Boudewijn T.H.M. Sleutjes

CMAP scan discontinuities: automated detection and relation to motor unit loss.

UNLABELLED Objective To evaluate an automated method that extracts motor unit (MU) information from the CMAP scan, a high-detail stimulus-response curve recorded with surface EMG. Discontinuities in the CMAP scan are hypothesized to result from MU loss and reinnervation. METHODS We introduce the parameter D50 to quantify CMAP scan discontinuities. D50 was compared with a previously developed manual score in 253 CMAP scans and with a simultaneously obtained motor unit number estimate (MUNE) in 173 CMAP scans. The effect of MU loss on D50 was determined with a simulation model. RESULTS We found a high agreement (sensitivity=86.8%, specificity=96.6%) between D50 and the manual score. D50 and MUNE were significantly correlated below 80 MUs (r=0.65, n=68, p<0.001), but not when MUNE was larger than 120 MUs (r=0.23, n=59, p=0.08). CONCLUSIONS Discontinuities in the CMAP scan as expressed by a decreased D50 are related to significant MU loss. The determination of D50 is objective, quantitative, and less time-consuming than both manual scoring and many existing MUNE methods. SIGNIFICANCE D50 is potentially useful to monitor neurogenic disorders and moderate to severe MU loss.

Electrically evoked multiplet discharges are associated with more marked clinical deterioration in motor neuron disease

Introduction: The aim of this study was to determine whether electrically evoked multiplet discharges (MDs) are related to severity of clinical deterioration in motor neuron disease (MND). Methods: Stimulated high‐density surface electromyographic (HDsEMG) recordings were performed in thenar muscles. Data were collected from 31 MND patients. MDs from the HDsEMG recordings were determined at baseline. ALSFRS‐R scores were obtained at baseline and at a maximum of 16 weeks follow‐up. Results: The presence of MDs was associated with progressive deterioration of ALSFRS‐R score (P = 0.02) and fine motor function (FMF) (P < 0.001). Patients who had a higher number of motor units that generated MDs (r = 0.61, P < 0.001) and patients who had a higher number of MDs (as percentage of applied stimuli) (r = 0.59, P = 0.001) had a more severe decline in FMF. Conclusions: Electrically evoked MDs are associated with more marked clinical deterioration in patients with MND. Muscle Nerve 53: 222–226, 2016

Excitability tests using high-density surface-EMG: A novel approach to studying single motor units

OBJECTIVE To study excitability of single motor units (MUs) using high-density surface-EMG. METHODS Motor unit action potentials (MUAPs) were evoked by submaximal stimulation of the median nerve at the wrist and recorded with a 9 × 14 electrode grid on the skin overlying the thenar muscles. For excitability tests of single MUs, the most optimal specific single-channel surface-EMG signal was selected based on the spatiotemporal profile of single MUs. RESULTS Axonal excitability measures were successfully obtained from 14 single MUs derived from ten healthy subjects. Selecting the optimal single-channel surface-EMG signals minimized interference from other single MUs and improved signal-to-noise ratio. The muscle fiber conduction velocity (MFCV) could also be derived from the unique spatiotemporal profile of single MUs. CONCLUSION High-density surface-EMG helps to isolate single MUAP responses, making it a suitable technique for assessing excitability in multiple single motor axons per nerve. SIGNIFICANCE Our method enables the reliable study of ion-channel dysfunction in single motor axons of nerves without any requirement for specific conditions, such as prominent MU loss or enlarged MUAPs due to collateral sprouting.

Identifying fasciculation potentials in motor neuron disease: A matter of probability

Introduction: Fasciculations, the spontaneous activity of single motor units (MUs) are characteristic, but nonspecific for motor neuron disease (MND). We aimed to identify MU discharge properties to optimally differentiate MND patients from healthy controls. Methods: High‐density surface electromyography recordings were performed in the thenar muscles during 10 min of rest. MU discharges were classified as “isolated” when the interspike intervals (ISIs) before and after were > 250 ms, “continual” when both ISIs were ≤ 250 ms, or as “other”. Results: In patients (n = 30) compared with controls (n = 14), more MUs were active (9 vs. 3, P < 0.001) and generated relatively more isolated discharges (35% vs. 10%, P = 0.01). Two or more MUs with isolated discharges occurred more frequently in patients compared with controls (24% vs. <1% of 10‐s windows, P < 0.001). Conclusions: More frequent occurrence of multiple MUs showing isolated discharges may improve identification of patients with MND. Muscle Nerve 53: 227–233, 2016

Diagnostic accuracy of electrically elicited multiplet discharges in patients with motor neuron disease

