Muscle weakness, usually proximal and symmetric, is the most common feature of PM and is seen in over 90% of patients. The onset of muscle weakness from PM is typically subacute, with patients reporting symptoms developing over a several-month time period. Common concerns include progressive difficulty lifting, kneeling, using the stairs, and raising arms. Other nonspecific symptoms include fatigue, weight loss, myalgia, arthralgia, dyspepsia, and dysphagia.
PM can manifest in a variety of organ systems. In addition to characteristic skeletal muscle weakness, many patients develop interstitial lung disease, dysphagia (due to involvement of skeletal muscle that makes up the upper third of the esophagus), and polyarthritis. Patients may also have myocarditis. It can be difficult to distinguish PM-related symptoms from those associated with PM and an autoimmune disease overlap syndrome, which presents diagnostic challenges.
There is no cure for PM, and patients' clinical courses can vary considerably. Patients with PM are at increased risk for myocardial infarction and ischemic stroke.
There are rare cases of juvenile PM.
M33.20 – Polymyositis, organ involvement unspecified
31384009 – Polymyositis
Differential Diagnosis & Pitfalls
- Inclusion body myositis
- Infectious myositis (viral or bacterial)
- Obstructive sleep apnea
- Anemia (see, eg, iron deficiency anemia)
- Malignancy – leukemia, lymphoma
- Amyotrophic lateral sclerosis
- Inflammatory bowel disease (eg, Crohn disease, ulcerative colitis)
- Polymyalgia rheumatica
- Muscular dystrophy (see, eg, Duchenne muscular dystrophy)
- Myotonic dystrophy
- Drug-induced myopathy
- Myasthenia gravis
- Lambert-Eaton myasthenic syndrome
- Human immunodeficiency virus disease-associated myopathy
- Metabolic disorders of carbohydrate and lipid metabolism
- Diabetic amyotrophy (see diabetic neuropathy)
- Systemic lupus erythematosus
Drug Reaction Data