OBJECTIVE To characterize clinical findings for polysaccharide storage myopathy (PSSM) in warmblood horses with type 1 PSSM (PSSM1; caused by mutation of the glycogen synthase 1 gene) and type 2 PSSM (PSSM2; unknown etiology). SAMPLE Database with 3,615 clinical muscle biopsy submissions. PROCEDURES Reported clinical signs and serum creatine kinase (CK) and aspartate aminotransferase (AST) activities were retrospectively analyzed for horses with PSSM1 (16 warmblood and 430 nonwarmblood), horses with PSSM2 (188 warmblood and 646 nonwarmblood), and warmblood horses without PSSM (278). Lameness examinations were reviewed for 9 warmblood horses with PSSM2. Muscle glycogen concentrations were evaluated for horses with PSSM1 (14 warmblood and 6 nonwarmblood), warmblood horses with PSSM2 (13), and horses without PSSM (10 warmblood and 6 nonwarmblood). RESULTS Rhabdomyolysis was more common for horses with PSSM1 (12/16 [75%] warmblood and 223/303 [74%] nonwarmblood) and nonwarmblood horses with PSSM2 (221/436 [51%]) than for warmblood horses with PSSM2 (39/147 [27%]). Gait abnormality was more common in warmblood horses with PSSM2 (97/147 [66%]) than in warmblood horses with PSSM1 (1/16 [7%]), nonwarmblood horses with PSSM2 (176/436 [40%]), and warmblood horses without PSSM (106/200 [53%]). Activities of CK and AST were similar in warmblood horses with and without PSSM2. Muscle glycogen concentrations in warmblood and nonwarmblood horses with PSSM1 were significantly higher than concentrations in warmblood horses with PSSM2. CONCLUSIONS AND CLINICIAL RELEVANCE Rhabdomyolysis and elevated muscle glycogen concentration were detected in horses with PSSM1 regardless of breed. Most warmblood horses with PSSM2 had stiffness and gait abnormalities with CK and AST activities and muscle glycogen concentrations within reference limits.

Objective-To determine whether alterations in myoplasmic calcium regulation can be identified in muscle cell cultures (myotubes) and intact muscle fiber bundles derived from Thoroughbreds affected with recurrent exertional rhabdomyolysis (RER). Animals-6 related Thoroughbreds with RER and 8 clinically normal (control) Thoroughbred or crossbred horses. Procedures-Myotube cell cultures were grown from satellite cells obtained from muscle biopsy specimens of RER-affected and control horses. Fura-2 fluorescence was used to measure resting myoplasmic calcium concentration as well as caffeine- and 4-chloro-m-cresol (4-CMC)-induced increases in myoplasmic calcium. In addition, intact intercostal muscle fiber bundles were prepared from both types of horses, and their sensitivities to caffeine- and 4-CMC-induced contractures were determined. Results-Myotubes of RER-affected and control horses had identical resting myoplasmic calcium concentrations. Myotubes from RER-affected horses had significantly higher myoplasmic calcium concentrations than myotubes from control horses following the addition of ≥ 2mM caffeine; however, there was no difference in their response to 4-CMC (greater than 1mM). Caffeine contracture thresholds for RER and control intact muscle cell bundles (2 vs 10mM, respectively) were significantly different, but 4-CMC contracture thresholds of muscle bundles from RER-affected and control horses (500µM) did not differ. Conclusions and Clinical Relevance-An increase in caffeine sensitivity of muscle cells derived from a family of related RER-affected horses was detected in vitro by use of cell culture with calcium imaging and by use of fiber bundle contractility techniques. An alteration in muscle cell calcium regulation is a primary factor in the cause of this heritable myopathy.

