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Objective: To describe the heterogeneity of electrodiagnostic (EDx) studies in Guillain-Barré syndrome (GBS) patients collected as part of the International GBS Outcome Study (IGOS)., Methods: Prospectively collected clinical and EDx data were available in 957 IGOS patients from 115 centers. Only the first EDx study was included in the current analysis., Results: Median timing of the EDx study was 7 days (interquartile range 4-11) from symptom onset. Methodology varied between centers, countries and regions. Reference values from the responding 103 centers were derived locally in 49%, from publications in 37% and from a combination of these in the remaining 15%. Amplitude measurement in the EDx studies (baseline-to-peak or peak-to-peak) differed from the way this was done in the reference values, in 22% of motor and 39% of sensory conduction. There was marked variability in both motor and sensory reference values, although only a few outliers accounted for this., Conclusions: Our study showed extensive variation in the clinical practice of EDx in GBS patients among IGOS centers across the regions., Significance: Besides EDx variation in GBS patients participating in IGOS, this diversity is likely to be present in other neuromuscular disorders and centers. This underlines the need for standardization of EDx in future multinational GBS studies., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 International Federation of Clinical Neurophysiology. Published by Elsevier B.V. All rights reserved.)

To investigate the facilitatory effect of thinking about movement on motor evoked potential (MEP) amplitude, we recorded MEPs in two test muscles during rest, with the subject thinking about contracting the test muscle but without subsequent contraction, and during 10% maximum voluntary contraction. Stimuli were delivered at 10% above resting motor threshold and at 90-100% stimulator output. H-reflexes, recorded in flexor carpi radialis, were obtained during rest and think conditions. MEP threshold was lower during the think condition (P = 0.004). At both stimulus intensities, median MEP amplitudes and areas were significantly (P < 0.001) larger during the think paradigm compared with rest. This effect was greater at the lower stimulus intensity. There was no significant difference in latency (P = 0.15). In 4/8 subjects, H-reflex amplitudes were mildly facilitated (P < 0.05) during the think condition. We conclude that thinking about movement without detectable EMG activity has a facilitatory effect on magnetic MEPs. The absence of a MEP latency shift between rest and think conditions and absence of a consistent increase in H-reflex amplitude suggests this effect occurs largely at the cortical level. In some subjects, however, an increase in spinal motoneuron excitability may also contribute.

We examined the inhibitory effect of a brief train of digital (D2) electrical stimuli at 4 times perception threshold on transcranial magnetic motor evoked potentials (MEPs) recorded from abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles ipsilateral to the side of D2 stimulation. We compared this to the inhibitory effect of ipsilateral D2 stimulation on averaged rectified EMG recorded at 10% maximum voluntary contraction and on F-responses and H-reflexes recorded from these same muscles. We also compared MEPs recorded following D2 stimulation just above perception threshold to MEPs following higher intensity D2 stimulation. As well, we assessed the effect of preceding D2 stimulation on MEPs recorded from a relaxed versus tonically contracted hand muscle. D2 stimulation elicited a triphasic response of modest MEP facilitation followed by inhibition and further facilitation. The duration and onset of MEP inhibition correlated with those of the initial period of rectified EMG inhibition, however, the magnitude of MEP inhibition was generally less than the magnitude of EMG inhibition, consistent with a greater inhibitory effect of digital afferents on smaller motor neurons. MEPs were not facilitated during the rebound of EMG activity (the E2 period) that usually followed the initial period of EMG inhibition (I1 period). The behavior of H-reflexes and F-responses following ipsilateral D2 stimulation suggested that inhibition of both EMG and MEPs is not mediated via presynaptic inhibition of Ia afferents, and that inhibition is augmented by descending rather than segmental input to spinal motor neurons. Tonic contraction of the target muscle during D2 stimulation decreased the inhibitory effect of the preceding digital stimulus possibly due to recruitment of larger spinal motor neurons less likely to be inhibited by cutaneous input.

To determine whether motor evoked potential (MEP) amplitude and area are accurate measurements of the magnitude of response to magnetic cortical stimulation, we simultaneously recorded the twitch and MEP in the first dorsal interosseous muscle of 8 normal subjects. Consecutive stimuli were delivered at increasing stimulus intensities (SI) or with increasing levels of background voluntary muscle contraction (BVC). There was stimulus to stimulus variability in MEP amplitude, area and twitch force. At low SI and at low levels of background contraction, there was a good correlation between twitch amplitude and MEP amplitude and area (r = 0.6-0.96, P < 0.005). Increasing either variable caused the correlation to decrease significantly (r = 0.02-0.31, P > 0.01). With increasing SI, MEP amplitude and area plateaued but twitch force continued to increase. A similar pattern was observed with higher levels of background muscle contraction although in some subjects a second increase in MEP amplitude and area was seen. Collision experiments demonstrated that the amplitude of the EMG activity resulting from repetitive motoneuron firing increased as SI was increased. This is due to multiple descending volleys which result in repetitive firing of some spinal motoneurons. Rapid, repetitive firing of some motor units is likely to result in phase cancellation and, therefore, the MEP amplitude, and to a lesser extent area, do not accurately reflect the net motor output.

