2025 Workshop Handout Bundle-W07C SFEMG Massey Sanders

W07C SFEMG Massey Sanders

Pdf Summary

Single Fiber Electromyography (SFEMG) is a specialized electromyographic technique that records action potentials (APs) from individual muscle fibers using a fine, side-ported electrode with a small recording surface (~25 µm). This selectivity enables assessment of two critical motor unit features: fiber density (FD) and neuromuscular jitter.<br /><br />Fiber density quantifies the local concentration of muscle fibers within a motor unit and is sensitive to muscle fiber topography rearrangements, which increase in neuropathies and some myopathies. FD is measured by counting time-locked APs exceeding 200 µV amplitude within a ~300 µm electrode recording territory at multiple muscle sites. Normal FD varies by muscle and increases with age, more prominently in distal muscles.<br /><br />Neuromuscular jitter represents the variability in latency between nerve stimulation and muscle fiber AP, reflecting the safety factor of neuromuscular transmission. Jitter can be measured during voluntary muscle activation or axonal stimulation. Axonal stimulation jitter studies are useful when voluntary activation is difficult, though technical expertise is essential to avoid artifacts. Voluntary activation jitter measurements involve recording pairs of time-locked APs from the same motor unit and calculating mean consecutive differences in interpotential intervals (MCD). Jitter values differ by muscle, increase with age, and very low jitter may occur in myopathies due to split fibers.<br /><br />SFEMG is particularly valuable in diagnosing neuromuscular transmission disorders such as myasthenia gravis (MG) and Lambert-Eaton myasthenic syndrome (LEM). MG patients show increased jitter even in clinically unaffected muscles; distribution may vary, especially in MuSK antibody-positive MG. In LEM, jitter and blocking improve with higher firing rates. SFEMG also detects reinnervation changes in neuropathies (increased FD and jitter) and neuromuscular abnormalities in myopathies.<br /><br />Conventional concentric needle (CN) electrodes can approximate SFEMG jitter measurements by filtering and using small electrodes, though sensitivity is lower and careful methodology with appropriate reference values is essential.<br /><br />SFEMG requires significant operator expertise, quality control at signal acquisition and analysis stages, and interpretation in clinical context. It complements repetitive nerve stimulation testing and routine EMG for comprehensive neuromuscular assessment. Guidelines and normative data for multiple muscles and electrode types guide proper application and interpretation of SFEMG results.

Keywords

Single Fiber Electromyography

SFEMG

fiber density

neuromuscular jitter

motor unit

myasthenia gravis

Lambert-Eaton myasthenic syndrome

neuromuscular transmission disorders

electromyographic technique

concentric needle electrodes