Dermatomyositis is an inflammatory disorder that primarily causes muscle weakness and a skin rash. As a systemic condition, dermatomyositis can also involve the lungs, joints, esophagus and sometimes the heart.

The exact cause of dermatomyositis is not clear, although the condition shares many similarities with autoimmune disorders. It is thought that autoantibodies may target the capillaries in muscles and disrupt the blood supply to muscles.

Dermatomyositis belongs to a class of disorders called the inflammatory myopathies and leads to muscle atrophy and muscle damage.

Some of the most important autoantibodies that have been detected in dermatomyositis include:

• Anti-synthetase ( targets tRNA synthetase enzymes)
• Antibodies against cytoplasmic enzymes, which catalyze the covalent binding of amino acids to tRNA
• Anti-hystidil-tRNA synthetase autoantibodies
• Anti-signal recognition particle antibodies
• Anti-Mi-2 antibodies

Diagnosis

Dermatomyositis may be diagnosed based on signs and symptoms, but other tests that may be used to confirm a diagnosis include:

Muscle biopsy

A sample of muscle tissue is removed and sent for laboratory analysis. This test may reveal the presence of inflammatory cells around muscle capillaries. This sample can also be checked for the presence of abnormal proteins and enzyme deficiencies.

Skin biopsy

A skin biopsy sample can be used to rule out other disorders and confirm a diagnosis, which may eliminate the need for a muscle biopsy.

Electromyograph

A needle electrode is inserted into the muscle to test the electrical activity, which can be abnormal in muscle disease. Different muscles can be tested to confirm disease distribution.

Blood analysis

Blood tests are performed to check for raised muscle enzymes, which can indicate muscle damage. Specific autoantibodies associated with the different forms of dermatomyositis can also be detected, which can aid treatment decisions.