may indicate the heterogeneity of pathological changes in different lung areas, that is, the early changes and the changes in the organic phase of diffuse alveolar injury can appear in different segments at the same time, which is more common in young patients. Due to the small number of cases in this group, whether it has a good diagnostic specificity remains to be further confirmed.
The results of this study showed that there was a positive correlation between the age and the size of lesion and the total number of segments involved in the lesion in the COVID-19 group. The correlation coefficient between the age and the size of lesion was 0.522, P < 0.001, and the correlation coefficient between the age and the total number of segments involved was 0.531, P < 0.001. In this group, the range of lesions in the first visit of the elderly patients was significantly larger than that in the young patients, and the number of involved segments was also significantly higher than that in the latter. This may be due to the rapid development of the disease in the elderly infected patients. The first CT scan has involved multiple segments, which is also one of the reasons affecting the prognosis of the patients.
The follow-up of COVID-19 group in the later period showed that the patients aged more than 60 years old had a lot of changes left in the lung in the later period, and the most serious manifestation was extensive fibrous cord shadow in the lung. In this group, 32 patients 19.4%) were all over 60 years old, suggesting that the prognosis of the patients over 60 years old was poor. The follow-up of 2-3 months showed that the remaining changes in the lung could still be absorbed, but the absorption was slow.