What are the typical exam and laboratory findings in polymyositis?

Exam and Laboratory Findings in Polymyositis

The primary diagnostic findings in polymyositis include proximal muscle weakness, elevated muscle enzymes (particularly creatine kinase), and characteristic inflammatory changes on diagnostic testing. 1

Clinical Examination Findings

• Muscle weakness:

  • Primarily affects proximal muscles (shoulders, upper arms, hips, thighs)
  • Symmetric distribution
  • Difficulty with activities like standing up from sitting, climbing stairs, lifting arms
  • True weakness rather than just pain 2, 1

• Absence of skin findings: Unlike dermatomyositis, polymyositis lacks characteristic skin manifestations 2

• Extramuscular manifestations:

  • Potential cardiac involvement (requires careful assessment)
  • Possible respiratory muscle involvement
  • Dysphagia in some cases 1

Laboratory Findings

• Muscle enzyme elevations:

  • Creatine kinase (CK): Markedly elevated, typically 5-50 times upper limit of normal (median 2650 IU/L, range 335-20,270 IU/L) 1
  • Aldolase: Elevated
  • Transaminases (AST, ALT): Often elevated
  • Lactate dehydrogenase (LDH): Elevated 2, 1

• Inflammatory markers:

  • Erythrocyte sedimentation rate (ESR): Often elevated
  • C-reactive protein (CRP): Often elevated 1

• Autoantibody testing:

  • Myositis-specific autoantibodies may be present
  • Anti-Jo-1 and other antisynthetase antibodies should be checked
  • Anti-SRP antibodies may indicate necrotizing myopathy 1

• Cardiac biomarkers:

  • Troponin should be checked to evaluate for cardiac involvement 2, 1

Diagnostic Testing

• Electromyography (EMG):

  • Shows muscle fibrillations indicative of myopathy
  • May reveal spontaneous fibrillation potentials, positive sharp waves
  • Characteristic myopathic motor unit potentials (short duration, low amplitude, polyphasic) 2, 1

• Magnetic Resonance Imaging (MRI):

  • Increased signal intensity on T2-weighted images
  • Edema in affected muscles
  • Useful for identifying affected muscle groups and guiding biopsy 2, 1

• Muscle biopsy (gold standard):

  • Endomysial inflammatory infiltrate predominantly of CD8+ T cells
  • Invasion of non-necrotic muscle fibers expressing MHC-I antigen
  • Absence of perifascicular atrophy (which would suggest dermatomyositis) 3

Common Pitfalls and Caveats

• Mistaking polymyositis for simple myalgia can delay critical treatment 1

• Failure to check CK levels in patients with muscle pain leads to incomplete evaluation 1

• Overlooking cardiac involvement, which can be life-threatening 2, 1

• Confusing polymyositis with other conditions that cause elevated CK:

  • Inclusion body myositis
  • Immune-mediated necrotizing myopathies
  • Metabolic myopathies
  • Endocrine disorders (e.g., hypothyroidism, hypocalcemia) 4
  • Muscular dystrophies 3

• Polymyositis is now considered relatively rare as improved diagnostic techniques have led to reclassification of many cases as other specific myopathies 5

Diagnostic Algorithm

1. Initial suspicion: Proximal muscle weakness without skin rash
2. Laboratory testing: CK, aldolase, AST, ALT, LDH, ESR, CRP, myositis-specific antibodies, troponin
3. Electrophysiologic testing: EMG to confirm myopathic pattern
4. Imaging: MRI of affected muscles
5. Definitive diagnosis: Muscle biopsy showing characteristic endomysial inflammatory infiltrate with CD8+ T cells invading non-necrotic muscle fibers

Remember that polymyositis is a diagnosis of exclusion that requires ruling out other causes of myopathy, including inclusion body myositis, necrotizing autoimmune myopathy, and metabolic or toxic myopathies 3, 5.