Pneumomediastinum overview

# Overview
Pneumomediastinum (from Greek pneuma - "air", also known as mediastinal emphysema)  is a condition in which air is present in the mediastinum. The condition was first described in 1819 by Rene Laennec and later in 1939, Louis Hamman described primary or spontaneous pneumomediastinum which is why it is also known as Hamman's syndrome. Macklin and Macklin, in 1944 provided a sound explanation for pneumomediastinum, based on experiments conducted on cats: the increase of alveolar pressure causes them to rupture, therefore releasing air which in turn migrates through the peribronchial and perivascular sheaths to the mediastinum. The condition can also result from direct physical trauma to the lung or to other parts of aerodigestive tract causing the air to leak into the chest cavity. The most common precipitating factor of pneumomediastinum is exacerbation of asthma. Spontaneous pneumomediastinum is more common in young men and pregnant women, with male to female ratio 8:1. Infants have high incidence rates of pneumomediastinum. Commonly, spontaneous pneumomediastinum presents with acute severe pain in the chest. Pneumomediastinum is diagnosed by radiologic studies such as X-ray or CT scan of chest and it is characterized by free air in the subcutaneous tissues and mediastinum. Pneumomediastinum is considered a benign condition and may resolve by itself in most of the cases. However, in case of complications such as subcutaneous emphysema or malignant pneumomediastinum surgical intervention becomes necessary.

# Historical Perspective
The condition was first described in 1819 by Rene Laennec. In 1939, Louis Hamman described primary or spontaneous pneumomediastinum which is why it is also known as Hamman's syndrome. Macklin and Macklin, in 1944 provided a sound explanation for pneumomediastinum, based on experiments conducted on cats: the increase of alveolar pressure causes them to rupture, therefore releasing air which in turn migrates through the peribronchial and perivascular sheaths to the mediastinum.

# Classification
Pneumomediastinum is classified according to cause into spontaneous pneumomediastinum and secondary pneumomediastinum. Both the categories are further classified according to their causes. Pneumomediastinum can also be classified according to the entry of air into the mediastinal cavity: Head/neck/upper respiratory tract, lower respiratory tract, gastrointestinal tract, or external sources.

# Pathophysiology
The pathophysiology of spontaneous pneumomediastinum is based on the existence of a pressure gradient between the alveoli and lung interstitium. Pneumomediastinum occurs when pressure rises in the lungs and causes the air sacs (alveoli) to rupture. Sudden increase in intrathoracic pressure due to a specific triggering event such as Valsalva maneuver, vomiting, asthma exacerbation, physical activity may lead to alveolar rupture and the consequent escape of air into the interstitium. Once the air is in the lung interstitium, it flows towards the hilum and the mediastinum along a pressure gradient between the pleural cavity and the mediastinum. Spontaneous neonatal pneumomediastinum may follow gas trapping associated with the aspiration of blood or meconium, neonatal respiratory distress syndrome, pneumonia, or the use of mechanical ventilation. Another possible mechanism is traumatic damage to the lungs or other nearby structures that allows air to leak into the center of the chest.

# Causes
Spontaneous pneumomediastinum may be caused without any underlying pathology. Secondary pneumomediastinum may be caused by blunt or penetrating trauma to the neck, chest or abdomen. Iatrogenic causes include disruption of the airways or GI tract during endoscopic procedures, intubation/extubation, central vascular access procedure, pleural cavity instrumentation, chest or abdominal surgery. Pneumomediastinum has also been associated with: Mycoplasma pneumonia, anorexia, obesity and pulmonary barotrauma in scuba diver, or an airplane passenger during rapid ascends or descends.

# Differentiating  from other Diseases
- Pneumomediastinum must be differentiated from other diseases that cause severe acute chest pain such as:
- Acute coronary syndrome
STEMI
Non-STEMI
- STEMI
- Non-STEMI
- Aortic dissection
- Pulmonary embolism
- Pneumothorax
- Hemothorax
- Esophageal perforation (Boerhaave's syndrome)
- Coronary artery dissection
- Aortic dissection

# Epidemiology and Demographics
Pneumomediastinum is a rare condition. Epidemiology of pneumomediastinum reflects the epidemiology of diseases associated with the condition. Spontaneous pneumomediastinum is more common in young men and pregnant women. Infants have high incidence rates of pneumomediastinum.

# Risk Factors
Factors that can increase the chances of pneumomediastinum include age: Infants and young adults are at high risk. Gender: Males have a higher incident rate than females. A tall, lean, male body habitus is considered a risk factor for pneumomediastinum. Preexisting pulmonary disease: People with lung diseases such as asthma, bronchiectasis, cystic fibrosis, COPD, interstitial lung disease, and cysts, are at higher risk of the pneumomediastinum.

# Natural History, Complications and Prognosis
Pneumomediastinum is considered a benign entity with good prognosis. The main symptom of pneumomediastinum is chest pain. The onset of the pain is sudden and acute or follows exacerbations of underlying pathology such as asthma. Spontaneous pneumomediastinum usually resolves by itself, but prolonged cases have also been reported (>2 months). There also have been incidences of recurrence.

# Diagnosis

## Diagnostic Criteria
There are no specific criteria associated with diagnosis of pneumomediastinum.

## Symptoms
The clinical presentation of pneumomediastinum depends on the causative or precipitating factor such as exacerbation of asthma, Boerhaave syndrome or pneumonia. Common symptoms of pneumomediastinum include severe, acute pain in the chest(pain may radiate to the shoulders or back), fever, subcutaneous emphysema, shortness of breath, neck pain, jaw pain, dysphonia, dysphagia, emesis or swelling of neck, face, chest, abdomen or shoulder.

## Physical Examination
In spontaneous pneumomediastinum, the patient appears normal. Patient with pneumomediastinum secondary to an exacerbation of asthma may appear distressed. On physical examination, the most pathognomonic sign of pneumomediastinum is Hamman's sign mediastinal crunch or click present on auscultation over the cardiac apex and the left sternal border synchronous with the heartbeat. Subcutaneous emphysema can also be detected in a patient with pneumomediastinum.

## Laboratory Findings
There are no specific laboratory findings associated with pneumomediastinum.

## Imaging Findings
CT scan of chest is the imaging modality of choice for pneumomediastinum. On radiological investigations such as CT scan or X-ray of chest, pneumomediastinum is characterized by free air in the subcutaneous tissues and mediastinum.

## Other Diagnostic Studies
There are no other diagnostic studies associated with pneumomediastinum.

# Treatment

## Medical Therapy
Pneumomediastinum is considered a benign condition. The first step in the treatment is the exclusion of any significant pathology causing pneumomediastinum and if diagnosed, treat the underlying conditions. In case of spontaneous pneumomediastinum without any complications, patients are required to be hospitalized for 24 hours for observational purposes.

## Surgery
Pneumomediastinum is considered a benign condition and does not require surgical intervention in uncomplicated cases. Surgical treatment varies from minor skin incision or chest tube drainage to major VATS (Video-assisted thoracoscopic surgery) or thoracotomy depending on the amount and location of air present in the mediastinum.

## Prevention
The primary mode of prevention in case of pneumomediastinum is to prevent and avoid precipitating factors. Children should be completely vaccinated, medical conditions associated with pneumomediastinum should be treated aggressively. All the medical or surgical procedures that can damage the tracheobronchial tree or gastrointestinal tract should be performed under ultrasound guidance.