Pneumomediastinum is defined by the presence of free air or gas within the mediastinum, the central compartment of the chest. This space is located between the two lungs and contains the heart, large blood vessels, trachea, and esophagus. Most cases of pneumomediastinum are benign and resolve on their own with conservative management.
How Air Enters the Mediastinum
Air accumulation involves a break in the air-containing structures of the lungs or airways. This breach allows air to escape into the surrounding tissues and ultimately track into the mediastinal space. The most common physiological explanation for this process is known as the “Macklin effect,” which describes a three-step sequence.
The process begins when a sudden, significant rise in pressure inside the lungs causes tiny air sacs, called alveoli, to rupture. This increased pressure can be generated by forceful activities like excessive coughing, strenuous vomiting, or performing a Valsalva maneuver. Once the alveoli break, the air escapes into the supporting connective tissue of the lung.
From the lung tissue, the free air then moves along the path of the pulmonary blood vessels and bronchi toward the center of the chest. This air spreads to the pulmonary hilum—the root of the lung—and finally enters the mediastinum. Pneumomediastinum is broadly categorized into two types based on its origin: spontaneous and secondary.
Spontaneous pneumomediastinum (SPM) occurs without an obvious external cause like trauma or surgery, often triggered by events like an asthma attack, forceful inhalation of recreational drugs, or severe coughing fits. Secondary pneumomediastinum is a result of a direct injury to the chest, such as blunt trauma, or a tear in the esophagus or trachea, often caused by medical procedures like endoscopy or intubation. This distinction is important because secondary cases may indicate a more serious underlying injury.
Recognizing the Symptoms and Confirmation
The most common complaint is sharp chest pain, typically felt behind the breastbone, which may radiate to the neck, back, or arms, often worsening when the patient takes a deep breath or swallows. Patients may also report shortness of breath, throat pain, or a change in their voice that sounds hoarse.
Air that has leaked into the mediastinum can sometimes track further into the soft tissues of the neck and face. This presents as a swelling that feels like a crackling sensation when the skin is touched, known as subcutaneous emphysema. A medical professional may also listen for a specific sound over the heart called Hamman’s sign.
Hamman’s sign is a crunching, rasping, or crackling noise synchronized with the patient’s heartbeat. This sound is caused by the heart compressing the trapped air bubbles within the mediastinum. While this sign is highly suggestive of the condition, it is not always present. Confirmation of pneumomediastinum relies on imaging techniques to visualize the free air.
A standard chest X-ray can often reveal the presence of air outlining the mediastinal structures, but a Computed Tomography (CT) scan is the more sensitive diagnostic tool. CT helps doctors rule out more dangerous conditions that can mimic the symptoms, such as a collapsed lung (pneumothorax) or a tear in the esophagus (Boerhaave syndrome).
Management and Recovery
For the majority of spontaneous cases, the condition is self-limited, and management involves conservative observation. The body naturally reabsorbs the trapped air over a period that typically ranges from a few days to a few weeks.
Supportive care is provided to manage symptoms, including rest and pain relief with appropriate medication. Supplemental oxygen may be administered, as breathing a higher concentration of oxygen can help speed up the reabsorption of the air. Patients are also advised to avoid activities that increase intrathoracic pressure, such as excessive coughing or straining.
The prognosis for spontaneous pneumomediastinum is excellent. Patients are typically monitored for a period to ensure their condition is stable and that no complications, such as tension pneumomediastinum, develop. Secondary pneumomediastinum requires immediate treatment of the underlying cause, which may involve surgical repair if a significant tear in the esophagus or trachea is identified.