Motor Unit Tracking Using High Density Surface Electromyography (HDsEMG) . Automated Correction of Electrode Displacement Errors.

INTRODUCTION: This article is part of the Focus Theme of Methods of Information in Medicine on "Biosignal Interpretation: Ad-vanced Methods for Neural Signals and Images". OBJECTIVES: The study discusses a technique to automatically correct for effects of electrode grid displacement across serial surface EMG measurements with high-density electrode arrays (HDsEMG). The goal is to match motor unit signatures from subsequent measurements and by this, achieve automated motor unit tracking. METHODS: Test recordings of voluntary muscle contractions using HDsEMG were performed on three healthy individuals. Electrode grid displacements were mimicked in repeated recordings while measuring the exact position of the grid. A concept of accounting for translational and rotational displacements by making the projection of the recorded motor unit action potentials is first introduced. Then, this concept was tested for the performed measurements attempting the automated matching of the similar motor unit action potentials across different trials. RESULTS: The ability to perform automated correction (projection) of the isolated motor unit action potentials was first shown using large angular displacements. Then, for accidental (small) displacements of the recording grid, the ability to automatically track motor units across different measurement trials was shown. It was possible to track 10 -15% of identified motor units. CONCLUSIONS: This proof of concept study demonstrates an automated correction allowing the identification of an increased number of same motor unit action potentials across different measurements. By this, great potential is demonstrated for assisting motor unit tracking studies, indicating that otherwise electrode displacements cannot always be precisely described.

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.

Decreased excitability of the distal motor nerve of young patients with type 1 diabetes mellitus.

OBJECTIVE: The compound muscle action potential (CMAP) scan is a novel neurophysiological technique that appears more sensitive in detecting peripheral motor neuropathy than conventional methods. This study explores the value of the CMAP scan for the detection of subclinical diabetic peripheral motor neuropathy. METHODS: In this cross-sectional pilot study, CMAP scanning of the peroneal nerve was performed in (i) 13 well-controlled patients (8-25 yr old) with type 1 diabetes mellitus (T1DM) duration between 2.5 and 5 yr; (ii) 17 patients (10-25 yr old) with a duration of T1DM of at least 10 yr, poorly controlled and/or with microvascular complications and (iii) 13 adults with T1DM and established clinical diabetic peripheral neuropathy (DPN). Various CMAP scan variables, including measures of axonal excitability and axonal loss and reinnervation, were compared between patients and healthy controls. RESULTS: Axonal excitability was significantly decreased in the young patient groups as compared to their controls. The CMAP scan measures of axonal loss and reinnervation differed only between patients with clinical DPN and their controls. CONCLUSIONS: Motor nerve axonal excitability seems to be reduced early in T1DM, even in well-controlled young patients, and probably before (irreversible) axonal damage occurs. These changes can be measured by the CMAP scan, which makes this a promising tool for detecting nerve dysfunction in T1DM.

Ultrasound-guided needle positioning near the sciatic nerve to elicit compound muscle action potentials from the gastrocnemius muscle of the rat.

The use of ultrasound-guided electrode positioning in near-nerve myography was investigated. This is a minimally invasive technique that allows repeated measurements to increase accuracy and hence decreases animal numbers. Ultrasound imaging of the sciatic nerve was performed in nine rats using a 55 MHz high-end transducer. Once visualised, a monopolar needle electrode was placed through the skin near this nerve. Upon stimulation, two surface electrodes, placed over the gastrocnemius muscle, recorded compound muscle action potentials (CMAPs). Reproducibility was tested having two teams of investigators perform the recordings consecutively. Reliability of the procedure was determined by comparing the ultrasound method to the conventional technique, which requires an incision through muscle and skin to expose the sciatic nerve. In all animals the sciatic nerve was visible on ultrasound images. Both methods showed CMAP latencies (duration was determined as the time interval between the onset latency and positive peak). The conventional method had a mean latency of 3.4±0.5 ms, our method had a mean latency of 3.3±0.5 ms. Reproducibility was excellent (observed latencies and amplitudes: 3.3 versus 3.3 ms and 25.6±5.1 mV versus 22.5±8.8 mV) resulting in a coefficient of variation for duration of 2.1% and for amplitude 6.7%. Interclass correlation coefficient was 0.828 for duration. Comparing the three different measurements no significant differences were found and our new method can therefore be considered reliable and comparable to the conventional method. Ultrasound-guided near-nerve needle positioning is a reproducible and reliable minimally invasive method for selectively eliciting CMAPs, which allows repeated CMAP measurements for studying nerve regeneration in rats.

Time varying neonatal seizure localization.

BACKGROUND: A common cause for damage to the neonatal brain is a shortage in the oxygen supply to the brain or asphyxia. Neonatal seizures are the most frequent manifestation of neonatal neurologic disorders. Multichannel EEG recordings allow topographic localization of seizure foci. OBJECTIVES: We want to objectively determine the spatial distribution of the seizure on the scalp, the location in time and order the dominant sources in the brain based on their strength. METHODS: In this paper we combine a method based on higher order CP-decomposition with subsequent singular value decomposition (SVD). RESULTS: We illustrate the abilities of the method on simulated as well as on real neonatal seizure EEG. CONCLUSIONS: The proposed method provides reliable time and spatial information about the seizure, gives a clear overview of what is going on in the EEG and allows easy interpretation.

Size does matter: the influence of motor unit potential size on statistical motor unit number estimates in healthy subjects.

