Repetitive nerve stimulation: effects of recording site and the nature of 'pseudofacilitation'

Revisiting the compound muscle action potential (CMAP)

The compound muscle action potential (CMAP) is among the first recorded waveforms in clinical neurography and one of the most common in clinical use. It is derived from the summated muscle fiber action potentials recorded from a surface electrode overlying the studied muscle following stimulation of the relevant motor nerve fibres innervating the muscle. Surface recorded motor unit potentials (SMUPs) are the fundamental units comprising the CMAP. Because it is considered a basic, if not banal signal, what it represents is often underappreciated. In this review we discuss current concepts in the anatomy and physiology of the CMAP. These have evolved with advances in instrumentation and digitization of signals, affecting its quantitation and measurement. It is important to understand the basic technical and biological factors influencing the CMAP. If these influences are not recognized, then a suboptimal recording may result. The object is to obtain a high quality CMAP recording that is reproducible, whether the study is done for clinical or research purposes. The initial sections cover the relevant CMAP anatomy and physiology, followed by how these principles are applied to CMAP changes in neuromuscular disorders. The concluding section is a brief overview of CMAP research where advances in recording systems and computer-based analysis programs have opened new research applications. One such example is motor unit number estimation (MUNE) that is now being used as a surrogate marker in monitoring chronic neurogenic processes such as motor neuron diseases.

Modulation of neuromuscular transmission using transcutaneous direct currents: An exploratory study

OBJECTIVE

To assess the in vivo long-lasting effects on neuromuscular transmission using transcutaneous stimulation with anodal and cathodal direct currents applied over the end-plate region (epDCS).

METHODS

An active DCS electrode was placed over the end-plate region of both abductor pollicis brevis and first dorsal interosseous muscles, with a reference electrode located on the forearm. Cathodal or anodal currents were applied (2.5mA during 15min). Repetitive nerve stimulation of the median and ulnar nerves at the wrist was performed before and after DCS: protocol A - 500 stimuli at 3Hz; protocol B - 30 stimuli at 30Hz. For both muscles, we measured changes in amplitude and area between the first and 4th compound muscle action potential (CMAP) and between the first and 500th CMAP (protocol A); and the change in amplitude and area between the first and 30th CMAP (protocol B).

RESULTS

Anodal current did not change any measurement. Using cathodal epDCS and median nerve testing, there was a larger increase in CMAP amplitude (p=0.046) and a smaller decrease in area (p=0.008) between the first and 30th response (protocol B). Using cathodal epDCS and ulnar nerve testing, there was a possible significant smaller amplitude decrease of the CMAP measured, between the first and fourth response (protocol A).

CONCLUSIONS

Cathodal transcutaneous direct currents over the end-plate may modulate end-plate function by increasing the release of quanta of acetylcholine (Ach) and/or the number of Ach receptors available. Future studies should address this topic.

Optimal stimulation settings for CMAP scan registrations

BACKGROUND

The CMAP (Compound Muscle Action Potential) scan is a non-invasive electrodiagnostic tool, which provides a quick and visual assessment of motor unit potentials as electrophysiological components that together constitute the CMAP. The CMAP scan records the electrical activity of the muscle (CMAP) in response to transcutaneous stimulation of the motor nerve with gradual changes in stimulus intensity. Large MUs, including those that result from collateral reinnervation, appear in the CMAP scan as so-called steps, i.e., clearly visible jumps in CMAP amplitude. The CMAP scan also provides information on nerve excitability. This study aims to evaluate the influence of the stimulation protocol used on the CMAP scan and its quantification.

METHODS

The stimulus frequency (1, 2 and 3 Hz), duration (0.05, 0.1 and 0.3 ms), or number (300, 500 and 1000 stimuli) in CMAP scans of 23 subjects was systematically varied while the other two parameters were kept constant. Pain was measured by means of a visual analogue scale (VAS). Non-parametric paired tests were used to assess significant differences in excitability and step variables and VAS scores between the different stimulus parameter settings.

RESULTS

We found no effect of stimulus frequency on CMAP scan variables or VAS scores. Stimulus duration affected excitability variables significantly, with higher stimulus intensity values for shorter stimulus durations. Step variables showed a clear trend towards increasing values with decreasing stimulus number.

CONCLUSIONS

A protocol delivering 500 stimuli at a frequency of 2 Hz with a 0.1 ms pulse duration optimized CMAP scan quantification with a minimum of subject discomfort, artefact and duration of the recording. CMAP scan variables were influenced by stimulus duration and number; hence, these need to be standardized in future studies.

Localization of frequency-dependent conduction block in carpal tunnel syndrome

Frequency-dependent conduction block (FDB) has been demonstrated in subjects with carpal tunnel syndrome (CTS) when the median nerve was stimulated at 30-Hz frequency proximal to the carpal tunnel (CT). However, it was still questionable whether FDB occurred within the region of the CT or more distally. High-frequency nerve stimulation (HFNS) was performed in controls and in CTS subjects while stimulating the median nerve proximal and distal to the carpal tunnel. The degree of FDB was measured by comparing the 20th thenar CMAP with the 1st following a train of 20 stimuli at 30-Hz. FDB occurred across the CT in 11/12 studies (91.7%) and across the distal portion of the palmar aponeurosis-distal terminal motor branches in 5 of the 11 studies (41.7%). These results suggest that the safety margin for impulse transmission can be impaired distal to the presumed site of injury in CTS.

