Pitfalls:
In the absence of lung sliding one must still consider the clinical context as other conditions can cause decreased or absent sliding, including:
- Large blebs
- Pleural adhesions
- Apnea
- Right main stem intubation (decreased movement on left chest)
- Dense consolidation: atelectasis, pneumonia, contusion
- Pleural effusion
In the case of apnea, main stem intubation and atelectasis often cause an artefact called the lung pulse that can be seen, differentiating these conditions from true pneumothoraces. A lung pulse is the slight motion of the pleura caused by the movement of the heart transmitted to the pleura and can only appear when the parietal and visceral pleura are opposed (video 4).
Video 4: Lung Pulse
Another cause of confusion is subcutaneous emphysema. Subcutaneous emphysema is generally associated with pneumothorax but the air in the subcutaneous tissues scatters the ultrasound beams creating a grey haze similar to lung. In this case it is often difficult to visualize the ribs or pleural line but pneumothorax can be assumed in the right clinical context (video 5).
Video 5: Subcutaneous emphysema
Rarely absent lung slide can be caused by pathology other than pneumothorax. Looking deep to the pleural line can also help distinguish absent lung sliding due to pneumothorax from absent slide due to other pathologies. On ultrasound, air appears as homogenous grey and regularly-spaced, echogenic, horizontal A-lines appear deep to the pleural line (figure 5). This pattern would be seen deep to a motionless pleural line in pneumothorax, large blebs and adhesions so the clinical context must be considered.
Figure 5: A-lines
Pleural effusions can be differentiated as a cause of absent lung sliding by anechoic fluid deep to the relatively hyperechoic pleural line. In dense consolidation caused by pneumonia, contusions and atelectasis not only is a lung pulse likely present but the lung deep to the pleural line will have absent a-lines and take on the appearance of a hypoechoic or organ-like structure.