Diagnosing muscle diseases

Diseases of muscle include acquired myopathies (eg immune-mediated myopathies) and inherited disorders (eg muscular dystrophies, congenital myopathies, metabolic myopathies, mitochondrial myopathies, channelopathies).

Acquired myopathies usually present as subacute onset of nonselective symmetrical weakness in the proximal (shoulder and hip girdle) muscles. Dysphagia can occur, especially in immune-mediated myopathies. In contrast to acquired myopathies, inherited muscle disorders often have a specific pattern of chronic, slowly progressive muscle weakness—the pattern may suggest a particular genetic cause.

Perform a complete physical examination that includes testing muscle strength and function, to identify the pattern and severity of muscle weakness. A standard scoring system for muscle strength is outlined in The standard Medical Research Council scoring system for muscle strength. A complete neurological examination and a complete systemic examination (including skin and joints) are also essential.

Table 1. The standard Medical Research Council scoring system for muscle strength

Muscle strength

Score

no contraction

0

contracts, but no movement

1

full range of movement (with gravity eliminated)

2

full range of movement (with gravity)

3

some resistance

4 [NB1]

normal power

5 [NB1]

Note: NB1: Gradations exist between scores 4 and 5.

Myopathies may be associated with a raised serum creatine kinase (CK) concentration, as well as raised alanine aminotransferase (ALT), aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) concentrations. A blood screen for autoantibodies (eg myositis antibodies) or endocrine abnormalities can help identify the cause of an acquired myopathy.

Further testing is best ordered and performed by experts, and could include electromyography, magnetic resonance imaging (MRI), muscle biopsy and genetic testing. Electromyography can confirm that the cause of the weakness is primarily muscle in origin, and exclude neurogenic causes or neuromuscular junction disorders. Electromyography and MRI can help guide the site for a muscle biopsy, and also show the pattern of muscle involvement. Often a muscle biopsy is needed to establish the diagnosis. The biopsy result needs to be correlated with the clinical picture. Genetic testing is best requested by experts, because they can provide genetic counselling and interpret complex results.

See additional advice on diagnosing idiopathic inflammatory myopathies.