S98. Pitfalls in jitter analysis with voluntary activation and electrical stimulation

Abstract

Jitter is increased in any condition with disturbed end-plate function, such as myasthenic conditions and ongoing reinnervation. The objectives are to identify some of the technical problems in assessing jitter with voluntary activation and electrical stimulation and in comparing results obtained with the two activation techniques. We measured jitter with concentric needle electrodes using conventional analysis techniques. To compare the results of voluntary activation and electrical stimulation, both activation techniques were used in the same muscle from 30 patients who had electrodiagnostic evidence of reinnervation from any condition in the tested muscle. Any disease causing chronic reinnervation was accepted, and most of them were from motor neuron disease or radiculopathy. With voluntary activation, errors are caused by: poor signal quality; a noisy baseline; selecting inappropriate time reference points on the signal; an irregular firing rate; signals with dual latencies, i.e., “flip-flop;” and using the wrong spike for triggering in multispike signals. With electrical stimulation, additional errors result from: insufficient stimulation intensity; abrupt change in stimulation rate; axon reflexes, and; rarely, F-responses and blink reflexes. Many pitfalls cannot be avoided during recording and can only be detected during post-processing. Comparing results from voluntary and electrical activation is very difficult to perform quantitatively, particularly in complex signals as seen in reinnervation. High jitter values in individual spikes can be missed in these complex signals, which introduces a bias towards more normal values. With voluntary activation, triggering on a spike from an abnormal end-plate in multispike potentials will overestimate the jitter. With electrical stimulation, increased jitter may be due to subliminal stimulation. Stimulation activation also increases the likelihood of activating many axons, which increases the risk of signal summation, with erroneous or impossible jitter estimation. Awareness of various pitfalls can help to improve the correct performance and interpretation of jitter recordings. Quantitative comparisons of results can be made within each activation method but not between results obtained with different activation modes.