Matthew D. Winter Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, FL, USA The peritoneum is a serous membrane that is primarily constructed of connective tissue and is broadly divided into the parietal and visceral peritoneum. The parietal peritoneum lines the abdominal cavity, pelvic cavity, and scrotum. The visceral peritoneum covers all or portions of the organs in the abdominal, pelvic, and scrotal cavities. Embryologically, organs descend ventrally into, or form within, the peritoneum, resulting in the creation of folds connecting peritoneum that are commonly referred to as the mesentery, omenta, or ligaments of these various organs. These resultant folds, spaces or cavities are also often the site of fat deposition [1]. The peritoneal space, then, is simply the potential space bounded by the peritoneal membranes that surround the abdominal organs. The cavities described above created by this peritoneal investment are essentially closed cavities, with no organs actually existing within the peritoneal space itself. The exception is an opening of the cranial/abdominal margin of the uterine tube, which in theory connects the peritoneal cavity through the female genital tract to the outside world. Therefore, during ovulation, an ovum may be considered to be, briefly, peritoneal in location [1]. Otherwise, the peritoneal space contains only a small amount of lubricating fluid to reduce friction between adjacent organs as well as between organs and the abdominal body wall. This small amount of fluid is typically not visible when imaging normal patients. The testes are covered with an investment of peritoneum, termed the vaginal process. The complex descension of the testes into the scrotum results in the creation of the cavity of the vaginal process. This cavity is continuous with the peritoneal space at the level of the vaginal ring. Similarly, the ovaries are covered with an investment of peritoneum, and are suspended in the peritoneal space via the mesovarium [1]. Organs that lie against the walls of the abdominal or pelvic cavities and have peritoneum covering only a single surface are considered retroperitoneal. The dorsal surface of the kidneys lacks a peritoneal covering, and may be separated from the sublumbar musculature by fat. Ventrally and cranially, the kidneys have a peritoneal covering. The ureters course caudally from the renal pelvis, and are also retroperitoneal, bounded dorsally by the psoas musculature and ventrally by the peritoneum, before diving ventrally through the lateral ligament of the bladder (and the broad ligament in females) and entering the urinary bladder. Finally, the adrenal glands are also considered retroperitoneal, contacting the retroperitoneal surfaces of the kidneys, renal hilus, and abdominal aorta. The right adrenal gland may be continuous with the adventitial lining of the caudal vena cava in dogs [1]. In a normal patient, serosal margin detail, or the ability to define the margins of abdominal organs on the radiograph, is dependent on the amount of fat in the cavities created by the peritoneal folds, such as the omentum and mesentery. This fat becomes interspersed between abdominal organs, providing contrast between adjacent soft tissue structures. On initial radiographic evaluation, serosal margin detail should be assessed. Specifically, evaluate the ability to visualize the serosal margins of the abdominal organs, and the amount of fat in the abdomen. Each patient should be treated individually, as there is significant variation in the amount of intraabdominal fat associated with body condition. Also, larger dogs produce more scatter radiation, reducing contrast resolution. In addition, brown fat in young patients does not provide the same degree of contrast as the yellow fat in more mature patients. In mature patients with moderate peritoneal and retroperitoneal fat, the margins of abdominal organs should be easy to delineate (Figure 22.1). Thin patients will have poor serosal margin detail (Figure 22.2). Detecting free peritoneal fluid in these patients can be challenging. Be sure to also consider the contour of the abdominal body wall in these patients. If an emaciated patient with poor abdominal serosal detail has a rounded, distended abdominal body wall, consider the possibility of free peritoneal fluid. Radiographically, the presence of fluid in the peritoneal space results in decreased serosal margin detail. The fluid in the peritoneal space will gradually border efface with the margins of abdominal organs, making them hard or impossible to see. With mild fluid, the margins of organs may look blurry or smudged rather than sharp and well defined. Progressively, the margins of organs in the vicinity of larger volumes of fluid become obliterated entirely (Figure 