5th June 2025

Investigations

The following investigations should be considered in dermatomyositis:

• Serum creatinine kinase (CK) – the most sensitive muscle enzyme test
• Serum aldolase – glycolytic pathway enzyme found mainly in skeletal muscle, brain and liver; not as specific or sensitive as CK, but can be elevated in dermatomyositis when CK is normal
• Muscle biopsy – should be performed in all suspected dermatomyositis cases 
• Electromyography (EMG) – recommended if raised CK and typical muscle biopsy findings are present
• Skin biopsy – required in patients that only present with skin manifestations to differentiate from other connective tissue diseases, biopsy appearance is similar to cutaneous lupus
• Antinuclear antibody (ANA) – positive in 80% of patients
• Anti-double-stranded DNA, anti-Ro, anti-La, anti-Sm, anti-ribonucleoprotein (RNP) – useful for differentiation from other connective tissue diseases
• Myositis-specific antibodies (MSAs) and myositis-associated antibodies (MAAs)

MSAs and MAAs are found in most dermatomyositis patients and are associated with different clinical subsets. They are not widely available, but can offer information regarding clinical courses and therapeutic responses.

• Anti-Mi-2 – directed against a helicase involved in transcriptional activation
  • Strongly associated with the classical presentation of dermatomyositis, but found in only 10% of patients
  • Anti-U1 ribonucleoprotein (RNP) and anti-Ku found in overlap syndromes that present similarly to anti-Mi-2 positive dermatomyositis
• Anti-transcription intermediary factor 1 gamma (TIF1-gamma)
  • Most commonly isolated antibody
  • Strongly associated with malignancy 
• Anti-signal recognition particle (SRP)
  • Found almost exclusively in polymyositis and only occasionally in dermatomyositis
  • When present, associated with acute severe, treatment-resistant necrotising myositis
• Anti-Jo-1 and other anti-amioacyl-tRNA synthetase antibodies (including anti-PL-7, anti-PL-12, anti-EJ, anti-OJ, anti-KS) -> anti-synthetase syndrome
  • Patients positive for the above present with a distinct clinical picture called anti-synthetase syndrome
  • Main clinical features include interstitial lung disease, fever, myositis, polyarthritis, mechanic’s hands, Raynaud’s phenomenon
  • Anti-Jo-1 associated with a higher frequency of myositis
  • The other are associated with a higher frequency of interstitial lung disease
• Anti-melanoma differentiation-associated gene 5 (MDA5, also known as CADM-140) – specific to clinically amyopathic dermatomyositis
  • Strongly associated with a rapidly progressive interstitial lung disease with poor prognosis
  • May present with tender palm papules and diffuse alopecia
• Anti-NPX2 
  • Strongly associated with juvenile-onset dermatomyositis; a high proportion of these patients suffer from cutaneous calcinosis
• Anti-PM-Scl
  • Found in patients with overlapping myositis and scleroderma