Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds: one syndrome, multiple aetiologies

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1 EQUINE EXERCISE PHYSIOLOGY 5 Equine vet. J., Suppl. 30 (1999) Exertional rhabdomyolysis in Quarter Horses and Thoroughbreds: one syndrome, multiple aetiologies STEPHANIE J. VALBERG*, J. R. MICKELSONt, ESTHER M. GALLANT*, JENNIFER M. MAcLEAY, LINNEA LENTZt and F. DE LA CORTE Department of Clinical and Population Sciences and tdepartment of Veterinary Pathobiology, College of Veterinary Medicine, University of Minnesota, St Paul, Minnesota , USA. Keywords: horse; muscle; myopathy; tying-up; exercise; polysaccharide storage myopathy; glycogen Summary The purpose of this study was to determine if chronic exertional rhabdomyolysis (ER) in Quarter Horses and Thoroughbreds represents one or several distinct myopathies. Eighteen Quarter Horses and 18 Thoroughbreds with ER were selected from cases presented to the Veterinary Hospital on the basis of a history of ER, assessment of muscle histopathology, and serum CK activity before and 4 h post exercise. In addition, 2 of 3 of the following parameters were evaluated: muscle glycogen concentrations, thyroid hormones (T3, T4), fractional excretion (FE) of sodium, potassium and chloride. The CK response to training, the metabolic response to a near maximal standardised exercise test (SET), blood glucose concentrations after an i.v. glucose challenge and a skeletal muscle in vitro caffeine contracture test were performed on 5 of the Quarter Horses, selected because of polysaccharide storage myopathy (PSSM), and 5 of the Thoroughbreds. Serum T3 and T4 were all within normal limits. Low FE of sodium and potassium were seen in <20% of Quarter Horses and Thoroughbreds. Four hours post exercise, CK was increased in 77% of Quarter Horses and 72% of Thoroughbreds with ER. Muscle glycogen concentrations in Quarter Horses with ER were significantly higher than in normal Quarter Horses and Thoroughbreds with ER. No Thoroughbreds, but 15/18 Quarter Horses with ER had abnormal polysaccharide accumulation in muscle biopsies consistent with a diagnosis of PSSM. PSSM Quarter Horses had higher CK activity during training than Thoroughbreds and higher glycogen utilisation with the SET. PSSM Quarter Horses also had significantly enhanced glucose clearance compared to normal Quarter Horses and Thoroughbreds with ER. Thoroughbreds with ER had significantly lower thresholds for caffeine-induced contracture than normal horses and PSSM Quarter Horses. It was concluded that there are multiple causes for exertional rhabdomyolysis. In Quarter Horses, rhabdomyolysis is commonly due to a glycogen storage disorder, PSSM, and is readily expressed in untrained horses. In Thoroughbreds, ER is commonly due to an underlying abnormality of muscle contraction. Rhabdomyolysis in Thoroughbreds, however, is only expressed intermittently when key stressors are present. 'Author to whom correspondence should be addressed. Introduction Recurrent muscle pain and stiffness with exercise occurs in many breeds of horses. The terms Monday Morning Disease, Azoturia, Fig I: PAS stain after amylase digestion of a gluteal muscle biopsy from a Quarter Horse with polysaccharide storage myopathy and a Thoroughbred with recurrent ER. Note the dark PAS positive inclusions resistant to digestion in the Quarter Horse (A) and the absence of any polysaccharide after amylase digestion in the Thoroughbred (B). Arrows indicate the central nuclei found in Thoroughbreds with ER.

