Metabolic myopathies
8 Interesting Facts of metabolic myopathies
1. Muscle cramps, pain, or myoglobinuria brought on by exercise suggests a metabolic myopathy.
2. Muscle symptoms with short bursts of high-intensity exercise and the second wind phenomenon are characteristic of a glycogen storage disease (GSD). McArdle disease is most common.
3. Muscle symptoms with prolonged low-intensity exercise and/or prolonged fasting suggest a defect in fatty acid oxidation. Carnitine palmitoyltransferase II (CPT II) deficiency is most common.
4. Elevated resting serum lactate level and/or ragged red fibers on muscle biopsy are characteristic of a mitochondrial myopathy (MM).
5. The most common metabolic myopathies associated with myoglobinuria are CPT II deficiency and McArdle disease.
6. The most common myopathies confused with polymyositis are acid maltase deficiency (Pompe disease) and the limb-girdle muscular dystrophies (LGMDs).
7. Children presenting with a muscle disease without rash almost always have a metabolic or genetic myopathy and not primary polymyositis.
8. If evaluation for a metabolic myopathy does not yield an answer, consider a metabolic presentation of a muscular dystrophy or RYR1-associated myopathy.
Metabolic myopathies are conditions that have in common abnormalities in muscle energy metabolism that result in skeletal muscle dysfunction. Primary metabolic myopathies are associated with biochemical defects that affect the ability of muscle fibers to maintain adequate levels of adenosine triphosphate (ATP). The three main categories of primary metabolic myopathies are GSDs, fatty acid oxidation defects, and mitochondrial cytopathies due to respiratory chain impairment. Because many of the enzyme defects are partial, these diseases can manifest at various ages from infancy through adulthood. Secondary metabolic myopathies are attributed to various endocrine and electrolyte abnormalities.
Metabolic myopathies can be divided into two groups based on clinical presentation: (1) the acute form manifesting repeated episodes of myalgias, exercise intolerance, hyperCKemia, contractures, rhabdomyolysis, and myoglobinuria with a normal examination between episodes, and (2) the progressive form with proximal and axial muscle weakness, and often accompanied by other organ system involvement such as the liver, heart, endocrine glands, or peripheral and central nervous systems (CNS).