Quantitative immunodiffusion in one dimension was performed in 6-mm Duran tubes containing a 1% Nobel agar solution and various dilutions of antisera. A series of dilutions of pure myoglobin in equine sera as well as plasma from horses with rhabdomyolysis were tested. Standard curves were prepared of the migration distance of the formed precipitate from the meniscus of the gel after 3, 6, 12, and 24 hours. The clearest line of precipitate was formed with a 1:20 dilution of antisera in agar. Standard curves were nonlinear and plasma myoglobin could be detected at 2 micrograms of myoglobin/ml or greater. The test was optimal, with an error of 5.6%, when read at 24 hours at approximately 25 C. Tubes with agar could be stored for 6 months at 4 C without affecting the accuracy of the test. The specificity of myoglobin for skeletal or cardiac muscle, and its rapid clearance from serum after muscle necrosis, make it ideally suited for evaluating acute muscle damage and for testing the susceptibility of horses for rhabdomyolysis following an exercise test.

The concentration of carbonic anhydrase III isoenzyme (cA-III) in serum samples from 216 clinically normal Thoroughbreds was determined by use of an enzyme immunoassay. The concentration range of cA-III was from 16.0 to 254.5 ng/ml (mean, 56.5 +/- 11.9 ng/ml). Significant differences were not detected according to age or sex. To confirm whether serum cA-III concentration was high in horses with muscle disease, serum samples of 11 horses with exertional rhabdomyolysis were analyzed by enzyme immunoassay. Their serum cA-III concentration was about 56 times (3,136 +/- 2,610 ng/ml) that of healthy Thoroughbreds. Concentration of cA-III was higher in horses with rhabdomyolysis that had been transiently recumbent than in horses with mild disease that were reluctant to move. Blood samples obtained serially from 6 horses with exertional rhabdomyolysis were studied. Serum activities of aldolase, creatine kinase, aspartate transaminase, and lactate dehydrogenase were high. Increases and decreases in concentration of cA-III were more rapid than that for aldolase, creatine kinase, aspartate transaminase, and lactate dehydrogenase activities; thus, cA-III may be clinically applicable as a diagnostic marker for muscle disease in horses.

In vitro twitch characteristics of the semimembranosus muscle were evaluated in 9 clinically normal horses, in 15 horses with chronic intermittent rhabdomyolysis (CIR) and in 2 horses with myotonia. Effects of phenytoin on in vitro muscle twitch and clinical signs of CIR and myotonia were evaluated in these same horses. Times to 90% relaxation were prolonged in the horses with CIR (mean +/- SEM, 186 +/- 5.9 ms) and in 2 horses with myotonia (197 and 177 ms) compared with those in clinically normal horses (mean +/- SEM, 146 +/- 2.1 ms). Horses with CIR also had significantly (P < 0.05) longer times to 50% relaxation, compared with clinically normal horses. In the group of horses with CIR, Standardbreds had significantly (P < 0.05) longer times to 90% and 50% relaxation, compared with Thoroughbreds. Times to 100% peak tension did not differ among the groups. Administration of phenytoin directly into a muscle preparation bath solution had no effect on muscle twitch properties. After the initial muscle biopsy, phenytoin was administered orally for 7 to 10 days to 4 horses with CIR, 2 myotonic horses, and 2 clinically normal horses before repeat biopsy from the same site in the contralateral semimembranosus muscle. Times to 90% relaxation decreased from 197 and 177 ms to 144 and 126 ms, respectively, in the 2 myotonic horses, from a mean of 192 (+/- 9) ms to 170 (+/- 9) ms in the 4 horses with CIR and remained unchanged (154 and 140 ms before vs 155 and 139 ms after treatment) in the 2 clinically normal horses. Phenytoin treatment of 8 horses with CIR was associated with excellent clinical response in 7; 1 horse became lame, which prevented evaluation of the drug, and the other horse with normal muscle twitch properties continued to have seasonally severe CIR. Of the 9 horses with CIR that were not treated, 4 were lost to evaluation, 3 continued to be affected (but 1 of these often performed well), and 3 were reported to perform satisfactorily. After 10 days of treatment, the 2 myotonic horses had no change in gait or myotonic dimpling and myotonic discharges persisted, although subjectively, they were slightly decreased. Phenytoin appears to be useful clinically for treating horses with CIR.