2 534. h 3 v Y QHl QH2 a Recurrent exertional rhabdomyolysis 100 I I I Days I I I I I Time (min) looooo 1 i = v Y 0 I I + TB1 B d- TB2-4- TB Days Fig 2: Four hour post exercise serum CK activity in 3 Quarter Horses with PSSM (A) and 3 Thoroughbreds with recurrent ER (B) undergoing 3 weeks of training for 30 midday. Note the higher CK activity from the beginning of training in Quarter Horses and the intermittent elevations in CK in Thoroughbreds after 10 days of training. I Fig 3: Plasma glucose concentrations during an intravenous glucose tolerance test in 5 Quarter Horses (PSSM) and 3 Thoroughbreds with recurrent exertional rhabdomyolysis. Note the significantly lower blood glucose concentrations in the Quarter Horses with PSSM compared to the Thoroughbreds. No significant differences in blood glucose concentrations were found between normal Quarter Horses, normal Thoroughbreds and ER Thoroughbreds (data not shown) ER-TB + PSSM-QH + Normal-TB + Normal-QH tying-up, chronic intermittent rhabdomyolysis and recurrent exertional rhabdomyolysis (ER) have all been used to describe this syndrome. Numerous aetiologies have been proposed including hypothyroidism, electrolyte depletion, lactic acidosis, glycogen storage disorders and altered muscle contractility (Waldron Mease 1979; Harris and Snow 1991; Beech et al. 1993; Valberg 1996). The search for one unifying aetiology for this condition has been unsuccessful, indicating the possibility of multiple causes. An inherited glycogen storage disorder termed polysaccharide storage myopathy (PSSM) has been described in Quarter Horses with recurrent muscle stiffness (Valberg et al. 1992, 1997). Horses with PSSM have enhanced glucose clearance after oral or i.v. glucose challenge, high muscle glycogen concentrations and an abnormal polysaccharide in skeletal muscle (Valberg et al. 1992; De La Corte et al. 1999). Using the diagnostic criteria of Valberg et al. (1992), PSSM was found to be specific to Quarter Horses, draft breeds, and related breeds (Valberg et al. 1997). Others, however, have suggested PSSM is an important cause of ER in many breeds including Thoroughbreds (Valentine et al. 1997). Recent findings of an abnormality in muscle contraction in Thoroughbreds with recurrent ER that is similar but not identical to malignant hyperthermia suggest that this may be a significant cause of ER in this breed (Lentz et al. 1999a; Ward et al. 1999). Clearly, controversy still exists with regard to the aetiology(s) of o 10.0 Caffeine (mmol/l) Fig 4: Contracture force normalised to the preceding tetanus versus caffeine concentration in muscle bundles from ER and normal Thoroughbreds as well as PSSM and normal Quarter Horses. Note the threshold for development of significant contracture force was lower for ER Thoroughbreds (1 mmoh!) compared to other horses (5 mmowl). Data represent a compositefrom 2 papers (Lentz 1999a and b). exertional rhabdomyolysis in various breeds of horses. The purpose of this study was to determine whether the syndrome in Quarter Horses and Thoroughbreds represents one or several distinct myopathies. Materials and methods Records from 400 horses that had muscle biopsies submitted to the Neuromuscular Diagnostic Laboratory at the University of Minnesota and the University of California, Davis, were searched to identify Quarter Horses and Thoroughbreds with ER. Criteria for inclusion in the present study included a minimum of 2 episodes of rhabdomyolysis confirmed by elevations in serum CK, examination by the primary author and assessment of frozen gluteal muscle sections stained with a