Objective To determine and compare the diagnostic accuracy of electrically elicited multiplet discharges (MDs) and fasciculation potentials (FPs) in motor neuron disease (MND). Methods Patients were eligible when they had MND in their differential diagnosis and were referred for electromyogram (EMG). Stimulated high-density surface EMG of the thenar muscles was performed on the same day as standard EMG examination. High-density recordings were analysed for presence of MDs and needle EMG of any muscle investigated in the cervical region for presence of FPs. Results Of the 61 patients enrolled in this diagnostic study, 24 patients were clinically diagnosed with amyotrophic lateral sclerosis (ALS) and 11 patients with progressive muscular atrophy (PMA). Another diagnosis was made in 26 patients. Sixteen patients in whom MDs were detected were diagnosed with either ALS (n=11) or PMA (n=5; sensitivity=47.1%, PPV=94.1%). MDs were detected in only one patient initially diagnosed with PMA, but in whom later on, multifocal motor neuropathy could not be excluded (specificity=96.2%). Electrically elicited MDs had a higher specificity than FPs (96.2% vs 53.9%, p<0.001, n=26) and lower sensitivity (47.1% vs 85.3%, p=0.002, n=34). When considering presence of MDs in MND as neurogenic EMG abnormality, lower motor neuron involvement of ≥1 EMG region increased from 50% to 73.5% (p=0.008, n=34). Conclusions Electrically evoked MDs are highly specific for ALS and PMA and are an early sign of lower motor neuron dysfunction.

Increased supernormality in patients with multiplet discharges: Evidence for a common pathophysiological mechanism behind multiplets and fasciculations

OBJECTIVE To determine whether there is a relation between electrically evoked multiplet discharges (MDs) and motor axonal excitability properties. We hypothesized that electrically evoked MDs share their underlying pathophysiological mechanism with fasciculations. METHODS High-density surface EMG and motor nerve excitability recordings of the thenar muscles were performed in 22 patients with motor neuron disease (MND) in their differential diagnosis and who were referred for EMG examination. RESULTS Supernormality (hyperexcitable phase following the refractory period) was significantly increased in patients with MDs (n=10) compared to patients without MDs (n=12) (25.5% vs 17.0%; p=0.02). Depolarizing threshold electrotonus differed significantly between both groups as well (TEdpeak, 76.6% vs 66.6%, p<0.01; TEd90-100ms, 51.7% vs 44.3%, p<0.01) CONCLUSIONS: Our findings imply that the same pathophysiological excitability changes are involved in generating MDs and fasciculations. Yet, MDs may be quantified more easily, and may be more specific for abnormal distal excitability than fasciculations, because fasciculations may originate along the motor axon as well as in the neuron cell body. SIGNIFICANCE MDs are potentially useful as objective measure of increased distal axonal excitability at individual motor unit level and might complement clinical studies in MND.

Sodium-potassium pump assessment by submaximal electrical nerve stimulation

OBJECTIVE Sodium-potassium pump dysfunction in peripheral nerve is usually assessed by determining axonal hyperpolarization following maximal voluntary contraction (MVC) or maximal electrical nerve stimulation. As MVC may be unreliable and maximal electrical stimulation too painful, we assessed if hyperpolarization can also be induced by submaximal electrical nerve stimulation. METHODS In 8 healthy volunteers different submaximal electrical stimulus trains were given to the median nerve at the wrist, followed by 5 min assessment of thresholds for compound muscle action potentials of 20%, 40% or 60% of maximal. RESULTS Threshold increase after submaximal electrical nerve stimulation was most prominent after an 8 Hz train of at least 5 min duration evoking submaximal CMAPs of 60%. It induced minimal discomfort and was not painful. Threshold increase after MVC was not significantly higher than this stimulus train. CONCLUSIONS Submaximal electrical stimulation evokes activity dependent hyperpolarization in healthy test subjects without causing significant discomfort. SIGNIFICANCE Sodium-potassium pump function may be assessed using submaximal electrical stimulation.

T106. Warming nerves for excitability-testing

Introduction Temperature affects excitability-variables measured from peripheral nerve. Excitability studies provide valuable insights into disease mechanisms in motor neuron disease and various neuropathies. Warming the nerve prior to the investigation helps to compensate for temperature variability, facilitates reproducibility, and improves the certainty of ascribing changes to pathophysiological mechanisms. For clinical purposes, therefore, it is important to determine how long a nerve has to be warmed for an excitability-variable to reach a stable value. We investigated which method of warming the nerve was quickest in reaching stable excitability variables. Methods In five healthy subjects, we compared three methods of warming the median nerve to 37 °C: (i) infrared lamp, (ii) water blanket, and (iii) water bath. Prior to warming the nerve, an excitability test was performed in all three methods to assess baseline values for distal motor latency (DML) and multiple excitability variables (strength duration time constant, rheobase, minimal and resting I/V slope, relative refractory period and super-excitability). During a one-hour warming period, the excitability-tests were repeated every 10 min from which the values for DML and excitability variables were determined. To asses which were affected by temperature in our subjects, we first investigated the temperature relation for all recorded variables. Temperature dependent variables were fitted by an exponential function from which the time-constant and asymptotic end-value were determined. A variable was considered to be stable when it had reached 95% of the asymptotic end-value. Results Temperature-dependency was found for DML, refractory period and super-excitability. Of the three warming methods, the water bath was the quickest in reaching stable values for DML (29 min, IQR 22–30), refractory period (25 min, IQR 23–33), and super-excitability (43 min, IQR 43–49). For the infrared lamp, DML reached stable values at 36 min (IQR 30–42), refractory period at 28 min, IQR 27–32), and super-excitability at 45 min, IQR 36–51). For the water blanket stable values were reached for DML at 36 min (IQR 23 – 52), for refractory period at 34 min (IQR 29–35), and for super-excitability at 45 min (IQR 25–71). Conclusion Warming the median nerve in a water bath is the most efficient way to warm the nerve for excitability-testing.

