Understanding disease spread in the abdomen requires a recognition of the differences between the peritoneum and extraperitoneum [1,2,3,4]. Peritoneal spread of disease has a distinct pattern dictated by its anatomy and flow pattern. The extraperitoneal space is defined as the space between the peritoneum and transversalis fascia [2, 5]. Posteriorly, the extraperitoneum is stratified by the anterior and posterior renal fascia into the pararenal space, perirenal space, and posterior pararenal space [2]. Disease spread within these spaces is dictated by fascial planes [6,7,8,9,10,11,12,13]. The subperitoneal space is an extension of the extraperitoneal space into the suspended abdominal viscera via their corresponding mesenteries [1,2,3,4,5, 14,15,16]. Subperitoneal spread of disease occurs along the avenues formed by the vessels connecting the intraperitoneal spaces and the mesenteries.

The peritoneal cavity is the potential space containing the suspended abdominal viscera and their mesenteries [2, 5, 16,17,18]. It is formed from the primitive coelom when the lateral mesoderm splits into the somatic and splanchnic layers [2, 18,19,20]. The somatic mesoderm covered by parietal peritoneum lines the coelomic wall. The splanchnic mesoderm lined by visceral peritoneum covers the suspended viscera and mesenteries (Fig. 1).

Illustrations of early abdominal embryological development demonstrate the layers of peritoneum and cavities forming over time from (a) through (c). Initially, the ventral recesses form in (a). Over time, the dorsal recesses and lesser sac form after organal rotation and mesentery elongation. A brown outline in a depicts the parietal layer and a light pink outline in a depicts the visceral layer. Blue shading reflects the greater sac and yellow shading show the lesser sac developing in (b) and (c). Images a–c reproduced with minor editing from “Meyers, M. A., Charnsangavej, C., Oliphant, M. (2011). Meyers' dynamic radiology of the abdomen: normal and pathologic anatomy. New York, NY: Springer” with permission from Springer

The transverse mesocolon divides the peritoneal cavity into the supramesocolic and inframesocolic portions [2, 19]. The supramesocolic recesses are divided ventrally by the falciform ligament. The right subphrenic space continues lateral to the liver and inferiorly to the subhepatic spaces (Figs. 2, 3). The subhepatic space continues posteromedial to the liver into the lesser sac via the epiploic foramen (Figs. 2, 3 and 4).

Schematic illustrating a coronal and b sagittal views depicting peritoneal anatomy and direction of flow in the peritoneal recesses. Anatomy includes the right subphrenic recess (1, pink), right subhepatic recess (2, orange), right paracolic gutter (3, black), left inframesocolic recess (4, blue), left paracolic gutter (5, light gray), right inframesocolic recess (8, dark gray), and left subphrenic recess (6, dark magenta). Ligaments depicted include the phrenicocolic ligament (black line) and falciform ligament (black dashed line). The foramen of Winslow (brown arrow) provides the only passage between the greater and lesser sac (7, yellow). Abbreviations include B-bladder, C-colon, D-duodenum, GS-greater sac, IVC-inferior vena cava, J-jejunum, L-liver, LS-lesser sac, P-pancreas, R-rectum, and S-stomach. Images a–b reproduced with minor editing from “Meyers, M. A., Charnsangavej, C., Oliphant, M. (2011). Meyers' dynamic radiology of the abdomen: normal and pathologic anatomy. New York, NY: Springer” with permission from Springer

Cases depicting the peritoneal spaces in patients while evaluating for a peritoneal dialysis leak. Axial CT section (a), coronal CT section (b), and sagittal CT section of the abdomen show contrast within peritoneal recesses. Important peritoneal anatomy includes the right subphrenic recess (1, pink), right subhepatic recess (2, orange), right paracolic gutter (3, white), left inframesocolic recess (4, blue), left paracolic gutter (5, light gray), and left subphrenic recess (6, dark magenta). Ligaments depicted include the phrenicocolic ligament (black dotted line), falciform ligament (black dashed line), and gastrosplenic ligament (black solid line). The foramen of Winslow (brown arrows) provides the only passage between the greater and lesser sac (7, yellow). Inferiorly, pelvic contrast opacifies the right and left medial (8 and 9 respectively) and lateral (10 and 11 respectively) inguinal recesses, as well as the supravesical recess (12) in (c) and (d). Abbreviations include L-liver, Sp-spleen, SB-small bowel, and St-stomach

Case continued depicting the peritoneal recesses in a patient evaluating for peritoneal dialysis leak in (a). Light blue arrows depict the direction of flow in the peritoneal recesses. A brown arrow approximates the course between the right subhepatic recess into the lesser sac via the foramen of Winslow. Coronal schematic (b) providing a correlate depicting flow in the peritoneal recesses. Image b reproduced from “Meyers, M. A., Charnsangavej, C., Oliphant, M. (2011). Meyers' dynamic radiology of the abdomen: normal and pathologic anatomy. New York, NY: Springer” with permission from Springer

The lesser sac is a peritoneal recess formed by the elongation and rotation of the dorsal mesogastrium [2, 5, 19] (Figs. 2,3). It lies posterior to the stomach and anterior to the transverse colon. The lesser sac is bordered by the stomach, duodenum, pancreas, spleen, as well as the gastrohepatic, hepatoduodenal, and gastrosplenic ligaments.

