Concentric/monopolar needle electrode modeling: spatial recording territory and physiologic implications

Bedeutung der Muskelsonographie in der Detektion von Faszikulationen bei der ALS

Bei der amyotrophen Lateralsklerose sind Faszikulationen häufig bereits in frühen Stadien in mehreren Körperregionen vorzufinden und haben daher Eingang in die entsprechenden Leitlinien und Diagnosekriterien gefunden. Während die invasive EMG-Diagnostik unverzichtbar zum Nachweis von akut- und chronisch-neurogenen Veränderungen des elektrischen Signalverhaltens motorischer Einheiten und zur Bestätigung von Faszikulationspotenzialen bleibt, bietet die Muskelsonographie ein hochsensitives Verfahren, um schnell und nicht-invasiv Faszikulationen in den verschiedenen Muskel-Etagen zu erfassen. In dieser Übersichtsarbeit stellen wir die bisherigen Daten zum Einsatz der Muskelsonographie zur Faszikulationsdetektion dar. Durch ihren Einsatz ermöglicht die Muskelsonographie im klinischen Alltag eine zielgerichtete und hierdurch aussagekräftigere EMG-Diagnostik. Aktuelle Forschungsstudien zielen darauf ab, Faszikulationen sonomorphologisch genauer zu charakterisieren, zu quantifizieren und als Verlaufsparameter zu untersuchen.

Detection radius of EMG for fasciculations: Empiric study combining ultrasonography and electromyography

OBJECTIVE

The aims of this study were to investigate the detection radius and sensitivity of EMG for fasciculations.

METHODS

Muscle ultrasonography was performed simultaneously to EMG recordings in patients with fasciculations in the context of amyotrophic lateral sclerosis. Ultrasonography and EMG parameters were analyzed for selected fasciculations.

RESULTS

A total of 381 fasciculations were detected by ultrasonography in 18 muscles of 10 patients. Out of these, 125 (33%) were EMG-negative. In contrast, none of the fasciculations detected by EMG were ultrasonography-negative. EMG detection probability decreased significantly with increasing distance from the center of the fasciculation. EMG detection rate was 98% when the EMG needle was located within the fasciculation and 50% at 7.75 mm distance from the fasciculation center. In addition, EMG detection depended significantly on cross-sectional area of the fasciculation and presence of neurogenic changes.

CONCLUSIONS

For detecting the same fasciculations, EMG is less sensitive than ultrasonography. EMG detection probability decreases sharply at a distance comparable to motor unit size.

SIGNIFICANCE

These results extend previous knowledge about superior sensitivity of ultrasonography for fasciculations. Moreover, our novel bimodal detection method provides first in vivo data about the EMG detection radius for fasciculations in a clinical setting.

Optimizing testing methods and collection of reference data for differentiating critical illness polyneuropathy from critical illness MYOPATHIES

INTRODUCTION

In severe acute quadriplegic myopathy in intensive care unit (ICU) patients, muscle fibers are electrically inexcitable; in critical illness polyneuropathy, the excitability remains normal. Conventional electrodiagnostic methods do not provide the means to adequately differentiate between them. In this study we aimed to further optimize the methodology for the study of critically ill ICU patients and to create a reference database in healthy controls.

METHODS

Different electrophysiologic protocols were tested to find sufficiently robust and reproducible techniques for clinical diagnostic applications.

RESULTS

Many parameters show large test-retest variability within the same healthy subject. Reference values have been collected and described as a basis for studies of weakness in critical illness.

CONCLUSIONS

Using the ratio of neCMAP/dmCMAP (response from nerve and direct muscle stimulation), refractory period, and stimulus-response curves may optimize the electrodiagnostic differentiation of patients with critical illness myopathy from those with critical illness polyneuropathy.

Increased neuromuscular transmission instability and motor unit remodelling with diabetic neuropathy as assessed using novel near fibre motor unit potential parameters

OBJECTIVE

To assess the degree of neuromuscular transmission variability and motor unit (MU) remodelling in patients with diabetic polyneuropathy (DPN) using decomposition-based quantitative electromyography (DQEMG) and near fibre (NF) motor unit potential (MUP) parameters.

METHODS

The tibialis anterior (TA) muscle was tested in 12 patients with DPN (65 ± 15 years) and 12 controls (63 ± 15 years). DQEMG was used to analyze electromyographic (EMG) signals collected during voluntary contractions. MUP and NF MUP parameters were analyzed. NF MUPs were obtained by high-pass filtering MUP template waveforms, which isolates contributions of fibres that are close to the needle detection surface. NF MUP parameters provided assessment of motor unit size (NF area), fibre density (NF fibre count) and contribution dispersion (NF dispersion) and neuromuscular transmission instability (NF jiggle).

RESULTS

DPN patients had larger (+45% NF area), more complex (+30% NF fibre count), and less stable (+30% NF jiggle) NF MUPs (p<0.05). No significant relationships were found between NF MUP stability and denervation, or strength; however NF MUP complexity was positively related to TA denervation in the DPN group (r=0.63; p<0.05). NF MUP complexity and instability were positively related in DPN patients (r=0.46; p<0.05).

