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Q: Doc, doc, got a middle-aged patient with subacute onset of proximal muscle weakness and CK of 10K. What could it be?
A: Any cutaneous or joint or respiratory issues? If it’s just muscle, it might be necrotizing autoimmune or toxic myopathy with that level of CK.
Q: He’s on statins, think it might be a statin myopathy?
A: Certainly a possibility. Let’s stop the statins, check HMGCR and SRP antibodies and repeat CK next week. Let’s get him back in clinic next week.
Q: Alas he’s back and he’s weaker. CK has gone up to 15K.
A: Unlikely statins then. Consider necrotizing autoimmune myopathy (NAM). Did HMGCR and SRP antibodies come back? HMGCR antibodies are positive in 40 percent of patients with NAM and SRP in 17 percent.
Q: Antibodies were negative.
A: Schedule a muscle biopsy - necrotic and regenerating fibers with little endomysial inflammation, and MHC1 and MAC staining on non-necrotic fibers with NAM. You do this to distinguish from other forms of myositis like polymyositis, dermatomyositis or overlap myositis.
Q: As you said! Necrotizing myopathy on muscle biopsy.
A: Then treat as per NAM with high dose IVIG or rituximab
A: If HMGCR or SRP antibodies were positive, then you could spare the muscle biopsy.
A: NAM isn’t typically associated with malignancy unlike polymyositis and dermatomyositis, but SRP antibodies are associated with cardiomyopathy. Check EKG and Echo.
Q: Do we have to test for other myositis specific antibodies?
A: If you suspect other etiologies - if there are skin manifestations, you can send a dermatomyositis panel. If there is other organ involvement, you can also check anti-synthetase antibodies.
A: Lupus and systemic sclerosis can also present with necrotizing myopathy so if the HMGCR and SRP are negative, consider ANA, anti-Sm, anti PML-scl, anti- RNP3, anti topimerase antibodies.
Q: What if he doesn’t respond to treatment?
A: Give him a 6 month trial of IVIG n physical therapy. If no improvement, consider genetic testing for LGMDs which may present similarly. LGMD muscle biopsy has inflammation and MHC upreg too! That’s all folks, welcome insights and thoughts, neuromuscular out.
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