PCI, also known as intramural gas, refers to gas within the bowel wall. Other names include pseudopneumatosis, intestinal emphysema, and bullous emphysema of intestines . In our discussion, PCI and intramural air will be used interchangeably. The condition was first documented by Du Vernoi in 1783 [1, 2]. It is not a common finding with little research on this topic, although in recent years there have been more case reports.
Based on current studies, there is no definite etiology of PCI. PCI appears to be associated with mechanical, bacterial, and biochemical processes. Some papers suggest an association of PCI with immunosuppressed patients [1, 3].
The mechanical theory proposes that increased intra-luminal pressure forces gas within the bowel lumen to breach the mucosal or serosal layers [1, 4, 5]. Once inside the mucosal layer, the gas travels along the mesenteric blood vessels. The gas may continue along the mesentery to distal sites along the entire bowel, which then accumulates to form cysts. Often, gas in the bowel lumen is produced by gastrointestinal pathologies as a result of ischaemia or inflammation. In a study of 50 patients with Crohn’s disease, 6 patients were found to have pneumatosis intestinalis while there were none in the control group (i.e. patients without Crohn’s disease) . However, the mechanical theory cannot explain how cysts persist after formation .
The pulmonary theory refers to increased intra-luminal pressure due to the respiratory system [2, 4, 6]. In patients with chronic obstructive pulmonary disease, constant coughing causes the alveoli to rupture. Gas from the alveoli dissects along the aorta within the mediastinum and through the diaphragm to the mesenteric blood vessels. Gas in the mesenteric blood vessels breaches the bowel wall and subsequently gets trapped in the bowel wall forming cysts. However, this theory fails to explain the presence of high levels of hydrogen within the intramural cysts .
In the bacterial theory, gas is introduced into the bowel lumen by pathogens. The gas again breaches the bowel wall integrity and seeps into the mucosal or serosal layer [1, 4, 5]. However, studies have shown that the gas in the cysts is sterile. If the cysts were to rupture, it causes pneumoperitoneum, and the extra-luminal gas is not known to cause peritonism. Hence, the bacterial theory may not explain the etiology of PCI.
Finally, the biochemical theory describes the production of hydrogen gas during metabolism of food groups, especially carbohydrate. The increased amount of gaseous by-product raises intra-luminal pressure and the gas is forced into the weakened bowel wall. Trapped extra-luminal gas accumulates to form cysts.
Several case reports have postulated a relationship of PCI with patients who are immunosuppressed . PCI has been found in patients on corticosteroids such as those with Crohn’s disease and ulcerative colitis [1–3, 8]. Others have found a relationship between PCI and those who have undergone chemotherapy . No explanation is available so far on how corticosteroids predispose to PCI.
Many hypotheses have been discussed in the literatures, while none can fully explain the etiology of PCI. At present, the condition is linked to several pathologies as described above. However, it must be stressed that PCI is not a disease but a physiological reaction to several conditions; hence, there is no single etiology [4, 9].
PCI is usually a benign, asymptomatic, and occult condition. Review of the existing literature shows that patients who are symptomatic present in a diverse manner as a result of the underlying cause of PCI or complications from PCI. Due to the vague symptoms, the condition is easily misdiagnosed as bowel ischaemia or infarction . In this case, what appeared to be pneumoperitoneum has been misdiagnosed as a perforated viscus. He was found to have small bowel obstruction caused by adhesion bands, and PCI was discovered incidentally during operation. Symptoms vary enormously amongst patients and are often vague [2, 6]. The majority of patients have abdominal pain, abdominal distension, nausea and vomiting, and a change in bowel habit [1, 4]. A small number of patients complain of excessive flatulence, tenemus, loss of appetite, and weight loss. These non-specific symptoms can easily lead to a misdiagnosis of irritable bowel syndrome. Physical examination can be completely normal or reveal mild generalised abdominal distension and tenderness. PCI can lead to complications like small or large bowel obstruction, volvulus, intussusception, and intra-abdominal haemorrhage. Generally, this benign condition is diagnosed incidentally during operation.
As illustrated in our case, PCI can easily mimic pneumoperitoneum on radiological imaging. Intramural air may appear as radiolucent shadows along the bowel lumen on X-ray . Extensive PCI or ruptured cysts may appear like free air under the diaphragm in an erect chest X-ray. CT being the more sensitive imaging may show circumferential gas collections outside the bowel lumen  or reveal life-threatening events related to PCI like bowel ischaemia . Two features of PCI have been described in the literature—cystic circumferential gas is associated with good prognosis, and linear gas is associated with poor prognosis [2, 11]. CT can also identify co-existing conditions like colitis, perforation, obstruction, and neoplasms. Although CT is deemed to be the most sensitive imaging modality, PCI can give the same appearance as intra-faecal gas, gas in pseudopolyps, and extra-luminal gas . Ultrasound shows high amplitude gas echoes with acoustic shadowing. Barium study shows protrusion of submucosa causing filling defects in the intestinal lumen. Endoscopically, PCI appears as submucosal cysts with pale or bluish tint. When biopsied, PCI may rapidly deflate with an audible hiss .
Due to diverse interpretation, imaging should not be used for definitive diagnosis. Up to 27 % of benign PCI was misdiagnosed as surgical abdomen resulting in unnecessary operation . Intra-operative finding of PCI is the most sensitive method of diagnosis.
Typical macroscopic appearance
Macroscopically, PCI is characterised by submucosal cysts varying from a few millimetres to centimetres in diameter [8, 10]. They may appear as a singular cyst or a branching pattern. Furthermore, they protrude into the bowel lumen. The cystic lesions have an outer layer with a bluish tint. Subserosal cysts are generally formed adjacent to the mesenteric blood vessels.
Typical microscopic appearance
PCI are true pseudocysts with no epithelial lining. Histiocytes and multinucleated giant cells line the submucosal or subserosal spaces and rarely the muscularis layer . These cells are commonly found in inflammatory conditions such as Crohn’s disease and colitis. The distinguishing feature for PCI is that the giant cells usually line a rounded or cleft-like space. Unlike inflammatory processes, a pathogen is usually not found in PCI. Another distinctive feature is the presence of round empty space in the submucosa resembling fat. Non-specific findings are inflammation, eosinophilia, gland disarray, vascular ectasia, and edema [10, 12].
An incidental finding of PCI is usually benign and does not require treatment. The important decision is recognising when to perform an emergency laparotomy and when to manage conservatively. Patients who do not require surgery can be managed with medical therapies like high flow oxygen, hyperbaric therapy, antibiotics and special elemental diets . Surgery is generally reserved for management of primary etiology, such as bowel obstruction and bowel ischaemia. Surgery may be elected in cases where medical therapies have failed. Laparotomy should be avoided in primary PCI as an unnecessary operation can lead to a fatal outcome [2, 10]. Uncomplicated PCI can be safely managed conservatively .
PCI is an uncommon and poorly understood condition despite being first described in 1738. Identifying the underlying pathophysiology of PCI could potentially avoid misdiagnosis and mismanagement. The majority of the published literatures are case reports. To date, there has been no randomised controlled trial and very few literature reviews. Wu and his colleagues published a systemic analysis in 2013. They also found no studies on epidemiology or randomised control trials on PCI. They performed a systematic review of 239 PCI cases and found that it is a condition more common in males than females and more frequent in large bowel than small bowel . Further studies are justified for every aspect of PCI from epidemiology, etiology, presentations, diagnosis, and treatments to prognosis.