Abdominal Ultrasound and Palpation per Rectum as Complementary Modalities in Diagnosing Equine Abdominal Pain

1. Introduction

The main goals when performing diagnostics on a horse with abdominal pain are to determine a diagnosis, choose an appropriate therapeutic plan (which may involve surgery), and provide the owner with a prognosis. Palpation of the abdominal contents per rectum has long been considered a mainstay in the diagnostic evaluation of horses with acute or chronic abdominal pain. It was once held that ultrasonographic evaluation of the abdomen had no utility in the diagnosis of acute abdominal pain in horses,¹ but that view has changed dramatically over the last 20 years. The ready availability of portable sonographic equipment and better protocols for rapid transabdominal sonography has made percutaneous ultrasound an increasingly utilized modality in the diagnosis of acute colic. Sonographic evaluation is especially helpful in evaluating colicky horses in which rectal palpation is impossible or unacceptably risky. In horses amenable to palpation per rectum, ultrasonographic evaluation offers additional information about parts of the abdomen unavailable to palpation. Rather than viewing these diagnostic modalities strictly independently of each other, however, it is useful to think about how they each contribute to answering questions about the state of the anatomy within the abdomen. Each one is a powerful diagnostic tool, but together, they provide information that neither could alone. 

2. Strengths and Limitations of Each

Abdominal palpation per rectum is useful for examination of the caudal aspect of the abdominal cavity. Indications include acute and chronic colic, fever of unknown origin, urogenital problems, reproductive evaluation, weight loss, inappetence, and other problems in which abdominal disease is suspected. Adequate restraint and the liberal use of lubricant are required minimums for performing a safe examination in any animal and the utility and safety of rectal palpation are often enhanced by the use of sedative, anesthetic, and antispasmodic medications.^2-4 No special equipment is required and skill is readily developed with practice and basic knowledge of the anatomy.

There are several limitations to the use of rectal palpation in horses. Palpation per rectum is not appropriate for very small or very fractious patients, in which circumstances of the safety of the patient and/or veterinarian is likely to be compromised. Very large horses may be also difficult to palpate, especially for smaller veterinarians. It is estimated that the caudal 1/3 of the abdominal contents are available for examination via palpation,³ but veterinarians palpating large horses or veterinarians with short arms may find that only 1/4 of the abdominal contents can be reached. In addition, a late term fetus, bladder distention, or colonic and cecal gas distention may obstruct palpation of other structures.

The most dreaded complication of rectal palpation is rectal tear. Specific breeds, horses >9 years of age, and mares have been shown to be at increased risk for this complication.⁴ The evaluation and treatment of rectal tears have been covered elsewhere in numerous excellent resources.^3-7 Although we as veterinarians are often more concerned about potential concerns to our patient’s well-being, another potentially serious consequence of rectal palpation of a horse is injury to the veterinarian through kick or crush injury. Care must be taken in providing the safest environment for both patient and practitioner.

Ultrasound is a portable noninvasive imaging modality. It is estimated that the peripheral 2/3 of the abdomen of an average horse can be sonographically imaged with a percutaneous technique.^8,9 Most abdominal sonography is performed percutaneously, but scanning per rectum can also augment the information gathered during rectal palpation, especially when the left kidney, urinary bladder, ureters, reproductive tract, or caudal abdominal vasculature are suspected as causes of abdominal pain. A good working knowledge of abdominal anatomy and image optimization is necessary to obtain diagnostic sonographic images in colic cases; that said, with practice and guidance, abdominal sonography is not difficult to learn. Newer protocols for scanning acute colic cases, such as the fast localized abdominal sonography of horses exam,¹⁰ provide guidance for performing rapid, consistent examinations that yield useful information in these cases.

As with any diagnostic modality, there are limitations to the application of sonographic evaluation to colic. Image quality can be affected by hydration, perfusion, and ambient temperature, as well as the horse’s skin thickness and density, degree of adiposity, and haircoat. Patient preparation, which includes grooming, clipping the hair if needed, and the application of gel or isopropyl alcohol, is necessary for adequate image quality. Gas within the bowel prevents sound wave penetration and so the greater the gas distention of the viscera, the fewer structures that may be imaged. Not all ultrasound units have battery power, so access to a power source may be required. In addition, although more affordable units and probe options are becoming increasingly available, there is cost involved in obtaining optimum equipment. However, even using a standard rectal probe, some useful information may be obtained.

3. Evaluation of the Large Colon and Cecum

Diseases of the large colon are common causes of colic and include impaction, spasm, displacement, volvulus, intussusception, and colitis/typhlitis. Impaction of the large colon, particularly the pelvic flexure, is easily palpated per rectum, as is impaction of the cecum.^1,3-5 Impactions are less likely to be identified using ultrasound than via palpation. Large colon displacement and volvulus comprised 1/3 of all cases requiring surgical intervention in one study.¹¹ For conditions such as displacement or volvulus of the large colon, the band direction of the colons may be palpated, although severe gas distention may confuse the findings.

In normal horses, palpation per rectum of the nephrosplenic area reveals the left kidney, the spleen, and the nephrosplenic ligament. In horses with nephrosplenic entrapment, the colon is palpable dorsal to the ligament and the spleen may feel enlarged and/or displaced medially and ventrally by the colon. Sonographic evaluation reveals gas-filled bowel dorsal to the spleen, with the view of the kidney partially or wholly obscured. The spleen is displaced ventrally and the dorsal border of the spleen has a straight horizontal border that extends from the paralumbar fossa to the 10th to 12th inter-costal space.⁸

Right displacement of the colon is characterized on rectal palpation as colonic bands coursing horizontally across the abdomen and the colon may be detected lateral to the cecum. However, severe gas distention in the colon can obscure the exam findings. The additional sonographic identification of colonic vessels on the right side of the abdomen is highly sensitive and specific for right dorsal displacement or 180° colon volvulus.¹²

In cases of vascular compromise and inflammation, sonographic assessment can add diagnostic and prognostic information, which is critical when providing information to clients leading to surgical decisions. Colonic wall thickness has been shown to be a predictor of colonic volvulus and may help to differentiate this condition from simple displacement.¹³ Colon wall edema may be present in cases of vascular compromise and tissue inflammation, which may be seen in chronic severe impaction or colitis; the changes in the latter scenarios are generally much less severe than in a strangulating lesion. Having information about colon wall health or compromise is useful when trying to form a prognosis to advise clients and set expectations. 

Colonic intussusception is rare, but ileocecal, cecocecal, and cecocolic intussusception have been reported in adult horses.¹⁴ On rectal palpation, the intussusception may be occult or manifest as a mass in the right abdomen.^1,3,4,14 Sonographically, a classic “target lesion” appearance is noted in the right paralumbar region. [...]