When would you suspect metabolic myopathy and what is your approach to initial workup?

Great question and really important to rheumatology practice. As always, a history and physical exam is the best initial diagnostic tool. It’s a little bit difficult because of the sheer number and heterogeneity of non-autoimmune myopathies that clinicians may encounter, but a few general principles that make me think about hereditary “myositis mimics” include:

• Presentation at a young age: As a caveat, some patients with hereditary myopathies may not be recognized until their 30s, 40s, or even later, especially if their disease is mild or only partially expressed (ie, a woman with an X-linked myodystrophy).
• Weakness or rhabdomyolysis precipitated by exercise or eating/fasting: Some patients may notice extremely dark urine post-workout even ...

The response by Dr. @Dr. First Last is excellent and nicely details all-important findings. I have a practical approach that I use in the clinic:

Suspect Metabolic Myopathy or Muscle Dystrophy if the answer is Yes to some or all of these. If yes, I would first do genetic testing of blood/saliva of neuromuscular panel available through many commercial labs like GeneDx, Invitae etc.

1. Refractory to immunosuppression
2. Diagnosis of Polymyositis
3. Lack of connective tissue features like ILD, polyarthritis, Raynauds, inflammatory rashes, Sicca, DM rashes, etc
4. Lack of sensory abnormality or neuropathy
5. No objective muscle weakness on examination at rest
6. Lack of Symmetric proximal > distal pattern of upper and lower extremity muscle weakness pattern
7. Lo...