Effect of phenytoin on the clinical signs and in vitro muscle twitch characteristics in horses with chronic intermittent rhabdomyolysis and myotonia.

This study investigates how the drug phenytoin impacts the physical symptoms and muscle behavior of horses with chronic intermittent rhabdomyolysis (CIR) and myotonia. The researchers found that horses with these conditions had longer intervals of muscle relaxation after muscle twitching. They also discovered the administration of phenytoin provided significant improvement in relaxation times for CIR and myotonic horses, but did not affect normal horses.

Study Process and Sample Groups

• This study was conducted on a total of 26 horses divided into three groups: nine were deemed clinically normal or “healthy”, 15 horses with chronic intermittent rhabdomyolysis (CIR), and two with myotonia.
• The in vitro twitching characteristics of the semimembranosus muscle were analyzed in all the horses, which involves studying the muscle’s response to stimulation in a controlled, lab environment.

Initial Findings

• Muscle relaxation times post-twitching were found to be longer in horses with CIR and myotonia than in the clinically normal – these times were measured up to 90% and 50% relaxation markers.
• Surprisingly, when comparing different breeds of horses with CIR, it was found that Standardbred horses had significantly longer relaxation times than Thoroughbreds.

Administration of Phenytoin

• Phenytoin, the drug under observation, was initially administered directly into a muscle preparation bath solution which showed no effect on muscle twitch properties.
• The drug was then administered orally for 7 to 10 days to the horses with CIR, myotonic horses, and the clinically normal horses. Following this treatment, a muscle biopsy was taken from the same site in the contralateral (opposite side) semimembranosus muscle of each horse.

Final Findings After Treatment

• Relaxation times dropped significantly in both the myotonic and CIR horses after the phenytoin treatment: from 197 and 177 ms to 144 and 126 ms in the myotonic horses; and from a mean of 192 ms to 170 ms in the CIR horses.
• In contrast, the relaxation times of the clinically normal horses did not change pre- and post-treatment, implying that phenytoin does not have an affect on horses without muscle tension disorders.