The extracellular waveform manifestations of the intracellular action potential are the quintessential diagnostic foundation of electrodiagnostic medicine, and clinical neurophysiology in general. Volume conduction is the extracellular current flow and associated voltage distributions in an ionic conducting media, such as occurs in the human body. Both surface and intramuscular electrodes, in association with contemporary digital electromyographic systems, permit very sensitive detection and visualization of this extracellular spontaneous, voluntary, and evoked nerve/muscle electrical activity. Waveform configuration, with its associated discharge rate/rhythm, permits the identification of normal and abnormal waveforms, thereby assisting in the diagnosis of nerve and muscle pathology. This monograph utilizes a simple model to explain the various waveforms that may be encountered. There are a limited number of waveforms capable of being generated in excitable tissues which conform to well‐known volume conductor concepts. Using these principles, such waveforms can be quickly identified in real time during clinical studies.

Electrodiagnostic (EDX) medicine and EDX testing are an extension of the history and physical examination tailored to the clinical scenario. A wide range of tests are available, and it is incumbent on the electrodiagnostician to select those relevant to the clinical circumstances. The testing procedure may change as data are acquired and new findings interpreted. High-quality consultations are rendered by physicians with experience and technical competence coupled with an understanding of the peripheral nervous system. This chapter provides an overview of EDX tests, the extent of such testing for different clinical scenarios based on guidelines, and the types of abnormalities one can expect to find with different neuromuscular disorders.

This investigation analyzes the temporal characteristics of maximal depolarization times for three waveforms: end-plate spikes, fibrillation potentials, and positive sharp waves (PSWs) to provide support for the electrode initiation hypothesis of PSW induction. The maximal depolarization times for PSWs are documented to comprise two distinct populations conforming to relatively short and comparatively longer maximal depolarization times. Those PSWs with short maximal depolarization times were found to be equivalent to end-plate spike maximal depolarization times, whereas those with longer times were comparable to fibrillation potentials. The PSW group with shorter maximal depolarization times was encountered more frequently. The combination of two distinct groups of PSWs with respective times comparable to end-plate spikes and fibrillation potentials supports the hypothesis that the majority of PSWs originate at the recording electrode during insertion, whereas a smaller population of PSWs arises as propagating fibrillation potentials that block at the recording electrode. Subcutaneous compared to intramuscular recordings from denervated muscle document that the recording electrode is necessary to both record and produce PSWs. Hence, this study confirms the proposed hypothesis that the majority of observed PSWs represent a suprathreshold single muscle-fiber discharge induced by, and originating in close proximity to, a perielectrode crushed membrane that then propagate away from the electrode; a smaller population of PSWs conform to that of a blocked fibrillation potential.

In this study we utilized a dual monopolar needle recording technique to assess propagated electromyographic insertional activity from the same single muscle fiber in order to characterize different categories of insertional activity. A total of six combinations of insertional activity were identified. Only two fundamental types of single muscle-fiber insertional discharge configurations were generated: biphasic initially-negative and monophasic positive. The propagated waveforms corresponding to these two insertional discharges were primarily triphasic initially-positive and, only rarely, monophasic positive. The monophasic positive insertional activity generated at the inserting electrode site is postulated to arise from a depolarization zone adjacent to a needle-induced peri-electrode membrane crush. The monophasic positive discharge was utilized as a model for positive sharp wave generation. It is postulated that the majority of positive sharp waves are initiated at the inserting electrode adjacent to a needle-induced zone of muscle membrane crush in contrast to the previous supposition that positive sharp waves are blocked fibrillation potentials.

tion techniques, it is important to pursue more specialized methods of evaluating the peripheral nervous system. Additionally, nerves requiring needle excitation and less commonly studied nerves are of importance. From time-to-time patients may present with lesions affecting specific sensory branches that yield small amplitude responses or require averaging techniques to better define the desired waveform. With the majority of nerve conduction studies described in this chapter, the difficulty lies not in the inherent technique or nerve, but more so in unfamiliarity. Most, if not all, of the techniques described in this chapter can be mastered with simple practice and repetition.