Silhouette Sign in Differential Diagnosis: A Key Radiographic Indicator, Especially in COVID-19 Pneumonia

INTRODUCTION

In radiology, the silhouette sign is a fundamental concept referring to the loss of a normal radiographic contour. Imagine a silhouette against a bright sky – it’s the dark outline that defines a shape. Similarly, in chest radiography, the silhouette sign, also known as the interface sign, describes how an intrathoracic opacity, when in anatomical contact with a border of the heart, aorta, or diaphragm, will obscure that border on an X-ray. Conversely, an opacity not touching these structures will not erase their outlines. This phenomenon is due to the differential attenuation of X-ray photons by structures of varying densities.

While the silhouette sign gained prominence through the work of radiologists Leo G. and Benjamin Felson, its initial description is attributed to Dr. Henry Kennon Dunham, highlighting its long-standing importance in chest radiography. Recognizing a positive silhouette sign, characterized by the obliteration of an expected anatomical outline, is crucial for accurate differential diagnosis in various thoracic pathologies. This article will delve into the silhouette sign, its clinical relevance, and its application in differential diagnosis, particularly in the context of a case involving COVID-19 pneumonia and pleural effusion.

CASE REPORT

A 44-year-old male presented to the hospital with an 8-day history of fever and productive cough, accompanied by shortness of breath (Modified Medical Research Council (MMRC) grade 2) for one day.

Initial clinical evaluation included an electrocardiogram and baseline laboratory investigations. Blood tests revealed lymphopenia and an elevated erythrocyte sedimentation rate, along with increased levels of inflammatory markers such as lactate dehydrogenase, C-reactive protein, and D-dimer.

Given the ongoing pandemic, COVID-19 infection was highly suspected. A reverse transcription polymerase chain reaction test confirmed SARS-CoV-2 positivity. Simultaneously, a chest radiograph was performed [Figure 1], which demonstrated a loss of the left lateral hemidiaphragm contour, a positive silhouette sign. Pleural thickening and obliteration of the left costophrenic angle were also observed. Subsequent high-resolution computed tomography (CT) of the lungs revealed patchy consolidations in the bilateral basal segments [Figure 2], consistent with viral pneumonia, and a loculated left pleural collection [Figure 3].

Further questioning of the patient revealed a history of extrapulmonary tuberculosis (Koch’s) with left-sided pleural effusion diagnosed a month prior. He was currently undergoing antitubercular therapy (ATT). This pre-existing condition explained the loculated nature of the pleural effusion.

The patient received treatment for COVID-19 according to national guidelines, and his ATT regimen was continued as per the National Tuberculosis Elimination Program (NTEP).

DISCUSSION

The silhouette sign is predicated on the principle that normal anatomical contours in a chest radiograph are visible due to the contrast in density between adjacent structures. For example, the distinct outlines of the diaphragm and heart are seen because of the density difference between the denser cardiac structures and diaphragm (appearing whiter on X-ray) and the less dense, air-filled lung tissue (appearing darker). When a lesion in the lung, adjacent to the diaphragm, achieves a similar density to the diaphragm, the normally visible border is no longer discernible – resulting in a positive silhouette sign.

Lesions located in the right middle lobe and lingula typically obscure the right and left borders of the heart, respectively. Similarly, pathologies in the lower lobes can obliterate the contours of the diaphragm and descending aorta. The differential diagnosis for conditions manifesting the silhouette sign is broad, encompassing atelectasis (lobar or segmental), aspiration pneumonia, pleural effusion, and pulmonary tumors.

In this specific case, the initial chest radiograph [Figure 1] was instrumental. The observed loss of the silhouette sign at the left lateral hemidiaphragm strongly suggested pathology in the left lower lobe. The concurrent obliteration of the left costophrenic angle further pointed towards pleural effusion, which was subsequently confirmed and characterized as loculated by Ultrasonography and CT-Thorax.

COVID-19 pneumonia primarily presents radiologically as atypical pneumonia or organizing pneumonia. While pleural effusions are not a typical primary finding in COVID-19, their prevalence has been reported variably.

A study by Tabatabaei et al. analyzing 120 symptomatic COVID-19 patients undergoing chest CT reported pleural effusion in approximately 17% of cases (20 out of 120). Another Iranian study involving 552 symptomatic COVID-19 patients detected pleural effusions in 7.6% of cases overall, with a significantly higher incidence in patients over 50 years of age (10% vs. 5.2% in younger patients, P = 0.037).

While widespread, COVID-19 continues to present with atypical manifestations. Loculated pleural effusion, as seen in this case, is not a commonly reported presentation in COVID-19. Pleural fluid loculations develop due to adhesions between the visceral and parietal pleura, preventing free fluid movement within the pleural space. Detection of loculations on plain radiography relies on the surrounding aerated lung tissue outlining the pleural opacity.

Crucially, tuberculous pleural effusion remains a significant differential diagnosis for loculated pleural effusion and extrapulmonary tuberculosis in general. In this case, the silhouette sign findings prompted a more detailed history review, revealing the patient’s pre-existing extrapulmonary Koch’s, which explained the loculated pleural effusion in the context of COVID-19 pneumonia.

CONCLUSION

In the ongoing COVID-19 pandemic, radiological findings are essential for clinical evaluation. This case underscores the continued relevance of the silhouette sign in differential diagnosis. Despite advancements in imaging technology, the silhouette sign, often perceived as a basic principle, remains a valuable diagnostic tool. In this instance, the observation of a loss of silhouette sign was pivotal in eliciting the history of extrapulmonary Koch’s in a patient presenting with COVID-19 pneumonia. This highlights the importance of revisiting and utilizing classical radiographic signs like the silhouette sign to enhance diagnostic accuracy, particularly in resource-constrained environments and as a rapid initial assessment tool.

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Keywords: Silhouette Sign Differential Diagnosis, chest radiograph, pleural effusion, COVID-19, lung pathology, thoracic radiology