Myositis is a group of related conditions of unknown cause that lead to muscle injury, resulting in subsequent inflammation. The characteristic diseases include, but are not limited to, dermatomyositis, polymyositis, necrotizing myopathy, and anti-synthetase syndrome. In addition to inflammation of certain muscle groups, the different types of myositis are defined based on other characteristic manifestations, such as rashes, difficulty swallowing, lung disease, and heart disease. In more recent years, new antibodies have been discovered that have allowed rheumatologists and scientists to better understand the different types of myositis, how they differ, and how to appropriately treat them.
Myositis affects individuals of all ages. The annual incidence of polymyositis and dermatomyositis ranges from 5-10 new cases per million. The average female-to-male ratio exceeds 2:1, with women predominating in this disease that occurs characteristically between ages 15 and 44 years.
These are immune-mediated processes, believed to be triggered by environmental factors in genetically susceptible individuals. The specific causes or triggering events remain unknown, but certain viruses have been strongly implicated. The myositis-specific antibodies are often distinct and lead to symptoms and clinical manifestations unique to that antibody phenotype.
The dominant clinical feature of myositis is symmetrical proximal muscle weakness. The weakness can be accompanied by systemic symptoms of fatigue, morning stiffness and fevers. Laboratory investigation reveals elevated levels of serum enzymes derived from skeletal muscle, especially creatine kinase (CK). Electromyography (EMG) demonstrates changes consistent with muscle inflammation, and a muscle biopsy can show inflammatory changes consistent with myositis. These manifestations can occur in a variety of combinations or patterns, and no single feature is specific or diagnostic. Other muscles, including the muscles that control swallowing, can also be affected. Certain forms of myositis can also affect other organs, such as the lungs and heart.
Polymyositis begins with gradual weakness over three to six months with no identifiable trigger. The shoulder and hip muscles are affected most severely. Weakness of neck muscles, particularly the flexors, occurs in about one-half of all patients, making it difficult to raise one’s neck off the bed or to stand up from a seated position. Difficulty swallowing may develop secondary to esophageal dysfunction or obstruction. Joint pain and swelling can also be present.
The clinical features of dermatomyositis include all those described for polymyositis, plus a variety of skin manifestations. These rashes may precede the onset of muscle weakness by a year or more. Skin involvement varies widely from person to person. Characteristic rashes can involve the knuckles of the hand (Gottron’s papules – Image Link), the eyelids (heliotrope rash), the upper back and neck (shawl sign), neck and upper chest (V sign), as well as the face.
The diagnosis of each different type of myositis, including polymyositis and dermatomyositis, includes a detailed clinical history, physical examination, lab testing, and other tests. A panel of myositis-specific antibodies may be checked to better characterize the disease as well. Other diagnostics that are often helpful in the diagnosis include an electromyography (EMG), MRI of the affected muscle, and a muscle biopsy. Certain forms of myositis develop in the setting of malignancy, so it is important to stay up-to-date on age-appropriate cancer screening, such as colonoscopy, mammogram, Pap smear, and prostate exam.
Treatment of myositis is largely empirical. Physical therapy plays an important role in maintaining and improving muscle strength. Corticosteroids, such as prednisone, are the standard first line medication for inflammatory myopathy. If a patient does not respond to corticosteroids, another agent is added, such as methotrexate or IVIG. Clinical improvement may be noted in the first weeks or gradually, over three to six months. In severe cases that involve the lungs, heart, or other organs, stronger immunosuppressive medications are often needed.