Pericardial Fat Pad Mimicking Cardiomegaly

 Pericardial Fat Pad Mimicking Cardiomegaly: Cause of Increased Cardiothoracic Ratio on Chest X-Ray

Introduction

An increased cardiothoracic ratio (CTR) on chest X-ray (CXR) is commonly interpreted as cardiomegaly. However, not all apparent cardiac enlargement reflects true cardiac pathology. One important and often overlooked cause is a pericardial fat pad, which can enlarge the cardiac silhouette without any increase in actual heart size. Recognizing this entity is essential to avoid misdiagnosis and unnecessary investigations.

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What Is a Pericardial Fat Pad?

Pericardial fat refers to adipose tissue located:

Epicardial fat: between the myocardium and visceral pericardium

Paracardial (mediastinal) fat: outside the parietal pericardium

When excessive, this fat can project over the cardiac borders on CXR, producing a falsely increased CTR.

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Why It Increases CTR on Chest X-Ray

CTR is calculated as the ratio of maximal transverse cardiac diameter to maximal thoracic diameter (normal ≤50% on PA view).

Pericardial fat pad increases CTR by:

Expanding the apparent cardiac borders

Creating smooth, symmetric enlargement of the cardiac silhouette

Mimicking global cardiomegaly despite normal chamber sizes

This is particularly misleading on:

AP films

Supine radiographs

Obese patients

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Typical Chest X-Ray Features

Smooth, globular cardiac outline

Symmetric enlargement

Clear lung fields

Normal pulmonary vasculature

No signs of heart failure (no congestion, no effusions)

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How to Differentiate From True Cardiomegaly

Feature Pericardial Fat Pad True Cardiomegaly

Cardiac chambers Normal Enlarged

Pulmonary congestion Absent Often present

Symptoms Usually asymptomatic Dyspnea, edema, fatigue

Echo findings Normal chamber size Dilated or hypertrophied chambers

CT/MRI Fat-density around heart Increased myocardial size

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Role of Echocardiography

Echocardiography is the key next step when CTR is increased:

Normal LV and RV dimensions

Normal systolic and diastolic function

Hypoechoic or echo-lucent space anteriorly (fat)

No pericardial effusion

A normal echocardiogram essentially rules out true cardiomegaly.

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CT and MRI: Definitive Diagnosis

CT chest: clearly identifies fat by low attenuation (−80 to −120 HU)

Cardiac MRI: fat appears hyperintense on T1-weighted images

These modalities confirm diagnosis when uncertainty persists.

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Common Clinical Associations

Obesity

Metabolic syndrome

Elderly patients

Chronic steroid use

Incidental finding during routine imaging

Pericardial fat volume also correlates with cardiometabolic risk, but isolated fat pad enlargement does not equal structural heart disease.

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Clinical Importance

Prevents mislabeling patients with heart disease

Avoids unnecessary cardiac medications

Reduces anxiety and downstream testing

Reinforces importance of correlating CXR with echocardiography

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Key Take-Home Points

Increased CTR on CXR is not synonymous with cardiomegaly

Pericardial fat pad is a common benign cause

Always correlate CXR findings with echocardiography

CT or MRI can confirm fat when diagnosis is unclear

Clinical context and imaging correlation are essential

🫀 Not All Cardiomegaly Is Real: Pericardial Fat Pad on Chest X-Ray

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