Tests and diagnosis

By Mayo Clinic Staff

Dermatomyositis is the most easily recognized of the inflammatory muscle diseases because of its characteristic rash. Occasionally, a rash alone may prompt a diagnosis of dermatomyositis (called amyopathic dermatomyositis), even if you don't have any muscle weakness.

In addition to assessing your signs and symptoms, your doctor may use other tests to confirm a diagnosis of dermatomyositis:

  • Magnetic resonance imaging (MRI). A scanner creates cross-sectional images of your muscles from data generated by a powerful magnetic field and radio waves.
  • Electromyography. A doctor with specialized training inserts a thin needle electrode through the skin into the muscle to be tested. Electrical activity is measured as you relax or tighten the muscle, and changes in the pattern of electrical activity can confirm a muscle disease. The doctor can determine the distribution of the disease by testing different muscles.
  • Muscle biopsy. A small piece of muscle tissue is removed surgically for laboratory analysis. In dermatomyositis, inflammatory cells surround and damage the capillary blood vessels in the muscle. A muscle biopsy may reveal inflammation in your muscles or other problems, like damage or infection. The tissue sample can also be examined for the presence of abnormal proteins and checked for enzyme deficiencies.
  • Blood analysis. A blood test will let your doctor know if you have elevated levels of muscle enzymes, such as creatine kinase (CK) and aldolase. Increased CK and aldolase levels can indicate muscle damage. A blood test can also detect specific autoantibodies associated with different symptoms of dermatomyositis, which can help in determining the best medication and treatment.
  • Skin biopsy. A small piece of skin is removed for laboratory analysis. The skin sample can confirm the diagnosis of dermatomyositis and rule out other disorders, such as lupus. If the skin biopsy confirms the diagnosis, a muscle biopsy may not be necessary.
Jul. 07, 2011