Introduction
Background
Polymyositis (PM), dermatomyositis (DM), and inclusion body myositis (IBM) are the major members of a group of skeletal muscle diseases called the idiopathic inflammatory myopathies. Clinical features, characteristic muscle biopsy findings, immune markers, and histopathologic findings differentiate these illnesses. No strictly defined diagnostic criteria for polymyositis or dermatomyositis exist; however, Bohan and Peter proposed the criteria most widely cited. These criteria include the typical rash of dermatomyositis, findings at history and physical examination that reveal symmetric proximal muscular weakness, elevated serum muscle enzyme levels, electromyographic evidence of myopathic abnormalities, and characteristic findings at muscle biopsy.
Histopathology of polymyositis showing endomysial mononuclear inflammatory infiltrate and muscle fiber necrosis.
Polymyositis and dermatomyositis have many shared clinical features. Both are inflammatory myopathies that present as symmetric muscle weakness that develops over weeks to months. Initial treatment with corticosteroids usually produces a response; however, nonresponders require further treatment. Both conditions may be associated with malignancies. Despite these similarities, muscle biopsy findings and characteristic skin findings of dermatomyositis reveal each as a distinct clinical entity.
Although classified as an inflammatory myopathy, inclusion body myositis shows minimal evidence of inflammation. This is the most common inflammatory myopathy in patients older than 50 years. It presents as an asymmetric, distal weakness and also has distinct biopsy findings. Studies so far have not yielded significant response to treatment. Inclusion body myositis is not be discussed further in this article.
Pathophysiology
In both polymyositis and dermatomyositis, immune-mediated muscle inflammation and vascular damage occur. In polymyositis, the immune system is primed to act against previously unrecognized muscle antigens. In dermatomyositis, complement-mediated damage to endomysial vessels and microvasculature of the dermis occurs.
Frequency
United States
Overall, the annual incidence of inflammatory myopathy is 1 case per 100,000 persons per year.
Mortality/Morbidity
- The 5-year mortality rate is 20%.
- Mortality is most often related to associated malignancy or pulmonary complications; however, elderly patients with cardiac involvement or dysphagia also have a higher mortality rate.
- Outcome of treatment varies widely and is related to clinical and histologic features.
Race
Polymyositis and dermatomyositis are more common among blacks. Estimated black-to-white incidence for polymyositis is 5:1 and for dermatomyositis is 3:1.
Sex
Both polymyositis and dermatomyositis are more common in females, with an approximate 2:1 ratio.
Age
- Polymyositis is a disease of adults, and it is rare in persons younger than 20 years.
- Although dermatomyositis is primarily a disease of adults, it also is observed in children, usually those aged 5-14 years.
Clinical
History
The history of patients with polymyositis (PM) or dermatomyositis (DM) typically includes the following:
- Symmetric proximal muscle weakness with insidious onset
- Muscles usually painless (Myalgias occur in fewer than 30% of patients.)
- Dysphagia (30%) and aspiration, if pharyngeal and esophageal muscles are involved
- Arthralgias may be associated
- Difficulty kneeling, climbing or descending stairs, raising arms, and arising from a seated position; weak neck extensors cause difficulty holding the head up; involvement of pelvic girdle usually greater than upper body weakness
- Characteristic rash of face, trunk, and hands seen in DM only
- Family history and medication history are important in excluding other causes of myopathy.
Physical
Findings at physical examination may include the following:
- Muscle tenderness on palpation
- Normal sensory test results and reflexes (Reflexes may be abnormal with advanced disease.)
- Muscle atrophy
- In dermatomyositis (DM), a characteristic heliotrope rash preceding or accompanying muscle weakness
- The heliotrope rash is a symmetric, confluent, purple-red, macular eruption of the eyelids and periorbital tissue. Edema may also be present.
- Other rashes seen with DM include erythematous nail beds and a scaly, purple erythematous papular eruption over the dorsal metacarpophalangeal and interphalangeal joints (Gottron sign).
- Violaceous erythematous confluent macular eruption over the deltoids, posterior part of the shoulders, and the neck is known as the shawl sign. A similar eruption of the V area of the anterior part of the neck and the upper part of the chest is known as the V sign.
- Extramuscular manifestations
- Cardiac - Congestive heart failure (CHF), arrhythmia
- Lung - Interstitial lung disease, pneumonia/aspiration
- Gastrointestinal - Dysphagia
- Joints - Arthralgias, symmetric arthritis
Causes
The specific etiology of polymyositis is unknown but is thought to be autoimmune.
- Muscle inflammation is immune mediated.
- Various hypotheses exist; some believe that a viral trigger causes autoimmune injury or that illness may reflect a paraneoplastic syndrome.
More on Polymyositis |
 Overview: Polymyositis |
| Differential Diagnoses & Workup: Polymyositis |
| Treatment & Medication: Polymyositis |
| Follow-up: Polymyositis |
| Multimedia: Polymyositis |
| References |
| Next Page » |
References
Hengstman GJ, van den Hoogen FH, van Engelen BG. Treatment of the inflammatory myopathies: update and practical recommendations. Expert Opin Pharmacother. May 2009;10(7):1183-90. [Medline].
Bronner IM, van der Meulen MF, de Visser M, Kalmijn S, van Venrooij WJ, Voskuyl AE. Long-term outcome in polymyositis and dermatomyositis. Ann Rheum Dis. Nov 2006;65(11):1456-61. [Medline].
Amato AA, Barohn RJ. Idiopathic inflammatory myopathies. Neurol Clin. Aug 1997;15(3):615-48. [Medline].
Chahin N, Engel AG. Correlation of muscle biopsy, clinical course, and outcome in PM and sporadic IBM. Neurology. Sep 26 2007;[Medline].
Christopher-Stine L, Plotz PH. Myositis: an update on pathogenesis. Curr Opin Rheumatol. Nov 2004;16(6):700-6. [Medline].
Dalakas MC, Hohlfeld R. Polymyositis and dermatomyositis. Lancet. Sep 20 2003;362(9388):971-82. [Medline].
Dalakas MC, Sivakumar K. The immunopathologic and inflammatory differences between dermatomyositis, polymyositis and sporadic inclusion body myositis. Curr Opin Neurol. Jun 1996;9(3):235-9. [Medline].
Hengstman GJ, van Engelen BG. Polymyositis, invasion of non-necrotic muscle fibres, and the art of repetition. BMJ. Dec 18 2004;329(7480):1464-7. [Medline]. [Full Text].
Koopman WJ. Polymyositis/dermatomyositis classification criteria. In: Arthritis and Allied Conditions. 1997:17-8.
Mastaglia FL, Ojeda VJ. Inflammatory myopathies: Part 1. Ann Neurol. Mar 1985;17(3):215-27. [Medline].
Nirmalananthan N, Holton JL, Hanna MG. Is it really myositis? A consideration of the differential diagnosis. Curr Opin Rheumatol. Nov 2004;16(6):684-91. [Medline].
Olsen NJ, Park JH. Inflammatory myopathies: issues in diagnosis and management. Arthritis Care Res. Jun 1997;10(3):200-7. [Medline].
Further Reading
Keywords
polymyositis, PM, dermatomyositis, DM, inclusion body myositis, idiopathic inflammatory myopathy, inflammatory muscle disease, immune-mediated muscle inflammation, proximal muscle weakness, dysphagia, aspiration, arthralgias, muscle atrophy, heliotrope rash, purple-red edematous periorbital eruption, scaly purple-erythematous papular eruption over knuckles, Gottron sign, conduction defects, arrhythmias, myocarditis, interstitial lung disease, aspiration pneumonia, skeletal muscle disease
