A number of conditions can mimic myositis, but clues that can point to the correct diagnosis are often present in cases involving the mimics, according to Lisa Christopher-Stine, MD.
For example, elevated levels of certain muscle enzymes are an important source of diagnostic information, Dr. Christopher-Stine, director of the Johns Hopkins Myositis Center, Baltimore, said at the Winter Rheumatology Symposium sponsored by the American College of Rheumatology.
Isolated elevations in aldolase can be seen in connective tissue–associated interstitial lung disease or in patients with fascial edema, and aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase (LDH), and creatine kinase (CK) levels can also be helpful, she explained.
The latter can also be elevated in the absence of muscle disease, for example, in healthy individuals following exercise. CK peaks at 24 hours after exercise before returning to baseline by 72 hours. In an experimental setting, a threefold increase in CK levels has been seen at 8-24 hours after exercise, Dr. Christopher-Stine said.
Trauma from causes such as intramuscular injection, electromyography (EMG), major surgery, or biopsy can also lead to increased CK levels. Motor neuron disease can also cause such increases. In one study, 75% of patients with amyotrophic lateral sclerosis had a mean twofold increase in CK levels, she said.
Asymptomatic CK elevations may also represent presymptomatic myopathies, type 1 or 2 macro-CK, manual labor occupations, or they may be idiopathic.
Race can play a role in CK levels as well. Black people tend to have higher CK levels than white people, she said, noting that one study of more than 10,000 adults showed that black race was strongly associated with CK, and that body composition largely explained differences in CK by age, but not by race/ethnicity ().
“So elevated CK may not herald any discernible illness,” she said.
Dr. Christopher-Stine described a case involving an otherwise healthy 30-year-old man with a CK level of 695 IU/L that was found incidentally. He had a desk job, no recent travel, and denied weakness, myalgias, joint pain, dysphagia, shortness of breath, and fevers. In this case, the elevated CK was felt to be secondary to his African American race given that other causes were ruled out.
Another case involved a 72-year-old man with left-arm pain. A cardiac event was ruled out, and CK was found to be about 4,500 IU/L. He reported “flare-ups” of diffuse swelling of the hands and feet. X-rays showed concerning signs of erosions. His transaminases and electromyogram were normal; he reported no weakness or myalgia; and an MRI showed no muscle edema. He was diagnosed with macro-CK, which refers to CK with an increased molecular weight. A clue to this diagnosis is a normal liver function test. In some cases, muscle/brain CK levels (CK-MB) are elevated and higher than total CK, she noted.
She presented an algorithm for the diagnostic work-up of patients presenting with elevated CK of unclear significance. Her recommended approach involves repeat CK assessment and a closer look at family history, medication, drug/toxin history, examination for weakness and neurologic abnormalities, and additional lab assessments in those whose levels remain elevated. In those in whom a diagnosis is not identified, the algorithm calls for observation every 3 months – including physical examination and labs – in asymptomatic patients with levels at less than five times the upper limit of normal, and further evaluation, including EMG with nerve-conduction velocity testing, muscle biopsy, and MRI in those with (or who later develop) marked elevation greater than five times the upper limit of normal and/or symptoms.