CAUSES OF PATHOLOGIC ECTOPIC GAS:
• Infection: Tissue infection with gas-forming bacteria (or emphysematous infection) is a very important source of gas that can affect any organ in the abdomen and pelvis either as a diffuse pattern of gas in the wall or the parenchyma of the structure involved,
as a well circumscribed abscess secondary to any infectious process or as presence of gas in the portomesenteric axis (usually as a secondary finding to the main infectious process).
These are severe infections,
most commonly seen in diabetic patients.
The clinical course can be severe and life-threatening if not recognized and treated promptly.[1-3]
Imaging findings:
o Cases of emphysematous cholecystitis Fig. 1 ,
emphysematous cystitis Fig. 2 ,
emphysematous pyelonephritis Fig. 3 and emphysematous pyelitis Fig. 4 are shown.
o Abscess in the context of a perforated cholecystitis Fig. 1 .
o An infected aortic endoprosthesis is also presented.
Fig. 5
• Necrosis: Another source of pathologic gas is caused by infarction with liquefactive necrosis,
which is usually seen as pneumatosis of the bowel wall and in the portomesenteric axis in mesenteric ischemia,
obstruction of the bowel or necrotizing enterocolitis,
among others.
It may also be present when extensive necrosis is seen in a solid organ.
Necrotizing fasciitis is an inflammatory infection located in the deep fascia,
with secondary necrosis of the subcutaneous tissues.
Imaging findings:
o Mesenteric ischemia and ischemic colitis are the most common causes of pneumatosis of the bowel wall and can also present gas in the portomesenteric axis.
[4,5] Fig. 6 Fig. 7
o Liquefactive necrosis of the spleen is seen in a patient with embolization of the splenic artery as a treatment for a severe hemolytic anemia.
Fig. 8
o Necrotizing fasciitis shows gas in the subcutaneous tissue,
thickening of the affected fascia,
fluid collections around the deep fascia and extension of the edema into the intermuscular septa and the muscles.
[6] Fig. 9
• Perforation of hollow viscera: When a hollow viscera wall ruptures,
the contents of the lumen (including gas) is expelled into the peritoneum cavity.
Pneumoperitoneum it´s a sign that must alert the radiologist to look for a pathologic cause such as inflammatory disease,
peptic ulcer disease,
bowel obstruction,
bowel suture dehiscence,
etc.
Imaging findings:
Cases of perforated gastric ulcer Fig. 10 and perforated mesenteric ischemia are presented.
Fig. 6
• Fistulas: Defined as an abnormal connection between two epithelium-lined organs or vessels that normally do not connect.
When a hollow viscera is one of the implicated organs,
the normally contained gas in the lumen of the viscera is also seen in the other affected organ by the fistula,
for example the aorta,
the biliary tree,
the bladder,
etc.
Imaging findings: A bilio-enteric fistula with a secondary biliary ileus is presented as case of pneumobilia.
Fig. 11
• Others: There are several conditions responsible for abnormal presence of gas in the abdomen within different locations (porto-mesenteric axis,
bowel wall and peritoneum cavity,
that don´t imply the three already mentioned,
such as: barotrauma,
asthma,
chronic obstructive pulmonary disease,
etc.
Imaging findings:
Barotrauma:Pneumatosis and rupture of the stomach wall due to scuba diving,
with subsequent pneumoperitoneum.
Fig. 12
CAUSES OF NON-PATHOLOGIC ECTOPIC GAS:
• Iatrogenic: Atmospheric air introduced at recent instrumentation or surgery.
[8] Imaging findings:
o Pneumoperitoneum may be found in recently operated patients,
especially after laparoscopy,
without implying a pathologic intraabdominal process.
Fig. 13
o Post-surgical normal gas in an aortic endoprosthesis.
Fig. 14
o Gastric and bowel dilatation (e.g upper and lower endoscopic procedures,
enemas) may produce pneumatosis of the bowel wall;
o Catheterization or drainages may introduce gas in the bladder,
blood vessels,
solid organs,
etc; Fig. 15 Fig. 16
o Biliary-enteric surgical anastomosis or drainage,
produce iatrogenic pneumobilia.
Fig. 17
• Spontaneous:
o Pneumatosis coli cistoides: Consists of air-filled cysts of various sizes in the submucosa and subserosa of the colon.
It can be focal or diffuse and it is usually asymptomatic but it can cause pneumoperitoneum as well as intermittent or persistent abdominal pain.
Imaging findings: Cases of pneumatosis coli cistoides as an incidental finding and a perforated one are shown.
Fig. 18
o Cholelithiasis “Mercedes Benz sign”: "Mercedes Benz" sign is a triradiate collection of nitrogen gas in gallbladder stones.
Crevices are created by shrinkage of cholesterol crystals in the stone.
Radiolucent fissures usually widest centrally radiating like points of star.
It should not be confused with pathologic gas in the gallbladder,
like in the case of an emphysematous cholecystitis.
Fig. 19
o Fibrous degeneration of intervertebral disc: Within time,
the intervertebral disc degenerates and produces nitrous oxide wich gives a gas appearance in diagnostic imaging.
Imaging findings: A case of a vertebral fracture with ruptured intervertebral disc.
Nitrous oxide is expelled and has migrated to the spinal channel and other adjacent structures as the retroperitoneum.
This finding may lead to believe that there might be and abdominal underlying pathologic cause.
Fig. 20