An elevated serum level of muscle enzymes like creatine kinase (CK) and aldolase are characteristic but not specific to dermatomyositis and polymyositis. However, they are useful initial tests when the diagnoses are suspected.
An elevated serum level of muscle enzymes like CK and aldolase are characteristic but not specific to dermatomyositis and polymyositis. However, they are useful initial tests when the diagnoses are suspected.
Myositis-specific antibodies like anti-Jo-1 antibody and anti-SRP (signal recognition particle) antibody can be detected in both these diseases. Anti-Jo-1 antibody is a type of antisynthetase antibody, a class of antibody that is related to an overarching condition called antisynthetase syndrome, that some patients with DM or PM may fall under.
Myositis-specific antibodies like anti-Jo-1 antibody and anti-SRP (signal recognition particle) antibody can be detected in both these diseases. Anti-SRP antibodies are almost always found in PM patients and often indicate severe, treatment-resistant disease progression. Other antibodies that may be identified in these conditions include anti-Mi2, anti-PM-Scl, and anti-Ku.
This larger class of antibody is detected in a variety of immune conditions; their presence is common in DM and PM patients and can be seen with immunofluorescent staining.
Electromyography, or EMG, is a diagnostic test that uses electrical energy to demonstrate muscular activity. Either a surface or needle electrode is placed on or within the muscle to trigger and measure electrical activity produced by a muscle; these measurements are then used to generate an electromyogram. Patients with DM or PM have weak muscle fibers, and therefore abnormal EMG findings, like spontaneous fibrillations and repetitive discharges. EMG serves to differentiate myopathies from neuropathic disorders like myasthenia gravis.
For a definitive diagnosis of PM or DM, a muscle biopsy provides positive confirmation. Biopsy for PM shows CD8+ T-cell infiltration, along with necrotic muscle fibers. Muscle biopsy for DM shows CD4+ T-cell infiltration along with perifascicular atrophy, and a skin biopsy may also support the etiology of the cutaneous features.
Corticosteroids suppress the immune system by upregulating anti-inflammatory proteins and downregulating pro-inflammatory proteins within cells. Systemic corticosteroid therapy for a duration of nine months to a year is a common first approach to controlling these myopathic diseases. Either prednisone or methylprednisolone is given in high doses initially, and then tapered off gradually depending on clinical response.
Methotrexate (MTX) is an immunosuppressive drug that inhibits dihydrofolate reductase. It may be used as an adjunct to corticosteroid therapy, and it decreases the need for high-dose steroids, thereby reducing their side effects.
Azathioprine (AZA) is another immunosuppressive drug that acts as an antimetabolite in order to interfere with DNA synthesis within a cell. Combination with glucocorticoids has been shown to be effective with improved long-term patient outcomes over single agent steroid therapy.
Intravenous immunoglobulin (IVIG) is a blood product used to slow down an aberrant immune system via several, but debated mechanisms. It has been shown to be effective in controlling treatment resistant dermatomyositis, but it is not effective for polymyositis.
Classified as an alkylating agent, cyclophosphamide cross-links DNA to trigger cell death. It is used for chemotherapy as well as autoimmune conditions. Side effects include myelosuppression and hemorrhagic cystitis, so its use to treat PM and DM should only be considered after second-line agents have failed.
Monoclonal antibody to the B-cell marker CD20, used as both a chemotherapeutic and immunosuppressive agent. When treatment to corticosteroid therapy plus MTX or AZA fails, rituximab is often used.
Cyclosporine and tacrolimus work by inhibiting the calcineurin cellular receptor, which subsequently prevents T-cell activation. These medications are considered in resistant PM and DM, and in cases involving interstitial lung disease.
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