IBM is “probably the No. 1 most-missed diagnosis” among patients referred for what is initially believed to be polymyositis, Dr. Christopher-Stine said.
“I used to think that this was missed at entry, that everybody [with IBM] had all of these criteria and that rheumatologists really didn’t understand this phenotype ... but some people morph into this,” she said, explaining that they often start out looking like they have polymyositis with proximal muscle weakness.
“They may even initially respond to steroids. And then they get this phenotype,” she said.
Older men are more likely to present with the phenotype from the beginning; women, in her experience, tend to present with what appears to be polymyositis, and then develop the phenotype over time, she noted.
An IBM diagnosis requires age over 30 years, but most patients are over 50, she said.
“This is the only one of the myopathies that is preferential to men,” she added, noting that it affects men twice as often as it does women.
The syndrome is characterized by proximal strength loss and muscle atrophy. Also, a finding that a patient’s knee extensors are weaker than their hip flexors is “a fantastic bedside sign” differentiating IBM from polymyositis, she said.
That’s not to say IBM patients don’t have hip flexor weakness, but their knee extensors usually are “considerably weaker by a grade strength or more” versus their hip flexors, she explained.
“It’s a very easy bedside test. In typical other myopathies we have this, but the knee extensors aren’t that weak in general, or they’re not as weak as the hip flexors,” she added.
Another sign is distal strength loss, particularly in the forearm and finger flexors.
“I was taught to have them make a fist; don’t have them make a fist,” she said, explaining that this recruits intrinsic muscles which basically allows cheating that may mask weakness.
Instead, ask them to flex just their distal interphalangeal joints by making a claw and using the fingers to pull against your fingers, she suggested.
Mixed myopathic and neuropathic features on electromyography also indicate IBM, she said.
Muscle biopsy may be helpful, but inclusions are seen in less than one-third of IBM patients.
“At times, we have had to biopsy three times to see them at all, and some people never show them, so you have to rely on your clinical acumen if you don’t see them,” she said.
Also, keep in mind that these patients are often labeled as having treatment-resistant polymyositis.
“Please, when somebody refers to you somebody that’s treatment resistant, that may be the case, but I want you to think maybe they’re treatment resistant because they don’t have that disease.”