Polymyositis and dermatomyositis can manifest with similar signs and symptoms. Look for the diagnostic markers that can help you make the correct diagnosis.
Polymyositis (PM) is one of a group of skeletal muscle diseases known as idiopathic inflammatory myopathies. When PM manifestations include a cutaneous rash, the disease is referred to as dermatomyositis (DM). In patients with both PM and DM, proximal symmetric muscle weakness develops slowly over weeks to months. Typically, both conditions also respond to corticosteroid treatment. PM and DM are associated with a wide variety of malignancies. Despite these similarities, some clinical features—in addition to the characteristic dermatologic findings—differentiate the two diseases.
PM and DM are rare disorders; prevalence rates are estimated at about 1 per 100,000 in the general population, with a female to male predominance of approximately 2:1. Peak incidence is in the fifth decade. The two diseases more commonly affect African-Americans. The estimated incidence of PM in African-Americans compared to whites is 5:1 and of DM, 3:1. The 5-year mortality rate is 20%; elderly patients and persons with cardiac and/or pulmonary involvement or dysphagia have a higher mortality rate compared to other persons with either disease.1
SIGNS AND SYMPTOMS
Despite the similar manifestations, PM and DM have clinical and pathological differences. Clinically, DM manifests as one of several skin conditions. DM is associated with a higher incidence of malignancy. Pathologically, DM is associated with immune complex deposition in the vessels, whereas PM appears to reflect a direct T-cell mediated muscle injury. Both conditions are idiopathic; however, PM can be a manifestation of another T-cell disorder, chronic graft-versus-host disease after bone marrow transplant.2
Clinical manifestations Bohan and Peter proposed five diagnostic criteria for PM/DM: (1) symmetric proximal 
muscle weakness; (2) elevated serum levels of muscle enzymes; (3) myopathic changes on electromyography; (4) characteristic muscle biopsy findings; and (5) appearance of a distinct rash, which will differentiate the diagnosis as DM.3 The diagnosis is made based on the presence of a constellation of symptoms rather than the presence of only one symptom. However, many patients who meet the Bohan and Peter criteria may actually have a different inflammatory myopathy, including rhabdomyolysis, muscular dystrophy, inclusion body myositis, or myositis associated with connective tissue disease.4 Viral and bacterial muscle infections also produce histologic evidence of muscle inflammation.
An alternative to the Bohan and Peter classification incorporates both clinical and serologic factors, which allows for a more accurate prediction of disease course and treatment response.5 This approach classifies inflammatory myopathies as PM; DM; overlap myositis, defined as myositis with an additional feature other than rash and/or the presence of myositis-specific autoantibodies, such as an antisynthetasis, signal recognition protein, and one of the scleroderma-associated autoantibodies; and cancer-associated myositis.
Muscle weakness The most common symptom of PM/DM is muscle weakness. Onset is normally insidious, with gradual worsening over weeks to months. In some cases, however, the onset may be very abrupt. The distribution of weakness is normally symmetric and proximal. Any distal muscle involvement is usually mild and does not cause specific muscle impairment. Myalgias and muscle tenderness occur in up to 50% of cases. These symptoms tend to be mild, unlike the more severe muscle pain seen in infectious myositis, the inherited metabolic myopathies, polymyalgia rheumatica, or fibromyalgia.
Muscle atrophy is not normally seen early in the course of the disease but may become profound in long-standing cases. Weakness of the oropharyngeal muscles or the striated muscle of the upper third of the esophagus may lead to dysphagia, nasal regurgitation, or aspiration. Esophageal involvement is more common in the elderly and may explain the higher incidence of bacterial pneumonia often seen in elderly patients with PM/DM.6
Rash DM manifests as one of several distinct rashes. Often the rash is transient and disappears before the onset of muscle weakness. The heliotrope rash is a symmetric, confluent, purple-red, macular eruption of the eyelids and periorbital tissue. Edema is often present. Although considered the most specific skin manifestation of DM, heliotrope rash is rarely encountered.
