Why is caffeine used in the in vitro contracture test (IVCT) for patients suspected of having malignant hyperthermia (MH) susceptibility?

Why Caffeine is Used in the In Vitro Contracture Test (IVCT)

Caffeine is used in the IVCT because muscle from patients susceptible to malignant hyperthermia (MH) demonstrates abnormally increased sensitivity to caffeine's contracture-inducing properties, making it a critical diagnostic agent for identifying MH susceptibility. 1

Mechanistic Basis for Caffeine Testing

Caffeine serves as a pharmacological challenge agent that exploits the underlying pathophysiology of MH:

• MH-susceptible muscle exhibits heightened contracture responses to caffeine at lower concentrations than normal muscle, a finding that has been the foundation of diagnostic testing since the early 1980s 1
• Caffeine acts on the calcium release mechanisms in skeletal muscle, triggering abnormal contractures in susceptible individuals at concentrations that produce minimal or no response in normal muscle 1
• The test measures the threshold concentration at which caffeine produces a sustained contracture of at least 2 mN (0.2 g) in baseline force 1

Standardized IVCT Caffeine Protocol

The European Malignant Hyperthermia Group established a rigorous protocol for caffeine testing:

• Caffeine concentrations are increased stepwise: 0.5,1.0,1.5,2.0,3.0,4.0, and 32 mmol/L 1
• Each concentration is administered after the maximal contracture plateau from the previous dose is reached, or after 3 minutes if no contracture occurs 1
• Caffeine must be used as free base (analytical grade), not caffeine citrate or benzoate, as these preparations produce different contracture responses due to pH and calcium concentration effects 2
• The muscle is NOT washed between successive caffeine concentrations (cumulative test) 1

Diagnostic Classification Using Caffeine

Caffeine testing contributes to the laboratory classification of MH susceptibility:

• Patients with abnormal caffeine responses alone are classified as MHSc (MH-susceptible to caffeine), previously termed MHE 1
• Patients with abnormal responses to both caffeine AND halothane are classified as MHShc 1
• All patients with abnormal caffeine responses should be considered clinically at risk for MH during anesthesia, regardless of halothane test results 1
• The maximal contracture at 2 mmol/L caffeine concentration must be reported in addition to the threshold concentration 1

Why Both Caffeine AND Halothane Are Necessary

The dual-agent approach maximizes diagnostic sensitivity:

• The IVCT achieves 99% sensitivity and 94% specificity for MH susceptibility by using both caffeine and halothane 3
• Some MH-susceptible patients respond abnormally only to caffeine, while others respond only to halothane 1
• Using both agents captures the full spectrum of MH susceptibility phenotypes that would be missed with a single-agent test 1, 3
• Combined testing has been validated over 30 years in more than 10,000 individuals worldwide 1

Critical Technical Considerations

Several factors affect the reliability of caffeine testing:

• Contractures less than 5 mN (0.5 g) are reproducible in less than half of tests, making responses near the 2 mN cutoff less scientifically reliable, though the clinical cutoff must remain unchanged for safety 4
• Larger contractures demonstrate better reproducibility, with correlation between contracture size and abnormal responses of 0.77 or greater 4
• The test must be completed within 5 hours from biopsy to maintain muscle viability 3
• Muscle specimens must meet specific dimensions (20-25 mm length, 2-3 mm thickness, 100-200 mg weight) for valid testing 1, 3

Alternative and Complementary Testing

While caffeine remains standard, research has explored other approaches:

• Ryanodine testing combined with caffeine or halothane may provide higher sensitivities (85.3-93.9%) compared to standard protocols, though this remains investigational 5
• Intramuscular caffeine injection with pCO₂ monitoring has been proposed as a minimally invasive alternative, showing pCO₂ increases to 63 mm Hg in susceptible patients versus 44 mm Hg in non-susceptible individuals 6
• However, the standard IVCT with caffeine and halothane remains the gold standard, as DNA testing is less sensitive and cannot replace it for primary diagnosis 1, 3