(E52) Delayed, Severe Rhabdomyolysis Following Uncomplicated Anesthesia in Six Warmblood Horses

Abstract: Background- Warmblood horses have not been reported to be at high risk for post-anesthetic myopathy.Objectives- Document the clinical presentation of severe rhabdomyolysis following uncomplicated anesthesia of 6 Warmblood horses with no history of myopathy.Animals- 6 Warmbloods, 5 geldings, 1 mare, mean age 10 years (range 4-9 years), weight 613 kg (range 550-703 kg).Methods- Records were reviewed and pre- and post- anesthetic bloodwork, management, muscle histopathology and post-mortem results compiled.Results - Severe rhabdomyolysis developed following uncomplicated anesthesia for exploratory celiotomy (3), cervical imaging (1), arthroscopy (1), and intraarticular injection (1). One horse never stood after anesthesia and a second stood but collapsed the following day. At time of euthanasia both horses had hyperlactatemia (12 and 21 mmol/L) and the latter had markedly increased creatine kinase (CK) activity (32559 U/L). Acute myodegeneration and recumbency developed 7 (CK 193273 U/L), 9 (CK >14000) and 10 days following anesthesia with prolonged recoveries in 3 horses that culminated in euthanasia. Macrophages, lymphocytes (2) and severe glycogen depletion (1) were evident in skeletal muscle. The surviving horse had prolonged recovery with rhabdomyolysis (CK 29800 U/L) evident 3 days post-anesthesia and focal myofibers had glycogen depletion at biopsy 13 days post-anesthesia. Histopathologic evidence of chronic myopathy (internalized myonuclei in mature fibers) was present in two horses. Conclusions and clinical importance- Severe rhabdomyolysis can occur in Warmblood horses with no history of myopathy up to 10 days after anesthesia. Fulminant anaerobic glycolysis during anesthesia could have caused the noted glycogen depletion and lactic acidosis.