How to evaluate suspected metabolic muscle disease

How do you clinically evaluate patients with suspected metabolic muscle disease?

The evaluation begins with a careful history and thorough physical exam. The problem of diagnosing metabolic myopathies is confounded because at rest, patients are usually asymptomatic and have normal physical findings. The importance of the physical exam is to pick up any additional abnormalities such as neurologic defects which might indicate MM. Patients with metabolic myopathies typically present with one of the following:

• Metabolic myopathies presenting with exercise intolerance, severe prolonged cramps, and red-wine or cola-colored urine (indicating myoglobinuria). If symptoms occur during strenuous brief exercise, then a GSD is most likely (e.g., McArdle disease) since glycogen is the main source of energy during brief exercise. If symptoms (usually myalgias and myoglobinuria without cramps) occur only after prolonged exercise and are worse during fasting , then lipid storage disease (e.g., CPT II deficiency) is more likely since free fatty acids are the most important source of fuel during prolonged exercise

• Metabolic myopathies that present with progressive muscle weakness indistinguishable from LGMDs or inflammatory myopathies include acid maltase, branching enzyme, debrancher, aldolase A, or carnitine deficiency.

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