An endotracheal tube can puncture the trachea or bronchi and cause subcutaneous emphysema. Infection
Air can be trapped under the skin in necrotizing infections such as gangrene, occurring as a late sign in gas gangrene, of which it is the hallmark sign. Subcutaneous emphysema is also considered a hallmark of fournier gangrene. Symptoms of subcutaneous emphysema can result when infectious organisms produce gas by fermentation. When emphysema occurs due to infection, signs that the infection is systemic, i.e. that it has spread beyond the initial location, are also present. Pathophysiology
Air is able to travel to the soft tissues of the neck from the mediastinum and the retroperitoneum (the space behind the abdominal cavity) because these areas are connected by fascial planes. From the punctured lungs or airways, the air travels up the perivascular sheaths and into the mediastinum, from which it can enter the subcutaneous tissues.Spontaneous subcutaneous emphysema is thought to result from increased pressures in the lung that cause alveoli to rupture. In spontaneous subcutaneous emphysema, air travels from the ruptured alveoli into the interstitium and along the blood vessels of the lung, into the mediastinum and from there into the tissues of the neck or head. Diagnosis
Significant cases of subcutaneous emphysema are easy to diagnose because of the characteristic signs of the condition. In some cases, the signs are subtle, making diagnosis more difficult. Medical imaging is used to diagnose the condition or confirm a diagnosis made using clinical signs.
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 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. 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. In 1994, M.T. Macklin and C.C. Macklin published further insights into the pathophysiology of  spontaneous Macklins Syndrome occurring from a severe asthmatic attack. The presence of subcutaneous emphysema in a person who appears quite ill and febrile after bout of vomiting followed by left chest pain is very suggestive of the diagnosis of Boerhaaves syndrome, which is a life-threatening emergency caused by rupture of the distal esophagus. Subcutaneous emphysema can be a complication of CO2 insufflation with laparoscopic surgery. A sudden rise in end-tidal CO2 following the initial rise that occurs with insufflation (first 15-30 min) should raise suspicion of subcutaneous emphysema. Of note, there are no changes in the pulse oximetry or airway pressure in subcutaneous emphysema, unlike in endobronchial intubation, capnothorax, pneumothorax, or CO2 embolism. Treatment
Subcutaneous emphysema is usually benign.