Peritoneal encapsulation (PE) is a general term used to describe three different entities: congenital peritoneal encapsulation (CPE), abdominal cocoon (AC), and sclerosing encapsulating peritonitis (SEP) .
Cleland was the first to describe CPE in 1868 as a rare congenital malformation resulting from the development of an accessory peritoneal membrane partially or completely encasing the small intestine and creating an accessory sac. To the best of our knowledge, less than 50 cases have been reported in the literature [1, 4].
Despite being poorly understood, few theories were suggested to explain the pathogenesis of CPE, and the most accepted one was postulated by Papez in 1932. Papez explained complex developmental pathogenesis, where adhesions develop between the linings of the physiological umbilical hernia and inferior duodenum which exerts a traction force over the sac harboring the mid-gut within the umbilical cord. Further adhesions develop between the peritoneal layers lining the sac and by the 12th week of gestation, when the hernia is reduced, an extraperitoneal accessory sac encasing the small bowel is formed .
CPE is usually asymptomatic, and the preoperative diagnosis is challenging as it is rarely complicated by intestinal obstruction. If small bowel obstruction is evident, plain radiographs probably will be able to detect the dilated small bowel loops or multiple air-fluid levels; however, CT may be able to detect the membranous capsule and the sac, as well as, any underlying complications . Mimics such as paraduodenal hernias can be excluded by their specific CT appearance. A left paraduodenal hernia usually appears as a cluster of small bowel loops in the left anterior pararenal space and posterior to the descending colon, inferior mesenteric vein, and ascending left colic artery, in contrast to peritoneal encapsulation which is anterior to these structures (Fig. 5). A right paraduodenal hernia has a typical appearance posterior to the superior mesenteric vessels .
The case presented shows that CT was able to detect a thin membrane encasing clustered and obstructed small bowel loops (Fig. 1). Also, its anterior relation to the stretched mesenteric vessels (Figs. 4 and 5), as well as the presence of the ascending left colic artery posterior to the sac (Fig. 5), precluded the diagnosis of internal paraduodenal hernias  and suggested an alternative diagnosis of accessory peritoneal sac encapsulating the small bowel.
The two other peritoneal encapsulation conditions that should be differentiated from CPE are abdominal cocoon (AC) and sclerosing encapsulating peritonitis (SEP). Both conditions are acquired but with different pathogenesis and etiology .
AC was first described by Foo et al. in 1978 . The etiology still poorly understood, and several theories were offered to explain the pathology. There is a general agreement that a status of subclinical peritonitis precedes AC. The underlying etiology includes retrograde menstruation with secondary viral infection, retrograde peritonitis, and various gynecological infections causing cell-mediated tissue damage [1, 7]. However, all theories failed to explain the fact that AC is twice more prevalent in males . AC is an idiopathic condition and characterized by the formation of a thick fibrotic membrane that encases the bowel, forming a sac or cocoon along with some internal adhesions. The formed sac has an appearance similar to scar tissue, and histologically, it is characterized by the formation of a thick fibrotic membrane. The preoperative diagnosis may be achieved by identifying the thick fibrous membrane surrounding the bowel loops, a condition that is best visualized on a CT scan, which gives more accurate information on the degree of obstruction and the types of bowel loops involved. However, sometimes the surrounding membrane may be very thin and difficult to identify on a CT scan, making preoperative diagnosis difficult .
SEP—also known as encapsulating peritoneal sclerosis—is an acquired secondary condition to underlying disease process and by far more common compared to the two other entities . It was first described in 1907 by Owtschinnikow . The pathogenesis is similar to AC and characterized by the formation of a thick, fibrotic membrane secondary to various inflammatory abdominal conditions. Histologically, the formed fibro-connective membrane demonstrates inflammatory cellular infiltrates and dilated lymphatics; this is a very important differentiating point as compared to the normal peritoneal mesothelial lining of CPE . The most common known etiology is peritoneal dialysis, and it has been estimated that 20% of peritoneal dialysis patients will develop SEP at 8 years after starting dialysis . Other causes such as peritoneal shunts, tuberculosis, malignancy, and systemic illness had also been reported such as systemic lupus erythematosus, sarcoidosis, and Mediterranean fever [1, 9]. CT features are variable, but usually a thick, enhancing membrane encapsulating bowel loops with or without peritoneal calcifications is identified. The formed sac is usually separated from the peritoneum but may have significant adhesions to the peritoneum and surrounding structures. Other imaging features such as peritoneal thickening, fluid loculations, and tethering of small bowel loops were considered pathognomonic for SEP .