Single muscle fiber discharge transformations: fibrillation potential to positive sharp wave

Volume conduction: Extracellular waveform generation in theory and practice

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.

EMG-Phänomene peripherer motorisch axonaler Übererregbarkeit

Bei der Nadel-Elektromyographie (EMG) besitzen Phänomene der vermehrten Erregbarkeit von Muskelfasern und von motorischen Axonen Bedeutung für die Diagnostik neuromuskulärer Erkrankungen. Zur motorisch axonalen Übererregbarkeit gehören spontane Phänomene wie Faszikulationen, spontane kontinuierliche Einzelentladungen der motorischen Einheit (SKEME), Myokymien, neuromyotone Entladungsserien und Krampi. Ferner gehören dazu reizinduzierte Phänomene wie manche A-Wellen, reizinduzierte komplex repetitive Entladungen oder tetanischen Spasmen bei Elektrolytstörungen. In der vorliegenden Übersicht wird der Kenntnisstand zu den verschiedenen Phänomenen motorisch axonaler Übererregbarkeit referiert. Ein Schwerpunkt liegt dabei auf den SKEME als neuem Mitglied der Gruppe spontaner Potenziale aus dem motorischen Axon.

Surgical Results of Microscopic Cervical Foraminotomy for Cervical Radiculopathy Presenting Drop Finger and Proposal of Classification Based on Drop Finger Patterns

Introduction

In drop finger, the extension of the finger is limited, although the wrist can be flexed dorsally. There have been no well-organized reports on drop finger pattern caused by cervical nerve root disorder. Moreover, diagnosis and treatment are delayed because of the inability to distinguish cervical radiculopathy from peripheral nerve disease. This study aimed to clarify the operative outcome of microscopic cervical foraminotomy (MCF) for cervical radiculopathy presenting drop finger and to investigate whether our classification based on drop finger patterns is useful retrospectively.

Methods

Overall, 22 patients with drop finger who underwent MCF were included. Grip power (GP) and longitudinal manual muscle test (MMT) score of each finger were examined. Drop finger patterns were classified as types I, II, and III. In type I, the extension disorders of the middle and ring fingers are severe and those of index and little fingers are mild. In type II, the extension disorders are severe from the little finger and slightly to index finger. In type III, the extension disorder is consistently severe in all fingers. Perioperative nerve root disorder and paralysis degree were investigated for all types.

Results

The mean GP was significantly postoperatively improved in all 22 patients. The mean MMT score would benefit from exact data for almost all muscles, except the abductor pollicis brevis at the last follow-up. However, pre- and postoperative paralyses were severe in type III patients. C7 nerve root disorder was confirmed in 5/6 type I patients and C8 nerve root disorder in 12/13 type II and 3/3 type III patients.

Conclusions

The operative results of MCF were relatively good, except in type III patients. As a certain tendency was confirmed between the drop finger types and injured nerve roots, our classification may be useful in reducing misdiagnosis and improving the operative results to some extent.

Positive sharp wave origin: Evidence supporting the electrode initiation hypothesis

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.

Propagated insertional activity: A model of positive sharp wave generation

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.

The difference between fibrillations and positive sharp waves is due to tissue filtering

Fibrillation potentials and positive sharp waves share many characteristics, and in general have the same clinical importance. Whether they are two species of the same potential or two different waveforms has been a long-standing controversy. The blocking hypothesis and inadvertent intracellular recording are two theories proposed to explain the difference between the two potentials, but neither is entirely satisfactory. In this study, waveforms with fibrillation potential configurations are modified by the standard filter settings on an electromyograph to attain positive sharp wave configurations. Single-fiber muscle action potentials were recorded at a low-frequency filter of 500 Hz and a high-frequency filter of 20 kHz, and had the appearance of a fibrillation potential. Changing the low-frequency filter and high-frequency filter to 0.2 Hz and 100 Hz respectively caused these same potentials to have a positive sharp wave configuration. Similarly, fibrillation potentials recorded from patients at a low-frequency filter and a high-frequency filter of 20 Hz and 10 kHz respectively had the appearance of positive sharp waves when the low-frequency filter and high-frequency filter were changed to 0.2 Hz and 500 Hz respectively. The authors propose that tissue filtering and the spatial relationship of the fibrillating fiber to the recording electrode determine whether the waveform will have a fibrillation potential configuration or a positive sharp wave configuration. The ability to model these waveforms artificially simply by changing the bandpass suggests that the passive electrical properties of the recording environment may suffice to explain much of the difference between fibrillation potentials and positive sharp waves.

