Reverse halo sign: Atypical chest CT findings in COVID-19 pneumonia

A 45-year-old male presented with low-grade fever (38.5C) and mild discomfort in chest since 3 days. He denied having cough or any other symptoms. On enquiry, he also gave history of loss of smell and taste since 5days. His laboratory investigations revealed slightly increased C-reactive protein (CRP- 6.5 mg/L; normal range <5.0 mg/L). he was hospitalised for further management.

High resolution non contrast CT scan of chest revealed multiple focal areas of subpleural and bronchogenic opacities and ground-glass attenuation in both lungs, predominantly involving peripheral basal segments of bilateral lower lobes. Few opacities showed central ground glass attenuation surrounded by mildly dense opacity giving appearance of REVERSE HALO in right lower lobe.

1. Background

A recent outbreak of Coronavirus disease-19 (COVID-19) had started in the Wuhan city of China’s Hubei province [1]. In short span of time it has spread to multiple countries all over the world with increased mortality.  The World Health organisation (WHO) declared it as a global health emergency on January 30, 2020 and then classified it as a pandemic on March 11, 2020 [2].

On Jan 7, 2020, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; previously known as 2019-nCoV), was identified as the causative organism which is the seventh known coronavirus to infect humans [1] with previous two remarkable outbreaks of severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

2. Clinical Perspective

Majority of the symptomatic patients present with lower respiratory tract infections [2,3]. Few patients also show gastrointestinal symptoms prior to respiratory symptoms. Symptoms are variable from mild to severe forms with fatality rate being 3% [2].

3. Imaging Perspective

Real-time polymerase chain reaction (RT-PCR) is the current standard diagnostic method used to diagnose COVID19 infection with sensitivity as low as 60-71% for detecting SARS-CoV-2 attributed to the low viral load present in test samples [4].

Chest CT shows approximately 56-98% sensitivity for COVID-19 even in the initial course of the disease and can be useful in correcting false negatives acquired from RT-PCR [4,5] and for the initial screening in individuals who are suspected to be have the virus [6].

The pattern of ground-glass and patchy consolidations with bilateral and peripheral lung distribution is considered to be the hallmark of COVID-19 infection [7].  These may be associated with smooth septal thickening (“crazy” paving pattern). These further progress into fibrosis or healing in chronic stage. The reverse halo sign typically occurs later in the course of disease suggesting that this CT finding correlates with the underlying pathophysiology of the disease process as it organises [2]. Such findings indicate that organising pneumonia is one of the mechanisms of lung injury [8].

4. Outcome

CT plays a vital role in this COVID-19 pandemic presently due to its easy availability and diagnostic value. It helps in early diagnosis and prognosis of the disease based on typical and atypical imaging patterns.

5. Take Home Message / Teaching Points

The atypical findings like reverse halo sign correlates with natural history of disease and its pathophysiology so these also need to be carefully evaluated besides the typical imaging features.