Pericardial Effusion Size Classification
A significant pericardial effusion is defined as >20 mm in maximum width during diastole, which represents a large effusion according to the European Society of Cardiology classification system. 1
Standardized Size Classification by Echocardiography
The 2015 ESC Guidelines establish the following thickness thresholds for pericardial effusion severity 1:
- Small effusion: <10 mm
- Moderate effusion: 10-20 mm
- Large effusion: >20 mm
This measurement should be obtained during diastole and represents the maximum width of the echo-free pericardial space 1. The American College of Emergency Physicians uses a similar classification, defining small effusions as <10 mm in diastole (often non-circumferential), moderate as circumferential with no part >10 mm, large as 10-20 mm in width, and very large as >20 mm and/or evidence of tamponade physiology 1.
Alternative Imaging Modalities
For CT and cardiac MRI, a pericardial width >4 mm is regarded as abnormal and indicates the presence of effusion 1. These modalities are particularly advantageous for depicting focal effusions and precisely quantitating the amount 1.
Critical Clinical Context
The hemodynamic significance is more related to the rapidity of fluid accumulation than to total volume alone 1. This means that even moderate effusions can cause tamponade if they develop acutely, while large chronic effusions may be well-tolerated.
Size-Based Clinical Implications
Small effusions (<10 mm): Generally have good prognosis and do not require specific monitoring or treatment if idiopathic 2
Moderate effusions (10-20 mm): Require echocardiographic follow-up every 6 months 2
Large effusions (>20 mm): Require more frequent follow-up every 3-6 months and consideration for drainage if subacute with signs of right chamber collapse 2. Large chronic idiopathic effusions carry a 30-35% risk of progression to cardiac tamponade 2
Etiology Correlates with Size
Research demonstrates that moderate to large effusions are more commonly associated with bacterial and neoplastic conditions 2. In one study of 322 patients with moderate or severe effusions, the most frequent causes were acute idiopathic pericarditis (20%), iatrogenic effusions (16%), cancer (13%), and chronic idiopathic effusion (9%) 3. Malignancy accounts for 38% of large pericardial effusions 4.
Important Measurement Considerations
Images should be documented digitally with detailed description in the echocardiographic report, including not only the extent but also the location of each measurement 1. This allows for accurate serial comparisons during follow-up.
Loculated effusions or those containing clots (e.g., post-cardiac surgery) may be difficult to diagnose using transthoracic echocardiography and may require transesophageal echocardiography 1.