F128. Excitability-tests using multi-channel contraction force recordings: Development of a novel setup to study distal motor excitability properties

Introduction In animal models of motor neuron disease downstream pathophysiological events were shown to occur at distal nerve terminal branches. Recently, distal excitability-tests were developed which have the potential to assess changes in ion-channel activity in these branches. In these tests, electrical conditioning and test stimuli are delivered to distal motor axon branches near the muscle. The muscle responses are then recorded by assessing threshold-changes for eliciting a given muscle force as CMAP-recording would be too much affected by electrical stimulus artifacts. With current methods insight into direction of force remains unknown when using a single force transducer. Therefore, we present a novel method to study distal excitability by monitoring force in multiple directions. Methods Excitability properties were obtained by stimulating the median nerve at wrist level and the motor point over the thenar muscles. Thumb contraction force was recorded using four transducers, which were configured in such a way to measure force in left–right and up–down direction. With this configuration the transducers enclosed a square in the middle which was used to position the interphalangeal joint of the thumb. Using multiple force transducers the optimal recording direction was determined by rotating the transducers to maximize the response in a single-channel and minimize the response in the others. For excitability testing, the single-channel with the maximum response was selected. Results Five excitability recordings were successfully performed in the median nerve at wrist level (n = 3) and motor point (n = 2) in a single test subject. The maximum baseline to peak force during supramaximal nerve and motor point stimulation was approximately 2–3 N. The mean threshold for 50% maximum force was 2.4 mA at wrist level, and right-shifted for the motor point with a mean threshold of 10.2 mA. The mean strength-duration time constant was 0.476 ms at the wrist level and 0.211 ms at the motor point. At the motor point, the current-threshold relationship could not be completed for hyperpolarizing conditioning stimuli over 50%, most likely due to high currents. In the recovery cycle, the thresholds from 2.5 ms to 100 ms were lower for motor point stimulation than stimulation at wrist level. Although minimal, small force responses were also observed perpendicular to main recording direction. Conclusion Measuring excitability with multiple force transducers is potentially useful to optimize distal excitability recording.

T35. Developing a mathematical model of the human peripheral myelinated axon: Effects of segmental demyelination and impaired sodium channel conductance

Introduction Immune-mediated neuropathies affect myelinated nerve fibers, including Schwann cells, axons, and the complex array of ion-channels and other proteins in their membrane. The downstream events leading to damage or dysfunction of these structures in human neuropathies are often impossible to investigate because the available experimental methods cannot assess the consequences of a single pathophysiological mechanism and are only applicable to lower mammalian myelinated axons. Knowledge of these mechanisms is essential for the development of treatments aimed at prevention of irreversible nerve damage. The present study is a first step in the development of a complex forward mathematical model of the human peripheral myelinated axon which should incorporate all available functional knowledge on higher mammalian or human myelinated axons obtained, for instance, by patch-clamp studies. Here we assessed activation threshold for electrical stimulation and conduction behavior by focussing on segmental demyelination and sodium channel dysfunction in human motor and sensory axons. Methods We applied an established peripheral myelinated axon model consisting of 40 nodes. To simulate human motor axons, we modified the nodal membrane dynamics to those used in human peripheral motor nerve excitability tests in the median nerve. Sensory axons were simulated by doubling the percentage of persistent sodium channels. Focal segmental demyelination was simultaneously applied on the paranodal, juxtaparanodal, and internodal regions surrounding the three middle nodes by gradually increasing myelin capacitance and conductance until conduction failure occurred. Subsequently, impaired nodal sodium channel function on the three middle nodes was simulated by gradually decreasing the nodal transient and persistent sodium conductance. Results Normal motor and sensory conduction velocity was approximately 53 m/s and 54 m/s. The activation threshold in the motor axon model was 2% higher than for the sensory axon model. At 70% demyelination, the motor and sensory conduction velocity dropped to 37 m/s. Conduction block occurred for both at 97% of segmental demyelination. With decreasing sodium channel conductance motor axons showed conduction block at a decrease of 94% and sensory axons at 95%. At the site of demyelination the activation threshold increased up to approximately 4-fold in motor and sensory axon model before conduction block occurred. For impaired sodium channel conductance the activation threshold increase was approximately a factor of 3. Conclusion Using the longitudinal myelinated axon model, we successfully explored the increase in activation threshold and conduction slowing and block due to demyelination and decrease in sodium channel conductance.