NAVLE Musculoskeletal

Equine Exertional Rhabdomyolysis Study Guide

Exertional rhabdomyolysis (ER), commonly known as "tying-up," "azoturia," or "Monday morning disease," is a syndrome characterized by exercise-associated skeletal muscle damage resulting in muscle pain, stiffness, and reluctance to move.

Overview and Clinical Importance

Exertional rhabdomyolysis (ER), commonly known as "tying-up," "azoturia," or "Monday morning disease," is a syndrome characterized by exercise-associated skeletal muscle damage resulting in muscle pain, stiffness, and reluctance to move. The term rhabdomyolysis literally means "dissolution of striated muscle." This condition has been recognized in horses for over 100 years and remains a significant performance-limiting or career-ending disorder for many equine athletes. ER represents a high-yield topic for the NAVLE due to its clinical significance, breed predispositions, and complex pathophysiology involving both sporadic and heritable forms.

The syndrome encompasses multiple distinct conditions with different underlying causes but similar clinical presentations. Understanding the classification, pathophysiology, diagnosis, and management of each form is essential for NAVLE success and clinical practice.

Sporadic ER Chronic (Heritable) ER
Definition: Single or infrequent episodes due to extrinsic factors Definition: Repeated episodes due to intrinsic muscle abnormalities
Causes: Overexertion beyond conditioning level, electrolyte imbalances, viral respiratory infections, heat/humidity, inadequate warm-up Types: PSSM Type 1 (GYS1 mutation), PSSM Type 2 (unknown cause), RER (calcium dysregulation), Malignant Hyperthermia (RYR1 mutation)
Breed Predisposition: Any breed; common in endurance horses Breed Predisposition: PSSM1: Quarter Horses, Drafts, Warmbloods; RER: Thoroughbreds, Standardbreds, Arabians
Prognosis: Excellent with rest and correction of underlying cause Prognosis: Good to guarded; requires lifelong dietary and exercise management

Classification of Exertional Rhabdomyolysis

Exertional rhabdomyolysis is broadly classified into two categories based on the underlying etiology:

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