Uses of Ultrasound in Equine Internal Medicine

Diagnostic Ultrasound

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Uses of Ultrasound in Equine Internal Medicine T . Douglas Byars, D.V.M., * and John Halley, M.V.B., M.R.C.V.S. t

Ultrasound examination allows the diagnostician to examine peripheral soft tissue s noninvasively to a depth sufficient to evaluate the size, shape, position, texture, and surrounding tissue or fluids of the majority of internal organs. Each major body system is amenable to specific ultrasound examinations. Visual diagnostics, guided biopsy techniques, and therapeutic placement of drainage devices should be considered routine uses of ultrasound in medical and surgical practices. The equipment utilized will vary from sector scanners, with or without M-mode capabilities, to linear units commonly used in broodmare practices. The versatility of each scanning unit is consistent with the experience and preference of the operator. Therefore, the most expensive equipment is not always better; less e xpensive equipment can be of equal diagnostic value if its functions are compatible with the operator's needs. In most cases, ultrasound can be used to complement physical diagnosis (for example, palpation and percussion) or as an ancillary to procedures such as radiography. Hair is rarely a barrier to ultrasound except when the horse's winter hair coat is fully developed. Therapeutic procedures for diseases such as pleuritis have become routine, if not relatively mandatory, for the proper management and evaluation in those practices presented with a significant number of cases of respiratory disease. In this issue, each body system is addressed in detail by authors with specialized experience using ultrasound. The following is a general overview of the uses of ultrasound in a spe* Diplomate , American Colle ge of Veterinary Inte rn al Medicine; Practitioner-Inte rnist,

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Hagyard-Davidson-McGee, Lexington, Kentucky. Private Practitione r, Springhill, Kill enaule, Co ., Tippe rary, Ireland.

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Table 1. Current List of Diseases of the Horse Utilizing Routine Ultrasound Diagnostics CARDIAC

UROGENITAL

Vegetative endocarditis Ruptured chordae tendineae Valvular calcification and thickening Valvular prolapse Septa! defects Myocardial calcification Pericarditis

Pregnancy and twins Hemorrhage and adhesions Renal lithiasis and hydronephrosis Renal enlargement Renal fibrosis Cystitis and abscessation Cystic and urethral calculi Testicular edema and fibrosis abscessation Omphalophlebitis

RESPIRATORY

Pleuritis and pleural effusions Pneumonia (consolidation, hepatization) Pulmonary abscessation ± diaphragmatic hernia

NERVOUS

Internal hydrocephalus

GASTROINTESTINAL

INTEGUMENT AND MUSCULOSKELETAL

Peritoneal effusions (peritonitis) Ascites and uroperitoneum Cholelithiasis Hepatomegaly and neoplasia Hepatic displacement Splenic displacement and neoplasia lieus Bowel displacements and distention Internal abscessation

Tendon edema and hemorrhage Tissue abscessation Neoplasia Ossifying myopathies Iliac thrombosis OCULAR

Periorbital abscessation or tumors

cialty internal medicine practice for horses. Table 1 is a list of conditions for which diagnostic ultrasound may be considered routine in equine medicine. The cardiovascular system is evaluated for either performance characteristics or the presence of disease. Ultrasound allows for the dynamic visualization of the heart valves, cardiac chamber size, gross myocardial textural changes or calcification, endocardial proliferation, the presence or absence of rightsided spontaneous contrast,3 and pericardia! thickening or pericardia! fluid accumulation. The scanhead must be placed between the ribs. Sector scanners allow for more variation of viewing angles and access to the semilunar valves, whereas linear units can be used to essentially visualize more singular planes of the A-V valves, ventricles, and interventricular septum. Most cardiac evaluations are subjective unless an M-mode is used to more definitively assess thickening of the heart valve 2 or a frame-by-frame video recorder is used to stop motion throughout the phases of the cardiac cycle. Septal defects can be evaluated by injecting saline intravenously (10 to 20 cc) and by then observing the microbubble contrast passing through the right heart. If right-to-left shunting occurs, the contrast may be ob-

