Influence of temperature on neuromuscular transmission in myasthenia gravis

Summary The effect of local cooling was studied in 28 patients with myastenia gravis. We stimulated the ulnar nerve with single stimuli and trains at 3/s for 2s and at 50/s for 1.5s. The compound muscle action potential (MAP), the muscle twitch and the isometric tetanic force of the adductor pollicis were registered. 1. At 3/s stimulation the pathological decrement of the MAP decreased after slight cooling. 2. The amplitude of the single MAP was higher at lower temperature when compared to normal temperature. The same increase is however to be found in healthy subjects. 3. After slight cooling, the maximum tetanic force was higher. However, the decrement of the force was higher also, therefore ruling out a practicable application of cooling for the patient. 4. After severe cooling (18–22°C) the tetanic force was much lower and in many cases a complete failure of the neuromuscular transmission occured.

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Zusammenfassung Der Einfluß lokaler Kühlung wurde an 28 Patienten mit Myasthenia gravis untersucht. Wir reizten den N. ulnaris mit einzelnen Impulsen und mit Serien von 3/s für 2s und von 50/s für 1.5s. Das Summenaktionspotential (MAP), die Muskelzuckung und die isometrische tetanische Kraft des Adduktor pollicis wurden registriert. 1. Bei Reizung mit 3/s war das pathologische Dekrement des MAP nach leichter Kühlung geringer. 2. Die Amplitude des MAP war größer. Diese Amplitudenzunahme tritt jedoch auch bei Gesunden nach Kühlung auf. 3. Nach leichter Kühlung war die maximale tetanische Kraft größer. Jedoch nahm die Kraft während der Reizung nach dem Maximum wieder schnell ab. Der praktische Nutzen einer Kühlung ist deshalb für den Patienten gering. 4. Nach starker Kühlung (18–22°C) war die maximale tetanische Kraft sehr niedrig. In vielen Fällen kam es zu einem vollständigen Versagen der neuromuskulären Impulsübertragung.

     

Effect of local cooling on excitation-contraction coupling in myasthenic muscle: Another mechanism of ice-pack test in myasthenia gravis

Objective

The ice-pack test is a convenient diagnostic testing procedure for myasthenia gravis (MG). We investigated the underlying mechanism of the ice-pack test performed on bilateral masseters.

Effect of maximal voluntary contraction on the amplitude of the compound muscle action potential: Implications for the interpolated twitch technique

The compound muscle action potential (M_MAX) during a maximal voluntary contraction (MVC) may be measured to determine if the motor nerve has been supramaximally stimulated during the interpolated twitch technique (ITT). Ten males performed isometric knee extension MVCs. M_MAX for the vastus medialis was recorded during MVC and rest. To examine the effect of stimulating electrode movement, the M_MAX of the thenar group and antidromic sensory nerve action potentials (SNAPs) to the third digit were recorded in a separate experiment. M_MAX during MVC was reduced by 18% (P < 0.0001) and 43% (p < 0.0001) for the quadriceps and thenar group, respectively. The SNAP amplitude was not different between rest and MVC (P = 0.18). Reduction of M_MAX during MVC suggests that some motor axons are refractory and unable to respond to a superimposed maximal stimulus. These results have implications for the sensitivity of the interpolated twitch technique. Muscle Nerve, 2010     

Influence of reference electrode position on the compound muscle action potential

ObjectiveWhen the compound muscle action potential (CMAP) is recorded in motor nerve conduction studies, the reference (E2) electrode can make a significant contribution to the CMAP. This study investigates the E2 recorded signal and its effect on CMAP measurements when E2 electrode is placed at different sites.MethodsThe CMAP was recorded using the active electrode on the muscle belly and 4 different E2 electrodes placed at distal and proximal sites. The signal recorded by each electrode was also measured using a reference electrode on the contralateral limb. Signals were recorded from the abductor pollicis brevis, abductor digiti minimi, tibialis anterior and biceps muscles.ResultsThe E2 recorded a smaller signal when it was placed near or off the proximal tendon or muscle origin. This affected CMAP latency, duration for tibialis anterior. Contrary to expectation, initial upward deflection was noted for E2 signal.ConclusionA proximal E2 position records a lower volume conducted signal and yields a CMAP that is more representative of the muscle over which the E1 (active) electrode is placed.SignificanceThe proposed ‘Proximal E2’ montage may be better suited to assess pathology.     

