Exercise intolerance with lactic acidosis

Exercise intolerance with lactic acidosis (also referred to as exercise intolerance due to mitochondrial complex deficiencies or mitochondrial myopathy with exercise intolerance and lactic acidosis) is a group of rare inherited metabolic disorders characterized by an inability to perform sustained physical activity due to impaired mitochondrial energy production in skeletal muscle. During exercise, affected individuals experience excessive fatigue, muscle pain (myalgia), muscle weakness, and disproportionate shortness of breath. A hallmark biochemical finding is elevated blood lactate (lactic acidosis) during and after physical exertion, reflecting the failure of oxidative phosphorylation and a compensatory shift toward anaerobic metabolism. The condition primarily affects skeletal muscle, though other high-energy-demand tissues such as the heart and brain may also be involved depending on the specific genetic defect. Multiple genetic causes have been identified, including mutations in genes encoding subunits or assembly factors of the mitochondrial respiratory chain complexes (e.g., MTCYB encoding cytochrome b of complex III, or genes affecting complex I, complex IV, or coenzyme Q10 biosynthesis). Some forms are caused by mutations in mitochondrial DNA (mtDNA), while others follow autosomal recessive inheritance patterns involving nuclear-encoded mitochondrial genes. The clinical severity can range from mild exercise limitation to significant disability. Diagnosis typically involves exercise testing demonstrating disproportionate lactic acidosis, muscle biopsy showing mitochondrial abnormalities (such as ragged red fibers or cytochrome c oxidase-negative fibers), biochemical analysis of respiratory chain enzyme activities, and molecular genetic testing. There is currently no curative treatment. Management is largely supportive and may include graded exercise programs, avoidance of metabolic stressors, and supplementation with agents such as coenzyme Q10, riboflavin, L-carnitine, or other mitochondrial cofactors, although evidence for their efficacy remains limited. Genetic counseling is recommended given the variable inheritance patterns.

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