WS6.3. Measuring jitter with concentric needle electrodes

Abstract

Although it is preferable to use single fiber electrodes (SFEs) to measure jitter, concentric needle electrodes (CNEs) are increasingly used instead because of concern about transmissible diseases and the cost of SFEs. Increasing the low frequency filter to 1 kHz or higher produces CNE signals that appear identical to SF action potentials (APs), but such CNE signals frequently represent summation of many SFAPs – to emphasize this, simple spikes recorded with CNEs that resemble SFAPs have been called “apparent single fiber action potentials” (ASFAPs). Jitter in summated signals recorded with CNEs is somewhat less than the jitter in SFAPs. Thus, although increased jitter in CNE studies indicates abnormal neuromuscular transmission, mild abnormalities may not be detected as well as with SFEs unless appropriate reference values are used. Note – fiber density measurements can only be made with SFEs. The following precepts should be followed if CNEs are used for jitter measurements: • Use the CNE with the smallest recording surface. Currently this is the DCF 25 electrode, which is available from several vendors. Note – because of their large pickup area, monopolar needle electrodes are even more likely to record superimposed APs and should not be used for jitter analysis. • Use a high pass filter setting of 1 kHz. Because this will reduce the amplitude of recorded signals, spikes with amplitude as low as 50 μV are acceptable for jitter measurement if they have a rise time less than 300 μsec. • Measure jitter only from simple spikes that resemble SFAPs (ASFAPs). Do not measure jitter from spikes with a notch or “shoulder,” as these are definitely produced by more than one AP. • Use reference values for the specific CNE that you are using. We recommend that each laboratory obtain results from a small group of control subjects and compare these to published reference values, using the same techniques that were used to acquire those reference values, i.e., the specific type of electrode, filter settings, and criteria of acceptable APs.