IS Case 326: Pneumomediastinum
Imaging Sciences URMC 2009
Publication Date: 2009-09-14
Many cases of pneumomediastinum are the result of air dissecting into the pulmonary interstitium and mediastinum. This sequence of events is sometimes referred to as the Macklin effect. The Macklin effect consists of alveolar base rupture with subsequent air tracking through the pulmonary interstitium with decompression into the mediastinum. Clinically, pneumomediastinum can be seen in up to 10% of blunt trauma to the chest. Most however are not associated with chest trauma and are the result of asthma related complications or secondary to other causes. These include sustained valsalva, cough, weightlifting, or inhalational drug use. Common presenting signs and symptoms of pneumomediastinum include; retrosternal chest pain, subcutaneous emphysema, neck pain, dyspnea, dysphagia, and precordial crunching sound at auscultation.
Radiographs are usually the initial modality when pneumomediastinum is discovered. Plain film findings include air outlining the mediastinal structures (aorta, trachea, bronchi, anterior and posterior heart). Other findings include pulmonary interstitial emphysema, perivascular halos, subcutaneous emphysema in the neck and chest wall, and thymic sail sign in pediatric patients. CT is more sensitive than chest radiographs in demonstrating pneumomediastinum. Chest CT can demonstrate pulmonary interstitial emphysema, pneumomediastinum, tension pneumomediastinum or pneumothorax, and visceral injuries such as esophageal tears. In esophageal tears, air usually collects around the esophagus at the level of the diaphragm. A large pneumomediastinum that progressively increases also suggests esophageal injury.
Differential considerations include pneumothorax, pneumopericardium, paratracheal air cysts, and mach lines. Pneumothorax will usually demonstrate apical air with shifting air on decubitus views. Pneumopericardium can be difficult to differentiate, but air should be confined to the pericardial space. Paratracheal air cysts should appear as a single or cluster of small air-filled cysts, usually in the right paratracheal region or the thoracic inlet.
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