CHAPTER 83 Imaging, Endoscopy, and Other Diagnostic Procedures for the Acute Abdomen
Acute abdominal pain, or colic, is commonly seen in horses, and the clinical signs of colic are often similar, irrespective of the various underlying causes. Because the causes of colic are numerous and usually, but not exclusively, involve the gastrointestinal tract, accurate and timely diagnosis is crucial for initiating appropriate medical and surgical treatment and achieving a successful outcome. A complete and methodic diagnostic approach that encompasses information obtained through history and physical examination, clinicopathologic testing, and diagnostic imaging is essential to achieve this goal.
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 findings can be used to predict severity, provide prognosis, rule out other disease processes, and aid in arriving at an accurate diagnosis.
Passing a nasogastric tube is a routinely performed diagnostic procedure that may also provide lifesaving treatment. Because they are unable to vomit, horses with physical or functional obstructive diseases of the stomach and intestinal tract may require decompression and lavage of fluid or ingesta from the stomach. The volume, color, consistency, odor, and pH of gastric reflux fluid can be characteristic for different conditions such as gastric impaction, proximal duodenojejunitis, or strangulating diseases of the small intestine, with larger volumes found in cases of longer-standing obstructions and proximal duodenojejunitis. Microbial culture of the sample may identify potential infectious agents, such as clostridial organisms.
Abdominal palpation per rectum should also be routinely performed in horses of adequate size and temperament. Appropriate restraint, with the aid of adequate sedation (xylazine 0.2-0.4 mg/kg or defomidine 0.005-0.02 mg/kg administered intravenously [IV]) and use of an agent to promote rectal relaxation (N-butylscopolammonium bromide, 0.3 mg/kg, IV) should be used to facilitate thorough examination and prevent injury to horse and examiner. Palpation is limited to the caudal portion of the abdomen, but abnormalities of the small intestine, cecum, large colon, small colon, spleen, urogenital tract, and mesenteric, sublumbar, and inguinal vessels and lymph nodes may be felt. Abnormalities in size, thickness, location, and contents of abdominal structures may be found, and these findings can direct further diagnostic tests, treatment, and diagnosis as well as provide a baseline for evaluation of response to treatment.
A complete blood cell count (CBC) and serum biochemical, blood gas, fecal, and abdominal fluid analyses should all be considered during evaluation of horses with colic. CBC, biochemical, and blood gas analyses can provide information and direct treatment regarding systemic health, cardiovascular status, organ function, and fluid and electrolyte imbalances. Results of these tests may also aid in diagnosis of specific causes of colic, such as infectious colitis, cholelithiasis, hemorrhage, urinary tract obstruction, or other conditions.
Examination of feces collected during rectal palpation should include gross examination for consistency, color, odor, and presence of mucus or sand; microbial culture for gastrointestinal pathogens; and microscopic examination of a flotation preparation or direct smears for endoparasites. Aseptic collection and analysis of abdominal fluid is valuable in determining abnormalities of the abdominal cavity. Gross characteristics, including relative volume, color, and turbidity, should be assessed. Examination of cell types, numbers, and reactivity and total protein, pH, glucose, and lactate content can differentiate between several diagnoses, including devitalized bowel, septic peritonitis, hemorrhage, and neoplasia. Abdominal fluid can also be submitted for microbial culture when septic peritonitis is suspected, and serial sampling may be used to monitor disease progression or response to treatment.
Abdominal ultrasonography in foals and mature horses is extremely useful, not only in horses with acute abdominal pain but also in those with recurrent colic and weight loss. Ultrasonography enables evaluation of structures that are relatively inaccessible to examination by other diagnostic imaging techniques, such as radiography and endoscopy, and subsequently has become a mainstay in diagnostic testing in horses with colic. Abdominal ultrasonography not only provides valuable structural information but also permits characterization of peritoneal fluid location, volume, and consistency and may guide other diagnostic techniques, such as abdominocentesis and biopsy.
Abdominal ultrasonography can be performed transcutaneously or transrectally, and techniques vary in terms of preparation required, equipment needed, and structures that may be identified. A combination of both techniques may be required to obtain optimal results. Transcutaneous ultrasonography requires lower-frequency transducers; most commonly, frequencies of 2.5 to 5 MHz are needed, but probes operating at 5 to 10 MHz may be useful in foals and smaller, thinner, mature horses. Use of a higher frequency allows better anatomic detail and resolution but less tissue penetration, whereas use of a lower frequency allows deeper tissue penetration but lower resolution. Convex or sector transducers are most appropriate and allow easier access and positioning between ribs. Ideally the horse’s hair should be clipped, the skin cleansed thoroughly, and coupling gel applied, although images can be obtained in most horses without clipping. The entire abdomen and caudal part of the 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 of 5 to 10 MHz. Preparation of the horse is similar for rectal palpation and necessitates 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 palpated during rectal palpation.