Dantrolene sodium (Dantrium) is used to control exertional rhabdomyolysis (ER, also called tying-up) in horses. Diagnosis of ER is typically confirmed by a finding of increased serum creatine kinase (CK), which is often greater than 1,000 IU at its peak (normal CK levels are around 200 IU). While Dantrium has been in use for about 20 years, there is little published data regarding its efficacy in horses. For this reason, John Edwards, BVSc, BSc, MSc, PhD, CertSHP, MRCVS, from Fenton Veterinary Practice in Pembrokeshire; Richard Newton, BVSc, MSc, PhD, FRCVS, from the Animal Health Trust, and Robert Pilsworth, MA,VetMB, BSc, CertVR, MRCVS, from Rossdale and Partners in Newmarket, United Kingdom, conducted a study to investigate the efficacy of Dantrium for controlling ER in racehorses.

Oral doses of 800 mg of Dantrium and equivalent volumes of placebo were prepared for 77 racehorses. Treatment coincided with a return to exercise after two days of strict stall rest. Each horse received either Dantrium or placebo one hour before exercise. Blood samples were collected at the time of treatment and six hours after exercise, which consisted of at least one hour of trotting. After a week, the alternative treatment was administered, so that each horse served as its own control.

There was an overall mean increase in serum CK of 104.8 IU in the placebo group after exercise. Dantrium-treated horses had a mean increase of 15.2 IU. This finding supports the conclusion that Dantrium can reduce the difference between pre- and post-exercise serum CK levels, thereby preventing ER in animals that are susceptible. In addition, no horse in the study treated with Dantrium developed ER, while three horses receiving the placebo (4%) developed ER following the exercise period. Again, this finding supports the use of Dantrium for the prevention of ER in susceptible horses.

This raises the question of whether all racehorses should be given Dantrium prophylactically (to avoid rather than treat problems). “My understanding,” Newton says, “is that it would be unnecessary and expensive to treat all racehorses. Dantrium is better used in a targeted manner for horses with a known history of tying-up