Ultrasonography of the Post-Foaling Mare
Rood and Riddle Equine Hospital, Lexington, KY, USA
Abdominal discomfort in the post-foaling mare may result from normal uterine contractions or be indicative of visceral compromise. The uterus and associated structures may be affected or non-reproductive viscera may be involved. Transabdominal ultrasonography is valuable because it is a non-invasive technique that allows assessment of peritoneal fluid quantity and quality, evaluation of visceral disposition, and estimation of visceral integrity. Transrectal ultrasonography, when appropriate, improves identification of structures distinguished by palpation per rectum.
A lower-frequency transducer (3.5 MHz) is preferred in larger horses as penetration of the ultrasound beam is enhanced, however smaller horses and foals can be adequately imaged with higher-frequency probes. Clipping of the abdomen and the use of ultrasound coupling gel may give the best image, but, in practical settings, wetting with alcohol and slicking down hair in the direction of growth to expel air is sufficient. The probe is then held perpendicular to the abdominal wall which is systematically covered in a dorsal–ventral direction and, when along the midline, in a cranial–caudal direction. The more cranial aspects of the abdominal cavity are located within the thoracic rib cage and imaging of this area uses a similar technique to that detailed for thoracic ultrasonography.
On the left side of the horse, the liver is adjacent to the diaphragm medially and lung fields ventrally between approximately the seventh and ninth intercostal spaces (ICS) . The liver is lateral to the spleen in this region, with the stomach visualized deep to this interface; however, the stomach may have variable contact with the body wall depending on degree of fill. The spleen is imaged over the majority of the left side of the abdominal cavity, extending medial to the body wall from the paralumbar fossa to the eighth ICS and, further cranially, medial to the liver as noted above. The spleen extends ventrally to the midline and may extend to the right ventral abdomen. Spleen size is variable. The left kidney lies medial to the spleen and is found from the sixteenth ICS through the paralumbar fossa. Gas-filled viscera may obstruct complete visualization of the left kidney.
On the right side of the abdominal cavity, the liver is from approximately the sixth to fifteenth ICS medially and from the lung to diaphragm ventrally . The right kidney is located dorsally and superficially in the same region, adjacent to the caudal-most aspects of the liver in the fourteenth to the seventeenth ICS region.
The intestinal tract is imaged via the caudal and ventral abdomen. Small intestine is variable in location, and may be visualized on the left or right side and is often seen on both. Ventral colon is adjacent to the ventral abdominal wall and identified by sacculations (haustra). Liver is not imaged from the ventral abdomen in the normal horse . The right dorsal colon can be visualized medial to the liver on the right side. The cecum is visualized in the right paralumbar fossa traversing caudally and ventrally before the tip trends cranially.
The bladder is located on the ventral midline of the caudal abdomen and visualization improves when it is urine filled. The non-pregnant uterus is not typically discernible with transabdominal ultrasound. Varying stages of pregnancy and uterine pathology can be noted depending on uterine size and disposition.
Portions of the intestinal tract may be visualized lateral to the spleen on the left side, with some appearing dorsal and lateral to the dorsal most aspect of the spleen. In the absence of content in the nephrosplenic space, this does not appear to cause problems.
Limitations of Transabdominal Ultrasonography
The limitations of transabdominal ultrasonography are a result of the size of the abdominal cavity relative to the penetrating ability of the ultrasound transducer, which depends on the nominal frequency.
Pathology, Abdominal Cavity
Ruptured Body Wall, Rectus Abdominis, and Prepubic Tendon
Ruptured body wall, ruptured rectus abdominis, and ruptured prepubic tendon cause abrupt changes in shape of the body wall. Subcutaneous edema, fluid, hemorrhage, and muscle fiber disruption are visible with ultrasonography (Figures 20.1, 20.2). Diagnosis of the specific tissue involved can be difficult . Prepubic tendon rupture results in ventral abdominal wall deviation, regional edema, and pain . Herniation of the abdominal wall has the potential to incarcerate gut .
Diaphragmatic hernia is a rare parturient complication  and prognosis is considered guarded. One review involving various horse signalments reported a 23% survival rate in diagnosed cases and 46% survival rate following surgical correction . Viscera reported involved include small intestine, large intestine, stomach, spleen, and liver [4,5]. With ultrasonography, strangulation of gut may be visible as thickened intestinal wall and increased peritoneal fluid. Intrathoracic intestine may be noted cranial to the diaphragm shadowing the lung due to gaseous content, and pleural fluid may be increased (Figure 20.3). An intraluminal air–fluid interface may be present .
Retroperitoneal Hemorrhage and Pelvicitis
With retroperitoneal hemorrhage into the pelvic cavity and pelvicitis, transrectal ultrasonography may detect fluid or fibrin caudal to the peritoneal reflection. Caudal vaginal or rectal trauma during parturition may be sufficient to cause hemorrhage, tissue necrosis, sepsis, or perforation. This may not be reflected in grossly observable changes in the adjacent peritoneal cavity.
Pathology, Peritoneal Cavity
Changes in the peritoneal fluid reflect the altered integrity of the viscera within the peritoneal cavity. These include ultrasonographically detectable changes in peritoneal fluid volume, echogenicity, and, sometimes, disposition. Increase in value of two or more fluid variables, including total protein, nucleated cell count, and percentage neutrophils, is considered significant .
Periparturient hemorrhage (Figures 20.4, 20.5, 20.6) can develop in mares of any age or parity, however, it is more common in older mares [8,9,10]. It was the second most common reason for emergency presentation of post-partum mares in one review . Ultrasonographic appearance will vary with rate of bleeding, volume of hemorrhage, and duration of time since cessation of bleeding. Hemoperitoneum has a characteristic ultrasonographic appearance in the initial stages, being cellular and swirling (smoke-like) in response to diaphragmatic and visceral movements. Within a few days, this progresses to clot formation ventrally with relatively anechoic fluid remaining dorsally. Hemorrhage may be acute and overwhelming, or may persist at a lower level over a number of days. While the genital tract is the most likely source in the periparturient mare, examination of the liver, spleen, and kidneys is prudent if a readily identifiable source is not present. Mesenteric disruption can also lead to hemoperitoneum.
Peritonitis (Figure 20.7) may result when intestinal bruising, strangulation, and ischemia progress to mural necrosis and rupture of the affected segment. The condition may progress over a number of days, with apparent abdominal discomfort and fever preceding overt signs of sepsis as bacterial translocation increases across devitalized gut wall. Spillage of gastrointestinal content precipitates a rapid onset of systemic deterioration. Peritonitis has also been reported secondary to bladder wall necrosis . Peritoneal fluid appears echogenic (cellular, flocculent, or turbid) and may, in advanced cases, contain fibrin strands, either free or attached to regional visceral and parietal peritoneal surfaces. Gas echoes or highly echogenic particles are consistent with a ruptured viscus. Abscess formation within the mesentery (Figure 20.8) may occur.
Uroperitoneum and Bladder Rupture
Normal, urine filled bladder (Figure 20.9) is visualized in the inguinal region transabdominally or transrectally. Uroperitoneum and bladder rupture (Figure 20.10) may occur if the bladder wall becomes traumatized and devitalized by compression against the pelvic brim during parturition . This is reported to be a rare occurrence . Uroperitoneum may result , with anechoic peritoneal fluid containing increased creatinine and calcium carbonate crystals . The bladder may be seen as folded or collapsed, however, the lesion is unlikely to be visualized.