Contracture test and histologic and histochemical analyses of muscle biopsy specimens from horses with exertional rhabdomyolysis.

This research focuses on the application of the halothane-caffeine contracture test and histologic and histochemical evaluations on biopsy specimens from horses experiencing muscle inflammation due to mild exercise. The study didn’t reveal any similarities between the contracture response in these biopsied muscles and malignant hyperthermia, contradicting the previous observation in one horse. The analysis also showed generally normal muscle fiber makeup and morphology in most horses, with one unique case of necrotic fibers.

Methodology and Findings

• The study involved obtaining biopsy samples from the cutaneous omobrachialis muscle, part of the pectoral muscle allowing movement in the shoulder, of 10 horses presenting with myositis due to lightweight exercise.
• Historically, one of these horses had shown indications of malignant hyperthermia-like contractures in a previous evaluation.
• This time around, the biopsy samples were put through a halothane-caffeine contracture test and histologic and histochemical reviews.
• In the contracture test, no muscle sample contracted when exposed to either 2 or 4% of halothane independently.
• The average concentration of caffeine in the presence of halothane was 5.23 +/- 0.5 mM for 2% halothane and 4.46 +/- 0.6 mM for 4% halothane. The values were, however, statistically indistinguishable.
• The contracture responses didn’t match contracture associated with malignant hyperthermia.

Further Observations

• The cutaneous omobrachialis muscle was made up mainly of type-II fibers, with only a few type-I fibers observed.
• The overall fiber form was normal for all but one horse, which showed necrotic, or dead, fibers.
• Most of the muscles (9 out of 10) displayed fibers that reacted positively for alkaline phosphatase activity, an enzyme linked to healing and growth. Some (3 out of 10) had ringed myofibers, typically seen in situations of muscle injury or disease.
• Three horses, one clinical case and two research horses, were given general anesthesia; two with a mixture of halothane and succinylcholine and one with halothane and a non-depolarizing muscle relaxant.
• A research horse experienced severe rhabdomyolysis—rapid degradation of skeletal muscle—due to anesthesia, coupled with an episode of muscle cramping two days post-anesthesia. However, the other two horses showed normal post-anesthetic behavior.