Subcutaneous emphysema overview

# Overview
Subcutaneous emphysema occurs when gas or air is present in the subcutaneous layer of the skin. Subcutaneous refers to the tissue beneath the cutis of the skin, and emphysema refers to trapped air.  Since the air generally comes from the chest cavity, subcutaneous emphysema usually occurs on the chest, neck and face, where it is able to travel from the chest cavity along the fascia.

# Causes
Subcutaneous emphysema can result from puncture of parts of the respiratory or gastrointestinal systems.  Particularly in the chest and neck, air may become trapped as a result of penetrating trauma (e.g., gunshot wounds or stab wounds) or blunt trauma. Infection  (e.g., gas gangrene) can cause gas to be trapped in the subcutaneous tissues. Subcutaneous emphysema can be caused by medical procedures and medical conditions that cause the pressure in the alveoli of the lung to be higher than that in the tissues outside of them.  Its most common causes are pneumothorax and an improperly functioning chest tube. It can also occur spontaneously due to rupture of the alveoli, with dramatic signs.  When the condition is caused by surgery it is called surgical emphysema. The term spontaneous subcutaneous emphysema is used when the cause is not clear.
Subcutaneous emphysema is not usually serious in and of itself, but the underlying causes, such as pneumothorax, can be.

# Natural History, Complications and Prognosis
Air in subcutaneous tissue does not usually pose a lethal threat; small amounts of air are reabsorbed by the body.  Once the pneumothorax or pneumomediastinum that causes the subcutaneous emphysema is resolved, with or without medical intervention, the subcutaneous emphysema will usually clear.  However, spontaneous subcutaneous emphysema can, in rare cases, progress to a life-threatening condition, and subcutaneous emphysema due to mechanical ventilation may induce ventilatory failure.

# Diagnosis

## History and Symptoms
Signs and symptoms of spontaneous subcutaneous emphysema vary based on the cause, but it is often associated with swelling of the neck and chest pain, and may also involve sore throat, neck pain, difficulty swallowing, wheezing and difficulty breathing.  When large amounts of air leak into the tissues, the face can swell considerably. In cases of subcutaneous emphysema around the neck, there may be a feeling of fullness in the neck, and the sound of the voice may change.The tissues surrounding SCE are usually swollen. If SCE is particularly extreme around the neck and chest, the swelling can interfere with breathing. The air can travel to many parts of the body, including the abdomen and limbs, because there are no separations in the fatty tissue in the skin to prevent the air from moving.

## Physical Examination
A significant case of subcutaneous emphysema is easy to detect by touching the overlying skin; it feels like tissue paper or Rice Krispies.  Touching the bubbles causes them to move and sometimes make a crackling noise.  The air bubbles, which are painless and feel like small nodules to the touch, may burst when the skin above them is palpated.

## Chest X Ray
On a chest radiograph, subcutaneous emphysema may be seen as radiolucent striations in the pattern expected from the pectoralis major muscle group. Air in the subcutaneous tissues may interfere with radiography of the chest, potentially obscuring serious conditions such as pneumothorax. It can also and reduce the effectiveness of chest ultrasound.  On the other hand, since subcutaneous emphysema may become apparent in chest X-rays before a pneumothorax does, its presence may be used to infer that of the latter injury.

## CT
Subcutaneous emphysema can also be seen in CT scans, with the air pockets appearing as dark areas. CT scanning is so sensitive that it commonly makes it possible to find the exact spot from which air is entering the soft tissues.

# Treatment

## Surgery
Although the underlying conditions require treatment, subcutaneous emphysema usually does not; small amounts of air are reabsorbed by the body.  However, subcutaneous emphysema can be uncomfortable and may interfere with breathing, and is often treated by removing air from the tissues, for example by using a chest tube.