The gastrohepatic and gastrosplenic recesses subdivide the left subphrenic space [5] (Fig. 5). The splenorenal recess is the posterior left supramesocolic recess formed by the splenorenal ligament [5]. The left supramesocolic recesses are separated from the right supramesocolic recesses by the falciform ligament ventrally, lesser sac medially, and inframesocolic recesses laterally the phrenicocolic ligament (Figs. 2, 3, 4, 5 and 6).

Case illustrating the left subphrenic space in a 36-year-old patient with history of gastric bypass and recent repair of gastro-gastric fistula presenting with left upper quadrant pain. a Axial CT and b coronal CT sections showing extraluminal enteric contrast and gas (yellow arrows) along the margin of the suture site in the left gastrohepatic recess (subphrenic recess) extending further into the left gastrosplenic recess (subphrenic recess) marked by a pink arrow. Expected flow of contrast depicted by dashed light blue arrows with inferior extent limited by the phrenicocolic ligament (dotted line)

Case illustrating the right subphrenic recess and paracolic gutter in a 54-year-old patient with history of peptic ulcer disease complaining of coffee ground emesis and severe pain. Axial CT section at a celiac artery level and b superior mesenteric artery level, as well as c coronal CT demonstrate extraluminal contrast and gas arising from a site at the second portion of the duodenum (yellow arrow), compatible with a perforated duodenal ulcer. Extraluminal contrast also extends into Morison’s pouch (posterior right subhepatic recess) (blue star), right subphrenic recess (blue arrow in image a), and inferiorly into the right paracolic gutter (blue arrow in image b). Dashed light blue arrows depict the expected flow of pathology from the site of perforation with superior extent limited by the phrenicocolic ligament (dotted line)

The right supramesocolic recesses continue to the right inframesocolic recesses by the right paracolic gutter (recess) (Figs. 2,3,6) [2, 5]. This recess is in continuity with the recesses between the small intestine and its mesentery (Fig. 7). The right paracolic gutter continues inferiorly to the pelvic peritoneal recesses. Ventrally, these recesses are divided by the urachus (medial umbilical ligament), the obliterated umbilical arteries (median umbilical ligament, and inferior epigastric vessels (lateral umbilical ligament) into the left and right inguinal recess and supravesical recess [2] (Fig. 3). Dorsally, these recesses are divided in the male by the urinary bladder and rectum to the rectovesical recess and in the female by the rectum, uterus, and urinary bladder to the rectovesical recess (Douglas pouch, cul-de-sac) and the uterovesical recess. The pelvic recesses continue lateral to the paravesical recesses and to the right and left paracolic gutters. Prior literature further details the pelvic peritoneal recesses [2, 5].

Case illustrating the left paracolic gutter in an 83-year-old patient s/p radical cystectomy and ileal-ileal anastomoses with abdominal pain. a Axial and b coronal CT sections demonstrates a jejunal perforation with extraluminal contrast, fluid, and gas in the left paracolic gutter (solid blue arrows) in a patient status post radical cystectomy and ileal-ileal anastomosis who presented with abdominal pain. Extraluminal contrast communicates with the peritoneum in the pelvis with flow of contrast depicted by dashed light blue arrows

Gravity predominately determines flow within these recesses with contribution from changes in intra-abdominal pressure and gastrointestinal peristalsis [2, 17, 19,20,21] (Figs. 2, 3). Note that fluid flow is unidirectional from interloop fluid in the inframesocolic recesses (Figs. 2, 3, 8), sequestered in the left subphrenic and posterior supramesocolic recesses (Figs. 2, 3, 5), and in the lesser sac (Figs. 3, 9).

Case illustrating the right inframesocolic recess in a 44-year-old patient with history of appendiceal carcinamatosis, right hemicolectomy, and omentectomy with small bowel resection 1 day ago. a Axial and b coronal CT sections show extraluminal contrast (blue arrows) in the inframesocolic recess (interloop fluid). Findings are compatible with perforated small bowel. Expected flow of contrast depicted by dashed light blue arrows

Case illustrating the lesser sac in a 48-year-old male while cutting trees a large branch struck his left flank with bleeding from the splenic artery. a Axial CT section and b coronal CT section shows a hematoma in the lesser sac (thick olive arrows). Surgery revealed bleeding from a splenic artery branch. Hematoma courses along the superior (SR) and inferior recesses (IR). Ligaments bounding the lesser sac include the gastrocolic ligament (GCL), gastrosplenic ligament (GSL), transverse mesocolon (TMC), and foramen of Winslow. Incidental note of imaged gastrohepatic (GHL) and hepatoduodenal ligament (HDL)

The anatomy of the peritoneal recesses and their flow pattern provides a distinctive spread pattern for the peritoneal disease. Examples include tumor penetrating an organ’s peritoneal lining (transperitoneal spread) entering the peritoneal cavity and spread within the recesses to seed the parietal peritoneum and the visceral peritoneum. Fluid and gas from a perforated viscus may spread diffusely through interconnection of many of the peritoneal recesses (Fig. 6). Ligamentous boundaries may limit spread. For instance, the phrenicocolic ligament limits communication between the left inframesolic and supramesocolic recesses (Figs. 5 and 8). Gas or fluid in the lesser sac localizes disease from the surrounding organs (Figs. 9, 10). Interloop fluid localizes pathology to the small intestine or mesentery (Fig. 8). Pathology from the stomach can localize to the left subphrenic space (Fig. 5).

Case illustrating the lesser sac in a 65-year-old patient with severe abdominal pain. Axial CT section shows pneumoperitoneum predominantly in the lesser sac (yellow arrow). At surgery, the patient was found to have a proximal duodenal perforation