Prevalence of denervation in paraspinal and foot intrinsic musculature

OBJECTIVE

The primary purpose of this investigation was to determine the prevalence of abnormal spontaneous activity (positive sharp waves (PSWs) and fibrillation potentials (FPs)) in selected lumbosacral paraspinal and foot intrinsic muscles in an asymptomatic healthy population.

DESIGN

This was a prospective assessment of 50 individuals without history or physical findings suggestive of peripheral neuromuscular disease whereby a monopolar needle electrode was located in the unilateral L4 and L5 paraspinal as well as abductor hallucis and extensor digitorum brevis muscles. These muscles were extensively evaluated for the presence of PSWs, FPs, and fasciculation potentials.

RESULTS

Ten subjects per decade from 20-59 yr and ten subjects from 60-80 yr comprised the 50 participants (28 women), resulting in a mean age of 45+/-15.9 (range, 20-76) yr. A single individual (prevalence, 2%) demonstrated fibrillation potentials in the extensor digitorum brevis, and FPs and PSWs were detected in two subjects' (4% prevalence) L4/L5 paraspinal muscles. Ninety-four percent of the subjects had fasciculation potentials in the abductor hallucis, whereas 60% had these waveforms in the extensor digitorum brevis. Only 6% of subjects had fasciculation potentials in the L4 but not L5 paraspinal muscles. All subjects demonstrated both prototypical and "atypical" appearing endplate spikes in all of the muscles examined.

CONCLUSIONS

We failed to confirm the previously reported prevalence of FPs and PSWs in both the paraspinal and foot intrinsic musculature. Atypical appearing endplate spikes, however, display configurations similar to FPs and PSWs and were present in all subjects. Failure to pay close attention to the discharge rate and rhythm of endplate spikes can lead to misinterpreting these waveforms as FPs and PSWs. It is likely that the previously reported high prevalence of spontaneous activity in healthy persons resulted from not fully appreciating the similarity between innervated and denervated spontaneous single muscle fiber discharge configurations.

Do endplate noise and spikes arise from normal motor endplates?

The concept that the endplate noise and endplate spike components of motor endplate potentials represent normal endplate potentials seems to be flawed. The morphology of the normal miniature endplate potentials described in the physiology literature is different from the morphology of the noise-like component of endplate potentials. This noise-like component is identified as normal in current electromyographic literature. There is strong experimental evidence that one source of the endplate noise component is grossly increased release (up to three orders of magnitude) of acetylcholine from the nerve terminal of that neuromuscular junction. The spikes can be accounted for by release of additional acetylcholine in response to mechanical stimulation by the electromyographic needle. Other possibilities exist.

Physiologic basis of potentials recorded in electromyography

The extracellularly recorded configuration of a single muscle fiber discharge is generally appreciated to be triphasic with an initially positive deflection. However, careful attention to waveform appearance during the electrodiagnostic medicine examination reveals that both innervated and denervated muscle waveforms may display a pantheon of configurations. Further, despite the fact that innervated and denervated single muscle fiber discharges arise from distinctly different intracellular action potential (IAP) configurations, their extracellularly recorded waveforms can appear quite similar, leading to potential misidentification and, hence, the possibility of an erroneous diagnostic conclusion. The least appreciated, but nevertheless critical, aspect of explanations for muscle waveform configurations is the relationship between the muscle fiber and recording electrode. Additionally, it is important to appreciate both the near-field and far-field aspects of single fiber and compound muscle action potentials. In this review, the leading/trailing dipole model is used to explain muscle waveform configurations in both innervated and denervated tissues.

