Single-fiber electromyography improvement with 3,4-diaminopyridine in Lambert-Eaton myasthenic syndrome

Single fiber electromyography

Single fiber electromyography (SFEMG) is a highly selective technique that permits assessment of individual muscle fiber action potentials (MFAPs). This selectivity is achieved with a specialized concentric needle electrode with a 25-μm diameter recording surface located in a side port 3mm from the needle tip. Additional selectivity is achieved with 500-Hz low-frequency filtering. An oscilloscope with a trigger and delay line enables identification of time-locked MFAPs within the same motor unit. SFEMG techniques allow assessment of two important features of the motor unit: jitter and fiber density (FD). Neuromuscular jitter is a direct measure of neuromuscular transmission and reflects the temporal variation in end-plate potentials reaching threshold to elicit a MFAP. SFEMG may be used to assess paired jitter with voluntary activation or by axonal stimulation of motor nerve branches to individual end plates. SFEMG is the most sensitive clinical test for neuromuscular junction disease and is often abnormal in clinically unaffected muscles in patients with myasthenia gravis (MG) and Lambert-Eaton myasthenia (LEM). Normal jitter findings in a clinically weak muscle exclude neuromuscular junction disease as a cause for weakness in that muscle. FD measurements assess the local concentration of muscle fibers within a motor unit and provide a sensitive in vivo assessment of reinnervation.

Clinical impact of single-fiber electromyography

The major clinical impact of single-fiber electromyography has been from its role in confirming, or excluding, the diagnosis of myasthenia gravis (MG). Jitter measurements also have a clinical role in demonstrating changes in disease severity in patients with MG and Lambert-Eaton myasthenic syndrome, in demonstrating subtle changes in motor unit architecture and physiology in patients with nerve and muscle diseases, and in demonstrating the remote effects of locally injected botulinum toxin. In addition to these clinical roles, the ability to identify the activity from single muscle fibers makes it possible to mark the discharges of single motor units. This, along with information gained by jitter and fiber-density measurements, has uniquely increased our understanding of motor unit organization and function in normal and disease states.