The prognosis and outcome of recovered COVID-19 patients are of great concern. Reexamination by CT scan is necessary to assess pulmonary lesions in these patients. The lung function of a small number of COVID-19 patients has been shown to be severely impaired. The CT manifestations of these recovered COVID-19 patients are closely related to their activities of daily living8,9,10.
The residual lesions found by pulmonary CT scan in the recovered COVID-19 patients usually include ground-glass opacities and interstitial shadows (interlobular septal thickening, subpleural linear shadow and irregular stripe shadows、reticular changes). Interstitial shadows do not occur alone, but generally following the ground-glass opacities. In our study, the ground-glass opacities were gradually absorbed and reduced in density. This was especially true when the lesions affected less than 2 lobes or when the lesions were located in less than 2 pulmonary segments and had lower density. The fibrous stripe shadows and subpleural linear shadows can also partially absorbed. However, the absorption is less likely for stripe shadows with a higher density. In some patients, the lesions remained hardly changed on repeat CT scan three months later. Antonio GE et al. Reported11 that stripe shadows in the pulmonary parenchyma, irregular interface sign, and traction bronchiectasis were the signs of fibrosis, which in turn were closely related to the patients’ age and gender. That is, residual intrapulmonary fibrotic lesions are more likely to be found in elderly males. According to the autopsy of COVID-19 patients, the early lesions included exfoliation of bronchiolar epithelium, cilia shedding, squamous metaplasia and atypically enlarged alveolar cells12. At the early stage of COVID-19, some patients with fever did not present with apparent exudative lesions upon chest X-ray or CT. The pulmonary shadows did not occur until 3-7 days later. This feature agrees with the fact that the early pathological lesions primarily occur in the bronchi without abnormal findings in the lungs. COVID-19 infection caused stimulation to the epithelial cells at the early stage, leading to intrapulmonary cell proliferation and squamous metaplasia. Patients with a course of disease shorter than ten days had hyaline membrane formation in the lungs, alveolar cell proliferation, and edema; those with a longer course of disease presented with diffuse alveolar damage13,14. Nicholls JM further divided the lesions into exudative, proliferative, and fibrotic stages15. Johkoh T et al. believed that the ground-glass opacities in the exudative stage of infectious pneumonia reflected the edema in the alveolar septum and the formation of the hyaline membrane in the alveolar wall. At the proliferative and fibrotic stages(15-30 days), alveolar and interstitial proliferation and fibrosis were observed.Therefore, the pathological changes accorded with the natural follow-up radiological results16,17.
During the follow-up period of 22−465 days, we found that the pulmonary function of convalescent patients with COVID-19 was damaged to some extent after discharge, which was related to residual ground glass opacity、reticular changes and fibrosis of CT. Especially in severe patients with ARDS and older male patients during hospitalization, pulmonary diffusion function decreased significantly, It is reported that the convalescent lesions of COVID-19 mainly exist in the alveolar wall, affecting the gas exchange through the alveolar capillary membrane, leading to the decline of diffusion function. In the early stage of rehabilitation in our study, the lung function of 89.83% of the convalescent patients recovered with the prolongation of the reexamination time. Only 12 patients' diffusion function did not fully recover to the normal level. At present, we continue to follow up the pulmonary function of the patients. Through the follow-up of chest CT after discharge, HRCT findings of covid-19 rehabilitation patients are closely related to their clinical manifestations, laboratory examination and pulmonary function. With the reduction of ground glass shadow、reticular changes and fiber strips on CT, residual lung lesions and lung function can be gradually improved, but interstitial shadow absorption is relatively slow.At the same time, we found that，given the rampant infectivity of covid-19 virus, the psychological pressure of rehabilitation patients is tremendous. With the substantial improvement of CT changes, it also accelerates the recovery of mental health of patients and makes them better integrate into normal social life18,19,20.