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAPs and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail, which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape. © 1996 John Wiley & Sons, Inc.     

Correlation between compound muscle action potential amplitude and duration in axonal and demyelinating polyneuropathy

Objective

To get a better understanding of pathophysiology in polyneuropathies (PNPs) by correlating compound muscle action potential (CMAP) amplitude with duration.

Methods

A total of 145 motor nerve conduction studies (MNCS) in 53 axonal and 132 MNCS in 45 demyelinating PNPs were analyzed. Peroneal and tibial MNCS were done by surface stimulation while for median and ulnar nerves near nerve or surface stimulations were used. CMAP amplitude and duration were compared in axonal and demyelination PNPs. Relationships between amplitude and duration of distally and proximally evoked CMAP were examined using regression analysis.

Results

CMAP amplitude was lower and duration was increased in all examined nerves in demyelinating PNPs than in axonal PNPs. In demyelinating PNPs, an inverse linear correlation between amplitude and duration was seen in distally and proximally evoked CMAP in all examined nerves. In axonal PNPs, there was no correlation in any of the nerves neither in distally nor in proximally evoked CMAP.

Conclusions

Distal CMAP duration and the relationship between CMAP amplitude and duration show supplementary electrodiagnostic potential in demyelinating PNPs.

Significance

More knowledge about the relation between amplitude and duration in axonal lesions and demyelination may help to reveal the pathophysiology in PNPs. Significant correlation between amplitude and duration in demyelination may suggest that the severe decrease in amplitude in demyelinating PNPs is probably due to the increase in duration secondary to temporal dispersion.     

Far-field potentials in the compound muscle action potential

It has long been believed that the compound muscle action potential (CMAP) in motor-nerve conduction studies (MCSs) records the action potential beneath the active electrode over the muscle belly. However, recent studies have revealed the contribution of the reference electrode to the CMAP, most prominent in the tibial nerve, followed by the ulnar nerve. This “reference electrode potential” is recorded when the conventional reference electrode distal to the muscle belly is connected to a proximal reference. It must be a far-field potential (FFP) considering its distribution, although the precise mechanism of its generation has not been clarified. The conventional theory of termination of the action potential at the muscle-tendon junction is insufficient. Regarding the ulnar CMAP, interosseous muscles mostly contribute to the FFPs. New understanding of CMAP based on the FFP theory may provide new insights into the interpretation of MCSs and related techniques, including motor unit number estimation.     

Impact of stimulus duration on motor unit thresholds and alternation in compound muscle action potential scans

ObjectiveTo investigate the impact of stimulus duration on motor unit (MU) thresholds and alternation within compound muscle action potential (CMAP) scans.MethodsThe stimulus duration (0.1, 0.2, 0.6, and 1.0 ms) in thenar CMAP scans and individual MUs of 14 healthy subjects was systematically varied. We quantified variability of individual MU’s thresholds by relative spread (RS), MU thresholds by stimulus currents required to elicit target CMAPs of 5% (S5), 50% (S50) and 95% (S95) of the maximum CMAP, and relative range (RR) by 100*[S95-S5]/S50. We further assessed the strength-duration time constant (SDTC). Experimental observations were subsequently simulated to quantify alternation.ResultsRS, unaffected by stimulus duration, was 1.65% averaged over all recordings. RR increased for longer stimulus duration (11.4% per ms, p < 0.001). SDTC shortened with higher target CMAPs (0.007 ms per 10% CMAP, p < 0.001). Experiments and simulations supported that this may underlie the increased RR. A short compared to long stimulus duration recruited relative more MUs at S50 (more alternation) than at the tails (less alternation).ConclusionsThe stimulus duration significantly affects MU threshold distribution and alternation within CMAP scans.SignificanceStimulation settings can be further optimized and their standardization is preferred when using CMAP scans for monitoring neuromuscular diseases.     