Frequency-dependent conduction block in carpal tunnel syndrome

Frequency-dependent conduction block (FDB) across segments of demyelination in response to high-frequency nerve stimulation has been well demonstrated in animals and has been explored in humans. However, attempts to demonstrate this phenomenon in sensory fibers involved in entrapment neuropathies have been unsuccessful. Therefore, we investigated the effects of high-frequency nerve stimulation in the median motor nerve in patients with carpal tunnel syndrome (CTS) with moderate to severely increased distal motor terminal latencies (MTL). As a group, the mean decrease in negative peak amplitude (npAmp) during 20 stimuli at 30-HZ frequency was significantly greater in CTS subjects (-11.3%) than in controls (+7.9%). The degree of FDB was greater when MTL was more prolonged (i.e., -4.9% at 5.0 ms and -25.3% at 9.4 ms) and FDB was more pronounced at higher stimulation frequencies (20 and 30 HZ). Our results suggest that the safety margin for impulse transmission is impaired in the motor axons of patients with a focal demyelinating lesion. These findings may correlate with the observation of weakness in the absence of conduction block in patients with entrapment neuropathies.

Physiologic changes of compound muscle action potentials related to voluntary contraction and muscle length in carpal tunnel syndrome

The affect of muscle length and voluntary contraction upon compound muscle action potentials (CMAPs) in subjects with carpal tunnel syndrome (CTS) has been evaluated. Twenty-five hands in a CTS patient group and 29 hands in a normal subject control group were studied. The CMAPs from the abductor pollicis brevis induced by median nerve stimulation at the wrist were obtained for five thumb positions: neutral, abduction for shortening with and without contraction, and adduction for lengthening with and without contraction. Upon muscle shortening with relaxation, CMAP duration decreased in both groups, whereas waveform amplitude increased in the control group and showed no significant change in the CTS group. Muscle shortening with contraction afforded decreased CMAP duration and increased CMAP amplitude in both groups. Upon muscle lengthening with relaxation, both groups showed a reduction in CMAP amplitude and an increase in CMAP duration. Upon lengthening with contraction, CMAP duration decreased in the control group; in contrast, the CTS group showed further amplitude reduction and the waveform duration returned to the neutral value. These results demonstrate that, in patients with CTS, physiologic CMAP summations by muscle shortening or contraction may be less effective, whereas decreases in amplitude and increases in duration may be accentuated by lengthening and contraction.

Compound muscle action potentials during repetitive nerve stimulation

When using repetitive nerve stimulation to examine neuromuscular transmission, the change in compound muscle action potential (CMAP) size is usually assessed by measurement of negative-peak or peak-to-peak amplitude. Technological developments now allow automatic measurement of CMAP area, but some patients show increment of CMAP amplitude and decrement of CMAP area. This study systematically analyzed the changes in these CMAP parameters in 23 neurologically healthy subjects. CMAPs were recorded when the ulnar nerve was stimulated at frequencies of 1, 2, 5, 10, 20, 30, and 50 HZ (five pulses per train). CMAP amplitude showed significant increment within a train when stimulus frequency was above 5 HZ (probably due to increased muscle-fiber conduction velocity), whereas CMAP area hardly changed at any frequencies. Measurement of CMAP area produces less ambiguous results than amplitude measurement in repetitive nerve stimulation studies.

Decrement in area of muscle responses to repetitive nerve stimulation

Measurement of the decremental muscle response to repetitive nerve stimulation (RNS) has low yields for the diagnosis of neuromuscular transmission defects compared with single fiber electromyography (SFEMG). We compared area and amplitude of muscle responses to RNS in 87 patients and 30 controls, using SFEMG as the reference standard. Decrement of response area provided additional diagnostic yields of 5.3% to 30% depending on the muscle examined and disease severity, and is recommended as a diagnostic adjunct to measurement of amplitude decrement during RNS.

Repetitive nerve stimulation of the nasalis muscle: technique and normal values

A technique of repetitive nerve stimulation of the facial nerve with recording from the nasalis and orbicularis oculi muscles was developed and tested in 15 healthy subjects. Quality criteria were that the stimulus artifact should return to baseline before the onset of the compound muscle action potential (CMAP), and that the CMAP should begin with a negative phase, be biphasic in shape, and have an amplitude of over 1 mV. Repetitive nerve stimulation was only performed if all four criteria were met, which was the case in all 15 subjects for the nasalis muscle, but in only 5 subjects for the orbicularis oculi muscle. The largest observed decrement was 9% in area. Reptitive nerve stimulation of the nasalis muscle is thus feasible, but its diagnostic utility remains to be established in patients with disorders of neuromuscular transmission.

A surface EMG analysis of sarcolemma excitability alteration and myofibre degeneration in Steinert disease

OBJECTIVES

To apply surface electromyography (EMG) using low and high stimulation frequencies, to establish the contribution of myotonia and/or dystrophy to muscle impairment in subjects affected by myotonic dystrophy (MyD).

METHODS

A motor point stimulation protocol, at 15 and 35 Hz, was carried out on the tibialis anterior (TA) of 25 MyD patients. These were subdivided into 3 subgroups, MyD3 (9), MyD4 (10) and MyD5 (6), on the basis of their TA MRC score. The surface myoelectric signal was recorded and the average rectified value of amplitude (ARV) was evaluated. In 4 MyD5 patients we simultaneously recorded EMG and force.

RESULTS

Each subgroup presented a characteristic ARV trend both at 15 and 35 Hz - increasing in MyD3 (like the controls), slightly decreasing in MyD4 and clearly decreasing in MyD5. The low frequency permitted a greater resolution of the statistical analysis. Two out of 4 patients showed an opposite trend of ARV with respect to the force, whereas the others presented a parallel decreasing trend.

CONCLUSIONS

The analysis of the ARV during a stimulated contraction permits the identification and quantification of the sarcolemma excitability alteration and/or the myofibre degeneration contributing to muscle impairment in MyD. The lower frequency is more sensitive and offers better inter-experiment repeatability.