Case reveal

Findings:

• Bilateral middle gluteal muscle, moderate/severe and diffuse IRU (yellow outline).

• Right tarsocrural joint, mild and diffuse IRU.
• Left proximal/mid aspect of the 2nd metatarsal bone, moderate, focal and linear IRU.
• Bilateral distal radial, mild and diffuse IRU at the physeal scar extending distally.

Conclusions:

• Severe rhabdomyolysis, affecting the middle gluteal musculature.
• Left MTII focal, linear increased uptake, likely consistent with focal trauma or exostosis.
• Suspected right tarsal osteoarthrosis.

Follow-up:

Testing:

• Cerebrospinal fluid (CSF) cytology: Normal cytology with slightly high protein concentration
• Cerebrospinal fluid Sarcocystis neurona ELISA and CSF:serum ratio analysis: Negative
• Cerebrospinal fluid Lyme multiplex (Osp A,C,F) and serum:CSF ratio: Normal
• Muscle enzyme testing (CK/AST):
  • Prior to exercise:
    • CK: 617 u/L (117-555)
    • AST: 347 u/L (148-322)
  • 4.5 hours after exercise:
    • CK: 1182 u/L
    • AST: 18095 u/L
• Semimembranosus muscle biopsy: normal
• Given the patient’s breed and history, the cause of the exertional rhabdomyolysis is almost certainly polysaccharid storage myopathy (PSSM).

Sent home with medical management, including a combination of nutritional and dietary modifications.  Daily caloric intake was reduced, and low level and frequent exercising was advised.

A little bit more…

• Rhabdomyolysis is also referred as “‘Monday morning disease’ or ‘tying up’ (Keen et al.).
• Multiple causes (not all are understood). The different classifications of rabdomyolysis are listed in the following table (Keen et al.):

• Exertional rhabdomyolysis is the most common. It may secondary to training beyond the horse’s fitness level, or may be seen in some horses laid off for a period of time (usually one or more days) without reduction in feeding. Excessive lactic acid build up in the musculature may result in muscle damage (Keen et al., Naylor et al.).
• Other less common muscle disease associated with signs of rhabdomyolysis:

• Recurrent/chronic exertional rhabdomyolsysis may be prevalent in thoroughbred race horses. The cause is less well understood. it may be associated with underlying vitamin or electrolyte imbalances or secondary environmental temperature changes. Muscle damage is thought to be due to an impairment in the ability of skeletal muscle to control intracellular calcium levels (Keen et al.).
• Polysaccharide storage myopathy affects primarily quarterhorses, draughthorses and warmbloods. Testing is warranted in those breeds if chronic rhabdomyolysis is suspected (Keen et al., Naylor et al.). 
• The severity of the clinical signs is widely variable and doesn’t always correlate with the severity of muscle damage present. Some horse’s gait may be mildly altered, while other horses may be unable to perform, present recumbent or less commonly die (Keen et al.).
• Muscles of both hindlimbs are usually affect, particularly the gluteal, femoral and lumbar muscles. In acute cases, the muscles often are painful, firm and thickened on palpation. Horses may present distressed, sweating and febrile with tachycardia and tachypnea (Keen et al.).

• Bloodwork and urinalysis are required at a minimum (increased serum concentrations of creatine kinase and aspartate aminotransferase observed). Muscle biopsies are indicated in chronic cases (Keen et al.).
• Nuclear scintigraphy is useful to screen for muscle damage, especially in chronic cases (Dyson et al., Carnicer et al.).

• Therapy varies between acute and chronic cases (Keen et al., Naylor et al.):
  • Acute: rest, fluid, pain therapy, sedation, corticosteroids, +/- calcium carbonate and calcium gluconate.
  • Chronic: Diet change, low impact exercise, +/- electrolyte/vitamin supplementation.

References:

• Keen, John. “Diagnosis and management of equine rhabdomyolysis.” In Practice 33.2 (2011): 68-77.
• Naylor, Rosie. “Managing muscle disease in horses.” In Practice 36.8 (2014): 418-423.
• Dyson, Sue. “Musculoskeletal scintigraphy of the equine athlete.” Seminars in nuclear Medicine. Vol. 44. No. 1. WB Saunders, 2014.
• Carnicer, David, et al. “Increased radiopharmaceutical uptake in skeletal muscle in 26 flat racing and endurance horses (2017–2021).” Equine Veterinary Education 36.8 (2024): 410-422.