吉兰-巴雷综合征复合肌肉动作电位随访观察分析

目的探讨吉兰-巴雷综合征(GBS)患者肌力变化与肌电图复合肌肉动作电位的关系。方法选取6例GBS患者,分别于入院以及发病2 m、3 m、6 m进行神经功能评分(Hughes评分)及肌电图检查,观察肌力与复合肌肉动作电位的变化特点。结果发病2 m、3 m、6 m Hughes评分结果较入院时明显好转(P<0.05);神经传导速度的好转于2 m出现(P<0.01),运动神经波幅于发病后3 m出现好转(P<0.05),至发病后6 m仍有部分神经未引出波形。结论神经功能的好转与肌电图的变化并不平行,电生理的恢复滞后于临床症状的恢复。     

Effects of evoked pain on the electromyogram and compound muscle action potential of the brachial biceps muscle

Muscle pain is often accompanied by a feeling of muscle fatigue and weakness. We examined the effect of experimental muscle pain on the electromyogram (EMG) during maximal voluntary contraction (MVC), and on the compound muscle action potential (CMAP) of the brachial biceps muscle. Twenty-one healthy subjects were injected intramuscularly with 0.1 ml/30 microg of the vanilloid receptor agonist (capsaicin) or 0.2 ml of 5% hypertonic saline. A Teflon-coated cannulated EMG needle was used to record the EMG interference pattern (IP) at MVC. The CMAP of the brachial biceps muscle was obtained by stimulation of the musculocutaneous nerve at the axilla using surface electrodes. Amplitude, mean frequency of the power spectrum, and turns/s of the interference pattern were reduced after pain induced by capsaicin or hypertonic saline. Latency, amplitude, and area-under-curve of the CMAP did not change after injection of either substance. Acute stimulation of muscle nociceptors thus produced a fatiguelike change in the interference pattern during MVC, possibly due to a decrease in motoneuron firing rate and increased muscle fatigability.     

Compound muscle action potential cartography of an accessory peroneal nerve

In daily practice, accessory peroneal nerves (APNs) are detected in less than the 18–25% of legs, as revealed by systematic searches. In one APN case, compound muscle action potential cartography showed that the APN was only apparent when the recording electrode was placed over a small lateral region of the extensor digitorum brevis muscle. Effects of recording site can explain why many APNs go unrecognized. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1331–1333, 1998.     

Action potential shape of rabbit masseter motor units and jaw angle.

Action potentials of rabbit masseter motor units (n = 42) were registered at different jaw angles to examine whether the shape of the action potential is related to length of muscle fibers in motor units and depends on the intramuscular location of the motor unit. Twitches were elicited by stimulating motoneurons in the trigeminal motor nucleus. During jaw opening (0-21 degrees), the duration of the action potentials increased by about 10%. Anteriorly located motor units showed an increase in duration larger than that of more posteriorly located units, which was probably due to a larger stretching of the more anteriorly located units.     

Prolonged compound muscle action potential duration in critical illness myopathy

Critical illness myopathy (CIM) is a frequent cause of generalized weakness in the intensive care unit. Prolonged compound muscle action potential (CMAP) durations have been described in this patient population, and this study presents further data on CMAP duration in normal controls and patients with CIM. The findings highlight the importance of testing multiple nerve muscle combinations in weak, critically ill patients. Recognition of this pattern, which has not been widely described, can facilitate the diagnosis of CIM. Muscle Nerve, 2009     

Contribution of reference electrode to the compound muscle action potential

In compound muscle action potential (CMAP) recording, the contribution by the reference electrode is considered to be much smaller than that of the active electrode. We tested this assumption by making quantitative measurements of the signals recorded individually by the active and reference electrodes. In the thenar (median nerve) and extensor digitorum brevis (peroneal nerve) muscles, the reference electrode did contribute less. In the hypothenar muscle (ulnar nerve), however, the signals recorded by active and reference electrodes were of similar amplitude. In tibial nerve conduction studies (NCS), the CMAP from the abductor hallucis (AH) muscle was recorded mainly by the reference electrode; the large-amplitude signal recorded by the reference electrode is attributed to volume-conducted activity from other muscles stimulated during the study. The onset latency of the potential recorded by the active and reference electrodes was similar despite significantly different distances from the stimulating site. Hence, the merits of using anatomic landmarks for defining the distal stimulation site are assessed. When the reference electrode makes a large contribution, the CMAP amplitude may not decrease commensurate with any wasting of the muscle under the active recording electrode, and the need to use another muscle for recording the CMAP for that nerve should be considered.     

