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Pneumomediastinum is the presence of free gas in the mediastinum, which is the space between the pleural cavities that contains the trachea and mainstem bronchi, esophagus, heart, vena cavae, and aorta. Other smaller structures (eg, vagus nerve, lymph nodes, vessels of the head, thoracic duct) also run through this region. 

Pneumomediastinum can be secondary to defects in the oropharynx, trachea, mainstem bronchi, lungs, esophagus, cervical fascial planes, or retroperitoneum. Trauma, infection, iatrogenic injuries (eg, intubation), migrating foreign bodies, emphysema, and neoplasia can lead to pneumomediastinum.1-4 Pneumomediastinum can subsequently progress to subcutaneous emphysema due to communication of the mediastinum with cervical fascial planes via the thoracic inlet.5

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Clinical Signs

Patients with pneumomediastinum are typically presented for witnessed trauma, dyspnea, or subcutaneous emphysema. Increased lung sounds may be present on thoracic auscultation, or lung sounds may be quiet if concurrent pneumothorax is present. Trauma is a common cause of pneumomediastinum; therefore, a complete physical examination should be conducted and a thorough patient history (eg, recent anesthesia, especially in cats) obtained. 

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Diagnosis 

A pneumomediastinum diagnosis is made via thoracic radiography (Figures 1 and 2). Air in the mediastinum outlines the cranial vena cava, dorsal and ventral tracheal walls, esophagus, and aorta. Gas may be present in the retroperitoneal space, cervical fascial planes, and subcutaneous space.2 If an obvious cause of pneumomediastinum is not apparent and the patient is stable, an oropharyngeal examination may be performed with the patient under sedation. Fluoroscopy with iodinated water-soluble contrast or endoscopy can be used to evaluate for esophageal lesions. Airway integrity may be assessed via endoscopy, although small lesions in the trachea may be missed. Advanced imaging (eg, CT) can be helpful in identifying the underlying lesion. 

Treatment

Strict cage rest and hospitalization to monitor for respiratory distress are recommended. Pneumomediastinum may progress to pneumothorax; in such cases, relieving pneumothorax via thoracocentesis is a priority.4 Thoracostomy tubes may be required in cases with significant recurrent pneumothorax. Subcutaneous emphysema should be aspirated only if it is causing discomfort. Analgesia should be provided for patients with trauma or other sources of pain. Oxygen therapy can speed up resolution of pneumomediastinum and subcutaneous emphysema by displacing nitrogen.

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Prognosis

Many patients with pneumomediastinum recover over several (≤10-14) days as the defect heals and the air is resorbed. Exploratory surgery to repair tears is likely indicated in cases that progress despite conservative management, in which very large tears that are unlikely to heal on their own are identified, or in which obvious mediastinitis is present. In these patients, the prognosis is more guarded.5

References and author information Show
References
  1. Stephens JA, Parnell NK, Clarke K, Blevins WE, DeNicola D. Subcutaneous emphysema, pneumomediastinum, and pulmonary emphysema in a young schipperke. J Am Anim Hosp Assoc. 2002;38(2):121-124.
  2. van den Broek A. Pneumomediastinum in seventeen dogs: aetiology and radiographic signs. J Small Anim Pract. 1986;27(11):747-757.
  3. Jones BR, Bath ML, Wood AKW. Spontaneous pneumomediastinum in the racing greyhound. J Small Anim Pract. 1975;16(1):27-32.
  4. Thomas EK, Syring RS. Pneumomediastinum in cats: 45 cases (2000–2010). J Vet Emerg Crit Care (San Antonio). 2013;23(4):429-435.
  5. Biller DS, Larson MM. Mediastinal disease. In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine. 7th ed. St. Louis, MO: Elsevier Saunders; 2010:1119-1124.
Authors

John Gagnepain

DVM University of Florida

John Gagnepain, DVM, earned his DVM from University of Georgia and his bachelor’s degree from Clemson University.

Gareth J. Buckley

VetMB, MRCVS, DACVECC, DECVECC University of Florida

Gareth J. Buckley, VetMB, MRCVS, DACVECC, DECVECC, is the medical director of the small animal hospital and a clinical assistant professor of emergency medicine and critical care at University of Florida College of Veterinary Medicine. His clinical and research interests focus on critical care, respiratory medicine, cardiopulmonary resuscitation, cardiovascular disease, and pharmacokinetics and pharmacodynamics. He graduated from University of Cambridge and completed a specialty internship, followed by a residency in emergency and critical care, at Tufts University before joining the University of Florida faculty.

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