The silhouette sign is a commonly used radiological terminology, useful in diagnosis. The term is a misnomer and the loss of silhouette sign is what is to be mentioned. Whenever there is a loss of silhouette sign in a chest radiograph the differential diagnosis of consolidation, pleural effusion, collapse. etc., needs to be considered.
Keywords: Chest radiograph, loss of silhouette sign, pleural effusion
| Introduction|| |
Dictionary meaning of silhouette – the dark shape and outline of someone or something visible in restricted light against a brighter background.
The silhouette sign was popularized by the Felson brothers, American radiologists; however, they themselves have quoted that Dr. H (Henry) Kennon Dunham was the first to describe the sign a decade earlier. It is described as an intrathoracic lesion touching a border of the heart, aorta, or diaphragm that will obliterate that border on the X-ray. An intrathoracic lesion not anatomically contiguous with a border of one of these structures will not obliterate that border.” The differential attenuation of photons by adjacent structures gives the silhouette. Loss of the anatomic outline is mentioned as a positive silhouette sign.
| Case Report|| |
A 44-year-old male patient was admitted for complaints of fever, cough with expectoration for 8 days, and shortness of breath ( Modified medical research council (MMRC) grade 2) for 1 day.
The patient was evaluated clinically, electrocardiogram baseline investigations were done. Complete hemogram revealed lymphopenia with raised erythrocyte sedimentation rate, with elevated levels of following inflammatory markers, lactate dehydrogenase, C-reactive protein, and D-Dimer.
In the current pandemic situation, the patient was suspected to have COVID-19 infection. Underwent reverse transcription polymerase chain reaction which tested to be positive. Simultaneously, the patient was evaluated radiologically, chest radiograph [Figure 1] was taken; this showed the left lateral hemidiaphragm, loss of contour, positive silhouette sign noted. There was pleural thickening, and the left costophrenic angle obliterated. High-resolution computed tomography (CT) of lungs revealed patchy consolidation in bilateral basal segments [Figure 2] likely secondary to Viral Pneumonia, loculated left pleural collection [Figure 3].
|Figure 1: X-ray - Chest radiograph showing left sided pleual effusion (left costophrenic angle is obliterated-[arrow] with pleural thickening)|
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|Figure 2: Patchy consolidation, ground-glass opacity noted in basal segments of bilateral lower lobes and lingular segments of left upper lobe|
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|Figure 3: Minimal loculated hypodense collection noted in left pleural cavity measuring 42 cc in volume|
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On probing the history patient revealed extrapulmonary Kochs-left side pleural effusion being diagnosed a month ago and was on antitubercular therapy (ATT), which explained the loculated pleural effusion.
Thus, patient was treated with COVID-19 treatment as per national guidelines. Further ATT was continued as per the National Tuberculosis Elimination Program (NTEP).
| Discussion|| |
The normal silhouette (contour) of the diaphragm and heart can be seen distinctly because of the differential attenuation by denser cardiac structures, diaphragm (white), and less dense adjacent lung tissue (black). When a part of the lung which is adjacent to the diaphragm is having its density same as that of the diaphragm cannot be seen separately. The lesions in the right middle lobe and lingula usually obliterate the right and left borders of the heart, respectively. Similarly, the lower lobe lesion obliterates the diaphragm and descending aorta. Diseases that can manifest the silhouette sign include atelectasis (segmental or lobar), aspiration, pleural effusion, and tumor.
In the given case, the patient's chest radiograph [Figure 1] revealed the loss of silhouette sign in the left lateral hemidiaphragm, giving a clue toward left lower lobe pathology. As the left costophrenic angle was obliterated, pleural effusion could be diagnosed by the chest radiograph. Further Ultrasonography Thorax and CT-Thorax showed a loculated pleural effusion.
The primary findings of COVID-19 on chest radiograph and CT are those of atypical pneumonia, or organizing pneumonia.,
The reported prevalence of pleural effusion in COVID-19 pneumonia has been variable in various publications.
According to the study by Tabatabaei et al., 120 consecutive symptomatic cases of COVID-19 infection who had undergone chest CT were enrolled in this retrospective study. The prevalence of pleural effusion was seen in approximately seventeen percent (20/120) of the cases.
However, the frequency of pleural effusions varies with age and disease severity. For example, in an Iranian study of 552 COVID-19 symptomatic patients, CT detected pleural effusions in 7.6% of cases overall, a percentage which was significantly higher in those over 50 years of age versus under (10% vs. 5.2%, P = 0.037).
With the worldwide spread of COVID-19, uncommon presentations are still being reported. However, there are hardly any studies that report loculated pleural effusion in COVID-19.
Pleural fluid loculations develop secondary to the presence of visceral-to-parietal adhesions that prevent fluid from falling to the dependent portion of the pleural cavity. Loculations can develop along any portion of the pleural cavity. The detection of loculations on plain chest radiography depends on the presence of surrounding aerated lung tissue that outlines the pleural opacity. Tuberculous pleural effusion is one of the most common forms of extrapulmonary tuberculosis. This is an important differential diagnosis for loculated pleural effusion. This gave a hint to reconsider the history and being incisive the diagnosis extrapulmonary Kochs was revealed by the patient.
| Conclusion|| |
In the current COVID-19 pandemic, clinical evaluation needs the aid of radiological findings, the importance of silhouette sign needs to be revisited and used in substantiating the diagnosis. In the last couple of decades, the silhouette sign and its usage is fading. In this case, the finding of loss of silhouette sign turned out to be an essential tool to elicit the history of extrapulmonary Kochs in a COVID-19 pneumonia patient.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient (s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
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Conflicts of interest
There are no conflicts of interest.
| References|| |
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Preethi Rajeshwari Gandhi,
No 95, 3rd Cross Nal Layout, Jalabhavan Main Road, Jayanagar 4th T Block, Bengaluru - 560 041, Karnataka
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2], [Figure 3]