Pneumomediastinum and subcutaneous emphysema

Pneumomediastinum is the presence of free gas in the mediastinum, the central compartment of the thoracic cavity, whereas subcutaneous emphysema refers to gas trapped under the skin. These conditions can occur in domestic animals due to various causes such as trauma or medical procedures, leading to air leakage from the respiratory system, particularly the trachea. These conditions are often detected through radiographic examination and can result in respiratory distress and other complications.

Terminology

Pneumomediastinum specifically refers to gas within the mediastinum, while subcutaneous emphysema is gas trapped beneath the skin. These two spaces can connect when air pressure within them is increased. A common cause is rupture of the trachea or an airway close to the mediastinum, allowing air to be forced into the mediastinum, thoracic inlet, and fascial planes.

Epidemiology

The incidence of pneumomediastinum and subcutaneous emphysema in domestic animals is difficult to quantify, as many cases may resolve without veterinary intervention. However, certain risk factors and associations can increase susceptibility.

Risk factors

Key risk factors include underlying respiratory diseases, trauma (such as bite wounds in small animals or iatrogenic during medical procedures like transtracheal aspiration). Tracheal rupture is the most common cause of diffuse subcutaneous emphysema.

Associations

There is a higher incidence of these conditions in certain situations, such as blunt thoracic trauma, overinflation of endotracheal tube cuffs during anesthesia, and neck trauma.

Clinical presentation

Animals with pneumomediastinum or subcutaneous emphysema may present with swelling under the skin that crackles when touched, respiratory distress, or lack of visible distress despite the presence of free air on radiographs. Occasionally, animals may exhibit dyspnea when the condition progresses to pneumothorax or if large volumes of subcutaneous emphysema are present.

Pathology

Pneumomediastinum and subcutaneous emphysema originate from air leakage from the trachea or lower respiratory tract. Subcutaneous emphysema may also result from direct trauma to the thorax with leakage of air from the lung.

Location

These conditions mainly appear in the mediastinum or subcutaneous spaces and can extend from the neck to the thorax and retroperitoneal space depending on the communication between these areas. If resulting from direct thoracic trauma, the emphysema tends to be localized to the site of trauma.

Macroscopic appearance

Grossly, affected animals exhibit pockets or layers of gas within the mediastinum or beneath the skin which can be visually detected through radiography.

Radiographic features

Radiographs will show radiolucent areas where the gas is present, with specific patterns of distribution.

Radiography

Lateral views are particularly helpful in detecting pneumomediastinum. The presence of air can make certain organs more visible and change the radiographic appearance of the mediastinum. The mediastinum will appear more radiolucent, with linear bands of air surrounding the serosal surfaces of the trachea and aorta. Air may also track into the retroperitoneum through the esophageal hiatus.

When the air extends to the subcutaneous planes secondary to airway rupture, the skin will appear to be separated from the body wall with intervening radiolucent air. The body size will be significantly enlarged if the amount of air is excessive.

In cases of direct thoracic trauma, subcutaneous emphysema is more localized and contained to the area of the wound.

CT

Computed tomography (CT) can offer a more detailed view of the gas distribution and may help diagnose more subtle cases or determine the exact cause.

MRI

Magnetic resonance imaging (MRI) is generally not the first-line diagnostic tool for these conditions but could theoretically show areas of air as signal voids.

The conditions present significant variability in clinical and radiographic features and are best confirmed through diagnostic imaging. Understanding predisposing factors and mechanisms can guide effective prevention and management strategies.

Radiology report

The report should detail the areas affected with particular attention to trauma and pneumothorax.

Items to include in a radiology report:

• Location of free air

• Pneumothorax

• Mediastinal shift

• Trauma to body wall

• Pulmonary contusions

Morphology

• Lucency in the mediastinal space indicating free air presence

Relationship to/involvement of

• Tracheal displacement or elevation

• Surrounding lung fields and heart contours

Extension

• To the cervical region, indicating potential subcutaneous emphysema, and to the retroperitoneal space

Treatment and prognosis

Treatment for pneumomediastinum and subcutaneous emphysema in domestic animals typically focuses on managing the underlying cause and providing supportive care to the animal. Standard treatments may include oxygen therapy, analgesics, and antibiotics if infection is suspected. Surgery is indicated when the subcutaneous emphysema is extensive or respiratory function is compromised.

Post-treatment imaging may show the resolution of free air in the affected compartments, however this can take time to resolve.

Complications

Complications that may occur from pneumomediastinum and subcutaneous emphysema include:

• Megaesophagus, which can lead to aspiration pneumonia

• Progressive respiratory distress

• Tension pneumomediastinum

History and etymology

The term "pneumomediastinum" originates from the Greek words "pneuma" meaning air, "mediastinum," which refers to the medial cavity of the thorax, and the suffix "-ium," denoting a condition. Similarly, "subcutaneous emphysema" describes the condition where air is trapped under the skin; "emphysema" comes from the Greek "emphysan" meaning 'to inflate'.

Differential diagnosis

When diagnosing pneumomediastinum and subcutaneous emphysema, other possible conditions should be considered:

• Pneumothorax

• Tracheal or esophageal rupture

• Foreign body

Practical points

• Close monitoring of respiratory status is essential in managing these conditions.

• Follow-up radiographs are important to assess the resolution or progression.

• Any intervention should be planned with consideration of the risk of introducing additional air into the affected compartments.