To understand fluid distributions and pathways for the spread of diseases, a good anatomic knowledge is required.

Anatomy

The peritoneum is a thin serous membranae made of mesothelium, a single layer of cuboidal cells. It is divided into the parietal layer which covers the anterior wall of the abdominal cavity, the retroperitoneum and the undersurface of the emidiaphragms; the visceral layer covers the abdominal viscera and forms mesenteries and omenta. Abdominal cavity is closed in males and open in females since the ostia of the oviducts provides a communication with the extraperitoneal space.

Ligaments consist of a double layer of peritoneum; they serve as support and connection between organs and contain arteries, veins and linfatic vessels.

Mesenteries are formed by a double layer of peritoneum that cover the walls of bowel loops reflecting at the root of the mesentery as a fixation on the peritoneal wall. They contain arteries, veins, linfatic vessels, nerves and fat tissue. Mesentery gives large mobility to the bowel loops and also fixation to the root in central abdome.

Omentum is a double layer of peritoneum that extends from the stomach and duodenal bulb to adjacent organs. The lesser omentum or lesser sac is made of two contiguous ligaments called the gastrohepatic and hepatoduo­denal ligaments, that connect the stomach to the liver.

Since ligaments and mesenteries contain arteries, veins and linfatic vessels, they represent of the main conduicts for the spread of diseases across the peritoneal cavity.

Peritoneal spaces and fluid dynamic

The intraperitoneal space is divided transversally into supramesocolic and inframesocolic space by the root of the transverse mesocolon, which is a peritoneal fold that attaches the transverse colon to the retro­peritoneum and contains the middle colic vessels (fig 1).

The right supramesocolic space is the most dependant site of the supramesocolic space, a frequent site of inflammatory or tumor implantation and is divided from the left by the falciform ligament.

The left supramesocolic space is divided from the left paracolic gutter by the phrenicocolic ligament,  representing an obstacle for the spread of diseases. This is one of the reasons why many pelvic infections are confinated to the right space.

The inframesocolic space includes the paracolic gutter, the root of mesentery and pelvic spaces.

Fluid collects at first in the pelvis, which is the most dependant region of the peritoneal cavity both in clino and orthostatic. The presence of fluid in the pelvis can suggest both a locoregional pathological process but also a distance disease. Infact, the Douglas sac is the most common localization for pelvic abscesses but also for metastatic implants. 

When fluid amount excedes, it goes up to the right paracolic gutter, to the Morrison’s space (right epato-renal space) as there are no fixations elements separating those two compartments. The right paracolic gutter is deeper than the left and lacks of boundaries, so that fluid can easily distributes. In patients with severe ascites, fluid spreads also in the mesenteries and in the left paracolic gutter.

Causes of ascites and differential diagnosis

Many pathological processes are caused or are associated with ascites. They are summarized in Table 1.

Imaging is helpful to localize a disease to a particular peritoneal space and formulate a differential diag­nosis on the basis of that location.

The position of a fluid collection can represent the main clue for its origin.

Subphrenic space: (fig 2)

- spreading from pleural effusion

- If present on the right, a prior liver surgery must be considered (bilomas). 

- If it is located on the left, consider spleen or pancreatic injuries. 

Omental bursa: (fig 3)

- primary pathological processes of the mesothelium

- stomach, duodenum, spleen, pancreas diseases

Mesentery:

fluid in the mesentery is common in inflammatory and vascular diseases of bowel loops, like in the small bowel obstructions. 

Pelvic space:(fig 4)

It is the most common site of fluid collection. Usually related to a pelvic organ. 

When searching for the main cause of ascites, is correct to analyze the following points:

• study in which compartment fluid is mainly located
• remember the main connections between compartments
• distinguish between peritoneal and extraperitoneal spaces
• search for the sorrounding organs, rather than the more distant
• evaluate the density and characteristics of fluid collection