What are the typical findings in Repetitive Nerve Stimulation (RNS) for myasthenia gravis?

Typical Findings in Repetitive Nerve Stimulation (RNS) for Myasthenia Gravis

The classic finding in repetitive nerve stimulation testing for myasthenia gravis is a decremental response (>10% decrease) in compound muscle action potential amplitude or area between the first and fourth responses, particularly after exercise, which reflects the characteristic neuromuscular junction transmission failure in this condition. 1, 2

Characteristic RNS Findings

• Decremental response of ≥10% in compound muscle action potential (CMAP) amplitude or area between the first and fourth responses at low-frequency stimulation (2-5 Hz) 1, 3
• More pronounced decremental response after brief exercise (post-activation exhaustion) 1, 4
• Decrement is most prominent in clinically affected muscles 5, 6
• Facial muscles (particularly nasalis and orbicularis oculi) show higher sensitivity in ocular myasthenia gravis compared to limb muscles 5, 6

Sensitivity by Disease Subtype

• Higher sensitivity in generalized myasthenia gravis (71-79%) compared to purely ocular myasthenia gravis (38-50%) 3, 5
• Sensitivity varies by muscle tested:
  • Facial muscles (orbicularis oculi): 66-79% sensitivity 5
  • Nasalis muscle: 52-85% sensitivity 5, 6
  • Trapezius muscle: 55-79% sensitivity 5
  • Distal limb muscles (e.g., hypothenar): 17-53% sensitivity 5, 6

Technical Considerations

• Low-frequency stimulation (2-5 Hz) is typically used to detect the characteristic decremental response 4, 1
• High-frequency stimulation (30-50 Hz) may show an incremental response in some cases, particularly in Lambert-Eaton myasthenic syndrome 1
• Testing multiple muscles increases diagnostic yield, particularly in ocular myasthenia 5, 6
• Early in the disease course, RNS findings may be normal or subtle 1, 3

Diagnostic Value and Limitations

• RNS has high specificity but moderate sensitivity for myasthenia gravis 2, 3
• Single-fiber electromyography (SFEMG) is more sensitive (>90%) than RNS (30-70%) 1, 2
• Increased jitter and impulse blocking on SFEMG correlate with decremental responses on RNS, representing the same physiological phenomenon 4
• A significant decrement on RNS is never found without increased jitter and impulse blocking on SFEMG 4
• RNS can be particularly valuable in myasthenic crisis, with abnormal findings in up to 92% of patients 7

Clinical Correlations

• The degree of decrement may not consistently correlate with antibody titers or current clinical severity 3
• However, there may be correlation between CMAP decrement and the worst recorded clinical status in a patient's disease course 3
• In myasthenic crisis, RNS can serve as a rapid diagnostic tool with high sensitivity (92%) 7

Pitfalls and Considerations

• RNS testing can be painful, particularly at high frequencies (30-50 Hz) 1
• Results are operator-dependent and technically challenging 1
• Early in disease, results may be normal despite clinical symptoms 1, 3
• Certain medications can affect results (e.g., anticholinesterases, aminoglycosides) 2
• Testing should be performed before administering acetylcholinesterase inhibitors or at least 12 hours after the last dose for accurate results 2