Effusion
A collection of fluid in a body space — a joint, the chest, or around the heart.
What it means
An abnormal build-up of fluid in a space that normally contains only a small amount.
- Pleural effusion — fluid between the lung and the chest wall. Causes breathlessness
- Joint effusion — fluid in a joint, often called water on the knee
- Pericardial effusion — fluid around the heart
- Middle ear effusion — glue ear, very common in children
The effusion is the finding, not the diagnosis. The question that matters is always what caused it.
In practice
On chest X-rays, ultrasound and CT reports, and in orthopaedic and ENT letters.
Often the first abnormality noticed, prompting the search for a cause.
Why it matters
Ask what caused it. An effusion with no explanation is an unfinished piece of work.
Fluid is often sampled where the cause is unclear — that is how the answer is usually found.
New breathlessness with a known pleural effusion needs urgent assessment, as does a hot, red, very painful joint with an effusion, which may be infected.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026