Gottron's sign, the most commonly occurring rash, is a symmetric, scaly, violaceous, erythematous eruption 
over the extensor surfaces of the metacarpophalangeal and interphalangeal joints of the fingers (see Figure 2). Lesions may also occur over the extensor surfaces of the elbows and knees, often mimicking psoriasis. Skin biopsy of these lesions reveals vascular ectasia alternating with areas of vascular dropout.
The shawl sign is a violaceous and erythematous, confluent macular eruption that appears over the deltoids, posterior part of the shoulders, and neck. The V sign is a similar eruption occurring on the “V” area of the anterior neck and upper part of the chest. Finally, mechanic's hands manifests as periungual erythema, abnormal nail bed capillaries, and an often painful roughening and cracking of the skin on the tips and lateral aspects of the fingers (see Figure 3).
Extramuscular symptoms Often patients with severe cases of PM/DM present with fever and weight loss. Nonerosive
inflammatory polyarthritis, Raynaud's phenomenon, and cardiopulmonary
abnormalities also may be present. Interstitial lung disease (ILD) is another clinical finding in patients with PM/DM. The presence of ILD indicates a poor prognosis for patients because of its potentially rapidly progressive nature.7 These patients experience dyspnea and cough before
the onset of muscle weakness. Myocardial involvement leading to heart failure is not a frequent occurrence. Signs and symptoms of PM/DM can overlap with the signs and symptoms of other connective-tissue diseases such as scleroderma, systemic lupus erythematosus, rheumatoid arthritis, and Sjögren's syndrome.
DIAGNOSTIC MARKERS
Muscle enzymes Creatine kinase (CK), aldolase, lactate dehydrogenase, aspartate aminotransferase, and alanine aminotransferase are the muscle enzymes routinely measured in patients presenting with myositis. At some point in the disease process, the level of at least one of these enzymes will be elevated, and most patients with PM/DM will have elevated levels of all the muscle enzymes.
Patients with PM/DM routinely have an elevated CK-myocardial band (MB) isoenzyme level either because of skeletal muscle inflammation with its antecedent expression of the CK-B chain or because of myocardial involvement. MI may be incorrectly suspected in this situation. A normal concentration of troponin I rules out cardiac involvement.
Autoantibodies Antinuclear antibodies are routinely elevated in patients with PM/DM. This finding is not specific for this disorder, however, and may overlap with myositis associated with another connective tissue disease.8
Autoantibodies directed against cytoplasmic RNA synthetases, other cytoplasmic proteins, ribonucleoproteins, and certain nuclear antigens have been noted in as many as 30% of patients with idiopathic inflammatory myopathy and are called myositis-specific autoantibodies.9 Only anti-histidyl-tRNA synthetase (anti-Jo-1 antibody) is a diagnostic marker. Anti- Jo-1 antibody is present in 20% of patients with PM/DM and is associated with ILD, arthritis, mechanic's hands, and Raynaud's phenomenon.
Electromyography The electromyogram (EMG) reveals evidence of increased membrane irritability in the form of a classic triad: increased insertional activity and spontaneous fibrillations, abnormal myopathic low amplitude and shortduration polyphasic motor unit potentials, and bizarre high-frequency discharges. These EMG findings are supportive, but not diagnostic, of PM/DM because similar findings can be seen in other infectious, toxic, or metabolic myopathies. However, EMG can differentiate between the inflammatory myopathies and other disorders that cause muscle weakness. Muscle enzyme levels must be measured before EMG because the EMG needle will falsely elevate the inflammation measured by these markers.
