Human studies demonstrate a sensitivity and specificity of 87% and 97% for lung ultrasound and 28% and 100% for thoracic radiography for identification of pneumothorax. The sensitivity and specificity of ultrasound to diagnose pneumothorax in companion animals is variable and likely depends on the experience of the sonographer, underlying pathology, patient positioning, scanning protocol, and criteria used to diagnose pneumothorax.

Pleura and Lung Ultrasound Findings That Rule Out Pneumothorax

Pneumothorax is ruled out when there is evidence the pleura (parietal lining of the inner chest wall and visceral outer lining of the lung) are in apposition to each other. The following sonographic findings confirm the pleura are in apposition below the probe; 1) detection of lung sliding and/or a lung pulse, and 2) detection of B-lines and/or lung consolidation (see below) arising from the pleural line. The absence of all these signs should “prompt” consideration of a pneumothorax, however, lung sliding is not always easy to detect, the lung pulse may be lost with aerated lung or not visible distal to the heart, and B-lines and lung consolidation are often not seen in healthy animals. In human medicine the positive predictive value of absent lung sliding for the detection of pneumothorax is 87% in the general population, falls to 56% in the critically ill, and to 27% in patients with respiratory failure. If the history and clinical exam are supportive, thoracocentesis is indicated when lung sliding, lung pulse, and lung pathology are not visible, particularly when assessed at locations where free gas is most likely to accumulate (widest part of the thorax with a patient in lateral recumbency, caudo-dorsal site with a patient in sternal or standing).

Pleura and Lung Ultrasound Findings That Rule in Pneumothorax

If the patient is sufficiently stable, the operator may rule in pneumothorax by finding; 1) the lung point and/or 2) an abnormal abdominal curtain sign. The lung point is the location within the thorax where the lung recontacts the parietal pleura and creates an intermittent or partial glide sign within a section of the ultrasound image when the patient breathes. The lung point is located by moving the probe from an area with no perceived lung sliding, to an area where lung sliding reappears within a region of the ultrasound window. Pleural space pathology tends to compress the lung around the hilus: to identify the lung point, slide the probe from a region of absent lung sliding towards the lung hilus (often from the caudo-dorsal lung regions towards the elbow). The lung point will not be seen if the lung fails to recontact the chest wall in cases of massive/complete pneumothorax, and has an overall sensitivity of 66%, and a specificity of 100% to detect pneumothorax in human patients. An experimental study of anesthetized dogs demonstrated a specificity of 100% and a sensitivity of 20% and 40% for the lung point to diagnose pneumothorax with volumes of 2 and 10 mL/kg, respectively.

Abnormal abdominal curtain signs, including asynchronous and double curtain signs, have been described in a clinical case series of dogs with confirmed spontaneous and trauma induced pneumothorax. These dogs were considered to have only mild to moderate degrees of pneumothorax, and extrapolation of these findings to larger volume pneumothorax is unknown. Of note, dogs with pneumothorax of mild to moderate volume had all 3 types of abdominal curtain signs (normal, double, and asynchronous) present over each hemithorax, with normal curtain signs visible in the ventral third of the thorax. The asynchronous curtain sign is defined as the cranial movement of a pneumothorax-induced curtain sign with concomitant caudal movement of abdominal contents during the inspiratory phase of the respiratory cycle, with caudal movement of a pneumothorax-induced curtain sign and cranial movement of the abdominal contents during the expiratory phase of the respiratory cycle. Therefore, it is not only the direction of movement of the curtain sign and visible abdominal structures that is important but also their direction of movement during the respiratory cycle. The double curtain sign is defined as 2 parallel vertical abdominal edge artifacts (pneumothorax induced) visible in the same sonographic window, or within close proximity to each other over the same hemithorax. Their appearance can vary, and they are sometimes identified as the sudden inspiratory visualization of abdominal soft tissue structures within an area that previously contained only A-lines and an absence of lung sliding. Alternatively, the double curtain sign may be visible throughout the respiratory cycle, appearing as abdominal soft tissue structures that are bound cranially and caudally by vertical air edge artifacts, which diverge away from each other on inspiration and converge toward each other on expiration. In addition, it is possible for both visible edge artifacts of the double curtain sign to slide caudally on inspiration in combination with the findings described earlier.

M-Mode Sonography and Pneumothorax

The use of M-mode, which detects motion over time, can be used to detect the motion of lung sliding. When the M-mode cursor is situated over the pleural line, 2 different patterns are displayed on the screen: the motionless portion of the thorax above the pleural line creates horizontal lines and the sliding below the pleural line creates a granular pattern, likened to a sand like pattern found at the beach. The resultant picture is one that resembles waves crashing onto the sand and is, therefore, called the seashore sign, which can be identified in normal lung. If the patient has an expiratory pause between respirations, then the seashore sign is temporarily lost and appears as a barcode sign. With pneumothorax, M-mode only shows 1 pattern of parallel horizontal lines above and below the pleural line, exemplifying the lack of lung sliding at the level of the pleural line. This pattern resembles a barcode or stratosphere sign. The use of M-mode has been suggested in people when the detection of lung sliding may be subtle using B-mode alone. When both the seashore and barcode sign are visible within the same window during the active phases of respiration, it is consistent with the B-mode finding of the lung point. Individual sensitivity and specificity of M-mode for detection of pneumothorax is rarely reported because it is generally considered in conjunction with B-mode to confirm the presence or absence of lung siding (e.g., lung sliding is reported regardless of which mode of ultrasonography is used to detect it). To the author’s knowledge, M-mode has not been clinically reported in dogs or cats for the detection of pneumothorax, although it was reported in the experimental canine study by Hwang and colleagues, where it was used in conjunction with B-mode to determine the overall presence of lung sliding.

Summary

Evidence in the human literature suggests PLUS is highly accurate at diagnosing pneumothorax, outperforming thoracic radiography, particularly when performed by experienced sonographers and when multiple PLUS criteria are considered. Although PLUS shows tremendous promise for the diagnosis of pneumothorax in small animals, study results are conflicting, likely because of species differences, lack of methodology standardization between studies regarding reference standards and PLUS criteria used to diagnose pneumothorax, limited training opportunities and expertise, and inclusion of only small study populations. Further research will likely provide insight into the true accuracy of PLUS and the various PLUS criteria to diagnose pneumothorax in small animal patients.

References

References are available upon request.