Subcutaneous emphysema is primarily caused by:

Subcutaneous emphysema occurs when air gets trapped beneath the skin, leading to swelling and a characteristic crackling sensation when touched. The primary cause of this condition is gas leakage from the mediastinum, which can occur due to various reasons, such as trauma, rupture of airways, or barotrauma as seen in diving accidents. When air leaks into the mediastinum, it can then migrate through the fascial planes and present as subcutaneous emphysema.

In contrast, high altitude exposure mainly leads to conditions such as altitude sickness, but it does not typically cause air to accumulate under the skin. Increased fluid in tissues would generally refer to edema, which is a different pathophysiological process and not directly linked to subcutaneous emphysema. Lung infections, while they can cause complications such as abscess formation, would not primarily lead to the same effects seen with gas leakage from the mediastinum. Thus, gas leakage from the mediastinum is the most direct and relevant cause of subcutaneous emphysema.