WS6.2. Stimulation single-fibre EMG: Methods and pitfalls (with demonstration)

Abstract

Single-fiber electromyography (SFEMG) is a technique introduced by Prof. Stålberg and his colleagues in 1970’s. SFEMG electrode has the smallest uptake area (0.3 mm in radius) among EMG electrodes, and therefore can identify action potentials of individual muscle fibers (single fiber action potentials; SFAPs) comprising the motor unit by applying a very high low- cut filter (500 Hz). Recently, disposable concentric needle is commonly used in place of reusable SFEMG needle (CN-SFEMG). By setting an even higher low-cut filter (1 or 2 kHz), individual SFAPs can be identified even using the concentric needle. There are two recording methods of SFEMG, during voluntary contraction (v-SFEMG) and following axonal stimulation (s-SFEMG). Both methods have merits and demerits, and specific pitfalls. Patient cooperation is necessary for v-SFEMG but not for s-SFEMG, which is therefore applicable for comatose patients or small children. Examination time is usually believed to be shorter in s-SFEMG (I don’t think that this is always true). This is because only one SFAP is necessary in order to measure jitter in s-SFEMG, although two SFAPs recoded simultaneously are necessary in v-SFEMG. However, v-SFEMG is probably more sensitive in detecting neuromuscular junction (NMJ) dysfunction in myasthenia gravis (MG). Since SFEMG is expected to be more sensitive than repetitive nerve stimulation test, the sensitivity must be the most crucial issue. Therefore, I prefer v-SFEMG and conduct only v-SFEMG in daily practice. For s-SFEMG, monopolar stimulating needle is inserted close to the motor nerve or terminal nerve twigs (facial nerve branch lateral to the external canthus for frontalis recording). CN needle is inserted within the target muscle. We search sharp SFAPs during 5 to10 Hz test stimulation. When good SFAPs are identified, jitter is measured during 10 Hz stimulation. Submaximal stimulation is the greatest pitfall in s-SFEMG. Therefore, the threshold current is determined for each SFAP and an intensity 20% over the threshold is used for recording jitter. Very often, the target SFAP is blurred by the contamination of other SFAPs that have been newly-recruited while increasing the current intensity. This Is the reason why s-SFEMG takes longer time than usually considered.