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served in both the right and left hearts simultaneously to confirm the diagnosis. Ultrasound units with Doppler capabilities can quantitate blood flow within the heart as well as the central vessels of other internal organs and the peripheral vasculature. Cardiac ultrasound diagnoses include valvular thickening, vegetative endocarditis, ruptured chordae tendineae, valvular prolapse, chamber enlargement, myocardial calcification, decreased contractility, and pericarditis. Ultrasound offers special diagnostic and therapeutic advantages in evaluating the respiratory system. Because air is a near-perfect reflector, penetration beyond air-filled lung does not occur. Observation windows between the ribs are utilized for evaluation of the lungs in a manner similar to that used for cardiac ultrasound except that a larger surface area of the thoracic cavity is covered. Normal visceral pleura is easily visualized as a brightly echogenic surface. The diaphragm can be observed medially in the most ventral chest region, with gasfilled bowel or liver just deep to the diaphragm on the ultrasound screen. The lung is a dynamic structure, and motion enables the operator to observe the sliding of the lung opposed to the relatively static chest wall. Lesions of the chest wall, pleural effusions, thoracic empyema, and superficial lung lesions such as abscessation, consolidation, infiltrative masses, and necrosis of the visceral pleural lining can be evaluated. Pulmonary abscesses adhered to the thoracic wall may be drained externally using ultrasound for guidance. Those abscesses not adhered will be free-sliding and not directly amenable to drainage. Similarly, pleural effusion and pockets can be sought and drained. Fibrinoid occlusion of chest drains can be assessed by observing fibrin floating in fluid around a nonproductive drain. The position of the drain can be identified by injecting enough air to fill the lumen of the drain. Ultrasound evaluation using linear scanners is different than sector scanners because the sound beams are rectangular rather than shaped like pie wedges. Sector scanners with maximal penetration (16 to 22 cm) can allow imaging of the mediastinum of the ventral thorax. The abdominal cavity can be examined with ultrasound for the presence of peritonitis, ascites, urine, internal masses (neoplastic, cystic, abscessation), internal organ size and shape, and visceral displacements. Bowel thickness and motility can be determined by using the gas or particulate echogenic gaseous fluid-filled lumen as a relatively dynamic landmark. Peristaltic activity is seen in the small intestines as rhythmic contractions causing a change in lumen size or as motion of two opposing

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serosal surfaces. Dark spaces between loops of bowel represent fluid consistent with ascites or peritoneal effusions. In severe cases of peritonitis, fibrin can be observed floating free or adhering to roughened or etched peritoneal surfaces. The area of fluid accumulation can be used as a preferred site for an abdominal paracentesis, especially in foals. The spleen is observed as a mottled tissue mass on the left side of the horse that may course over the ventral midline. Remote vascular structures are seen as dark "holes" or oblique tubular structures coursing through the spleen. Deviations from the normal architecture or thickenings of the capsule signal the presence of neoplastic infiltrations, abscesses, or extracapsular adhesions. The liver should be similarly examined slightly more cranially in the abdomen just caudal to the diaphragm bilaterally. Lung shadows may momentarily obliterate portions of the liver during the inspiratory phases of respiration. The tissue appears more distinct and denser than the spleen; the walls of the portal veins are more echogenic than those of the hepatic veins. Excessive echogenicity seen as white masses casting dark acoustic shadows may represent calculi or choleliths within the canaliculi or the common bile duct. Nodularity or diffuse mottling indicates either focal areas of inflammation or neoplastic infiltrations. Fluid-filled structures, other than vasculature, can represent cysts or hematomas. Abscesses are usually observed as denser fluid-filled structures that can occasionally be compartmentalized. Cysts, hematomas, and abscesses can be situated subcapsularly or within the liver parenchyma. A freeze-frame evaluation will allow the practitioner to choose the best site and penetration technique for biopsy of the liver. The kidneys and lower urinary tract are important structures to visualize with ultrasound. The ureters, bladder, and urethra can be examined trans rectally for the presence of stones, strictures, thickening of the lumen walls, or mural lesions of the bladder. The perinea! region allows for external evaluation of the urethra in males. The bladder can also be visualized externally in foals. Uroperitoneum is readily observed as excessive abdominal fluid in foals and is observed only rarely in adults. Renal tissue normally changes density at the corticomedullary junction. An enlarged renal pelvis with decreased renal parenchyma suggests the presence of hydronephrosis. The renal pelvis is seen as a dark area centrally located within the kidney. The renal pelvis may be mineralized, contain urinary sediment, or harbor nephroliths seen as echogenic structures casting acoustic shadows through the deeper tissues. Kidneys are usually acutely enlarged whenever inflam-

