Tag Archives: Lung

The Shred Sign

As we have seen previously, lobar consolidations tend to be well-defined, often accompanied by a small pleural effusion. However, small subpleural consolidations (of course, ultrasound does not detect if they are not subpleural) are separated from the surrounding aerated lung by an irregular margin, like a torn paper called the ‘shred sign’. The brightness is because of the air, which scatters the ultrasound beam. It is also known as the fractal line. Here is an illustrative image.

Another example

Sometimes, the consolidation might appear anechoic mimicking a pleural effusion. But the presence of shred sign as in this example is specific to consolidation.

Lung Mass

Occasionally, you may stumble upon a lung mass while evaluating for B lines or other common pathologies. Particularly, ultrasonography is good at detecting primary and metastatic lung masses adjacent to the pleural surface. They typically appear as hypoechoic areas that are distinct from consolidated or hepatized lung, with normally aerated lung appearing in the scan field with respiration. Consolidation and fluid bronchograms may be been seen adjacent to the mass.

It is interesting to note that ultrasonography has been shown to have better sensitivity and specificity (89% and 95% respectively) for assessing chest wall involvement by a lung tumor compared to CT scan. Extension of the tumor beyond the parietal pleura into the chest wall can be determined if the mass breaches the pleura and stops moving with respiration. If trying to find this, use a high resolution linear probe. Spectral Doppler may be helpful in differentiating malignant versus benign masses but it is beyond the scope of NephroPOCUS.

Lung contusion

While nephrologists are not the first line of contact for trauma patients, we are often asked to evaluate the need for renal replacement therapy in these patients. It is important that we are aware of the sonographic findings seen in lung contusion so that we don’t confuse them with interstitial syndrome. As mentioned before, B-pattern in two or more sonographic lung zones bilaterally is suggestive of interstitial syndrome and this term encompasses pulmonary edema of various causes. Usually the pleural line is regular in these cases unless there is underlying lung disease such as pulmonary fibrosis.

Lung contusion is typically associated with B-lines in conjunction with pleural line irregularities, subpleural hypoechoic areas and may be localized pleural effusion. Unless the trauma is diffuse, these findings are localized to certain lung zones. The B-lines are frequently confluent, which means, too numerous and difficult to identify each line separately. In addition, Z-lines can be seen: lines that arise from the pleural line and fade away vertically and do not reach the edge of the screen.

It is also important to note that the contusion can be associated with pneumothorax, particularly in cases of rib fracture. Presence of B-lines almost always rules out pneumothorax even when you are not sure about absent pleural sliding. It is because there is air in between pleural layers, which does not allow B-lines to form. If you notice absent pleural sliding and suspect pneumothorax, look for ‘lung point’, which is the junction between normally sliding pleura and the pneumothorax with absent sliding. It is virtually diagnostic of pneumothorax.

This jellyfish is not atelectasis!

Previously, we talked about atelectasis and pneumonic consolidations. Here is a unique case shared by Dr. Kylie Baker @kyliebaker888, where misdiagnosis could have lead to deleterious consequences.

A 70-year-old man with a history of substance abuse presented with left sided chest pain and presyncope. Right lung POCUS was normal and so was the left anterior scan. However, left posterior zone demonstrated anechoic pleural effusion and a mobile echogenic structure (***) that appeared to be an atelectatic lung vs. pneumonia (causing pleurisy) at first glance. Cardiac windows were difficult to obtain.

CT scan of the chest demonstrated ruptured thoracic aorta and left hemothorax, which means the echogenic structure on ultrasound was a blood clot and not lung tissue.

How to differentiate?

A blood clot appears homogeneous and does not have air bronchograms (dynamic or static) unlike the lung tissue. Moreover, there will not be any demonstrable blood flow within the structure. As mentioned before, pneumonia > atelectasis tends to be hyperemic. So, pay attention to detail.

The jellyfish sign: atelectasis

In moderate to large pleural effusions, it’s not uncommon to see lung atelectasis on the ultrasound. This collapsed lung within the effusion moves with respiration and appears like a jellyfish.

Important things to note:

Jellyfish sign favors atelectasis over pneumonia as the collapsed lung can change its shape with respiration whereas pneumonia is more firm.

It also favors transudative etiology of the effusion as the viscosity of exudative effusions tend to hamper mobility of the lung tissue.

Here are more examples

Pneumonia versus atelectasis: the differentiation can be difficult

As mentioned before, hepatization of the lung i.e., lung looking like liver tissue can occur in both pneumonia and atelectasis. Dynamic air bronchograms, when present, point toward pneumonia but static air bronchograms can be seen in both conditions. The differentiation can be very difficult at times and the management should be guided by the clinical picture.

Dynamic air bronchograms are not always as obvious as previously demonstrated. The probe angle needs to be adjusted as necessary to differentiate out of plane lung motion from moving air bronchograms. Other things to note:

  • More pleural effusion and less consolidated tissue suggest collapsed lung i.e., relaxation or passive atelectasis.
  • More consolidated tissue with less effusion, especially when the patient has fever or signs suggestive of infection points toward pneumonia even in the absence of dynamic air bronchograms.
  • Fibrin strands or loculated pleural effusion suggest infectious etiology.
  • Increased color flow in the consolidated tissue favors pneumonia while little to no flow suggests atelectasis.
Dynamic air bronchograms better seen in some planes. Note the Doppler flow in this case of pneumonia. Image courtesy: Dr. Lars Mølgaard Saxhaug

Here is another example of pneumonia with dynamic air bronchograms that require careful observation. Color flow is overall increased though there is some interference from tissue motion while the patient is breathing rapidly.

Pleural effusion and consolidation with fibrin stranding suggestive of infectious etiology. Image courtesy: Dr. Rohit Patel

Also note that small subpleural consolidations do not show typical dynamic air bronchogram pattern. For example, this lung ultrasound image obtained with the straight linear array probe (5–13 MHz) over right anterior chest demonstrates lung sliding and a small subpleural hypoechoic area with ragged margin separating it from the surrounding normal lung. This is described as the “shred sign” because of its distinctive irregular boundary with the normal lung.

B lines do not always indicate pulmonary edema

Diffuse B-line pattern can be seen in pulmonary edema of various causes, interstitial pneumonia and diffuse parenchymal lung disease (e.g. fibrosis). Whether you are dealing with pulmonary edema or fibrosis largely depends on the clinical context. However, findings such as irregular, fragmented pleural line, sub pleural abnormalities appearing as small hypoechoic areas and nonhomogeneous distribution of B-lines favor fibrosis. On the other hand, “focal” B-line pattern may be seen in pneumonia, atelectasis, pulmonary contusion, pulmonary embolism, pleural disease and malignancy.

The following loops were obtained from upper anterior zones on each side (R/L). While they show B-line pattern, the lower lung zones were normal and the patient was breathing comfortably on room air. This is a patient with interstitial lung disease and the B-line pattern represents fibrotic process. Also note that the pleural line is irregular.