From the Guidelines
Diagnosing myositis requires a comprehensive approach that includes a thorough medical history, physical examination, laboratory tests, imaging, and sometimes muscle biopsy, with the goal of preventing irreversible muscle damage and improving outcomes. The diagnostic process typically begins with a thorough medical history and physical examination, looking for characteristic symptoms like muscle weakness (especially in the shoulders, hips, and neck), fatigue, muscle pain, and difficulty with activities like climbing stairs or rising from a chair.
Key Diagnostic Steps
- Laboratory tests are essential, particularly checking for elevated muscle enzymes such as creatine kinase (CK), aldolase, AST, and ALT, which indicate muscle damage, as recommended by 1.
- Inflammatory markers like ESR and CRP may also be elevated, and autoantibody testing is crucial, including myositis-specific antibodies (anti-Jo-1, anti-Mi-2, anti-SRP) and myositis-associated antibodies (anti-Ro/SSA, anti-La/SSB).
- Electromyography (EMG) can detect abnormal electrical activity in affected muscles, while MRI can visualize muscle inflammation and rule out other conditions, as noted in 1.
- Muscle biopsy remains the gold standard for definitive diagnosis, showing characteristic inflammatory cell infiltration and muscle fiber degeneration.
Additional Considerations
- In some cases, additional tests may be needed to identify underlying causes or associated conditions, such as cancer screening for dermatomyositis patients.
- Early diagnosis is important as prompt treatment with immunosuppressive medications like prednisone (starting at 0.5-1 mg/kg/day) can prevent irreversible muscle damage and improve outcomes, as suggested by 1.
- The use of other immunosuppressants and IVIG may also be indicated, as they are used for treatment of polymyositis/dermatomyositis, as discussed in 1.
From the Research
Diagnosing Myositis
To diagnose myositis, a combination of clinical assessment, laboratory investigations, and imaging findings is necessary. The following steps can be taken:
- Clinical assessment: A thorough clinical evaluation, including a detailed family history, should be undertaken in any patient with presumed idiopathic inflammatory myopathy 2.
- Laboratory investigations: Electromyography and creatine kinase measurements should be performed to support the diagnosis 2.
- Imaging findings: MRI plays an important role in the diagnosis and evaluation of patients with suspected myositis, and can help to differentiate it from other muscle disorders 3, 4.
- Muscle biopsy: A muscle biopsy remains essential in achieving an accurate diagnosis and guiding treatment 5, 2.
Differential Diagnosis
It is essential to consider the differential diagnosis of myositis, as other muscle disorders can have similar clinical and imaging features. These include:
- Muscular dystrophies: Can have clinical and muscle histopathologic overlap with inflammatory myopathies 2.
- Inclusion body myositis: Less responsive to immunotherapy and can be mistaken for other forms of myositis 6, 2.
- Deep venous thrombosis: Can have similar imaging features to myositis 4.
- Diabetic myonecrosis: Can have similar clinical and imaging features to myositis 4.
- Neoplasms: Can have similar imaging features to myositis 4.
Diagnostic Criteria
The diagnostic criteria for myositis have been refined over time, and it is essential to use strict histopathologic criteria to diagnose polymyositis and dermatomyositis 5. The Bohan and Peter's criteria are now obsolete, and more recent guidelines should be used to diagnose and manage myositis 4, 5.