Introduction

The abdomen extends from the diaphragm to the pelvis. The extra abdominal structures consist of the body wall, the caudal thorax, the included portion of the vertebral column, and included portions of the pelvis and pelvic limbs. These structures are important to evaluate for alterations in body condition, body wall compromise, for osseous lesions that may reflect degenerative or aggressive diseases, and soft tissue or fat opaque masses. Evaluation of the osseous structures is covered in the musculoskeletal section. Evaluation of the diaphragm is covered in the thoracic section. The reader is referred to these respective sections for review of these components of the abdominal body wall assessment.

Body Wall Anatomy

The cranial abdominal cavity is separated from the thoracic cavity by the diaphragm. The abdominal wall consists of the different abdominal muscles, fascia, and fat. The lateral and ventral margins of the abdominal body wall consist of the muscle, fascia, fat, and cutaneous tissues. Specifically, the musculature of the abdominal body wall includes the external abdominal oblique muscles, the internal abdominal oblique muscles, the rectus abdominis muscles, the transverse abdominal muscles, and the cutaneous trunci. These are also connected via various fascia and aponeuroses [1]. Fat is interspersed between these layers, within the fascial planes.

The cranial margin of the abdomen is the diaphragm, and the caudal margin is continuous with the pelvic canal at the pelvic inlet. The pelvic canal is bounded caudally by the pelvic diaphragm which consists of the coccygeus and levator ani muscles. Dorsally, the abdominal body wall includes the osseous components of the vertebral column and the paraspinal musculature.

Lymph nodes are also present in the inguinal region, and should be evaluated on imaging studies. The superficial inguinal lymph nodes are located in the furrow between the ventrolateral abdominal body wall and the muscles of the thigh. These lymph nodes drain the caudoventral abdominal body wall, including the caudal mammary glands in the female, and the prepuce and scrotum in the male [1]. These can occasionally be seen in the inguinal fat of feline and canine patients, even in normalcy, and may indeed be visible when enlarged.

Ventrally in the female, mammae and mammillae are present, and can enlarge during various stages of estrus and parturition in the intact dog and cat (Figure 21.1). These can also be the source of mass lesions, including neoplasia, infection, and abscessation.

Hernias

Congenital hernias of the diaphragm are covered in the thoracic section. The most common congenital hernia of the abdominal wall is an umbilical hernia, where a small amount of fat/omentum is herniated into an opening at the umbilicus and extends into the subcutaneous tissues (Figure 21.2). A larger hernia at the umbilicus is called an omphalocele. In this congenital anomaly, there is intestinal tract or other abdominal organ herniation outside the abdominal wall at the umbilicus. Traumatic ruptures of the abdominal wall occur commonly post trauma with varying degrees of abdominal organ herniation (Figure 21.3). Herniation of organs into the inguinal canals can occur post trauma as well.

One of the features to evaluate radiographically is the presence of subcutaneous gas. This can be post trauma, postoperative surgical, secondary to abscess formation or fluid administration. The location and extent of the subcutaneous emphysema would be seen as a reflection of the location and degree of trauma. Ultrasound and CT can be useful to identify and characterize any herniated abdominal contents (Figure 21.4) and to better define abdominal body wall defects (Figure 21.5).

In intact male dogs, caudal perineal hernias can occur. Herniation of the urinary bladder, prostate gland or other intestinal segments can occur through the pelvic canal and into a perineal hernia due to the weakening of the perineal muscles by the testosterone levels in the male dog.

Masses

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