CONCLUSIONS

DPN is associated with neuromuscular transmission instability and MU remodelling that can be assessed using DQEMG.

SIGNIFICANCE

DQEMG-derived NF MUP parameters may be useful in identifying patients in early stages of neuromuscular dysfunction related to DPN.

Models and simulations in electromyography

In electromyography, one assesses the pathophysiology on the basis of the waveform characteristics of the recorded signal. This requires detailed knowledge of the relationship between the waveform generators and the waveform measurements. Models and computer simulations can be used to explore this relationship in an efficient manner. Combining models with experimental methods will allow us to define new measurements and new rules of interpretation. This is discussed with some of the models developed for electromyography signal analysis.

Concentric needle EMG versus macro EMG I. Relation in healthy subjects

OBJECTIVES

The relation between motor unit action potentials (MUAPs) recorded via a macro needle electrode (MA-MUAP, MA-EMG) and MUAPs recorded via a concentric needle electrode (CN-MUAP, CN-EMG) is under debate. In particular it is not known to what degree CN-MUAP variables reflect the electrical properties of a motor unit.

METHODS

CN-EMGs and MA-EMGs of the right brachial biceps muscle were recorded from 40 healthy subjects (23 women and 17 men) aged 17-83 years and CN-MUAP and MA-MUAP variables were cross-correlated.

RESULTS

CN-MUAP duration was positively and significantly correlated with CN-MUAP area (r=0.52), rate of polyphasia (r=0.45), MA-MUAP amplitude (r=0.47) and MA-MUAP area (r=0.45). CN-MUAP amplitude was positively and significantly correlated with the rate of polyphasia (r=0.39) and the fibre density (r=0.45).

CONCLUSIONS

CN-MUAP duration appropriately reflects the motor unit's electrical activity and may substitute MA-MUAP area and amplitude.

Motor unit sound in needle electromyography: assessing normal and neuropathic units

Motor unit action potentials (MUPs) recorded by a monopolar needle electrode in normal and neuropathic muscles were computer-simulated. Five experienced electromyographers acted as examiners and assessed the firing sounds of these MUPs without seeing them on a display monitor. They judged whether the sounds were crisp or close enough to accept for the evaluation of MUP parameters and whether, when judged acceptable, they were neuropathic-polyphasic. The examiners recognized motor unit (MU) sound as crisp or polyphasic when the MUP obtained was 0.15-0.2 mm from the edge of the MU territory. When the intensity of the sound decreased, they were unable to perceive it as crisp. When the intensity exceeded the saturation level of loudspeaker output, the sound was perceived as polyphasic, but the wave form of the MUP was not. When the frequency of the neuropathic MUP was lowered, the examiners were unable to determine whether the MUP was polyphasic. MUPs recognized as acceptable for evaluation can be distinguished by listening to MU sounds. The audio amplifier gain must be appropriately adjusted for each MUP amplitude in order to assess whether an individual MU sound is crisp or polyphasic before MUP parameters are measured on a display monitor.

Relation between maximum discharge rates on electromyography and motor unit number estimates

To improve quantitative assessment of motor unit recruitment by standard concentric needle electromyography (CNEMG), hypothenar muscles of 22 healthy subjects, 18 with denervation, and 10 with a myopathy were studied. Discharge rates of motor units were measured in CNEMG recordings comprising action potentials of, at most, 4 motor units. Motor unit number estimation (MUNE) was done using the manual incremental method. In controls, the upper 95% limit of the discharge rate was 16.2/s. In all subjects, a strong nonlinear correlation between the number of motor units and the maximal discharge rate was found (r = 0.88, P < 0.0001). Increased discharge rates were found in all but one of the paretic muscles with denervation, but in none of the myopathic muscles. Measurement of the discharge rate is a simple and reliable procedure. If the discharge rate is high in a hypothenar muscle, loss of motor units can be inferred. Moreover, the discharge rate value gives an estimate of the number of motor units in that muscle. Thus, we suggest that maximal discharge rate be included in electromyographic reports.

Determinants of motor unit action potential duration

OBJECTIVE

Motor unit action potential (MUAP) recordings are modeled by means of a single muscle fiber simulation program, to define two key subcomponents comprising the complete physiologic MUAP duration. A number of defining properties of these subcomponents are further developed.

METHODS

A single muscle fiber simulation program is utilized with various muscle fiber lengths and conduction velocities to generate near-field and far-field waveforms.

RESULTS

Two key subcomponents to the total physiologic single muscle fiber and hence MUAP duration are identified. One, defined as the near-field component, is directly dependent upon muscle fiber hemi-length. The other, defined as the far-field component, is independent of fiber length, but matches the internal action potential in duration. Both the near-field and far-field components are inversely dependent upon intracellular action potential conduction velocity. Additionally, temporal dispersion among the individual fibers contributing to a MUAP must be included in the overall MUAP duration calculation.

CONCLUSIONS

It is hoped that this approach to MUAP duration may allow a more complete appreciation of the components contributing to the MUAP, than permitted by the empirically derived values for MUAP duration presently under clinical use.

