Myositis should be suspected in primary health care if muscle weakness and possibly elevated muscle enzyme values are found.
Patients with severe symptoms should be referred to hospital as emergency cases if they suffer from severe dyspnoea or serious swallowing problems, for example.
Medication and rehabilitation can influence the course and prognosis of polymyositis, dermatomyositis and necrotizing autoimmune myopathy.
Inclusion body myositis is a chronically progressive disease. Its treatment consists of rehabilitative and supportive care, and there is no course-altering medication available.
All patients benefit from regular training to maintain muscle fitness and strength.
Classification
The most common subtypes are inclusion body myositis (IBM), polymyositis (PM), dermatomyositis (DM) and necrotizing autoimmune myopathy (NAM).
Rarer types of myositis include those associated with an infection (influenza A or B, HIV, dengue, toxoplasma) or cancer, for example.
Myositis may occur as a symptom of a generalized connective tissue disorder.
The antisynthetase syndrome associated with polymyositis may in addition to myositis include interstitial pulmonary disease, arthritis, fever, Raynaud's syndrome Raynaud's Phenomenon (RP) or White Finger Disease and "mechanic's hands" (dry, cracking fingertips).
Symptoms
Usually subacute or chronic proximal muscle weakness in upper and lower extremities, with onset after the age of 45 and causing difficulty with activities such as walking up and down stairs and getting up from a squatting position
Dermatomyositis causes skin symptoms (see in more detail below here).
Systemic symptoms, such as slight fever, fatigue and weight loss may occur.
There may be muscle pain and tenderness.
Some patients have respiratory symptoms (cough and dyspnoea). These are due to either interstitial lung disease associated with the underlying disease, aspiration pneumonia or weakened respiratory muscles.
Some patients have difficulty swallowing, which may be quite significant.
In addition to the skin and lungs, the disease may also affect joints, the heart or the gastrointestinal tract.
Joint symptoms include joint pain and nonerosive arthritis.
Cardiac symptoms are rare but patients may have conduction disorders or develop cardiac failure.
In addition to swallowing problems, gastrointestinal symptoms such as reflux oesophagitis, diarrhoea, constipation and abdominal pain, are common.
Workup and diagnosis
If myositis is suspected, the following laboratory tests should be performed in primary health care: CK, basic blood count with platelet count, ESR, CRP, TSH, creatinine and anti-nuclear antibodies.
The basic blood count with platelet count is usually normal, CRP normal or slightly elevated, and ESR may be either normal or elevated.
ENMG can be performed in primary health care if readily available.
The diagnosis should be confirmed in specialized care.
If rheumatic or antibody positive myositis or dermatomyositis is primarily suspected, patients should be referred to a rheumatologist. If IBM is suspected, they should be referred to an outpatient neurology clinic.
The diagnosis is based on clinical picture, elevated CK levels and muscle MRI and ENMG.
Muscle biopsy forms the cornerstone of diagnosis.
Tests for myositis-specific autoantibodies in specialized care (S-myositis test) are important for classification of the disease, choice of treatment, and assessment of prognosis.
Further studies, such as cardiac or pulmonary imaging and functional tests, should be planned individually depending on the clinical picture.
The most common myopathy in people over 50, with a prevalence of approximately 2.2-7.1/105
The clinical picture helps in differential diagnosis: slowly progressive (asymmetric) weakness of the quadriceps and arm muscles; distal muscle symptoms at an early stage of disease
CK levels vary from normal to ten times normal.
More than half of the patients have swallowing problems at some stage of the disease.
There may be weakness of respiratory muscles; in the presence of such symptoms, spirometry and/or polysomnography should be performed.
Main findings in muscle biopsy: inflammation, myodegeneration and protein accumulation, as well as mitochondrial pathology
Polymyositis (PM)
Subacute proximal, symmetric muscle weakness without skin findings
Prevalence approximately 9.7/105
Differential diagnosis particularly versus other types of myositis (DM and IBM) and hereditary myopathies is important.
CK levels may be normal but they are usually 10 to 50 times normal.
Some patients have antisynthetase antibodies (e.g. Jo-1).
Early-stage muscle MRI shows oedematous changes in T1 sequences in the absence of atrophy or accumulation of fat.
Muscle biopsy shows cytotoxic CD8+ cell invasion.
There is a slightly increased risk of cancer.
Dermatomyositis (DM)
Occurs in both children and adults, with a prevalence of approximately 5.8/105
Acute onset muscle pain and fatigue preceded by or simultaneously with skin changes
Purple periorbital dermatitis
Dermatitis on the chest or upper back
Gottron's papules on the knuckles (erythematous purplish lesions on the extensor sides of metacarpophalangeal and proximal interphalangeal joints)
Dilated capillaries in fingertips; capillaries can be examined by video capillariscopy.
Particularly in patients with antisynthetase antibodies, thickened skin and cracking fingertips ("mechanic's hands")
CK levels vary from normal to 50 times normal.
Some patients have dermatomyositis-associated antibodies (S myositis test): anti-Jo-1, anti-MDA-5, anti-Mi-2. Anti-TIF-1ɣ antibodies are associated with a malignant tumour.
Muscle biopsy shows perivascular, perimysial and perifascicular inflammation; invasion of CD4+ cells
Other possible organ manifestations: interstitial lung disease, heart, blood vessels, gastrointestinal tract
Due to the increased risk of cancer (breast, ovarian, lung and prostate cancer, in particular), annual screening for 3 years following diagnosis
Necrotizing autoimmune myopathy (NAM)
More common than polymyositis, representing about 19% of all inflammatory myopathies in adults
Muscle weakness of acute onset either without any predisposing factors or associated with
viral infection
cancer
connective tissue disease, such as systemic scleroderma Systemic Sclerosis
Treatment strategy, prognosis and further follow-up depend on the diagnosis.
All patients benefit from regular training maintaining muscle fitness and strength.
Individual rehabilitation plans should be made in multidisciplinary cooperation (physician, physiotherapist, rehabilitation advisor).
IBM is a chronically progressive disease. Its treatment consists of rehabilitative care, and there is currently no medication available influencing the course of the disease.
In DM and PM, response to immunosuppressive treatment is good.
In NAM, response to immunosuppressive treatment varies.
Patients with dermatomyositis should avoid sunshine (protective clothing, high sun protection factor in sun protection products). For skin symptoms, a topical glucocorticoid or calcineurin inhibitor may be used, as necessary.
Follow-up and prognosis
If the response to treatment is poor, the correctness of the diagnosis should be reassessed.
Response to treatment is monitored based on the patient's condition (patient's and physician's general assessment of disease activity), muscle strength testing and, as necessary, CK level, muscle MRI and autoantibody test.
If the CK level is elevated, elevated aminotransferase (AST , ALT), LD and TnT levels are often also seen in the absence of liver or heart damage.
With treatment, the prognosis is good, with the probability of 10-year survival exceeding 90%.
Long-term follow-up in specialized care is necessary if the disease is even slightly active.
If the disease has been at a calm stage for a long time and demanding immunosuppressive medication is no longer needed, patients can be monitored in primary health care, where their general state and muscle condition can be monitored, measuring CK levels, as necessary, and performing other laboratory tests depending on the medication (see Rheumatoid arthritis > Safety monitoring tests for drug therapy Rheumatoid Arthritis).
If reactivated myositis is suggested during follow-up, specialized care should be consulted.
References
Dalakas MC. Inflammatory muscle diseases. N Engl J Med 2015;372(18):1734-47. [PubMed]