Factors leading to the development of PI include bacterial production of gas, intraluminal GI gas compositions, increased intraluminal pressure, pulmonary gas tracking through vessels communicating with the thorax, and mucosal disruption. PI has been linked to bacterial infections of the GI tract in humans including C difficile, Klebsiella, and Whipple disease. 14-18 In animal models, C difficile within the walls of rat intestine results in the appearance of pneumocysts, or discrete collections of submucosal gas, which are the hallmark feature of PI. 30 It is thought that direct invasion of bacteria into intramural spaces can cause PI in humans, although bacteria have yet to be directly isolated from the pneumocysts. Translocation of luminal gas into pneumocysts found in PI is theorized to be driven by differences in partial pressures. 31 The concentration of hydrogen within the intestinal lumen is high due to bacterial production. Hydrogen, diffusing along its partial pressure gradient between the lumen and blood, accumulates within the intestinal wall and causes the formation of pneumocysts. This phenomenon has been hypothesized to explain the tendency for pneumocysts to form around the mesenteric vasculature.
Gas from the lumen can also be forced into the intestinal wall during an abrupt increase in intra-abdominal pressure, such as that seen with forceful vomiting. The final possible origin of the gas is the lungs, as PI has been associated with lung disease. It was previously thought that gas from ruptured alveoli tracks along mediastinal vessels, below the diaphragm, and into the mesentery. 30 Newer theories argue that increased intra-abdominal pressure, typical of patients with obstructive lung disease and frequent coughing, is the driver of PI by the mechanism previously described. 32-34 Additionally, mucosal disruption leads to increased permeability and allows accumulation of gas within the intestinal walls. Mucosal abnormalities have been described in histopathologic studies of patients with PI and associated with conditions known to compromise mucosal integrity, such as immunodeficiencies, inflammatory bowel disease, and the receipt of cytotoxic chemotherapy. 10,12,19-23
Our patient likely had mucosal disruption due to his gastrostomy tube as well as increased intraluminal pressure from recurrent vomiting, contributing to translocation of otherwise normal intraluminal gas. The presence of portal venous gas, as seen in this case, has historically portended a worse prognosis, with 37% mortality in one series. 7,35,36 However, portal venous gas as well as pneumoperitoneum occur in benign etiologies of PI as well. It is thought that this occurs due to rupture of the submucosal pneumocysts through the wall opposite the intestinal lumen and thus does not result in a direct communication between the intestinal lumen and the peritoneal cavity. 12
PI is not a diagnosis but a manifestation of an underlying disease. As such, the treatment of PI is targeted toward the underlying condition. Of note, the pattern and extent of PI seen on imaging has not been shown to correlate with the severity of the underlying pathologic process. 35,37 Instead, assessment of the patient and their clinical trajectory should determine the appropriate treatment. The decision facing the clinician when PI is discovered is whether urgent surgery is indicated, as is the case in mesenteric ischemia, bowel necrosis, or intestinal perforation, conditions known to be associated with PI. Otherwise, there is no definitive treatment for PI. Bowel rest is almost universally pursued. There are reports of treating with supranormal levels of supplemental oxygen, maintaining arterial partial pressure of oxygen above 300 mm Hg, with a face mask and 8 L/min flow rate. 38,39 The proposed mechanisms of benefit include establishing a favorable diffusion gradient for intramural gas to exit the pneumocysts as well as creating an inhospitable, aerobic environment for hydrogenproducing anaerobic enteric bacteria. A prudent approach for most cases of PI is conservative management with bowel rest and supplemental oxygen unless there is a definitive indication for urgent surgical intervention, such as peritonitis, abdominal sepsis, or perforation. 40,41 Management recommendations suggest that up to 50% of cases can be successfully managed nonoperatively. 42