RheumatologyTier 2Disease (DEADMAN)

Myopathies - metabolic

Description

Metabolic myopathies are inherited disorders of energy metabolism affecting muscle - distinct from the inflammatory myopathies (polymyositis, dermatomyositis) in mechanism, typically presenting with exercise intolerance and recurrent rhabdomyolysis rather than progressive proximal weakness

Individually rare; the two most clinically important for exertional rhabdomyolysis are McArdle disease (glycogen storage disease) and CPT2 deficiency (fatty acid oxidation disorder), covered in depth in their own dedicated notes

Defects in glycogenolysis/glycolysis (e.g. McArdle disease - myophosphorylase deficiency) or fatty acid oxidation (e.g. CPT2 deficiency) impair the muscle's ability to generate ATP under specific metabolic demand conditions - the specific exercise trigger pattern (brief high-intensity vs prolonged/fasting-associated) reflects which fuel pathway is defective

Exercise intolerance, cramping, and recurrent rhabdomyolysis triggered by a specific, reproducible pattern of exertion (rather than the progressive, exercise-independent proximal weakness of inflammatory myopathy) is the key distinguishing clinical feature
Creatine kinase is often persistently mildly elevated between episodes, with marked elevation during acute rhabdomyolysis; specific diagnostic testing (forearm exercise test, genetic testing) as detailed in the McArdle disease and CPT2 deficiency notes

Avoidance of the specific triggering exercise pattern, and management of acute rhabdomyolysis episodes when they occur (hydration, renal function monitoring) - management is condition-specific (see McArdle disease and CPT2 deficiency for detail)

McArdle disease, CPT2 deficiency, acute kidney injury from myoglobinuria, malignant hyperthermia (a distinct but sometimes confused entity - a pharmacogenetic anaesthetic reaction, not a primary metabolic myopathy)

Distinguishing a metabolic myopathy from an inflammatory myopathy by the pattern of symptoms (trigger-specific exercise intolerance vs progressive weakness) avoids inappropriate immunosuppression for a condition that would not respond to it and instead requires exercise/dietary modification

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