Utility of the distal compound muscle action potential duration for diagnosis of demyelinating neuropathies

To assess the significance of distal compound muscle action potential (CMAP) duration for diagnosis of demyelinating neuropathies, electrophysiologic data were reviewed from 471 subjects, including 145 normal controls, 60 patients with chronic inflammatory demyelinating polyneuropathy (CIDP), 205 with other neuropathy, and 61 with amyotrophic lateral sclerosis (ALS). The duration of distally evoked CMAP was measured in the median, ulnar, tibial, and peroneal nerves. Optimal cut-off values were calculated with receiver-operating characteristic (ROC) curves. In comparison of normal controls and CIDP patients, ROC analyses showed the sufficient area under the curves (82-93%). When the cut-off values in the detection of demyelination were determined as the point with 98% specificity vs. normal on the ROC curves (median, 6.6 ms; ulnar, 6.7 ms; peroneal, 7.6 ms; tibial, 8.8 ms), the sensitivity was 77% for CIDP, with a specificity of 90% vs. ALS and 95% vs. diabetic neuropathy. The distal CMAP duration is a useful index for the detection of distal demyelination. We suggest the above cut-off values for each nerve as one of the electrodiagnostic criteria for demyelinating neuropathies, preferentially affecting the distal nerve terminals, such as CIDP.     

The contribution of the interosseous muscles to the hypothenar compound muscle action potential

The contributions of the various ulnar‐innervated muscles of the hand to the hypothenar compound muscle action potential (CMAP) were estimated by directly stimulating individual muscles and by analyzing CMAP shape changes resulting from manipulations that changed individual muscle lengths. The results show that the first peak of the negative phase of the hypothenar CMAP comes from the hypothenar muscles, but that the second peak is due to a large volume‐conducted potential from the interosseous muscles. The interosseous contribution affects both the amplitude and the area of the CMAP, and makes these parameters sensitive to changes in the configuration of the fingers and the temperature gradient in the hand. To reduce the interosseous contribution, a “balanced reference” consisting of two reference electrodes, one over each tendon, is proposed. © 1999 John Wiley & Sons, Inc. Muscle Nerve 22: 6–15, 1999     

Compound action potential of sensory tail nerves in the rat

Assessment of the conduction velocity of motor fibers of the rat tail nerves has been used by some authors in the past, but very little is known about the sensory fibers. In 10 adult rats, weighing between 320 and 380 g, responses from the nerves and muscles of the tail have been recorded after stimulation at its root and tip. It was found that stimulation of the tip involved mainly sensory fibers, of which two main groups could be identified. One faster group, conducting within the range of 38-27 m/s, and one slower group with range 14-7 m/s. The bipolar recording configuration was found to be optimal for sensory recording. Stimulation of the tail root evoked a motor response, which was preceded by a very small neurographic activity, due to the fastest sensory fibers conducting antidromically. The conduction velocity of motor fibers was calculated to be approximately 19 m/s. Distance traveled by the volley can be assessed with excellent precision on the tail nerves; hence the calculated conduction velocities are highly reliable and reproducible. We propose that the tail nerves may be a useful tool for evaluation of conduction velocity of Abeta and Adelta afferents. As the technique is just minimally invasive, the test can be repeated a number of times in animals under chronic experimental conditions.     

Comparison of single-fiber and macro electrode recordings: Relationship to motor unit action potential duration

The factors contributing to the duration of a motor unit action potential (MUAP) are believed to be well known, with both manual measurements and computer simulations agreeing with respect to MUAP durations approaching 10 ms. In this investigation, it is clearly demonstrated that use of a wide-open amplifier bandpass combined with signal-to-noise ratio enhancement results in MUAP durations approaching 30 ms recorded with either a macro or single-fiber electrode. Why the clinically recorded MUAP duration differs significantly from these physiologic durations is discussed. A hypothesis is presented whereby the major contributing factor toward MUAP duration is the total time of action potential transmembrane current flow along the muscle fiber from end-plate zone to musculotendinous junction. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1381–1388, 1997