Pneumothorax (ultrasound)

Pneumothorax is a serious potential consequence of blunt thoracic trauma and, if misdiagnosed, it may quickly become life-threatening.

For a discussion on epidemiology, clinical presentation, pathology, and treatment and prognosis please see the main pneumothorax article. 

Radiographic features

CT is considered the gold-standard in the diagnosis of pneumothorax. Thoracic ultrasound has more sensitivity than a supine chest radiograph (see: supine pneumothorax) for the identification of pneumothorax after blunt trauma. The other advantage of ultrasound is that it can be used at point-of-care. 

Ultrasound

The normal lung interface with pleura shows lung sliding with z-lines, which appear as vertical comet tails running down from the pleural surface. In pneumothorax, this sliding is absent and so are the comet tail artifacts from the pleura. The sight of even a single B-line excludes pneumothorax at that location. There is also loss of the lung pulse, the subtle lung oscillation in tandem with cardiac contraction, which is especially important when trying to distinguish between right mainstem intubation (loss of sliding on the left, but lung pulse is present) and a left sided pneumothorax (no sliding or lung pulse). Free intrathoracic air will then reflect incident ultrasound waves, obscuring the visceral pleura beneath; while all dynamic signs will be abolished, A-lines will be seen extending into the far field.

Visualizing the junction between sliding lung and absent sliding is known as the lung point sign and is near 100% specific for pneumothorax. One may use the mid-axillary line as a landmark for subsequent semi-quantification of the pneumothorax volume. It is not able to be found in all pneumothoraces (sensitivity is around 65%) especially large pneumothoraces where the lung is collapsed and there is globally absent sliding. 

On M mode, classical signs for the gray scale imaging are seen:

• seashore sign: normal lung sliding
• barcode/stratosphere sign: pneumothorax 

History and etymology

The use of ultrasound to diagnose pneumothorax was first described in a veterinary medical journal in 1986 ⁴. 

Differential Diagnosis

The bilateral, anterior discovery of sliding lung is sufficient to rule out pneumothorax in the vast majority of cases. Bilateral absence of B-lines/comet tail artifacts from the pleura with loss of the lung pulse and sliding, and subsequent discovery of a lung point in a typical location is quite specific for the presence of a pneumothorax. The differential for absent lung sliding can be broadly divided into three categories ⁷:

• symphysis/increased adhesion between pleural layers
  • chemical pleurodesis
  • pleural fibrosis
  • interstitial lung disease
  • pneumonia
  • acute respiratory distress syndrome
• decreased ventilation
  • apnea
  • mucus plugging
  • endobronchial intubation
  • obstructive lung disease
• loss of pleural apposition
  • pneumothorax
  • pleural effusion

Unilateral loss of sliding has a shorter differential diagnosis, and may be most suggestive of ⁵:

• endobronchial (typically right mainstem bronchus) intubation, in the context of chest trauma and recent endotracheal intubation 
  • a lung pulse will be present, as the visceral and parietal pleura are still apposed
  • ultrasound may be used, real-time, to "guide" the endotracheal tube proximally, confirming position with the bilateral return of lung sliding and diaphragmatic excursion
• complete atelectasis
  • loss of lung sliding occurs early, accompanied by an accentuated ipsilateral lung pulse and diaphragmatic elevation with decreased excursion
• hydropneumothorax
  • the characteristic "hydro-point" defined by an A-line pattern with absent sliding abutting a sharply demarcated, anechoic collection