myopathy
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Few comments
1- the muscle biopsy is a gold standard for the diagnosis in your case , as few other things aprat of mitochodrial dysfunction could cause a proximal myopathy and respiratory muscle weakness with more frequent myalgia
2-Myalgia is generally a rare symptom in myopathy and suggests orthopedic and rheumatic causes especially if noprmal CK!
3- CK could falsely be normal in case of end stage myopathy with atrophied muscles or if using steroids or if the cause is hyperthyroidism
In brief, yes you should see a rheumatologist again
Bob