Pneumatosis

• Pneumothorax, air or gas in the pleural space
• Pneumomediastinum, air or gas in the mediastinum
  • Also called mediastinal emphysema or pneumatosis/emphysema of the mediastinum

Low magnification micrograph of pneumatosis intestinalis in bowel wall.

• Pneumoperitoneum (or peritoneal emphysema), air or gas in the abdominal cavity. The most common cause is a perforated abdominal viscus, generally a perforated peptic ulcer, although any part of the bowel may perforate from a benign ulcer, tumor or abdominal trauma.
• Pneumatosis intestinalis, air or gas cysts in the bowel wall
• Gastric pneumatosis (or gastric emphysema) is air or gas cysts in the stomach wall[6]

Pneumarthrosis is the presence of air in a joint. Its presentation on radiography is a radiolucent cleft often called a vacuum phenomenon, or vacuum sign.[7] Pneumarthrosis is associated with osteoarthritis and spondylosis.[8]

Pneumarthrosis is a common normal finding in shoulders[7] as well as in sternoclavicular joints.[9] It is believed to be a cause of the sounds of joint cracking.[8] It is also a common normal post-operative finding at least after spinal surgery.[10] Pneumarthrosis is extremely rare in conjunction with fluid or pus in a joint, and its presence can therefore practically exclude infection.[8]

• 
  X-ray of a hip with hip replacement and pneumarthrosis, in this case aseptic.
• 
  A vacuum sign, or vacuum phenomenon, is a normal finding on shoulder X-rays.

CT scan of subcutaneous emphysema.

Subcutaneous emphysema is found in the deepest layer of the skin. Emphysematous cystitis is a condition of gas in the bladder wall. On occasion this may give rise to secondary subcutaneous emphysema which has a poor prognosis.[11]

Pneumoparotitis is the presence of air in the parotid gland caused by raised air pressure in the mouth often as a result of playing wind instruments. In rare cases air may escape from the gland and give rise to subcutaneous emphysema in the face, neck, or mediastinum.[12][13]