How would you approach a young patient with bilateral lower extremity muscle weakness and positive anti-Smith, dsDNA, RNP, Raynaud’s, and pericardial effusion but normal muscle enzymes?

The timeline of weakness may be helpful, but another possible cause of weakness in the setting of normal CK would be an inflammatory demyelinating polyneuropathy (can be acute or chronic). EMG and NCS would be helpful in evaluating this.

Since the muscle biopsy is pending, I'd base my diagnosis on the muscle biopsy results.

However, in this scenario, I'd have a high suspicion for myositis (muscle weakness + muscle edema on MRI):

Lupus myositis is rare, and most studies evaluating it require high muscle enzymes. I am not aware of any studies assessing muscle enzyme normal lupus myositis.

Perifascicular muscle inflammation (as occurs in dermatomyositis, Mathur T et al) is more apt to have normal muscle enzymes. So, I would not be sur...