What is Single-Fiber Electromyography (SFEMG)?
Single-fiber electromyography (SFEMG) is a highly specialized electrodiagnostic technique that records action potentials from individual muscle fibers to assess neuromuscular junction transmission and motor unit reinnervation patterns. 1
Technical Specifications
SFEMG achieves exceptional selectivity through:
- Specialized concentric needle electrode with a 25-μm diameter recording surface located in a side port 3mm from the needle tip 1, 2
- High-frequency filtering at 500-Hz low-frequency cutoff to isolate individual muscle fiber action potentials (MFAPs) 1
- Oscilloscope with trigger and delay line enabling identification of time-locked MFAPs within the same motor unit 1
Key Measurements
SFEMG assesses two critical parameters:
1. Jitter
- Definition: The temporal variation in end-plate potentials reaching threshold to elicit a muscle fiber action potential 1
- Clinical significance: Direct measure of neuromuscular transmission reflecting the variability in arrival time of action potentials between consecutive discharges 2, 3
- Expression: Measured as the mean of absolute consecutive differences (MCD) of latency between time-locked potentials 3
- Abnormal criteria: Study considered abnormal if 10% of fiber potential pairs exceed normal jitter or have impulse blockade, and/or jitter exceeds normal limits 4
2. Fiber Density (FD)
- Definition: Average number of muscle fibers belonging to the same motor unit within the recording area 1, 2
- Clinical significance: Provides sensitive in vivo assessment of reinnervation and motor unit spatial arrangement 1, 2
Recording Methods
SFEMG can be performed using two approaches:
- Voluntary activation (volitional SFEMG): Recording during slight voluntary muscle contraction 3
- Axonal stimulation (stimulated SFEMG): Recording by electrical stimulation of motor nerve branches to individual end plates 1, 3
Clinical Applications
SFEMG is the most sensitive clinical test for neuromuscular junction disease 1, 2, 3, particularly:
- Myasthenia gravis (MG): Often abnormal in clinically unaffected muscles; recommended when repetitive nerve stimulation is normal but high suspicion remains 4
- Lambert-Eaton myasthenic syndrome (LEM) 1, 2
- Other neuromuscular junction pathology 2, 3
Diagnostic Performance
Normal jitter findings in a clinically weak muscle exclude neuromuscular junction disease as a cause for weakness in that muscle 1. In low prevalence populations, when strict criteria are applied (requiring 3 jitter pairs be abnormal or mean jitter elevated), SFEMG demonstrates 100% sensitivity and 100% specificity 5.
Important Caveats
Non-Specific Findings
Increased jitter is not specific to neuromuscular junction disorders and can occur in:
- Ongoing reinnervation (neurogenic conditions) 6, 3
- Some myopathic conditions 6
- Unstable conduction in motor nerves and muscle fibers (extrajunctional jitter) 3
Botulinum Toxin Effects
Jitter remains abnormal after botulinum toxin injections, even in muscles remote from the injection site, and can persist for 6 months or more—this complicates detection or exclusion of abnormal neuromuscular transmission 6.
Laryngeal Application Limitations
While SFEMG can be applied in laryngeal muscles, the accuracy of standard diagnostic criteria has not been examined in these muscles 4. The rarity of myasthenia gravis affecting laryngeal muscles can lead to misdiagnosis 4.
Alternative Recording Methods
Concentric needle electrodes (CNE) can measure jitter with similar sensitivity to SFEMG electrodes 6. However, fiber density cannot be measured with CNE, requiring conventional electromyography to detect neurogenic or myogenic abnormalities in muscles with increased jitter 6.