Radiological findings for diagnosis of SARS-CoV-2 pneumonia (COVID-19)

Abstract

On 31st December 2019, China reported that several cases of pneumonia of unknown origin had occurred in Wuhan (Hubei Province), the cause of which was identified on 7th January 2020: a new coronavirus initially named 2019 novel coronavirus (2019-nCoV). On 11th February, the World Health Organization (WHO) renamed the new virus SARS-CoV-2 due to the similarities between the cause of the severe acute respiratory syndrome that broke out in 2003 and COVID-19 (Coronavirus Infectious Disease 2019).1 On 30th January, WHO declared the outbreak a Public Health Emergency of International Interest, and a pandemic on 11th March. As of 21st March 2020, 311,314 cases have been reported in 188 affected countries, with 13,079 deaths as a result of the infection and 95,800 people recovered (https://www.worldometers.info/coronavirus/). The statistics probably reflect a bias toward the most serious cases, which have more contact with the health system. However, many cases may be undiagnosed, as they are asymptomatic patients who do not go to a health center or mild symptomatology cases that are not confirmed due to the limited availability of diagnostic tests in many countries, such as Spain. Therefore, these statistics are likely to underestimate mild and asymptomatic cases, with the pandemic having a higher prevalence.2 COVID-19 infection can develop as a mild, moderate, or severe illness, including severe pneumonia, acute respiratory distress syndrome (ARDS), sepsis, and septic shock. The incubation period ranges from about 5 days (interval: 4–7 days) to a maximum of 12–13 days. Mild cases may develop flu-like symptoms: high fever, myalgia, fatigue, and respiratory symptoms, especially dry cough, with possible progression to pneumonia. The onset is usually less abrupt than in the flu, and upper respiratory symptoms seem minor or absent.2, 3 The main source of infection is other patients already infected with SARS-CoV-2. This is transmitted through respiratory droplets (aerosols > 5 μm) over short distances (1.5–2 m) when patients cough, speak or sneeze, and through contaminated hands when in close contact with the mouth, nose, or bulbar conjunctiva. Prolonged contact represents the highest risk, with casual contacts being less likely to cause contagion. There can be infections from asymptomatic patients and even from people in the incubation period of the disease.2, 4 Transmission through infected surfaces is likely, with SARS-CoV-2 recently reported to remain on plastic and stainless steel up to 72 h.5 The diagnosis of COVID-19 disease is made through reverse transcription polymerase chain reaction [RT-PCR]) with the detection of nucleic acids of SARS-CoV-2 or by the virus's genetic sequencing. Pharyngeal or nasopharyngeal smear samples, sputum, stools, or blood may be used. The virus can also be isolated in airway epithelial cell cultures, but this is done exclusively in specialized laboratories.6 Therefore, RT-PCR screening is regarded as the gold standard laboratory test for the diagnosis of COVID-19. False negatives have been documented in some cases, likely due to inappropriate viral sample material or technical problems during nucleic acid extraction.7, 8, 9 The incidence of COVID-19 has been observed to be higher in men than in women, for which the influence of the X chromosome and sex hormones on innate and adaptive immunity has been suggested as a possible explanation.10, 11 The respiratory system is the first to be affected by COVID-19, so in suspicious cases it is common to request a chest X-ray (CXR) as the first imaging test. However, its diagnostic performance in the initial stages of the disease is limited, since it has been reported that pathological findings identifiable on chest computed tomography (CCT) may not be detected on X-rays.7, 12, 13 This fact, together with the initial circumstances of the epidemic, when the accumulation of suspected cases exceeded the availability of RT-PCR tests, led to certain working groups adopting CCT as a diagnostic test in the absence of RT-PCR, leading to an unprecedented number of CCT studies to try to characterize a novel infectious disease. The CCT obtained very good results in these studies, observing that the pathological findings of the CCT can occur even before the symptoms7, 12, 13 and be diagnostic in patients with initial RT-PCR false-negative results.7, 8, 14 Therefore, it was concluded that CCT is a very valuable tool to diagnose COVID-19 infection, both in the initial assessment of lung involvement and for its follow-up.7, 15, 16 This article aims to identify the characteristics of this rapidly progressive viral pneumonia on imaging tests, both on CXR and CCT. The characteristic imaging findings of the different stages of the disease, the evolution of the findings associated with the progression of the disease or with clinical improvement, and the variation in the prevalence of the different patterns according to the age of the patients are also explained.