3 ~ Stephanie J. Valberg et al. 535 TABLE 1: Muscle histopathology in 18 Quarter Horses (QHs) and 18 Thoroughbreds (TB) with recurrent exertional rhabdomyolysis No. QHs No. TBs Acute necrosis 4 5 Macrophage infiltration 6 0 Myotubes 1 2 Central nuclei 4 17 Subsarcolemmal vacuoles 14 0 PAS + inclusions 15 0 Atrophy 1 1 PAS +: periodic acid Schiff s positive staining. minimum of haematoxylin and eosin, and periodic acid Schiff s stains. In all horses to be included, measurements of serum CK before and 4 h following min trotting on a lunge-line was required. The exercise challenge was performed at a time when horses were in training and had experienced episodes of rhabdomyolysis within the last month. In addition, at least 2 of 3 of the following parameters had to be measured for inclusion in this study: 1) muscle glycogen concentrations (17/18 QH; 14/18TB); 2) triiodothyronine (T3; 12/18 QH; 16/18 TB) (IRA method) and tetraiodothyronine (T4; 14/18 QH; 16/18 TB) concentrations (chemiluminescence method); and 3) renal FE of sodium, potassium and chloride (QH 14/18; TB17/18) (Harris and Snow 1991). No standardisation of diet occurred prior to electrolyte analysis. Muscle histochemistry was scored for the presence of necrosis, macrophage infiltration, myotubes, central nuclei in mature myofibres and abnormal polysaccharide inclusions. Individual glycogen concentrations, CK activity, T3, T4 concentrations, and FE of electrolytes were compared with normal reference ranges (Harris and Snow 1991). Means were compared between Quarter Horses (QH) and Thoroughbreds (TB) using an unpaired Student s t test or median test. Further studies were performed on 5 of the Quarter Horses and 5 of thethoroughbreds with ER that were subsequently donated to the University of Minnesota. All Quarter Horses were diagnosed with PSSM on the basis of polysaccharide inclusions in muscle biopsies. The Quarter Horses ranged in age from 5-11 years (mean 7) and comprised 3 mares, 1 gelding and 1 stallion. Rhabdomyolysis was so frequent in all Quarter Horses that it prevented them from training successfully. The Thoroughbreds were all mares age 4-12 years (mean 7). Three had recurrent episodes that prevented them from racing successfully and 2 were riding horses that had intermittent episodes of rhabdomyolysis. Effect of training Serum CK activity was determined 4 h after treadmill exercise during a 3 week training period for the 3 Quarter Horses and 3 Thoroughbreds most severely affected by ER. Creatinine kinase (CK) was compared between Quarter Horses and Thoroughbreds using a one-way ANOVA. Horses were kept idle at the University for at least 4 months and were accommodated to the treadmill before training. Each training session lasted for 30 min or until muscle stiffness was evident. Training sessions for Quarter Horses differed from Thoroughbreds due to the lack of exercise tolerance in PSSM Quarter Horses. Quarter Horses walked for 1 week, did 2 min intervals of walk and trot for 1 week and 2 min intervals of walk, trot and canter for 1 week. TABLE 2: Mean f s.e. thyroid hormone (T3 and T4) concentrations and fractional excretion of electrolytes In Quarter Horses and Thoroughbreds with recurrent exertional rhabdomyolysis Quarter Horses Thoroughbreds Normal values T3 ng/dl 83.6 f * T4 pg/dl 2.5 i i O-2.6 FE sodium YO 0.25 i i o FE potassium YO 56.7 f f FE chloride Yo 0.82 * i Maximum speed was 6 m/s. Thoroughbreds performed intervals of walk, trot, canter and gallop on Monday and Friday with a maximum speed of 10 m/s on a 6% slope for 3 weeks. On Tuesday and Thursday, horses performed intervals of walk trot and canter with maximum speed of 9 m/s. On Tuesday, horses trotted for 30 rnin after a 4 rnin walk (MacLeay et al. 1999b). Standardised exercise tests (SET) Prior to any training studies, the 5 Quarter Horses and 5 Thoroughbred horses donated to the university performed a SET on a 6% slope. Horses were adapted to the