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.
If a patient does not meet this criterion of diagnosis, they must have a family history of NF2, and present with a unilateral vestibular schwannoma and other associated tumors (cranial meningioma, cranial nerve schwannoma, spinal meningioma, spinal ependymomas, peripheral nerve tumor, spinal schwannoma, subcutaneous tumor, skin plaque). This being said, more than half of all patients diagnosed with NF2 do not have a family history of the condition. Although it has yet to be included into clinical classification, peripheral neuropathy, or damage to the peripheral nerves, which often causes weakness, numbness and pain in the hands and feet, may also lead to a diagnosis of NF2. In children, NF2 can present with similar symptoms, but generally causes "visual disturbances (cataracts, hamartomas), skin tumors, mononeuropathhy (facial paresis, drop foot), symptomatic spinal cord tumors, or non-vestibular intracranial tumors". Prenatal
Bilateral vestibular schwannomas are diagnostic of NF2. Postnatal
Ferner et al. give three sets of diagnostic criteria for NF2:

Bilateral vestibular schwannoma (VS) or family history of NF2 plus Unilateral VS or any two of: meningioma, glioma, neurofibroma, schwannoma, posterior subcapsular lenticular opacities
Unilateral VS plus any two of meningioma, glioma, neurofibroma, schwannoma, posterior subcapsular lenticular opacities
Two or more meningioma plus unilateral VS or any two of glioma, schwannoma and cataract.Another set of diagnostic criteria is the following:
Detection of bilateral acoustic neuroma by imaging-procedures
First degree relative with NF2 and the occurrence of neurofibroma, meningiomas, glioma, or Schwannoma
First degree relative with NF2 and the occurrence of juvenile posterior subcapsular cataract.The criteria have varied over time.