Amyotrophic lateral sclerosis (ALS)
We studied 70 patients (41 men and 29 women) with definite (12), probable (51) or possible (7) ALS as defined by the revised El-Escorial  and Awaji criteria . None had coincidental polyneuropathy, diabetes or ulnar neuropathy. Those older than 75 years, with disease duration greater than 24 months, with weak 1st dorsal interosseous muscle (1st DI) of the right hand on clinical examination, with respiratory distress lying down or with cognitive change, were excluded. The median age was 62 years [range 30–75 years, 1st–3rd interquartile range (IQR) 52–69 years]. The disease was of upper limb onset in 11 patients, lower limb onset in 30 patients, bulbar onset in 28 patients and axial onset (drop-neck) in 1. Disease duration before study entry ranged from 2 to 24 months (median 14 months, 1st–3rd IQR 8–18 months). All the patients showed clinical progression in subsequent follow-up. Of these 70 patients, 14 were excluded from the protocol because they could not cooperate with the experimental protocol, which required minimal muscle contraction (10 subjects) or because no fasciculation potential (FP) was detected by needle EMG (4 subjects). The 1st DI muscle of the right hand was studied in each patient. Patients were evaluated within the diagnostic workup period, before riluzole treatment was started.
Benign fasciculation syndrome (BFS)
Eighteen subjects (15 men and 3 women) were studied (median age 53.5 years; range 29–68; 1st–3rd IQR 50 to 60 years). These subjects had all been symptomatic for several years, and all had been followed for more than 5 years by one of the authors (MdC). In all of them, motor conduction studies, sensory action potentials and regularly repeated EMG were normal, including motor unit potential analysis in the right 1st DI muscle that was selected for investigation. In addition, the FPs were of normal morphology .
In all the subjects, in both experimental groups, preliminary conventional concentric needle EMG was performed in the 1st DI muscle. In the group of patients with a diagnosis of ALS, the 1st DI muscles were subgrouped into those with neurogenic motor unit potentials (MUPs) on quantitative analysis and/or presence of fibrillations and positive sharp waves (fibs-sw) (ALS-1) and those in whom quantitative analysis revealed MUPs of normal morphology and no fibs-sw (ALS-2). Four needle positions were used for the conventional study of MUPs and fibs-sw. For detection of the presence of FPs, a 2-min recording was made in the four different sites .
A Keypoint-Net device was used (Dantec-Natus, Denmark) for all investigations. Motor (both ulnar nerves at wrist, below and above elbow, and both peroneal nerves, including F-waves) and sensory nerve fibers (both ulnar and both sural nerves) were assessed to exclude peripheral neuropathy and ulnar nerve lesion. For motor studies, standard amplifier filter settings of 20 Hz and 10 kHz were used. The latency measurements were performed with a gain of 200 μV/division. Sensory responses were recorded by bar electrodes using filter settings of 20 Hz–2 kHz, and a gain of 10 μV/division. A ground electrode was placed at the wrist. Following routine evaluation of the 1st DI muscle, we introduced two disposable concentric facial needle electrodes (recording area 0.017 mm2; Dantec-Natus) connected to two different channels of the Keypoint device (double-EMG recordings). Filter settings were 500 Hz–10 kHz. Data was saved on the hard-disk of the device for offline analysis.
One concentric facial needle electrode (needle A) was placed in the most lateral part of the muscle (closer to the thumb) and the other (needle B) was placed more medially. The two needle electrodes were placed ≥25 mm apart, perpendicular to the long axis of the limb, 2–3 mm deep in the muscle, and in the territories of different motor units . The position of the needle electrodes was maintained using a tape to hold the cables on the patient´s arm. To ensure that the needles were recording different motor units, we performed two tasks. We required that one or both of these tests confirmed that the needle electrodes were recording from different motor units. We first asked the patient to make a very slight voluntary contraction of the muscle and noted whether the voluntarily activated motor units (≥50 μV) could be recorded from only one of the two electrodes (Fig. 1). Secondly, the ulnar nerve was stimulated at threshold intensity at the wrist and elbow to confirm that the response from a single motor unit was recorded from only one needle electrode (Fig. 2). A single motor unit response was supported by the observation of an all-or-none response at near-threshold axon stimulation (Fig. 2). A similar surface stimulation technique was used by others in stimulation SFEMG studies of single motor units . We have previously validated the method [11, 13]. Following confirmation that the needles were recording from the territories of different motor units at two different loci, a 2-min recording period was used to detect FPs in both motor unit territories . Typically, each facial needle electrode recorded from 2 to 3 easily recognisable and separable motor units. The total number of FPs was calculated in the recordings from each of the two sites in each muscle studied in the ALS and BFS groups of subjects. We followed the definition of the American Academy of Electrodiagnostic Medicine  and accepted as FP the electric activity with the configuration of a motor unit activation potential, but occurring spontaneously with the muscle at rest. For this study, we accepted FPs only if their amplitude was greater than 100 μV. The motor point, located proximally in this muscle , was avoided.
In each of these double-EMG recordings, a sweep speed of one second/division (Fig. 3a) was used to detect and quantify FPs (number of FPs during the 2-min recording in each channel) [11, 13]. The morphology of the recorded FPs was evaluated using a sweep speed of 20 ms/division (Fig. 3b). After the 2-min recording of the muscle at rest, without allowing movement of the needle electrodes, each patient was asked to perform a very slight contraction of the 1st DI muscle in order to voluntarily activate a single motor unit. Online analysis with offline review was used to compare the morphology of each FP with the morphology of the voluntarily activated motor unit (Fig. 4). The comparison was done by careful visual inspection, taking into account that during slight contraction the relationship between the electrode’s recording surface and nearby muscles fibres changes a little, implying a very minor difference in amplitude of the potential (<20 %), which was considered in the analytical process; however, we required a similar potential duration and number of phases.
All subjects gave written informed consent as required by the local Research Ethics Committee.
In each of the three groups of patients (ALS-1, ALS-2 and BFS), we calculated the number of FPs recorded from each of the two needle sites, and the difference between the pairs of recordings in each subject. The percentage differences in these paired groups of data were not normally distributed. These percent differences were therefore ranked and the 20 % of pairs of recordings with the least differences were discarded as not being sufficient to warrant further analysis. This 20 % cut-off derived from a preliminary study of ten ALS patients, in whom 20 % was the largest variation observed in the number of FPs in a single site in two consecutive 2-min recording periods. The remaining 80 % of paired recordings (with more marked asymmetry in the number of FPs between sites) were analysed to determine the relation of the electrode site with more frequent FPs and the site of the first recorded motor unit potential (electrode A or B) following voluntary activity. These categorical relationships were evaluated using the Fisher exact test for significant differences. A p value <0.05 was considered significant. The median number of FPs in each group was compared using the Kruskal–Wallis test.