26th January 2024

Diagnosis 

Dermatomyositis

Most patients with myositis will have one or more muscle enzymes elevated, such as creatinine kinase, lactate dehydrogenase, aldolase, aspartate aminotransferase or alanine aminotransferase. 

Blood Tests: In dermatomyositis, there are specific autoantibodies that may be found, such as Anti Mi-2, which is associated with cutaneous disease but seems to have a lower risk of associated malignancy. Anti-melanocyte differentiation-associated protein 5 (anti-MDA5) is associated with rapidly progressive interstitial lung disease and characteristic skin changes, including ulcerations, Gottron’s papules, painful palmar and macular papules and non-scarring alopecia. 

The diagnostic criterion generally used is the 2017 EULAR/ACR classification criteria. To make a diagnosis of dermatomyositis, there needs to be features such as the previously mentioned skin changes (heliotrope rash and Gottron’s papules or Gottron’s sign) AND symmetrical, progressive muscle weakness (usually the proximal upper extensors and/or lower extensors or neck flexors relatively weaker than extensors. 

Muscle Biopsy: If this is required, the histopathological changes would show inflammation in the perimysial region (around the muscle bundles) and include CD4+ cells. 

Polymyositis

Polymyositis is seen in patients with muscle weakness without characteristic skin changes. 

Laboratory Tests: These may reveal similar results to that of other inflammatory myopathies, and as such, a muscle biopsy is frequently required for diagnosis. 

Muscle Biopsy: Histopathology, in this case, would show predominantly cellular infiltrates in the endomysial and inflammatory cells invading individual muscle fibres. Myocyte injury is mediated by CD8+ cytotoxic T lymphocytes that surround and invade the myofibers. 

Inclusion Body Myositis 

Inclusion body myositis also presents without any skin changes, and thus, diagnosis is dependent upon laboratory investigations, electromyography and muscle biopsy. 

Blood Tests: Autoantibodies against cytoplasmic 5′-nucleotidase 1A (cN1A) may help distinguish IBM from other forms of myositis. 

Neurophysiology studies: Changes in electromyography include fibrillation potentials, bizarre high-frequency repetitive discharges and short, small, low-amplitude polyphasic motor unit potentials. 

Muscle biopsy: This is likely to be diagnostic in many cases and histologically has characteristic features of sarcoplasmic ‘rimmed’ vacuoles that are red-rimmed on modified trichrome stain and blue-rimmed on H&E stain.