• Air within the mediastinal space, rendering normally invisible structures visible on radiography.
• Normally, only the heart, aorta, caudal vena cava and tracheal lumen (not extraluminal borders) should be seen. Other structures are not discerned due to smaller size or because they are surrounded by structures of similar radiographic density.
  The mediastinum contains the heart, trachea, major blood vessels, esophagus, lymph nodes, vagus nerve, phrenic nerve, thoracic duct and thymus.
• Cause: idiopathic, post-traumatic eg traumatic jugular venipuncture, iatrogenic, neoplastic or infective.
• Signs: subcutaneous emphysema around head/neck, dyspnea/cyanosis seen only in severe cases.
• Diagnosis: radiography.
• Treatment: most cases resolve spontaneously however pneumomediastinum may coexist with other abnormalities, eg pneumothorax.
• Prognosis: good.

Presenting signs

• Subcutaneous emphysema, especially around the head/neck region.

• Dyspnea/cyanosis is uncommon - only in severe and rapidly developing cases.
• Dysphagia may occasionally be seen.

Acute presentation

• Pneumomediastinum alone does not often present as an acute entity.
• In conjunction with pneumothorax or tension pneumothorax, the animal may present cyanotic and in severe respiratory distress/collapse.

Special risks

• There may be risks associated with concomittant abnormalities, eg pneumothorax, causing severe respiratory compromise.