8 Peritoneal Cavity

Definition

●Capillary space between the parietal and visceral peritoneum

Synonyms

●Peritoneal cavity

●Greater peritoneal cavity

●Intraperitoneal space

●Abdominal cavity

●Intra-abdominal space

The abdominal organs (viscera) are invested by a thin serous membrane, the so-called visceral peritoneum. Its continuation lining the ab-

dominal wall is known as the parietal peritoneum. The peritoneum continuously transudes serous fluid, thus enabling the organs lined by the visceral peritoneum to glide over each other. Since the peritoneum constantly reabsorbs this peritoneal fluid, imaging modalities cannot demonstrate any significant amount of fluid. Thus, the peritoneal cavity usually exists as a “peritoneal gap” and it is this space that can be visualized sonographically only in pathological conditions. A true space or cavity exists only if fluid or another substrate collects in the gap between the visceral and parietal peritoneum (intraperitoneal position).

Ultrasound Morphology

Ultrasonography of the Peritoneal

Cavity

●Sonographic accessibility

●Preferred localization for fluid and/or gas collections

●Classification as to intraperitoneal, extraperitoneal, and retroperitoneal location

●Retroperitoneal space

The peritoneum itself cannot be demonstrated by ultrasound since it does not have a large enough impedance gradient. However, in ultrasound studies of the viscera it seems to be an integral part of their boundary layers, e. g., the hepatic surfaces, the boundary layer between the liver and the right kidney, the splenic surfaces, and the inner lining of the abdominal wall, and therefore appears as a hyperechoic band investing the capsule of the organ. When the visceral relations change (e. g., between liver and right kidney) the “gliding” motion of the peritoneum becomes quite evident (Fig. 8.5).

Visualization of the peritoneal space implies the presence of pathology, but there is always an exception to the rule: in menstruating young women a scant amount of free fluid can be demonstrated in the retrouterine (Douglas) pouch. The same is true in patients after abdominal surgery.

Fluid collections. Fluid collections are seen as anechoic/hypoechoic structures between the abdominal viscera. Owing to gravity, fluid will always collect in the dependent areas, i. e., in the posterior parts of the body while recumbent and in the lower abdomen while upright (Fig. 8.6).

Preferred localizations for fluid (anechoic/ hypoechoic) collections are ( 8.1a–f):

●Perihepatic right

●Perihepatic left

●Subhepatic right (between the right lobe of the liver and the right kidney = hepatorenal recess, Morrison’s pouch)

●Subphrenic

●Perisplenic

●Omental bursa

●Retrouterine pouch, perivesical

●Between the bowel loops (characteristic for ascites)

●Pericolic (“paracolic gutter”)

●Below the abdominal wall (knee–elbow position)

Gas collections. The accumulation of free in- tra-abdominal gas may result in the formation of hyperechoic structures. Physics dictates that gas will rise, and therefore it will be found in locations different from those for liquid, i. e., in the more superior regions such as the subphrenic recess (Fig. 8.7; see 8.1 g–i).

Knowledge of these preferred localizations helps to identify substrates and recesses: when the patient is repositioned, liquid will gravitate to the more dependent parts of the abdominal cavity while gases will rise.

Extraperitoneal or intraperitoneal location.

Determination whether a formation is intraperitoneal, extraperitoneal, or retroperitoneal may be quite difficult. Visualization of the organ surfaces is the most decisive factor in this correlation; in the case of fluid collections in the gastrointestinal tract, precise correlation requires demonstration of the stomach or bowel wall.1 Differential diagnosis becomes especially problematic when the liquid or gas does not relocate upon patient repositioning. This repositioning does not produce any significant alterations in extraperitoneal and retroperitoneal formations.

Differentiating intraluminal air within the GI tract from hyperechoic intra-abdominal structures (gas collections) can become rather difficult ( 8.2a,b).

Retroperitoneal space. Here too this space as such may only be demonstrated by the organs it contains or by collections of foreign substrates such as fluid, gas, or solid tissue. The retroperitoneal space is delineated by correlating it with the retroperitoneal organs, e. g., the kidneys, large vessels, or pancreas (8.2c–h).2

a Perihepatic: anechoic film anterior and lateral to the right hepatic lobe. Ascites due to alcoholic cirrhosis. The amount of ascites may be estimated by the thickness of the film.

d Perisplenic: anechoic film around the spleen, particularly at the hilum. In the presence of a very small amount of fluid only hypoechoic organ surfaces will be visualized.

b Morrison’s pouch (hepatorenal recess, subhepatic recess): already there is a small amount of free fluid, seen as an anechoic gap between the liver and kidney. Malignant ascites in ovarian cancer.

e Retrouterine pouch: in the lesser pelvis even minute amounts of fluid can be demonstrated. In this longitudinal plane the uterus is bathed by the fluid.

c Subhepatic: free fluid below the liver in the right transverse plane; ascites (transudate) in right cardiac failure. The hyperechoic delineation anterior to the kidney corresponds with the peritoneum plus the renal capsule.

f Interenteric: in the presence of large amounts of free intra-abdominal fluid the bowel loops float in the fluid and weave around like underwater creatures (“sea anemone phenomenon”).

Fig. 8.8 Minute fluid collection between the bowel loops (several milliliters) resembling an obtuse angle.

Fig. 8.10 When “guesstimating” the amount of free fluid, the distance between the surface of the liver and the lateral abdominal wall is a good marker.

Fig. 8.9 “Sea anemone phenomenon”: in the case of large amounts of diffusely distributed free fluid the bowel loops will float unencumbered in the fluid.

Fig. 8.11 The amount of free fluid in the lesser pelvis (retrouterine pouch) may also be “guesstimated” when viewed in the longitudinal plane. Since this is a dependent region of the body, even minute amounts of fluid can be detected here.

Fig. 8.12 Ultrasound-guided fine-needle paracentesis in the differential diagnosis of ascites. The tip of the needle is visible as a small echogenic structure (arrow) anterior to the bowel loops. Ultrasound guidance will help avoid injuries to intra-abdominal structures.

Quite often the clinical data and the patient’s history, supplemented by ultrasound morphology and the pattern of distribution, are not enough to confirm the precise type of ascites. Ultrasound-guided fine-needle paracentesis is an efficient and proven method with few potential complications that can help ascertain the final diagnosis. For diagnostic purposes it uses needles of up to 3 Fr (1 mm diameter, fine-needle). Larger needles are employed in therapeutic procedures.

Ultrasound targeting. Ultrasound targeting helps to avoid injury to vascular structures on the inside of the abdominal wall (e. g., internal caput medusae, Fig. 8.43b) as well as dry aspiration caused by the tip of the needle adhering to tissue. Furthermore,

it permits atypical needle insertion sites in the left lower quadrant (inverse McBurney’s point). Although any site may be selected, the distance to the target should be as short as possible and also should avoid any obstacles, such as the bowel, omentum, etc.

Laboratory studies. If fluid can be aspirated, thus confirming the suspected ultrasound diagnosis of “free fluid,” it first undergoes gross examination; this will allow the ascites to be classified as transudate/exudate, blood, bile, pus, urine, or chyle. Then the liquid substrate is subjected to additional cytological, bacteriological, and other laboratory studies.

One aspiration is enough for the laboratory panel, but in the case of peritoneal carcino-

matosis cytology sometimes requires several aspirations and work-ups. The sensitivity depends directly on the number of aspirations. If a malignant fluid collection is suspected clinically and the cytology is negative, the fine-needle aspiration has to be repeated. For differential diagnosis, the following laboratory parameters are cardinal