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

[EMG phenomena of myogenic hyperexcitability]

The type, distribution pattern and time course of spontaneous muscular activity are important for the diagnostics of neuromuscular diseases in the clinical practice. In neurogenic lesions with motor axonal involvement, pathologic spontaneous activity (PSA) is usually reliably detectable by needle electromyography (EMG) 2-4 weeks after occurrence of the lesion. The distribution pattern correlates with the lesion location. The focus of the present work is the description of the different forms of PSA in myogenic diseases.

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.

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.

Cervical paraspinal spontaneous activity in asymptomatic subjects

Paraspinal fibrillation potentials or positive sharp waves (PSWs) reportedly may be the only electrodiagnostic abnormality in patients with radiculopathies. However, spontaneous activity may be present in muscles of asymptomatic subjects. To determine whether abnormal spontaneous activity in the cervical paraspinal muscles is clinically relevant, we evaluated the prevalence of such activity in asymptomatic individuals and examined its relationship with age. Sixty-six asymptomatic subjects underwent a needle electromyographic examination of the paraspinal muscles bilaterally at the C5/6 and C6/7 levels. All of the recorded potentials were captured and their firing rates and waveforms were evaluated. The potentials were considered to be abnormal if reproducible trains of PSWs or fibrillation potentials were present. Eight of the 66 (12%) subjects showed PSWs, five bilaterally; none showed fibrillation potentials. A statistical analysis for the effect of age could not be performed due to the small sample size. Because electromyographic cervical paraspinal abnormalities can be found in asymptomatic subjects, caution should be exercised when attributing the etiology of neck pain to radiculopathy if the only electrodiagnostic findings are electromyographic cervical paraspinal abnormalities.