22.3). Fat in the region of mild peritoneal fluid may look heterogeneous, with soft tissue streaks interspersed with fat in the mesentery, omentum, or retroperitoneal space. Occasionally, this is referred to as fat stranding. Loss of detail can be focal or generalized, depending on the cause. Regional inflammation, such as that caused by pancreatitis, can be localized to the right cranial abdominal quadrant. The presence of large volumes of fluid, such as that seen with hypoproteinemia, severe uroabdomen, or severe hemorrhage, often creates generalized loss of serosal margin detail. If localized, often this may provide direction to a particular organ or organ system as the source of pathology. Normal body cavity fluid, including peritoneal fluid, has low cell and protein concentrations, typically less than 3000 cells/μL and less than 2.3 g/dL respectively [2]. Effusions typically occur due to increases in the amount of fluid entering the cavity, or from decreased removal. The nature of the fluid varies with etiology. Peritoneal effusion may retain the characteristics of normal peritoneal fluid. Alternatively, it may have increased total nucleated cell count (TNCC), increased total protein, and may contain atypical cells. Identifying the type of fluid may help with differentials. Traditionally, effusions have been classified as transudates or exudates based on alterations in cell count and protein, but parameters are variable. Transudates have been subdivided into pure transudates (1000–1500 cells/μL; <2.5 mg/dL protein) and modified transudates (TNCC 1000–5000 or 7000 cells/μL; >2.5 mg/dL protein). Exudates have been variably described as having a total protein >2.0, 2.5, or 3 g/dL and TNCC greater than 3000, 5000, or 7000. A simplified classification scheme has been proposed, with a cutoff between of 3000 cells/μL considered useful in the discrimination of transudates and exudates. Further, dividing transudates into low‐ and high‐protein categories using a threshold of 2.5 g/dL was deemed more useful in determining differential diagnoses [2]. Radiography is insensitive to the underlying cause of effusions. The presence of an increased amount of fluid in the peritoneal and/or retroperitoneal spaces results in reduced serosal margin detail due to border effacement (Figure 22.4). Recall that fluid and soft tissue have the same opacity, and therefore the margins of structures in contact with the fluid will not be visible. The type or classification of the effusion cannot be distinguished radiographically. Still, generalized differentials for effusion identified radiographically are limited, and would include hemoabdomen, uroabdomen, septic abdomen, neoplastic/malignant effusion, or bile peritonitis. In some cases, localizing reduced serosal margin detail to a particular region of the abdomen may assist in prioritizing differentials. For example, focally reduced serosal margin detail in the right cranial abdomen may implicate the pancreas, gall bladder, right liver, stomach, or duodenum as potentially abnormal. When synthesized with patient information and other clinicopathologic data, differentials may be reprioritized. FIGURE 22.1 Normal serosal margin detail. Right lateral (A), left lateral (B), and ventrodorsal (C) projections of normal serosal margin detail in a large‐breed dog with good to increased body condition. Observe the normal margins of the liver, spleen, and intestinal loops. The left kidney is well visualized on the left lateral; the cranial pole of the right kidney is border effaced with the caudate process of the liver. On the ventrodorsal projection, it is more difficult to delineate specific organs, especially centrally, due to superimposition of more structures and thicker patient dimension. Left lateral projection (D) of a cat that has average body condition with normal serosal margin detail. Note the exquisite detail seen in cats, where most organs are surrounded by fat and easily defined. FIGURE 22.2 Normal serosal margin detail in a thin patient. Right lateral (A) and ventrodorsal (B) images of a young dog with poor body condition. Note the protrusion of the spinous process resulting in an undulating cutaneous margin (open arrowheads), the lack of subcutaneous fat, and the absence of falciform and retroperitoneal fat. The ventral margin of the abdomen is not rounded, but is tucked and tapers to the pelvic canal (white arrow). As a result of poor body condition, it is difficult to delineate margins of intestinal segments (white arrowhead), or any other abdominal organs.
CHAPTER 22
The Peritoneal and Retroperitoneal Space
Normal Anatomy
Imaging Findings
Serosal Margin Detail in the Normal Patient
Causes of Altered Serosal Margin Detail
Effusion
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