[Spontaneous electromyographic activity. Practical importance]

Spontaneous activities are a major semeiologic sign in electromyography. The present article deals with the different aspects recorded in practice in normal and pathological cases. There are two types of spontaneous activities, those related to motor unit hyperactivity (fasciculations and myokymia) and those related to the hyperactivity of one or more muscle fibers: fibrillations, positive sharp waves, myotonic discharges and complex repetitive discharges. In the first case the lesion is located in the axone and in the second in the membrane of the muscle fibers; All theories related to the cells' abnormalities share a common feature: spontaneous activities result from abnormal firing of the membrane action potential of muscular fibers. This functional abnormality may results from different types of lesions within the cells' membrane and determines the aspects of spontaneous activities. Impaired function of muscular cells' membranes can be produced by denervation or lesion of the membrane structure itself. The latter can be multiple and linked with the membrane proteins (such as laminine or dystrophin as in AIDS diseases) or with ion channel disturbances. Multiple membrane cell alterations may produce the same kind of spontaneous activity; for instance, myotonic discharges have the same morphology in Thomsen and Steinert's disease despite their different mechanisms and fibrillations seen in denervations and myopathies. The practical consequences are discussed and a new classification of these spontaneous activities is proposed.

Configuration of normal and abnormal non-volitional single muscle fiber discharges

OBJECTIVE

This investigation utilizes a single muscle fiber simulation to compare and contrast single muscle fiber waveform configurations arising from innervated and denervated tissue taking into account possible tissue-electrode interactions.

METHODS

Intracellular action potentials (IAPs) from innervated and denervated muscle tissue are simulated. The extracellular waveform configurations as recorded from the fiber's midpoint (endplate in innervated tissue), halfway between the midpoint and fiber termination, as well as fiber termination for both innervated and denervated single muscle fibers are examined. Further, two types of muscle fiber terminations are assessed: (1) sealed end effect; and (2) compressed end effect.

RESULTS

Irrespective of different types of IAPs, recordings from the fibers' middle, halfway between the midpoint and termination, as well as from the sealed end, revealed similar configurations. However, for the innervated fiber's compressed termination, a monophasic positive waveform was derived while the denervated fiber's compressed termination generated a prototypical positive sharp wave.

CONCLUSIONS

It is hypothesized that the needle electrode can no longer be considered a passive recording device but may interact with the fiber so as to generate a sealed end or compressed end effect. Depending upon the type of needle-fiber interaction and the electrode's location with respect to the IAP's generation site, a limited number of potentials with specific configurations will be recorded for both innervated and denervated tissue. Further, depending upon the type of needle-tissue interaction, innervated muscle fibers can generate non-volitional waveforms with configurations similar to those recorded from denervated tissue. It is no longer sufficient to merely consider waveform configuration when attempting to define positive sharp waves and fibrillation potentials, but it is important now also to consider firing rate and rhythm.

Hybrid fibrillation potentials and positive sharp waves

Fibrillation potential configurations are characterized as initially positive triphasic waveforms, whereas positive sharp waves appear biphasic with an initial positive deflection. Careful observation of single muscle fiber discharges in denervated muscle, however, can reveal many different-appearing and stable firing waveforms that resemble a bifid positive sharp wave or some form of combined fibrillation potential and positive sharp wave. In this investigation, a number of atypical-appearing single muscle fiber discharges are hypothesized to arise from particular interactions between the muscle fiber and recording electrode. Single muscle fiber potentials are modeled as originating from a single denervated muscle fiber's former endplate and midfiber region as well as from the fiber's tendinous termination for both a compressed and sealed end effect. The modeled waveforms' appearance corresponds well to those obtained clinically and the necessary interpotential summated templates' temporal domains are feasible for action potential termination at the electrode with subsequent reinitiation beyond the proposed peri-electrode compressed region. It is hypothesized that the majority of hybrid waveforms are the result of a single muscle fiber action potential terminating at a recording electrode while also initiating a "skipped" activation of the muscle fiber past the electrode resulting in the summation of two distinct time-locked waveforms.

Some observations on fibrillations and positive sharp waves

Electromyographic recordings of fibrillation potentials (FPs) and positive sharp waves (PSWs) demonstrate transformation of FP to PSW and vice versa, atypical firing patterns, changes in waveform shape and amplitude, and time-locked potentials. The etiology of the waveform characteristics of FP and PSW is discussed based on abnormal propagation in a small section of muscle fiber that is "damaged" by the needle. The results of simple computer simulations are described.