treadmill for 2 weeks prior to the study. The tests for Quarter Horses and Thoroughbreds have been described previously (Valberg et al. 1999; MacLeay et al. 1999b). Briefly, horses warmed up for 10 rnin at intervals of walk (1.8 ds), trot (QHs 3 or TBs 4.5 ds), and canter (QHs 6 or TBs 7 ds). Further 2 rnin increments were added such that horses reached a heart rate of 200 beatdmin or refused to exercise at higher speed. The increments for Quarter Horses were 7.5 m/s for 1 horse, 7.5 and 8.5 m/s for 2 horses and 8.5 and 9.5 m/s for 1 horse. All Thoroughbreds performed the same 2 extra increments at 10 and 11 m/s. Muscle biopsies were obtained from the gluteus medius using a percutaneous needle biopsy from a standardised site before and immediately after exercise through the same incision (Lindholm and Piehl 1974). One to 2 mg samples of muscle were freeze dried, dissected free of blood, fat and connective tissue and analysed for glycogen and lactate fluorometrically (Lowry and Passonneau 1973). Intravenous glucose tolerance tests An i.v. glucose tolerance test (0.5 mgkg bwt) was performed without fasting in 5 PSSM Quarter Horses, 3 ER Thoroughbreds, 4 normal Quarter Horses or crossbreds and 6 normal Thoroughbreds as previously described (De La Corte 1999). ANOVA for repeated measures was used to compare groups. Caffeine contracture testing Following local anaesthesia, a small strip of external intercostal muscle was removed intact from tendon to tendon from 5 PSSM Quarter Horses, 5 RER Thoroughbreds, 4 normal Quarter Horses or crossbreds and 5 normal Thoroughbreds donated to the University. Muscle samples were placed in oxygenated physiological saline solution and intact muscle fibre bundles were isolated as previously described (Lentz et al. 1999a). Muscle bundles were mounted between horizontal platinum electrodes in a water-jacketed chamber at 3840 C and allowed to equilibrate. Caffeine was used to stimulate calcium release

4 536 Recurrent exertional rhabdomyolysis TABLE 3: Serum creatine kinase (CK) activity 4 h after min of submaximal exercise in 18 Quarter Horses (QHs) and 18 Thoroughbreds (TB) with recurrent exertional rhabdomyolysis CK (u/l) Number of QHs Number of TBs < > >loo >2000 s 10, >10, , from the sarcoplasmic reticulum directly. Each bundle was exposed to all caffeine concentrations (0.1,0.2,0.5, 1.0,2.0,5.0, 7.5 and 10 mmol/l) in sequence, and both twitch and contracture forces were recorded. Aliquots from a freshly made caffeine stock solution were mixed into the muscle bath to yield the desired final concentration. At least 5 minutes elapsed between incremental caffeine additions to allow the contracture generated to reach a plateau before addition of the next concentration. Twitch and contracture forces were recorded with a Statham UC-2 transducer and compared for various caffeine concentrations using ANOVA. Muscle bundles did not differ significantly in maximum tetanic tension or cross-sectional area. Results are expressed as mean * s.e. Significance was set at P<0.05. Results Horses Eighteen horses of Quarter Horse-related breeds (Quarter Horses, American Paint Horses and Appaloosas) and 18 Thoroughbreds with ER met the inclusion criteria. The Quarter Horses comprised 13 mares, 3 geldings and 2 stallions, mean age 6.2 * 0.8 years (range 2-16). The Thoroughbreds comprised 19 mares and 1 gelding, mean age 6.7 * 0.9 years (range 2-16). Muscle histochemistry Fifteedl8 Quarter Horses had PAS positive inclusions and subsarcolemmal vacuoles consistent with a diagnosis of PSSM. The PAS positive inclusions were only partially digested when preincubated in 0.35% amylase for 10 min (Fig la). Abnormal polysaccharide accumulation was not found in muscle biopsies from any Thoroughbreds (Fig lb). The most