Motor unit action potential duration and muscle length

Motor unit action potential (MUAP) components are investigated by means of single fiber computer simulations and clinical measurements. The single fiber simulations have essentially full bandwidth without noise, whereas the clinical measurements were made with a 3-10,000-Hz bandwidth utilizing approximately 1000 averages to reduce noise optimally. These parameters allow the recording of a MUAP's complete "physiologic" duration including its very slow onset and termination. The simulation results demonstrate a constant waveform onset regardless of the electrode's recording location along the fiber. A far-field potential is initiated when the action potential encounters the muscle fiber's termination. The simulated waveform's and clinically recorded MUAP's near-field component extends between the potential's onset and its corresponding far-field potential's onset. This near-field component's duration should vary with fiber length, and this prediction is clinically confirmed by measuring three different muscle lengths. The far-field potential reveals a constant duration, independent of fiber length, and appears to be associated with the muscle fiber's intracellular action potential duration. A more complete understanding of the components contributing to MUAP duration should provide a more fundamental basis for quantitative clinical MUAP duration measurements.

Motor unit action potential components and physiologic duration

Motor unit action potentials (MUAPs) recorded from the same motor unit at two distances along the biceps brachii muscle with monopolar needle electrodes at high amplifier gains (20 microV/division) and averaged 2000-3000 times reveal total potential durations of 39.6 +/- 4.6 ms. In addition, the terminal segment for each of these two MUAPs contained a late far-field potential with a mean duration of 23.8 +/- 4.1 ms. Computer simulations of MUAPs suggest that this long-duration positive far-field mirrors the true morphology of the intracellular action potential (IAP), which is monophasic positive, possessing a terminal repolarization phase approaching 30 ms. This investigation suggests that the MUAP's physiologic duration is directly proportional to the muscle fiber length and the IAP's duration, which becomes manifest as a positive far-field potential when the IAP encounters the musculotendinous junction and slowly dissipates. The leading/trailing dipole model is used to explain qualitatively this study's quantitative clinical and computer simulation findings.

The size index as a motor unit identifier in electromyography examined by numerical calculation

A computer simulation was performed to investigate the size index as a motor unit identifier in electromyography. The size index calculated from the amplitude and area of the simulated motor unit action potential (MUP) was plotted against the distance between the needle electrode and current source to show how the index changes as a function of the distance. The index of the MUP also was plotted against the number of muscle fibers belonging to a single motor unit, the size of the motor unit territory, and the diameter of the muscle fibers in order to establish the major determinants of the index. The index was relatively constant for the distance less than 2 mm between the needle electrode and closest edge of the current source. It changed logarithmically with the number of muscle fibers and with the diameter of the fibers.

The physiological origin of the slow afterwave in muscle action potentials

OBJECTIVE

Both intramuscularly-recorded motor unit action potentials (MUAPs) and surface recorded MUAPs and compound muscle action potentials (CMAPs) have slow afterwaves which can contribute as much as half their measured duration. This study tested the hypothesis that the slow afterwave has its physiological origin in the negative afterpotential of the muscle fiber intracellular action potential (IAP).

METHODS

We investigated the slow afterwave in MUAPs and CMAPs from brachial biceps, tibialis anterior, first dorsal interosseous, thenar and hypothenar muscles in 15 normal subjects, and using computer simulations.

RESULTS

The slow afterwaves did not match the time constant of the amplifier's high-pass filter, and so were not filtering artifacts. They lasted long after propagation had terminated at the muscle/tendon junction, and so were not due to the temporal or spatial dispersion of propagating single-fiber potentials. Their amplitude and polarity varied with the recording site as predicted by computer simulations that modeled the IAP as having a negative afterpotential. They also changed with double-pulse stimulation and decreasing temperature in ways consistent with the results of intracellular studies of the IAP negative afterpotential.

CONCLUSIONS

The presented results support our hypothesis that the slow afterwave is a manifestation of the IAP negative afterpotential.

Concentric needle recording characteristics related to depth of tissue penetration

This study investigates the influence of tissue penetration depth as it relates to a concentric needle electrode, particularly delineating regions where the cannula potential predominates over the core potential. The regions of cannula predominance is studied by means of a standard and 20 times enlarged physical model of an electromyographic concentric needle electrode in a homogeneous volume conductor by delineating the zero isopotential which partitions where the core potential predominates versus where the cannula potential predominates. Clinical studies in muscle tissue are used to test and confirm results from the enlarged physical model. At shallow electrode insertions equivalent to 4 mm, the concentric needle model records a net negative potential, which is a region where the cannula predominates, from a distant positive dipole at the same depth compared with a net positive potential for penetration depths exceeding 4 mm. The clinical portion of this study verifies the bipolar nature of the concentric needle electrode in detecting motor unit action potentials (MUAPs) with primarily an initial positive onset irrespective of recording depth. Refinements to the conceptualization of the nature and detection of MUAPs are discussed which are consistent with all the findings of the clinical and model study.