Dermatomyositis and polymyositis are a group of inflammatory muscle conditions characterized by varying degrees of progressive muscle weakness.
The most common symptom is muscle weakness, usually affecting those muscles closest to the trunk of the body (proximal – shoulders/upper arms and hips/thighs). In some patients, the muscles ache and are tender to touch. More advanced findings include difficulty rising from a seated position, climbing stairs or lifting objects over one’s head. Trouble with swallowing may occur. The disease has a gradual onset and generally begins in the second decade of life. Polymyositis rarely affects persons under the age of 18. The disease may be associated with other collagen, vascular, autoimmune, or infectious disorders.
Symptoms can begin slowly or suddenly and may include:
- Weakness, especially in the upper arms, shoulder and hips and thighs.
- Muscle and joint pain
- Weight loss
- Low-grade fever
- Hand tremors
- Shortness of breath, even at rest
- Difficulty swallowing.
Dermatomyositis differs from polymyositis clinically by having skin involvement. Some dermatomyositis patients may develop a violet to deep reddish-purple rash over their eyelids and bumpy, red rash on their chest, upper back or finger joints.
Evaluation begins with a detailed history and physical exam aimed at identifying the time course of symptoms, distribution of muscle weakness and muscle strength exam. Several studies may be ordered to help confirm the diagnosis, these include:
The inflammation causes a breakdown of muscle cells, so you may have a blood test (CPK or Aldolase level) to look for abnormal amounts of broken cells. Other types of tests might include:
Creatine kinase and/or aldolase levels in the blood – product of the breakdown of muscle cells
MRI to identify the exam muscles affected by inflammation and breakdown.
Electromyography, or EMG, a test that looks at muscle strength and function.
Biopsy of the involved muscle – this is the gold standard for diagnosis. Biopsy will identify the type of inflammation and classification of inflammatory muscle disease (ie. dermatomyositis or polymyositis)
The goal for treatment is prevent further muscle weakness by the reduction of inflammation and breakdown of muscle cells. Treatment generally consists of a steroid medications called prednisone. For patients in whom prednisone is not effective, immune system modifying medications such as azathioprine and methotrexate may be prescribed. Intravenous immunoglobulin or ritixumab infusions are used in more severe or refractory cases.
Physical therapy is recommended to all patients to avoid muscle atrophy.
The prognosis and response to therapy varies greatly. Some patients respond quickly, while those patients refractory to treatment may progress to develop significant disability. A significant factors for muscle strength recovery is the level of disability at the time of diagnosis and treatment initiation. Death is rare but may occur in patients with severe and progressive muscle weakness, swallowing difficulty, malnutrition, or respiratory failure.