What is the difference in Creatine Phosphokinase (CPK) levels between myositis and Guillain-Barré Syndrome (GBS)?

Creatine Phosphokinase (CPK) Levels in Myositis vs Guillain-Barré Syndrome

In myositis, CPK levels are typically markedly elevated, while in Guillain-Barré Syndrome (GBS), CPK levels are generally normal or only mildly elevated. This key laboratory distinction helps differentiate between these two neurological conditions 1.

Myositis CPK Characteristics

• Markedly elevated CPK levels: A hallmark laboratory finding in inflammatory myositis
• Mechanism: Direct muscle inflammation and damage leads to release of muscle enzymes into circulation
• Clinical correlation: CPK elevation typically correlates with disease severity and muscle weakness
• Range: Often significantly elevated, can reach thousands of units/L
• Monitoring value: Used to track disease activity and treatment response

Myositis presents primarily with true muscle weakness, particularly in proximal extremities, with difficulties in standing up, lifting arms, and moving around 1. In severe cases, patients may develop a fulminant necrotizing course with rhabdomyolysis, which can further elevate CPK levels dramatically.

Guillain-Barré Syndrome CPK Characteristics

• Normal or mildly elevated CPK levels: CPK is typically within normal limits in GBS 1, 2
• Exception cases: About 27% of GBS patients may show some CPK elevation 3
• GBS with elevated CPK profile:
  • More likely to be male
  • Higher rates of antecedent infections
  • Higher anti-GM1 antibody positivity
  • Less sensory involvement (hypoesthesia)
  • Less cranial nerve involvement
  • Less urinary retention
  • Electrophysiological patterns consistent with acute motor axonal neuropathy (AMAN) 3

Differential Diagnostic Value

This CPK distinction is particularly valuable when differentiating between:

1. Polymyalgia-like syndromes vs. Myositis: In polymyalgia-like syndromes, CPK levels should be within normal limits, differentiating this condition from myositis 1.

2. GBS vs. Myositis: When evaluating a patient with weakness, normal CPK levels point toward GBS rather than myositis 1, 2.

3. GBS variants vs. Myositis: Some GBS variants may have overlapping clinical features with myositis, but CPK levels help distinguish between them 1, 2.

Clinical Pearls and Pitfalls

• Rare coexistence: In rare cases, myositis and GBS can coexist, particularly following specific infections like Mycoplasma pneumoniae 4. In these cases, CPK will be elevated.

• Delayed CPK elevation in GBS: Some GBS patients may develop elevated CPK levels about three weeks after disease onset, potentially due to denervation-induced muscle hyperexcitability 5.

• Diagnostic algorithm:

  1. Assess pattern of weakness (proximal vs. distal, symmetric vs. asymmetric)
  2. Check deep tendon reflexes (typically absent/reduced in GBS)
  3. Measure CPK levels
  4. Perform electrophysiological studies
  5. Consider CSF analysis (albuminocytologic dissociation in GBS)

• Treatment implications: The distinction is critical as treatment approaches differ significantly between myositis (corticosteroids and immunosuppressants) and GBS (IVIg or plasma exchange) 2, 6.

Remember that while CPK levels provide valuable diagnostic information, they should be interpreted in the context of the complete clinical picture, including neurological examination, electrophysiological studies, and other laboratory findings.