common pathological feature in Thoroughbreds was the presence of mature muscle fibres with centrally located nuclei (Fig 1 b). This was not unique to the Thoroughbreds, however, and was found in 4 Quarter Horses, 3 of which had no abnormal polysaccharide in muscles samples (Table 1). Muscle glycogen concentrations Mean muscle glycogen concentration for the 15 Quarter Horses with PSSM was 825 * 60 mmolkg (range mmol/kg). Muscle glycogen concentrations for the 3 other Quarter Horses were 520, 590 and 543 mmol/kg. The glycogen concentrations in PSSM Quarter Horses was significantly higher than the mean for RER Thoroughbreds of 564 * 32 mmolkg (range mmolkg). Mean * s.e. glycogen concentrations for normal Quarter Horses are 464 i 47 mmollkg and normal Thoroughbreds 545 * 58 mmolkg in our laboratory. T3, T4 and fractional excretion of electrolytes The mean T3 and T4 activities in Quarter Horses were not significantly different from Thoroughbreds (Table 2). No Quarter Horses or Thoroughbreds had T3 and T4 activities below the normal range. There was no difference in FE of sodium, potassium and chloride between Quarter Horses and Thoroughbreds. FE sodium was below the normal range in 2 Quarter Horses (1 PSSM) and 1 Thoroughbred. One other Quarter Horse and 2 other Thoroughbreds had Fe potassium below the normal range (Table 2). Three Thoroughbreds had Fe sodium >1.0 but 4.4%. Serum creatinine was normal in all horses. FE chloride was above the normal range in 2 Quarter Horses without PSSM and 1 Thoroughbred with no FE chloride values below normal. CK response to a single submaximal exercise challenge The post exercise CK was >500 ull in 14/18 (77%) Quarter Horses and 13/18 (72%) Thoroughbreds. A similar distribution of CK activity 4 h post exercise was seen between Quarter Horses and Thoroughbreds (Table 3). Clinical signs varying from slight stiffness to reluctance to move occurred in 10 Quarter Horses and 5 Thoroughbreds. CK response of Quarter Horses and Thoroughbreds to training Serum CK was significantly higher in the 3 Quarter Horses vs. the 3 Thoroughbreds during training. The Quarter Horses had high CK from the beginning of training (Fig 2). Two Quarter Horses with consistently elevated CK were rarely not able to exercise for more than 15 min without showing signs of muscle fasciculations, tucked up abdomen and stretched out stance. Serum CK activity was normal in the Thoroughbred horses until after 10 days training (Fig 2). Intermittent elevations in CK up to 30,870 u/l were then observed in 2 Thoroughbreds without obvious clinical signs. SET The average increase in CK 4 h after exercise for Quarter Horses was 878 * 335 u/l compared to 199 * 67 for Thoroughbreds (Table 4). Variations in post exercise CK activity were wide for Quarter Horses with values between 522 and 6230 ull. All Thoroughbreds had CK activity <595 u/l. Only one horse (a Quarter Horse) showed evidence of muscle stiffness following exercise. Mean muscle glycogen concentrations were significantly (P<O.Ol) higher before and after the exercise test in Quarter Horses compared to Thoroughbreds and they utilised significantly more glycogen during exercise (Table 4). Muscle lactate concentrations were significantly higher at rest, but not after exercise in the Quarter Horses compared to Thoroughbreds. Intravenous glucose tolerance test A similar increase in blood glucose concentration occurred in Quarter Horse and Thoroughbreds after glucose administration, but values then declined faster in PSSM Quarter Horses. Mean glucose concentrations were significantly lower in PSSM Quarter Horses than in ER Thoroughbreds during the i.v. challenge (Fig 3). No significant differences were found between normal Quarter Horses, normal Thoroughbreds or ER Thoroughbreds.

