As with evaluation of any condition, obtaining signalment and a complete history and performing a thorough physical examination are essential. Information regarding age, duration of clinical signs, husbandry, and other related information can provide pertinent clues to differential diagnoses. Physical examination parameters can predict severity, provide prognosis, rule out other disease processes, and aid in arriving at an accurate diagnosis.

Abdominal ultrasonography in foals and mature horses is extremely useful for evaluating not only acute abdominal pain but also recurrent colic and weight loss. Ultrasonography allows evaluation of structures relatively inaccessible to other diagnostic imaging techniques, such as radiography and endoscopy, and subsequently has become a mainstay in equine colic diagnostics. Abdominal ultrasonography provides valuable structural information as well as characterization of peritoneal fluid location, volume, and consistency and may guide other diagnostic techniques, such as abdominocentesis and biopsy.

Abdominal ultrasonography may be performed transcutaneously or transrectally, and techniques vary in preparation, equipment needed, and structures that may be identified. A combination of both techniques may be required for optimal results. Transcutaneous ultrasonography requires lower frequency transducers. Most commonly, use of a transducer with frequencies of 2.5 to 5 MHz is necessary, but frequencies in the range of 5 to 10 MHz may be useful in foals and smaller, thinner mature horses. Generally, use of a higher frequency yields better anatomic detail and resolution, but lesser penetration; a lower frequency yields deeper penetration but lower resolution. Convex or sector transducers are most appropriate and allow easier access and positioning between ribs. Ideally, the haircoat should be clipped, the skin cleansed thoroughly, and coupling gel applied, although images may be obtained in most horses without clipping. The entire external abdomen and caudal thorax should be imaged. Structures that may be imaged include the stomach, duodenum, jejunum, ileum, cecum, large colon, liver, spleen, and urogenital tract. Transrectal ultrasonography is performed with a linear or small convex transducer with a frequency range of 5 to 10 MHz. Preparation of the horse is similar to what is undertaken for rectal palpation and requires adequate restraint, sedation, rectal relaxation, lubrication, and evacuation of feces from the rectum. Structures that may be imaged are similar to those that may be felt during rectal palpation. Doppler ultrasound is another technique that can be used in abdominal ultrasound. It may be used to detect propulsive motility and can differentiate peristaltic from mixing (nonperistaltic) activity in the jejunum and aid in diagnosis of ileus or intestinal obstruction.

The intestinal wall of most of the intestinal tract has a five-layered sonographic appearance, including a hyperechoic serosa, hypoechoic muscularis, hyperechoic submucosa, hypoechoic mucosa, and hyperechoic mucosal interface (gas and ingesta). Exceptions include the stomach and ileum. The ileum has a seven-layered appearance created by the additional muscle layer. The total thickness of the normal intestinal wall ranges from 2.00 to 3.75 mm throughout the small and large intestines. Total wall thickness may be decreased with intestinal distension and may be increased with infiltrative diseases such as enteritis, strangulation, or neoplasia. Gas contents in the gastrointestinal tract produce a hyperechoic pattern and acoustic shadowing, fluid contents appear hypoechoic, and ingesta appears hyperechoic and heterogenic without acoustic shadowing.

Consideration of all of the ultrasonographic findings and changes over time is paramount to distinguishing between different diseases. The convenient and noninvasive nature of ultrasound also makes it a useful diagnostic test in monitoring the horse for resolution of abnormal findings and for determining prognosis after surgery. For instance, on serial ultrasound of the abdomen after exploratory celiotomy, the small intestine may be seen to be hypomotile with hypoechoic contents, findings consistent with mild postoperative ileus or enteritis. In the first week after surgery, it is not uncommon for the bowel to be mildly thickened, even in structures not involved in the primary lesion. In horses in which a strangulating lesion of the large colon is identified intraoperatively, a prolonged postoperative period of colonic wall involution, as measured on ultrasound, is associated with a poorer prog­nosis and increased risk for multiple-organ dysfunction syndrome.

In emergency situations, it is important to perform a quick and thorough ultrasonographic examination in horses with colic. A standardized approach such as the fast localized abdominal sonography (FLASH) technique allows efficient detection of major intraabdominal abnormalities. With this technique, several standard topographical locations are examined and evaluated for the most common abnormalities in horses with acute abdominal pain. With experience, the FLASH method takes about 10 minutes, and it may be performed simultaneously with other procedures.