Postanesthetic rhabdomyolysis in 7 warmblood horses

To describe the clinical findings, outcomes, and muscle histopathology in warmblood horses that developed severe rhabdomyolysis in the perianesthetic period despite remaining stable while under general anesthesia.

7 warmblood horses, 6 geldings and 1 mare, with a median age of 9 years (range, 4 to 18 years) and median weight of 615 kg (range, 550 to 703 kg). Records from the Valberg Neuromuscular Diagnostic Laboratory and Michigan State University were reviewed (2016 to 2023) to identify warmbloods with postanesthetic myopathy (PAM).

Warmblood horses with no history of myopathy developed PAM after remaining stable while under general anesthesia. Five of 7 horses were in regular work prior to anesthesia, and activity level was unknown in 2 horses. Time to standing in recovery was prolonged, 3 horses were euthanized due to persistent recumbency, and rhabdomyolysis recurred in 4 horses 5 to 11 days after anesthesia, with 1 surviving. Horses had muscle stiffness, pain, and sweating and struggled to remain standing. As PAM developed, serum creatine kinase activity and lactate concentrations (12 ± 7 mmol/L; n = 5) were markedly increased.

At necropsy, histopathology revealed complete glycogen depletion (5 of 7), acute myodegeneration (6 of 7), and chronic active myodegeneration of representative skeletal muscle samples. A semimembranosus biopsy obtained 14 days after anesthesia from the survivor had rare glycogen-depleted fibers.

Warmblood horses are susceptible to fatal PAM characterized by acute myodegeneration, lactic acidosis, and muscle glycogen depletion that occurs up to 11 days after anesthesia. In horses with delayed recovery after anesthesia, monitoring for 2 weeks after anesthesia, including assessment of serum creatine kinase activity and blood lactate, could potentially improve outcomes.