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EQUINE VETERINARY JOURNAL Equine vet. J. (1991) 23 (5)347-352

Scintigraphic identification of skeletal muscle damage in horses 24 hours after strenuous exercise ELISABETH MORRIS, H. J. SEEHERMAN, M. W. OCALLAGHAN, S. H. SCHELLING, MARY ROSE PARADIS

and R. S. STECKEL Tufts University School of Veterinary Medicine, 200 Westboro Road, N. Grafton, Massachusetts 0 1536, USA.

Summary The uptake of the bone-seeking radiopharmaceutical 99mTc-MDP by damaged skeletal muscle in horses is evaluated. Twenty-four hours following strenuous exercise, 109 racehorses with a history of inadequate athletic performance and subtle lameness were imaged using scintigraphic techniques. Ten horses (9.2 per cent) demonstrated abnormal uptake of the radioisotope within skeletal muscles. A muscle biopsy from one of theses horses confirmed that the muscles with increased scintigraphic activity had histologic evidence of rhabdomyolysis. This technique allows localisation and relative quantification of muscle damage and is a valuable aid in the evaluation of the athletic horse. Introduction RHABDOMYOLYSIS is characterised by necrosis of skeletal muscle with release of muscle cell contents into the plasma. It can occur as a focal disorder involving only one muscle or as a more generalised process involving multiple muscle groups. Documented causes of rhabdomyolysis in man include alcoholism, trauma, seizures, sepsis, hyperthermia, toxicity, defective carbohydrate or lipid metabolism and over-exertion (Gabow, Kaehny and Kelleher 1982). Exertional rhabdomyolysis of varying severity has been reported in equine athletes (Lindholm, Johansson and Kjaersgaard 1974; Arighi, Baird and Hulland 1984). In the most severe form, a horse suffers pain, muscular stiffness and exhibits sweating, tachycardia, tachypnea and palpably hard skeletal muscles. An elevation of muscle enzymes is found in the serum and myoglobinuria also may occur (Watanabe ef al 1978). In human athletes, a less severe, localised form of the disease has been reported (Lentle, Percy, Riga1 and Russel 1978; Matin, Lang, Caretta and Simon 1983; Haseman and Kriss 1985). Presumably the equine athlete also suffers from localised exertional rhabdomyolyis resulting in only vague signs of exercise intolerance or lameness. Diagnosis of the disease process may be difficult, because serum creatine kinase (CK) levels may be elevated only marginally for short periods of time, discoloration of urine due to myoglobinuria may not be visible and no muscle hardness may be evident. It is well known that bone-seeking radiopharmaceuticals will localise in damaged myocardial and skeletal muscle (Parkey, Bonte and Meyer 1974; Wynne and Holman 1980). This paper reports on the value of the radiopharmaceutical

99mTc-methylene-diphosphonate (99mTc-MDP), a radioisotope used clinically for skeletal imaging, as a means of identifying and localising acute muscle damage in the horse.

Materials and methods The evaluation protocol Horses used in this study were presented to the Tufts University School of Veterinary Medicine Sports Medicine Program for clinical evaluation of poor racing performance. Routine physical examination and flexion examination at the walk and the trot in hand were followed by a high speed treadmill (Sato Treadmill, AB, Sweden) lameness evaluation. Training the horses to run comfortably on the treadmill took 20 to 30 mins usually. For the test, the horses were equipped with racing tack consisting of a head check, harness and hobbles for pacing Standardbreds, head check and harness for trotting Standardbreds and a racing bridle for Thoroughbreds. Following a 3 min warm-up at 4.5 m/sec (equivalent to a 6 min mile) the horses were observed at three increasing speeds: a 1.8 m/sec walk (15 min mile), a 4.5 m/sec jog, trot or canter (6 min mile) and a racing gait between 9 m/sec and 13.5 m/sec (3 and 2 min miles), depending on the horse's previous training history and level of performance. The fastest speed was sufficient to maintain a heart rate in excess of 200 beats/min (about 85 per cent of maximal heart rate). Each speed was maintained for about 3 mins and video recordings were taken. This protocol was repeated three times and the horse was recorded from the front, the side and the rear. The following day, skeletal scintigraphy was performed as described later. Two affected horses were re-imaged 72 h after exercise to determine the length of time following muscle damage that the radioisotope would continue to accumulate in the damaged muscles. One horse returned for a second complete evaluation 14 days later to determine whether rhabdomyolysis would recur. Horses From January 1988 to June 1990, 109 racehorses underwent skeletal scintigraphy 24 h after strenuous treadmill exercise. Eighty-five were Standardbreds (average age, 3.5 years) and 25 were Thoroughbreds (average age, 4.1 years). All had a history of poor athletic performance suspected to be localised to the musculoskeletal system and all demonstrated gait asymmetry when working on the treadmill. All horses were involved in training and/or racing at the time of the evaluation.

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Diagnostic musi~uloskeletalimaging For scintigraphic evaluation of the skeleton, 175 mCi of the bone-seeking radionuclide 99mTc-MDP was administered by intravenous (iv) injection 4 h prior to imaging. Imaging was performed by a Large-Field-of-View gamma camera (Siemens Gammasonics Inc., Des Plaines, Illinois) mounted on a suspension system that allowed movement in three orthogonal planes. The gamma camera was interfaced to a microdot imager and a dedicated computer system. Involvement of soft tissue also was investigated in two horses by iv injection of 150 mCi of 99mTc-pertechnetate, a radionuclide highly bound to albumin, which allows analysis of relative perfusion/accumulation in soft tissue. Imaging was performed 30 mins following injection using the gamma camera described earlier.

Serum chemistry analysis of muscle enzymes Prior to and 6 h following strenuous treadmill exercise, serum samples were obtained for analysis of creatine kinase (Kodak Ektachem DT, Eastman Kodak Company, Rochester, New York).

Muscle biopsy Twenty-four hours after scintigraphy and 48 h after exhaustive exercise, a muscle biopsy was taken from the left gluteal TABLE 1: Results of 99mTc-MDP scintigraphy of horses with abnormal skeletal muscle uptake

Horse No Details -1 4-year-old

S mare

2 3 4

3-year-old S mare 3-year-old S mare 2-year-old S mare

5

3-year-old S stal

6

4-year-old S stal

7

3-year-old S geld 3-year-old T geld 3-year-old

8 9 10

T mare

6-year-old T geld

Areas of maximal 99mTC- Serum CK' Pre Post MDP muscle uptake L semitendinosus L biceps femoris

ND

L gluteals faint uptake in R semitendinosus R biceps femoris R gluteals R gluteals

ND

R gluteals > L gluteals

232

10,050

bilateral gluteals R semimembranosus semitendinosus bilaterally symmetrical gluteal uptake L semimembranosus R long digital extensor R lateral digital extensor R gastrocnemius L gluteals

277

14,000

843

6,040

406

4,660

177

5,050

R gluteals > L gluteals

112

2,900

R gluteal L and R triceps brachii

364

33,000

longissimus dorsi R semitendinosus

337

8.440

'Levels prior to exercise and 6 h following strenuos exercise. CK is expressed as iu/litre. Normal values in our laboratory are 10 to 400 iu/litre. Differences between pre- and post exercise values are significant in all cases (P

Scintigraphic identification of skeletal muscle damage in horses 24 hours after strenuous exercise.

The uptake of the bone-seeking radiopharmaceutical 99mTc-MDP by damaged skeletal muscle in horses is evaluated. Twenty-four hours following strenuous ...
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