The Peritoneal Cavity, Retroperitoneum, and Abdominal Wall
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Early detection of peritoneal changes is essential for the management of several neoplastic and non-neoplastic conditions in veterinary patients. The peritoneal and retroperitoneal spaces are normally included in abdominal MDCT examination. Contrast-enhanced MDCT scans with near-isotropic or isotropic resolution are necessary for fine evaluation of the abdominal cavity in dogs and cats. Isotropic imaging and post-processing with dorsal and sagittal reformation can fully delineate the peritoneal and retroperitoneal spaces and extent of diseases.
The abdominal region is the large part of the body extending from the diaphragm to the pelvis. It includes the abdominal and pelvic cavities, bound by bone and muscle. The peritoneum is the largest and most complexly arranged serous membrane in the body. It forms the lining of the peritoneal cavity, which extends in abdominal cavity and contains the gastrointestinal tract; major abdominal organs, such as the liver, spleen, pancreas, urinary bladder, ovaries, and uterus; and many nerves, vessels, and lymph nodes. The peritoneum extends caudally in the pelvic cavity in males and females (as vaginal tunic and vaginal process, respectively). It is a serous membrane, divided into the parietal peritoneum, which covers the inner surface of the abdominal wall (the abdominal fascia) and the pelvic and scrotal cavities; the visceral peritoneum, which covers the abdominal, pelvic, and scrotal organs; and the connecting peritoneum, which extends between organs or between an organ and the parietal peritoneum, forming peritoneal folders that are divided into mesenteries, omenta, and ligaments. The purpose of the peritoneum is to provide a frictionless surface over which the viscera can move. It also serves as a site of fluid transport. A capillary film of serous fluid separates the parietal and visceral layers of the peritoneum from one another and lubricates the peritoneal surfaces. The retroperitoneal region extends dorsal to the peritoneal and pelvic cavities from the diaphragm to the pelvic inlet. It is bordered dorsally by the vertebrae and paraspinal muscles and laterally by the muscular abdominal and pelvic walls. The kidneys, ureters, and adrenal glands are considered to be retroperitoneal organs. The descending aorta, caudal vena cava, lumbar lymph nodes, cisterna chili, lymphatics, fat, and much of the abdominal wall musculature are also are also located in the retroperitoneum. The retroperitoneal cavity communicates cranially with the dorsocaudal mediastinum and caudally with the pelvic canal. Familiarity with these connections is important because diseases can spread easily among different body regions. MDCT studies performed with isotropic imaging and dorsal and sagittal reformation can fully delineate the peritoneal and retroperitoneal spaces and extent of diseases.
2 MDCT Imaging Strategies
The peritoneal and retroperitoneal spaces are normally included in abdominal MDCT examination. The scan range for such examination generally includes the diaphragmatic crura cranially and the pelvic outlet caudally, but not the perineal and scrotal regions. These regions must be included in examinations in cases of specific clinical indications (e.g., pelvic mass) or for comprehensive evaluation of the retroperitoneal space.
On MDCT, normal peritoneum appears as a fine, thin, and thus barely detectable structure. In humans, contrast-enhanced MDCT is the primary imaging modality for the assessment of peritoneal pathology. Early detection of peritoneal changes is essential for the management of several neoplastic and non-neoplastic conditions in veterinary patients. Contrast-enhanced MDCT scans with near-isotropic or isotropic resolution are necessary for fine evaluation of the abdominal cavity in dogs and cats. Most peritoneal and retroperitoneal pathological processes originate in the peritoneal and retroperitoneal organs. Therefore, a multiphasic approach (including the arterial phase and portal venous phase) provides adequate results in most instances. Performance of a third delayed series may be helpful for interpretation of pelvic pathological processes (e.g., prostatic, rectal, and vaginal diseases).
3 Disorders of the Peritoneum and Retroperitoneum
3.1 Inflammatory Peritoneal Disorders
3.2 Neoplastic Peritoneal Diseases
Mesotheliomas originate from the cells covering the serosal cavities (pleura, pericardium, peritoneum, and tunica vaginalis). They are very uncommon in animals, representing just 0.2% of all canine tumors. Mesotheliomas are usually malignant and may involve one or all cavities.
3.3 Retroperitoneal Diseases
3.4 Large Abdominal Masses
The veterinary literature contains no studies on large and huge abdominal masses or the interpretation of MDCT images of such masses. However, MDCT is often required to characterize these masses and establish eligibility criteria for resectability. Large abdominal masses may involve the abdominal wall and peritoneal or retroperitoneal cavity.
Cranially, the abdominal wall is composed of osseocartilaginous structures, including the sternum, ribs, and costal cartilages. Muscles and fascial layers compose the ventral and lateral abdominal walls. They support and protect the intraperitoneal content and extend to the retroperitoneal space. Masses of the abdominal wall may be subcutaneous or involve one or more deep structures.
Nonneoplastic and neoplastic conditions may affect the abdominal wall, and both types can require MDCT assessment. Nonneoplastic abdominal wall lesions include abscesses, phlegmonous lesions, and abdominal wall hernias (see below in this chapter). Tumor lesions include various benign (e.g., lipoma) and malignant (e.g., sarcoma) conditions.
3.5 Abdominal Hernias
Abdominal hernias are protrusions of intra-abdominal contents through defects in the diaphragm or abdominal wall. Abdominal hernia types in small animals include diaphragmatic (hiatal hernia [HH] , pleuroperitoneal, and peritoneopericardial), umbilical, and inguinal hernias. Internal abdominal hernias are reported rarely, but their occurrence is probably underestimated. Perineal hernias are encountered commonly in MDCT examinations performed to evaluate pelvic trauma or masses.
Diaphragmatic hernias include HH and pleuroperitoneal and peritoneopericardial hernias. HH is a congenital condition in which organs of the abdominal cavity herniate through the esophageal hiatus into the thoracic cavity (see the chapter entitled “The Mediastinum and Neck”). Pleuroperitoneal hernia is a defect in the dorsolateral diaphragm with the herniation of abdominal viscera into the thoracic cavity. Acquired pleuroperitoneal hernias are commonly observed after blunt trauma in dogs and cats. Such hernias associated with congenital conditions are rarely reported on because affected animals die shortly after birth.
- Llabrés-Díaz F. The retroperitoneum. Chapter 5. In: O’Brien R, Barr FJ, editors. BSAVA manual of canine and feline abdominal imaging. Gloucester: BSAVA Publications; 2009. p. 40–8.Google Scholar
- Ragetly GR, Bennett RA, Ragetly CA. Septic peritonitis: etiology, pathophysiology, and diagnosis. Compend Contin Educ Vet. 2011;33(10):E1-6. quiz E7.Google Scholar