Dermatomyositis (DM) and polymyositis (PM) are idiopathic inflammatory myopathies, characterized by the shared feature of muscle inflammation with associated proximal skeletal muscle weakness (although an amyopathic form of dermatomyositis exists). Dermatomyositis, in addition, is also associated with a variety of characteristic skin manifestations.
Dermatomyositis is relatively rare, and presents differently in adults and children. Only in the adult form is there an association between the disease and an occult systemic malignancy. The frequency of this association varies in different series, but may be as high as 2.5% and is found more commonly in males, men in their sixth to eighth decades.
Question patients regarding the duration, mode of onset, location, and severity of weakness. Ask of their ability to carry out various activities they commonly perform, such as climbing stairs, getting up from a chair, and carrying groceries or other objects. Assess the severity and distribution of myalgia, and presence or absence of significant stiffness. True muscle weakness should be distinguished from complaints of fatigue or shortness of breath with exertion and from limitation due to joint disease.
Examine the skin, muscles, and joints. A thorough examination of the skin should be performed with particular attention to the scalp, face, eyelids, hands, fingers, extensor aspects, and joints. A general physical examination should be performed, with particular attention to the heart and the lungs should be carefully auscultated for evidence of interstitial disease. A thorough joint examination should detect any signs of inflammatory arthritis. A detailed neurologic and neuromuscular examination is critical to determine the severity and distribution of weakness and of muscle tenderness, as well as the presence or absence of other abnormal neurologic findings.
The typical rash of dermatomyositis is a macular erythema on the face, particularly marked in the periorbital area, where it is of heliotropic (blue-violet) hue. Examine the dorsum of hands and fingers, also often involved, for characteristic linear erythema. Nail-fold haemorrhages may be present, albeit reflective of any connective tissue disorders. An erythema on the neck and upper chest can progress to poikiloderma (dappled skin), consisting of alternating telangiectasia with atrophy. Photosensitivity is also commonly seen.
Associated with a 10% risk of malignancy (i.e., ovarian cancer, GU cancers, breast, lung, gastric, etc.)
Possible positive autoantibodies:
- ANA
- anti-Jo-1 (anti-histidyl transfer RNA synthetase) correlates with pulmonary fibrosis, Raynaud’s, and polyarthritis
- anti-Mi-2, correlates to classic DM (shawl sign, cuticle change, Gottron’s papules/sign, favourable diagnosis
- anti-SRP, correlate with cardiac disease, poor prognosis
- anti- Ku, correlate with sclerodermatomyositis
- anti-PL 12, correlate with pulmonary disease1
Muscle weakness is very variable. It most commonly affects the proximal muscle groups of all four limbs, so that early weakness tends to be noted on climbing stairs or performing tasks with the hands at or above shoulder level. When severe, respiratory embarrassment can result from involvement of intercostal and diaphragmatic muscles.
In addition to general laboratory testing, test for muscle enzymes, including creatine kinase and aldolase. A chest X-Ray should be performed in all patients with suggestive findings, to help detect the presence of pulmonary involvement, such as interstitial lung disease. Pulmonary function tests and chest computed tomography (CT) scanning are indicated in patients with pulmonary symptoms or with abnormal chest X-Ray demonstrating interstitial lung disease.
The diagnosis of dermatomyositis and polymyositis generally depends upon the presence of characteristic clinical and laboratory findings, including symmetric proximal muscle weakness and elevated muscle enzymes. Diagnosis often rests upon further information from one or more diagnostic studies, selected based upon the clinical presentation and initial findings:
Muscle biopsy: dermatomyositis and polymyositis are respectively distinguished and also differentiated from other forms of myopathy by their histopathologic findings; both dermatomyositis and polymyositis include muscle fibre necrosis, degeneration, regeneration, and an inflammatory cell infiltrate. Perifascicular atrophy and fibrosis are characteristically seen in dermatomyositis, but may be absent early. This is a perimysial infiltrate and includes CD4+ types but predominantly (30-90%) of plasmacytoid dendritic cells rather than T cells. In polymyositis, the cellular infiltrate is predominantly within the fascicle, with inflammatory cells invading individual muscle fibers. Myofiber injury appears to be mediated directly by CD8+ cytotoxic T-lymphocytes that surround and invade myofibers.
Skin biopsy: characteristic light-microscopy skin biopsy findings of dermatomyositis are very similar to those seen in systemic lupus erythematosus.
Electromyography: Characteristic abnormalities on EMG can support a diagnosis of inflammatory myopathy although changes are not diagnostic of dermatomyositis and polymyositis, with similar findings occurring in various infectious, toxic, or metabolic myopathies.
Magnetic resonance (MR) imaging: can demonstrate areas of muscle inflammation, oedema with active myositis, fibrosis, and calcification. Although MRI can assess large areas of muscle, avoiding problems with sampling error, it is, however, non-specific and may not distinguish the changes of an inflammatory myopathy from those that occur in rhabdomyolysis, muscular dystrophy, or metabolic myopathy.
Atypical features that may suggest an alternate diagnosis include:
- asymmetric or distal presentation of weakness
- intermittent symptoms
- painful muscles
- marked muscle atrophy
- family history of muscle disease
- history of medication that could be associated with myopathy
- neuropathic symptoms or findings
Ultimately, dermatomyositis and polymyositis must be distinguished from other conditions that cause muscle weakness, with or without muscle enzyme elevation. This includes other inflammatory myopathies, motor neuron disease, myasthenia gravis, and the muscular dystrophies, as well as a variety of inherited, metabolic, drug–induced, endocrine, and infectious myopathies. None of these disorders are associated with the skin lesions that are characteristic of dermatomyositis. Additionally, myalgia tends to be mild in dermatomyositis and polymyositis, unlike that seen in polymyalgia rheumatica, fibromyalgia, and viral or bacterial myositis.
In those patients suspected of dermatomyositis and polymyositis based upon proximal muscle weakness, elevated muscle enzymes, and a non-specific rash, confirm a diagnosis based upon muscle biopsy findings. An EMG as well as muscle biopsy should be performed in those patients who have atypical findings or in whom the diagnosis is uncertain, to exclude inclusion body myositis, a metabolic myopathy, or a dystrophy. Further studies to exclude malignancy in confirmed cases of dermatomyositis and polymyositis depend upon an individualized assessment of risk for particular disorders. Patients with clinical evidence of other organ involvement, including in particular those with cardiac, pulmonary, or esophageal disease, should also undergo further evaluation to characterize abnormalities that may be present.
References
- Johnston, Ronald B. Weedon’s Skin Pathology Essentials. 2nd Edn. Elsevier, 2017.
- MacKie, Rona M. Clinical Dermatology: An Illustrated Textbook. 3rd Edn. Oxford: Oxford University Press, 1991.
- Miller, Marc L., Targoff, Ira N., Shefner, Jeremy M, Callen, Jeffrey and Curtis, Monica Ramirez. “Diagnosis and differential diagnosis of dermatomyositis and polymyositis in adults.” Dec 14, 2015. Available at http://www.uptodate.com/…is-and-polymyositis-in-adults?topicKey=RHEUM%2F15653&elapsedTimeMs=1&view=print&displayedView=full. Accessed Feb 8, 2016.
