Propagated insertional activity: A model of positive sharp wave generation

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.

Fibrillation potential onset in peripheral nerve injury

INTRODUCTION

Fibrillation potentials are an accepted electrical marker of muscle denervation that occur in axonal nerve injury. Clinically, they are used to determine the type of, and prognosis for nerve injuries. The time of occurrence after nerve injury plays a critical role in clinical decision making. This study explores the evolution of the generally accepted guideline that fibrillation potentials occur 1 to 4 weeks after axonal nerve injury.

METHODS

Pubmed, Ovid, and EMBASE, and current textbooks were reviewed. References were recursively followed back to the initial description of fibrillation potentials.

RESULTS

The majority of our understanding regarding the timing of onset of fibrillation potentials appears to arise from animal experiments in the mid-20th century.

CONCLUSIONS

Despite frequent use in human clinical care, published evidence for the 1 to 4 week guideline comes almost entirely from animal studies. An appreciation of this background and resulting limitations aids clinical application of this guideline.

Clinical Characteristics and Analysis of CLCN1 in Patients with "EMG Disease"

Background and Purpose

While the etiology and clinical features of "EMG disease" - which is characterized by diffusely increased insertional activity on needle electromyography (EMG) in the absence of neuromuscular disease - are not well known, some authorities believe it may be a form of myotonia congenita (MC). The aims of this study were to determine the clinical features of EMG disease and its relationship with CLCN1 mutations in patients.

Methods

The detailed clinical and electrophysiological features of EMG disease were evaluated in six patients. All 23 coding exons and exon-intron boundaries in CLCN1 gene were analyzed by direct sequencing to detect nucleotide changes.

Results

The common clinical symptoms of EMG disease were chronic muscle stiffness or generalized myalgia, which were aggravated in a cold environment. Four patients complained of action myotonia several times a year. Short trains of provoked positive sharp waves were documented on needle EMG, but myotonic discharges, fibrillation potentials, and fasciculations were not. Increased insertional activity was identified at the asymptomatic muscles studied. One novel heterozygous mutation was identified in one patient following genetic testing for CLCN1 mutations (c.1679T>C, p.Met560Thr).

Conclusions

The clinical features of EMG disease might be quite similar to those of MC, but CLCN1 mutation was found in only one subject. It is thus difficult to accept that EMG disease lies within the phenotypic spectrum of MC. Additional testing is needed to verify the pathogenetic cause of the diffusely increased insertional activity associated with this condition.

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.