“When someone refers you [a patient with suspected] polymyositis, I want you to do a checklist in your head and say, ‘Have I thought about these five things?’ ”, director of the Johns Hopkins Myositis Center, Baltimore, said at the Winter Rheumatology Symposium sponsored by the American College of Rheumatology.
The five most common diagnoses in patients labeled as having polymyositis are immune-mediated necrotizing myopathy (IMNM), overlap with other rheumatologic conditions, antisynthetase syndrome, inclusion body myositis (IBM), and muscular dystrophy, she explained.
“You may say, ‘look, it’s all what you call it,’ but I think we need to be a little bit more careful in what we call it,” she said.
Patients with IMNM present with clinical symptoms similar to those seen in polymyositis and dermatomyositis – mainly proximal muscle weakness.
However, there are some important differences, both clinically and histologically, Dr. Christopher-Stine said.
“Look for higher [creatine kinase (CK)] levels,” she said. “In the thousands, usually multiple thousands ... like 5,000, 10,000, 2,000 ... that’s when you’re thinking about a necrotizing phenotype before you even look at the biopsy.”
CK levels will usually be under 30,000 U/L in IMNM, she noted, adding that data increasingly suggest that the extensive muscle necrosis in IMNM explains the elevated CK levels versus those seen in other myopathies.
Myalgias also tend to be more prominent in IMNM than in polymyositis.
“These folks hurt,” she said, noting that IMNM patients tend to have more extensive muscle atrophy and functional disability. “Many will be wheelchair bound within 9 months of diagnosis; it’s not subtle.”
The most important tool for making an IMNM diagnosis is muscle biopsy; look for prominent myocyte necrosis and a relative paucity of lymphocytes, she advised.