Pleural effusions occur when transudate or exudate builds up in the base of the lungs. They are best viewed from the PLAPS-point where the lower lobes are viewed. Point of Care Ultrasound (POCUS) is over 90% sensitive and specific for the detection of pleural effusions.
In this section you will learn how to recognize pleural effusions, calculate pleural effusion size, look for all of the common ultrasound signs.
Calculating Pleural Effusion Size
The simplest way to estimate pleural effusion volume is to use the formula by Balik et al. It involves measuring the maximal effusion diameter ( in millimeters) between the diaphragm and base of the lung in a supine patient (see Figure Below) and multiplying that number by 20.
Pleural volume (mL) = (measured distance in mm) x 20.
Ultrasound Signs of Pleural Effusions
The PLAPS point is the most specific and sensitive view used to diagnose pleural effusion. If you have a patient with a suspected pleural effusion, the following signs/findings that can help you arrive at a proper diagnosis:
- Spine Sign
- Jellyfish sign
- Sinusoid Sign
- Quad Sign
- Plankton Sign
- Hematocrit Sign
- Loculated Pleural Effusions
Spine Sign
You will find a Spine sign at the PLAPS point along with a pleural effusion.
In a normal lung, you should be able to see the spine up until the edge of the diaphragm, but never passing the diaphragm. This is because the lung’s air above the diaphragm prevents any sound waves from passing.
However, in pleural effusions, sound waves can pass through the pleural fluid allowing the spine to be seen above the diaphragm.
Jellyfish Sign
You will find the Jellyfish sign at the PLAPS-point, located slightly above the diaphragm, in the RUQ. Recall that the PLAPS-point is the key location to find pleural effusions. The “Jellyfish Sign” occurs when a consolidated lung is seen floating in the pleural effusion.
Sinusoid Sign
- You can turn on M-mode to look for the Sinusoid sign (looks like a sine wave). It is caused by the parietal and visceral pleura moving closer and further apart while the patient breathes. (White arrows point to the lung line/visceral pleura while black arrows point to the pleural line/parietal pleura). It is equivalent to an M-mode view of the jellyfish sign.
Quad Sign
- A pleural effusion has an anechoic appearance often delineated by the pleural line, the rib shadows, and the lung line, called the “Quad Sign.”
Plankton Sign
Now that you know how to detect a pleural effusion on point of care ultrasound (POCUS), you can further differentiate pleural effusions into transudative or exudative. Though most effusions are transudative, exudative effusions can have the plankton sign, as seen below. The plankton sign shows an effusion with swirling, hyperechoic debris.
Hematocrit Sign
The hematocrit sign refers to the echogenic layering of material in a pleural effusion. This can be due to exudative effusions or a hemothorax (Chichra A).
Transudative vs Exudative Pleural Effusion
Here are some differences between transudative and exudative pleural effusions:
| Transudative Pleural Effusion | Exudative Pleural Effusion | |
| Ultrasound Findings | – Absent plankton sign/hematocrit sign – Typical effusion signs | – Present plankton sign/hematocrit sign -Typical effusion signs |
| Fluid Quality | – Transudate with low protein and cell count – Normal glucose levels | – Cloudy transudate with high protein and cell count – Light’s Criteria: Pleural fluid protein >0.5 and LDH >0.6 or LDH – Low glucose levels |
| Mechanism | – Increased capillary hydrostatic pressure and/or decreased capillary oncotic pressure (low albumin). | – Increased vascular permeability from inflammation/infection/malignancy |
| Common Etiologies | – Heart failure, nephrotic syndrome, liver cirrhosis | – Trauma (hemothorax), bacterial infections, lymphatic obstructions, malignancies, pulmonary embolism |
Loculated Pleural Effusion
- Loculated (or septated) pleural effusions are most often seen in exudative effusions and describe any effusion with fluid divided into “pockets.” They can be caused by infections, abscesses, scarring, or fibrosis in the pleural cavity that complicates proper fluid drainage.
The image below shows an image of a loculated pleural effusion that lies above the diaphragm.