5 Stephanie J. Valberg eta[. 537 TABLE 4: Mean * 8.8. muscle glycogen and lactate concentrations before and after a near-maximal standardised exercise test as well as before and 4 h post exercise activity for 5 Quarter Horses (QH) and 5 Thoroughbreds (TB) with recurrent ER Glycogen (mmovkg) Lactate (mmol/kg) CK (u/l) QH TB QH TB QH TB Pre *96* 476 f f 12" 23* i i21 Post 831 *76* 333 f i i f f 80 *Indicates significant difference between breeds. Caffeine contracture testing The threshold for development of a muscle contracture was 5 mmovl caffeine for PSSM and normal Quarter Horses as well as normal Thoroughbreds (Lentz et al. 1999a,b) (Fig 4). In contrast, the contracture threshold was 1 mmol/l for all RER Thoroughbreds, significantly lower than the threshold for the PSSM Quarter Horses, normal Quarter Horse and normal Thoroughbreds. Discussion The purpose of this study was to determine if all horses with ER had one unifying aetiology or if multiple, possibly breedspecific, aetiologies exist for recurrent ER. Using conventional techniques such as physical examination, blood chemistry profile, T3 and T4 concentrations, and FE of sodium, potassium and chloride, it was not possible to identify any significant differences between Quarter Horse and Thoroughbreds with ER. Hypothyroidism was not identified in any horse and <20% of horses of either breed had significantly low fractional excretions of sodium and potassium. It is possible that subtle functional alterations could be detected in some ER horses if TRH stimulation or evaluation of other electrolytes were performed. Serum CK activity has copventionally been used to diagnose ER. A similar prpportion of Quarter Horses and Thoroughbreds with recent episodes of ER had elevations in serum CK when a single exercise challenge was performed on a lunge-line. Different serum CK responses were, however, noted for the first 3 weeks of treadmill training when CK activity was determined daily in 3 unfit Quarter Horses with PSSM and 3 unfit Thoroughbreds. Increased CK and clinical signs of rhabdomyolysis occurred consistently in 2/3 unfit PSSM Quarter Horses, whereas, CK increased intermittently without clinical signs in unfit RER Thoroughbreds. More frequent episodes of rhabdomyolysis in PSSM horses were also supported by muscle histopathology. Twice as many Quarter Horses (10/18) had acute myonecrosis and macrophage infiltration than Thoroughbreds. A number of myopathies have been described in man that are associated with high muscle glycogen concentrations and increased CK with exercise (DiMauro and Tsujino 1994). A few of these myopathies are also characterised by abnormal polysaccharide accumulation. Accumulation of abnormal polysaccharide and glycogen in skeletal muscle was the clearest distinguishing feature between Quarter Horses (W18 horses) and Thoroughbreds (011 8 horses). Mean muscle glycogen concentrations were at least 1.5 times higher in the Quarter Horses with PSSM compared to the Thoroughbreds. Subsequent studies of ER by other investigators have led to the diagnosis of equine polysaccharide storage myopathy in many breeds including Thoroughbred horses. In these studies, however, the presence of abnormal polysaccharide was not a necessary diagnostic criteria for PSSM (Valentine et af. 1997, 1998). A few Thoroughbred horses in the present study had high muscle glycogen concentrations (range for 4 TBs mmolkg); however, no abnormal polysaccharide was present in any of these horses. Therefore, using the presence of abnormal polysaccharide as the diagnostic criteria for PSSM, this specific myopathy is common in Quarter Horses with ER, but does not appear to be a prevalent cause of ER in Thoroughbreds. Most glycogen storage disorders are caused by an inherited deficiency of a glycogenolytic or glycolytic enzyme that impairs glycogen/glucose metabolism and lactate production (Cardinet 111 and Holliday 1979; DiMauro and Tsujino 1994). PSSM horses, however, were able to metabolise glycogen to lactic acid during the SET. Glycogen accumulation occurred in PSSM in the face of normal glycogen metabolism. Therefore, a glucose tolerance test was performed to determine if enhanced blood glucose clearance and muscle glycogen synthesis could be responsible for polysaccharide accumulation in the Quarter Horses. Insulin is released immediately following glucose administration and skeletal muscle acts as the major insulinresponsive glucose sink (Gould 1997). The results of the glucose tolerance test and results published