Muscle biopsy The definitive tool for establishing a diagnosis of PM/DM, as well as for ruling out other causes of myopathy, is the muscle biopsy. The biopsy should be obtained from a clinically weak muscle, most often the quadriceps or deltoid. Caution should be exercised to avoid sampling a severely weak muscle, atrophied muscle, or muscle from the EMG site. A biopsy specimen obtained from the muscle contralateral to the muscle found abnormal by EMG increases the diagnostic yield.
The histologic features of PM/DM include muscle fiber necrosis, degeneration and regeneration, and an inflammatory cell infiltrate. A discussion of the specific histologic findings that differentiate PM from DM isbeyond the scope of this article.
Imaging studies MRI and magnetic resonance spectroscopy (MRS) are useful for evaluating myopathies.10 MRI can reveal areas of muscle inflammation and edema with active myositis and fibrosis. MRI also reduces biopsy sampling errors by visualizing severely involved or necrotic areas in the large muscles. MRS reveals the decreased muscle metabolism found in abnormal muscle.11 As clinicians gain more experience with MRI and MRS, these imaging studies will replace the more invasive procedures used for diagnosing myopathies. The case study illustrates how these markers and test results are used to come up with the diagnosis of PM/DM.
Differential diagnosis Diseases that mimic PM/DM, both with and without elevated muscle enzymes, include the motor neuron diseases (such as amyotrophic lateral sclerosis), the muscular dystrophies, myasthenia gravis, hypothyroidism-related myopathy, rhabdomyolysis, and inclusion-body myositis. Drug and alcohol induced myopathies, acute viral infections, inherited metabolic myopathies, and muscle weakness resulting from chronic disease also mimic PM/DM.
TREATMENT
A prolonged duration of illness prior to diagnosis, dysphagia, increased severity of weakness, malignant disease, and respiratory and/or cardiac involvement are features associated with a poor outcome.12,13 Initial therapy for PM/DM is glucocorticoids. Prednisone, 0.5 to 1.5 mg/kg/d, has been shown to improve strength and preserve muscle function.14 Muscle enzyme normalization occurs in approximately 6 weeks,whereas improvements in muscle strength may take 2 to 3 months to become evident. Once sufficient muscle strength is attained, the prednisone dosage is tapered gradually; muscle enzyme levels, muscle strength, and extramuscular signs of a relapse of PM/DM, as well as signs of glucocorticoid toxicity, are monitored throughout the treatment course. If there is no significant improvement in symptoms after 3 months, treatment failure or an alternative diagnosis should be considered.
If treatment failure is suspected, adding azathioprine or methotrexate may improve outcomes. For more resistant disease, IV immune globulin, cyclosporine, and/or tacrolimus, alone or in combination, may offer a benefit. Data on the efficacy of other agents such as cyclophosphamide, chlorambucil, tumor necrosis factor inhibitors, and B cell-depleting treatments (rituximab) are limited. Clinicians should note that these alternatives are not FDA-approved for resistant PM/DM.
PATIENT EDUCATION
Patients taking glucocorticoids for several months need counseling on an increased risk of osteoporosis, the need to avoid direct sunlight, and the importance of exercise. An early referral to a physical therapist is warranted. A weekly dose of a bisphosphonate, in addition to calcium and vitamin D supplementation, may be appropriate when therapy begins. Patients presenting with cricopharyngeal muscle weakness should be educated about the risks of aspiration. They should elevate the head of their beds and be referred for consultation
with a speech therapist. These patients need to maintain a diet of semithick foods and be aware that they may eventually require tube feeding.
