Radiology Case
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Radiographic findings: : The patient is intubated with the cuff inflated. There is moderate to severe subcutaneous emphysema surrounding the thorax (a), neck (a), and the right flank Gas is present dissecting the fascial planes of the neck and mediastinum enhancing serosal detail appreciated mostly on the lateral view (arrow). There is dorsal displacement of the cardiac silhouette from the sternum by a gas opacity. The left lung lobes are retracted from the thoracic wall and present an increased interstitial opacity. There are two larger pockets of gas, one between the liver and diaphragm and the other around the right kidney (arrow heads); in addition, there is also a small amount of gas diffusely present in the peritoneal cavity.
Radiographic impression: Pneumomediastinum, pneumothorax, pneumoretroperitoneum, pneumoperitoneum, and subcutaneous emphysema.
Comments: A basic review of mediastinal anatomy is helpful in understanding pneumomediastinum, the etiologies, and the progression. The mediastinum is in the mid- sagittal plane of the thorax and is formed by reflections of the parietal pleura. The cranial border is the thoracic inlet, the caudal one is the diaphragm, the dorsal margin is the thoracic vertebrae, and the ventral margin is the sternebrae. The esophagus, trachea, major thoracic vessels, heart, and lymph nodes are positioned within the mediastinum.
A diagnosis of pneumomediastinum is made when mediastinal structures usually invisible on thoracic radiographs can be seen (cranial vena cava and cranial aortic branches, etc). The causes include spontaneous pneumomediastinum (idiopathic, cough, cysts), transtracheal washes, tracheal or bronchial rupture (over inflation of the endotracheal cuff, traumatic intubation, tracheal laceration or trauma), esophageal rupture (foreign body, trauma, penetrating wound), mechanical ventilation (alveolar rupture), tracheostomy, percutaneous placement of large central venous or arterial catheters, and other causes of airway rupture (thoracic trauma).
Gas from a tracheal or esophageal rupture or other cause mentioned above dissects along fascial planes into the mediastinum allowing a gas soft tissue interface to form. This interface allows the visualization of previously indistinguishable structures. The gas in the mediastinum can migrate into the subcutaneous tissues causing subcutaneous emphysema, or through a defect in the mediastinal pleura causing pneumothorax, or through the aortic hiatus causing pneumoretroperitoneum, or through the esophageal hiatus (or a defect in the peritoneum) causing pneumoperitoneum. In rare instances, a pneumopericardium can be present.
Treatment should be directed at the primary etiology if it can be identified. If dyspnea is present, thoracocentesis is generally warranted. If the subcutaneous emphysema becomes uncomfortable, it can be drained percutaneously. If the gas accumulation is persistent or increases, endoscopy or contrast radiography to identify a tracheal or esophageal rupture may be necessary. Surgery may be indicated when rupture is present In this case, it is suspected that endotracheal cuff over inflation, traumatic intubation, or overzealous ventilation was the primary etiology.
References
Ettinger SJ, Feldman EC, Textbook of Veterinary Internal Medicine, Vol.
1, 4th ed., WB Saunders, 1995, pp.834-36.
Dr. John Feleciano, DVM, DACVR
Dr. Renee Leveille, DVM, ACVR