previously confirmed that PSSM horses have enhanced blood glucose clearance. Insulin concentrations as well as insulin tolerance tests showed that PSSM horses have increased insulin sensitivity (De La Corte et al. 1999). PSSM appears to be inherited in Quarter Horses, possibly as an autosomal recessive trait (Valberg et al. 1995). Therefore, PSSM seems to be a novel, heritable form of glycogenosis in Quarter Horses, involving enhanced glucose transport and glycogen synthesis. Treatment for this disorder includes a combination of a low soluble carbohydrate, fatsupplemented diet and regular daily exercise to keep horses fit and metabolise glycogen stores (Valberg et al. 1997) The only consistent histopathological feature of Thoroughbreds with ER in this study was the presence of centrally located nuclei in mature muscle fibres and occasional evidence of muscle degeneration. Central nuclei were present in 17/18 Thoroughbreds, and 4 Quarter Horses (including 1 with PSSM). Centrally located nuclei are considered a nonspecific pathological finding present in many disorders (Cumming et al. 1994). In recurrent ER, central nuclei may be due to continuous subclinical muscle regeneration. The most specific abnormality in Thoroughbreds with recurrent ER was the abnormally low threshold for contracture of intact intercostal muscles in the presence of caffeine. Caffeine contracture tests in PSSM horses were similar to those of normal Quarter Horses and normal Thoroughbreds (Lentz et al. 1999b). Caffeine contracture tests are used to identify susceptibility to malignant hyperthermia in man with a relevant family history, prior to undergoing halothane anaesthesia (Mickelson and Louis 1996). In addition to halothane, stress and exercise may precipitate rhabdomyolysis

6 538 Recurrent exertional rhabdomyolysis with malignant hyperthermia. One of the Thoroughbred horses tested in this study previously had a post anaesthetic myopathy (CK >500,000 ua) and all Thoroughbreds had intermittent episodes of rhabdomyolysis with exercise. A proportion of human subjects and all swine with malignant hyperthennia have an inherited defect in the calcium release channel (Mickelson and Louis 1996). However, functional assays of the calcium release channel in isolated membrane preparations have not revealed any abnormalities in Thoroughbreds with recurrent ER (Ward et al. 1999). Therefore, the underlying defect in recurrent ER differs from that in swine with malignant hyperthermia. A genetic relationship among Thoroughbreds with recurrent ER, including those in this study, has indicated a dominant mode of transmission (MacLeay et al. 1999a). Therefore, it appears that in a family of Thoroughbreds, ER is due to a novel defect in muscle contraction that may be attributed to an unknown abnormality in intracellular calcium regulation. Since all 5 Thoroughbred horses (3 racehorses, 2 riding horses) in this study had identical contractile defects it seems likely that this abnormality is present in many other Thoroughbreds with ER. In conclusion, ER is a syndrome that has several aetiologies. A common cause of recurrent ER in Quarter Horses is PSSM, which is best diagnosed by identifying abnormal polysaccharide in frozen muscle sections. In contrast, recurrent ER in a proportion of Thoroughbreds appears associated with a heritable defect in muscle contraction. Specific diagnosis of this form of ER is done practically by history, physical examination, exercise testing and the finding of numerous central nuclei in muscle biopsies. The caffeine contracture test was a very sensitive test for recurrent ER in Thoroughbreds; however, the requirement of an intact muscle and a specialised laboratory limit its practical application. Acknowledgement Funding from the American Quarter Horse Association, the American Horse Show Association, the Grayson Jockey Club Research Foundation, Morris Animal Foundation and the Minnesota Equine Research Centre is gratefully acknowledged. References Beech, J.. Lindborg, S., Fletcher, J.E., Lizzo, F., Tripolitis, L. and Braund, K. ( 1993) Caffeine contractures, twitch characteristics and the threshold for calcium induced calcium release in skeletal muscle from horses with chronic intermittent rhabdomyolysis. Res. vet. Sci. 54, Cardinet, G.H. and Holliday, T.A. (1979) Neuromuscular disease of domestic animals: a summary of muscle biopsies from 159 cases. Ann. N. Y. Acad. Sci. 317, Cumming, K, Fulthorpe, J., Hudgson, P. and Mahon, M. 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