Malignancy can manifest with PM/DM, so age appropriate cancer screening should begin at presentation and continue annually thereafter. The more common malignancies found in patients with PM/DM are cervical, ovarian, lung, pancreatic, bladder, gastric, and nasopharyngeal cancers and non-Hodgkin lymphoma.15
CONCLUSION
Inflammatory myopathies are acquired muscle disorders of unknown etiology that manifest as proximal muscle weakness. The diagnostic evaluation includes measuring muscle enzyme levels and myositis-specific autoantibodies. EMG, muscle biopsy, and imaging studies are used to diagnose PM/DM and, when appropriate, to differentiate between PM/DM and other myopathies. Treatment involves glucocorticoids to reverse the inflammatory process and, when the disease does not respond to treatment, adding other pharmacotherapeutic agents as necessary. PAs should keep in mind that some malignancies are associated with PM/DM. Therefore, appropriate patient monitoring and education is an important part of the overall treatment regimen. JAAPA
Jason Astrin is a physician assistant at Mercy Medical Center, Baltimore, Maryland. He has indicated no relationships to disclose relating to the content of this article
DRUGS MENTIONED Azathioprine (Azasan, Imuran)
Chlorambucil (Leukeran)
Cyclophosphamide (Cytoxan, Neosar)
Cyclosporine (Gengraf, Neoral, Sandimmune)
Methotrexate (Rheumatrex, Trexall)
Prednisone
Rituximab (Rituxan)
Tacrolimus (Prograf)
REFERENCES
1. Tymms KE, Webb J. Dermatopolymyositis and other connective tissue diseases: a review of 105 cases. J Rheumatol. 1985;12(6):1140-1148.
2. Parker P, Chao NJ, Ben-Ezra J, et al. Polymyositis as a manifestation of chronic-graft-versushost disease. Medicine (Baltimore). 1996;75(5):279-285.
3. Bohan A, Peter JB. Polymyositis and dermatomyositis (first of two parts). N Engl J Med. 1975; 292(7):344-347.
4. van der Meulen MF, Bronner IM, Hoogendijk JE, et al. Polymyositis: an overdiagnosed entity. Neurology. 2003;61(3):316-321.
5. Troyanov Y, Targoff IN, Tremblay JL, et al. Novel classification of idiopathic inflammatory myopathies based on overlap syndrome features and autoantibodies: analysis of 100 French Canadian patients. Medicine (Baltimore). 2005;84(4):231-249.
6. Marie I, Hatron PY, Levesque H, et al. Influence of age on characteristics of polymyositis and dermatomyositis in adults. Medicine (Baltimore). 1999;78(3):139-147.
7. Selva-O'Callaghan A, Labrador-Horillo M, Muñoz-Gall X, et al. Polymyositis/dermatomyositisassociated lung disease: analysis of a series of 81 patients. Lupus. 2005;14(7):534-542.
8. Reichlin M, Arnett FC Jr. Multiplicity of antibodies in myositis sera. Arthritis Rheum. 1984; 27(10):1150-1156.
9. Love RA, Leff RL, Fraser DD, et al. A new approach to the classification of idiopathic inflammatory myopathy: myositis-specific autoantibodies define useful homogenous patient groups. Medicine (Baltimore). 1991;70(6):360-374.
10. Reimers CD, Finkenstaedt M. Muscle imaging in inflammatory myopathies. Curr Opin Rheumatol. 1997;9(6):475-485.
11. Scott DL, Kingsley GH. Use of imaging to assess patients with muscle disease. Curr Opin Rheumatol. 2004;16(6):678-683.
12. Carpenter JR, Bunch TW, Engel AG, O'Brien PC. Survival in polymyositis: corticosteroids and risk factors. J Rheumatol. 1977;4(2):207-214.
13. Dankó K, Ponyi A, Constantin T, et al. Long-term survival of patients with idiopathic inflammatory myositis according to clinical features: a longitudinal study of 162 cases. Medicine (Baltimore). 2004;83(1):35-42.
14. Drake LA, Dinehart SM, Farmer ER, et al. Guidelines of care for dermatomyositis. American Academy of Dermatology. J Am Acad Dermatol. 1996;34(5, pt 1):824-829.
15. Mebazaa A, Boussen H, Nouira R, et al. Dermatomyositis and malignancy in Tunisia: a multicenter national retrospective study of 20 cases. J Am Acad Dermatol. 2003;48(4):530-534.