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mation, hemorrhage, or edema involves the renal parenchyma. Chronic enlargements and lobulations can be seen with pyelonephritis, neoplasia, or, rarely, previous hemorrhage. The left kidney is located medial to the caudal third of the spleen, whereas the right kidney is more cranial and attached more firmly in the retroperitoneal space. With the aid of ultrasound, the renal pelvises may be flushed percutaneously or indwelling drainage tubes may be placed to bypass occluded lower urinary tracts. Abdominal structures that are filled with fluid include abscesses and hematomas. Both may have thick walls and be interloculated with fibrous strands. If these masses are adhered to the body wall, they may be considered amenable to external drainage or needle aspiration. Common abscessation sites that warrant ultrasound examination are the umbilicus and urachus of foals or perivaginal region of mares. Random scanning of the abdomen can be performed transrectally as well as externally in more mature horses. Transrectal examination also allows for the visualization of the terminal aorta, its mesenteric branches and iliac bifurcations. Thrombic lesions or aneurysms may be located if the operator can successfully utilize either a sector or linear scanner to the depth of full-arm extension and slowly trace these vessels caudally. Ultrasound examination of the reproductive tract is routinely performed in equine practice for the diagnosis of pregnancy and the detection of twins. However, veterinarians should be prepared to look for problems such as abscesses, cysts, hematomas, and adhesions in both the male and female reproductive systems. Ultrasound examinations can be performed via the rectum or vagina in mares. In males, transrectal examination can be used to examine the urogenital system. One can look for calculi of the male urethra by scanning from the bladder sphincter to the external perineum via rectal examination. The perinea! region is then scanned externally. In intact males, the ultrasound transducer can be placed against the scrotal region for the evaluation of scrotal edema, the testes, and occasionally, the presence of a herniated intestine. The neurlogic system is rarely amenable to ultrasound examination, because most of this system is encased within bone. One report utilized ultrasound as an adjunct in the diagnosis of hydrocephalus in a foal. 1 Ultrasound is routinely used for examination of the central nervous system in human neonates; the sound is directed through the open fontanelles, which serve as windows through the skull. Thus far, this has not been found to be practical in the foal due to the anatomic difference. Miscellaneous soft tissues that are scanned with ultrasound

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include muscle tissues, which are scanned for abscesses, cellulitis, or hematomas. Tendons can be similarly examined for edema, hemorrhage, or gross disruption offibers. Joint effusions may be examined to assess the amount of intrasynovial fibrin and determine the feasibility of subsequent arthrotomy or simple needle lavage. Ultrasound should be viewed as a major contribution to veterinary medicine that provides a noninvasive means of diagnosis. The selection of ultrasound units has increased and the costs have decreased sufficiently to make them economically practical. Linear units as well as more expensive sector scanners with or without cardiac M-mode capabilities are commercially available. The ease with which most ultrasound interpretation can be learned allows practitioners to immediately increase their diagnostic capabilities and improve assessment of patients and client services. Veterinarians should utilize all available opportunities in clinical practice to become proficient in ultrasound imaging and then apply this information to the disciplines of veterinary practice.

REFERENCES J. H., Reed, S. M., Rantanen, N. W., et al.: Congenital internal hydrocephalus in a quarter horse foal. J. Equine Vet. Sci., 3:5, 154-164, 1984. 2. Pipers, F. S., and Hamlin, R. L.: Echocardiography in the horse. J. Am. Vet. Med. Assoc., 170:815-819, 1977. 3. Rantanen, N. W., Byars, T. D., and Hauser, M.: Spontaneous contrast and mass lesions in the hearts of race horses: Ultrasound diagnosis preliminary data. J. Equine Vet. Sci., 4:5, 220-223, 1984. 1. Foreman,

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