CT imaging has a vital role in the diagnosis and management of patients with COVID-19 infection. It allows objective evaluation of the lung lesions, which provides a better understanding of the disease pathogenesis .
In general, CT findings of viral pneumonia are diverse and usually affected by the immune status of the host and the underlying pathophysiology of the viral pathogen . Upon entering the pulmonary cells, coronaviruses cause cell damage and pathological changes through direct cytotoxic effects and immunopathogenic effects. These changes are characterized by diffuse alveolar damage (DAD), interstitial mononuclear inflammatory infiltrates, hyaline membrane disease, and desquamation consistent with acute respiratory distress syndrome (ARDS) .
Most types of viral pneumonia show bilateral distribution with multiple lung lobes involvement . The lesions in our study showed bilateral distribution in 80% of cases, and unilateral distribution in 20% of cases. For craniocaudal involvement, the lesions were mainly involving the upper lung lobes in 9.1% of cases, the middle lung lobe, and lingula in 2.7% of cases, the lower lung lobes in 30% cases, and showed diffuse involvement in 58.2% of cases. The lesions showed perihilar distribution in only one case (0.9%), peripheral distribution in 74.5% of cases, and diffuse central and peripheral in 24.5% cases. Our results as regards lesion distribution are in line with the previous studies [6, 7, 11,12,13,14,15,16,17,18,19]. The bilateral peripheral basal distribution which is often seen in many viral pulmonary diseases could be explained by the small size of pathogenic microorganism particles and its brisk way to reach peripheral tissue and attack the alveolar epithelium.
Based on previous reports, the most frequently observed CT features with coronaviruses affection are diffuse airspace opacities which present as GGO, consolidation, or mixed GGO and consolidation [9, 20,21,22,23]. The pathological basis of the airspace opacities in viral pneumonia is diffuse alveolar damages including intra-alveolar edema, fibrin, and variable cellular infiltrates with a hyaline membrane that is usually present early in the course of coronaviruses affection [9, 21]. In a study done by Henckel et al. for 14 confirmed cases of COVID-19 with antemortem CT and autopsy correlation, the histopathological observations of airspace opacities were consistent with diffuse alveolar damage associated with capillary dilatation and congestion . They attributed consolidation and bronchial wall thickening to superimposed acute bronchopneumonia.
In our study on confirmed cases of COVID-19, the most common pattern was multifocal ground glass opacity found in 72.7% of our cases while single ground glass opacity was found only in 1.8%. Mixed GGO with consolidation was identified in 24.5% of the cases and predominant consolidation in 7.3%. Associated bronchial wall thickening was observed in 18.2% of cases. The lesions’ shape varied from round to irregular or confluent patches.
Our findings as regards typical CT features of COVID-19 are matching the other recently published reports about COVID-19 imaging [6, 15, 17,18,19, 24,25,26].
With the progression of the disease, the congestion of alveolar septal capillaries and exudation of the fluid into the interstitium cause interstitial septal thickening with linear opacities. Thickened interlobular and intralobular lines in combination with a ground glass pattern are called a crazy-paving pattern. In our study, we included CT exams done within 10 days from the first complaint. Linear opacities and thickening of interstitial septa were observed in 40% of our cases and crazy-paving pattern was observed in 9.1% of cases.
Atypical CT features of COVID-19 are not discussed in depth in most of the previous studies. The atypical imaging features of COVID-19 reported by previous studies include tree in bud and centrilobular nodule, cavitation, vascular thrombosis, predominant perihilar ground-glass opacity, pleural thickening and effusion, pneumothorax, and mediastinal or hilar adenopathy [27,28,29].
The incidence of pulmonary nodules in COVID-19 as reported by previous studies is ranging from 3 to 13% . In our study, we had two cases that showed pulmonary nodules (1.8%). The nodules were associated with other typical findings in both cases. Lung cavitation is an uncommon finding in COVID-19 pneumonia and usually seen in the late stage . Based on autopsy reports, the cavitation in COVID-19 pneumonia is usually caused by diffuse alveolar damage, intra-alveolar hemorrhage, and parenchymal necrosis [31, 32]. We had 3 cases of lung cavitation in our study (2.7%) with all detected within short duration less than 4 days.
Pleural effusion is relatively rare in cases of viral infection. A study by Shi et al. indicated that the prevalence of pleural effusion varies depending on the stage of the disease. They reported a prevalence of 13% in the third week of illness . Another study suggested that the presence of pleural effusion is a poor prognostic indicator for COVID-19 patients . We had 5 cases with pleural effusion in our study (4.5%) and two cases of pleural thickening (1.8%). The amount of pleural effusion was mild to moderate, 4 of them showed associated predominant consolidation, and one was associated with a mixed pattern of GGO and consolidation. Since we excluded cases with other comorbidities from our cohort, pleural effusion in our cases can be attributed to COVID-19 infection or superimposed bacterial infection. All the cases with pleural effusion in our study presented within 4 days after the presenting complaint and 3 of them were older than 50 years old.
Intravascular microthrombi were noticed in patients with COVID-19, and the combination of DAD and thrombosis is associated with rapid deterioration of clinical conditions in severe COVID-19 cases . In our study, vascular dilatation was detected in 18 cases and pulmonary embolism was detected in one case. The increased incidence of pulmonary embolism in cases of COVID-19 could be attributed to the cytokine storm which causes a release of proinflammatory cytokines that predispose to coagulopathy .
The study of temporal changes in the CT findings of COVID-19 pneumonia can help in understanding the disease pathogenesis and prediction of disease prognosis. A study done by Pan et al. revealed that chest CT showed the most extensive disease almost 10 days after symptom onset . Previous studies indicated that the lesions were limited to single or multiple areas and were distributed along the sub-pleural areas in the early phase of the disease. With disease progression, the lesions increased in number and extended gradually from the periphery to the center of the lung . In our study, there was a positive association between the number of lesions and bilateral involvement of the lung with the duration of presenting complaint. In addition, the peripheral distribution of the lesions was associated with a short duration of presenting complaint (< 4 days) while diffuse pattern of distribution was associated with a relatively longer duration of presenting complaint (≥ 4 days). As regard the association between typical imaging lesions and duration of presenting complaint, no significant association could be detected between multifocal GGO, predominant consolidation, mixed pattern, or linear opacities and the duration of presenting complaint. The later results are non-concordant with the findings reported by Pan et al. . They found that pulmonary consolidation is rare in the early stages of COVID-19 and increased with the progression of the disease. We cannot argue phases of disease progression as all CT scans included in our study were done relatively earlier in the disease course; however, we confirm that all typical findings can occur in the early stage. The crazy-paving pattern showed a positive association with increased duration of presenting complaint (p value = 0.002). All CT exams showing crazy-paving pattern were done after 5 days from the initial complaint. This finding is consistent with the reported findings in previous studies [15, 17,18,19, 25, 29].
Based on concurrent literature, older age, and co-existing comorbidities might be risk factors for the poor prognosis of COVID-19 patients . In our study, we found an association between bilateral lesion distribution and diffuse lung involvement with the older age group (≥ 50 years old). Our cases showed more extensive lesions in the older age group (≥ 50 years old) than the younger age group (< 50 years old). This finding is matching the results reported by Chan et al. . On the other hand, no significant association could be detected in our study between typical CT patterns and increased patient age. This contradicts what was reported by Li et al. that younger patients tended to have GGO while older patients tended to have more pulmonary consolidation. They admitted the presence of consolidation as a sign of a bad prognosis in elderly patients .