We quantitated the size and configuration of compound muscle action potentials (CMAPs) in 266 nerves (66 median, 67 ulnar, 71 tibial and 62 peroneal) of 72 patients with acute inflammatory demyelinating polyneuropathy (AIDP) initially studied within 19 days of symptom onset. Results were compared with criteria for CMAP abnormalities, including criteria for abnormal negative peak duration and desynchronisation, derived from a control population of 50 median, ulnar, tibial and peroneal nerves. Other motor conduction abnormalities including minimal F response latency were also examined. We also analysed patterns of CMAP abnormality, peak disability and outcome for AIDP patients who had at least 3 motor nerves evaluated at first electrophysiologic study. Amongst AIDP nerves, low amplitude of the distal CMAP, usually with prolonged distal latency, was much more common than an abnormal fall in CMAP amplitude between stimulus sites. Using our CMAP criteria more than half of these low amplitude distal responses showed prolonged negative peak duration of desynchronisation or both, consistent with demyelination. Of the 47 AIDP patients who had 3 or more nerves initially studied, 37 (78.7%) had at least 1 motor nerve with a distal CMAP showing evidence of temporal dispersion. In addition, those with at least 75% of motor nerves showing a pattern of low amplitude of the distal CMAP without a further significant fall in amplitude between stimulus sites had greater peak disability and a poorer outcome. Assessment of temporal dispersion of the distal CMAP should be included in electrophysiologic criteria for acute demyelination. In addition, for some patients with AIDP patterns of CMAP amplitude abnormality amongst motor nerves are present early in the illness and may provide prognostic information.

We examined F wave mean and minimum latency, mean and maximum amplitude, duration, persistence and chronodispersion in 241 nerves from 78 patients with Guillain-Barré syndrome (GBS) and 162 nerves from 43 patients with chronic inflammatory demyelinating polyneuropathy (CIDP). Results were compared with normal criteria derived from 72 median, 73 ulnar and 73 tibial control nerves, to determine the relative diagnostic sensitivity of the various F wave parameters. F wave abnormalities were found in 92% and 95% of nerves of patients with GBS and CIDP respectively. Absence of F responses or prolongation of minimum and mean latency were the most frequent abnormalities in both groups. Forty-five (11.2%) nerves overall had absent F responses with normal compound muscle action potential (CMAP) amplitudes and no significant fall between stimulus sites, consistent with isolated proximal conduction block. Forty-four nerves (23.7% of nerves in which F waves were present) fulfilled minimum F latency criteria for acquired demyelination . Eighty-one (20.1%) nerves had normal conventional motor nerve conduction studies and abnormal F responses, not all of which were identified by assessing only F absence or minimum latency. Severity of F wave abnormalities did not correlate with clinical outcome. Our findings confirm the high frequency of proximal nerve lesions in early GBS and CIDP, not all of which are associated with distal motor conduction abnormalities, and suggest that assessment of multiple F wave parameters, in particular chronodispersion, mean latency and mean amplitude (in addition to absence and minimum latency), increases the yield of F wave studies.

We studied the effect of stimulus intensity, coil size, mental alertness and prestimulus muscle contraction on the variability of motor evoked potentials (MEPs) produced by magnetic cortical stimulation (MCS). In 5 healthy subjects we delivered MCS either with a circular coil centered at the vertex or a figure-8 coil centered over the motor cortex hand area, recording from first dorsal interosseous. With the subject at rest or exerting 5% maximum voluntary contraction, 30 consecutive stimuli were given at 4 stimulus intensities (SIs) in 10% increments above resting motor threshold. Concurrent mental arithmetic constituted mental alertness. Spectral analysis was performed on data from 300 consecutive stimuli. The variability of MEP response size was inversely related to stimulus intensity, prestimulus voluntary muscle contraction, the recruitment of motoneurons and the size of the field generated by the magnetic coil. The MEP variability was larger than and not correlated with the variability of the H-reflex. Fast Fourier transformation and cross-correlation analysis did not identify a consistent dominant frequency, suggesting that the variability in MEP size is essentially random. We suggest that the variability in MEP response is caused by constant, rapid, spontaneous fluctuations in corticospinal and segmental motoneuron excitability levels. Any maneuver that raises this level or increases the probability of motoneuron firing will decrease MEP variability.