OBJECTIVE: The statistical method of motor unit number estimation (MUNE) assumes that all motor unit potentials (MUPs) have the same size. The present study aims to evaluate the consequences of this assumption as well as its implications for the validity of statistical MUNEs. METHODS: We performed statistical and multiple point stimulation (MPS) MUNE with an array of 120 electrodes on the thenar muscles of 15 healthy subjects. These recordings allow isolation and quantification of the effect of non-uniform MUP size on MUNE, because the differences in submaximal CMAP size (and, hence, in MUNE) between electrodes are due almost entirely to differences in (summed) MUP size. RESULTS: We found no correlation between statistical and MPS MUNEs. Statistical MUNEs proved very sensitive to small variations in the "bandwidth" (variance) of the response series; MUNEs from electrodes only 8mm apart could deviate by as much as 60%. This variation in bandwidth resulted from spatial (and, hence, size) differences between the contributing MUPs. CONCLUSIONS: Statistical MUNEs are very sensitive to violation of the uniform MUP-size assumption, to an extent that blurs any correlation with MPS MUNE in healthy subjects. SIGNIFICANCE: Statistical MUNE cannot be used to detect mild to moderate motor unit losses.

Neonatal seizure localization using PARAFAC decomposition.

OBJECTIVE: The description and evaluation of two EEG-based algorithms for automatic and objective determination of the seizure location in the neonatal brain as it is reflected on the scalp. METHODS: Each algorithm extracts the electrical potential distribution of the seizure over the scalp using the higher-order canonical decomposition or Parallel Factor Analysis (PARAFAC), also referred to as the CP model. This model decomposes a tensor in a sum of rank-1 components. The two algorithms differ in the way the tensor is constructed and in the type of activity they are able to extract. While the first method extracts oscillatory seizure activity, the second extracts spike train activity. RESULTS: We compared the seizure localization results of 21 seizures from 6 neonates with post-asphyxial hypoxic ischemic encephalopathy, with that based on the visual analysis of the EEG by a clinical neurophysiologist. There was a good agreement between the two methods in the localization of seizure onset in all. CONCLUSION: The techniques presented in this paper are robust, objective methods to determine neonatal seizure localization. They can be a useful tool for neonatal EEG analysis and for continuous brain function monitoring. SIGNIFICANCE: The proposed algorithms significantly improve neonatal seizure localization and monitoring.

Automated neonatal seizure detection mimicking a human observer reading EEG.

OBJECTIVE: The description and evaluation of a novel patient-independent seizure detection for the EEG of the newborn term infant. METHODS: We identified characteristics of neonatal seizures by which a human observer is able to detect them. Neonatal seizures were divided into two types. For each type, a fully automated detection algorithm was developed based on the identified human observer characteristics. The first algorithm analyzes the correlation between high-energetic segments of the EEG. The second detects increases in low-frequency activity (<8 Hz) with high autocorrelation. RESULTS: The complete algorithm was tested on multi-channel EEG recordings of 21 patients with and 5 patients without electrographic seizures, totaling 217 h of EEG. Sensitivity of the combined algorithms was found to be 88%, Positive Predictive Value (PPV) 75% and the false positive rate 0.66 per hour. CONCLUSIONS: Our approach to separate neonatal seizures into two types yields a high sensitivity combined with a good PPV and much lower false positive rate than previously published algorithms. SIGNIFICANCE: The proposed algorithm significantly improves neonatal seizure detection and monitoring.

Heart rate changes are insensitive for detecting postasphyxial seizures in neonates.

We studied heart rate (HR) changes during 169 seizures (mean 12 per patient, range 8 to 18) in 14 neonates with severe birth asphyxia. HR changes were found in 21 seizures (12.4%) in eight patients (HR increases in four, decreases in one, and both patterns in three patients), suggesting the existence of neonatal cerebral hemispheric connections with brainstem autonomic regulatory centers. HR monitoring appears to be insensitive for detecting postasphyxial neonatal seizures.

Three-layer volume conductor model and software package for applications in surface electromyography.

On comparing multichannel surface electromyographic measurements of the m. biceps brachii with simulations performed with a previously developed two-layer volume conduction model, we found substantial discrepancies. To incorporate an apparent distorting effect of the skin tissue, the model was extended to three layers. This new model describes the potential resulting from an eccentric bioelectric source in a finite, cylindrical, and anisotropic volume conductor consisting of three layers, representing muscle, subcutaneous fat, and skin tissue. This contribution presents the governing mathematical equations of the three-layer volume conductor model as well as our approach to their solution. A comparison of various models shows that the three-layer model best describes measured potential distributions. Furthermore, we present the "ANVOLCON" (analytical volume conductor) software package. This package has been developed to facilitate the use of the model for scientific and educational purposes and is freely available from http://www.mbfys.kun.nl/knf

Propagation disturbance of motor unit action potentials during transient paresis in generalized myotonia: a high-density surface EMG study.

Patients with autosomal recessive generalized myotonia, or Becker's disease, often suffer from a peculiar transient paresis. As yet, the relationship between this transient paresis and the defect in the gene encoding for a voltage gated Cl- channel protein in the muscle membrane of these patients is unclear. In order to gain a better understanding of the electrophysiological properties of the muscle fibre membrane in these generalized myotonia patients, we have studied transient paresis with a novel high-density surface EMG (sEMG) technique. We conclude that the transient paresis is explained by a deteriorating muscle membrane function, ending in conduction block and paresis. Multi-channel sEMG during the period of force decline in transient paresis shows a decrease in peak-peak amplitude of the motor unit action potentials from endplate towards tendon. This disturbance increases with time and place, indicating a deteriorating membrane function, and ends in a complete blocking of propagation within seconds. Spatiotemporally, this leads to a V-shaped sEMG pattern. In a more general sense, this contribution shows how spatiotemporal information, available through non-invasive high-density sEMG, may provide novel insights into electrophysiological aspects of membrane dysfunction.

Surface EMG models: properties and applications.

After a general introduction on the kind of models and the use of models in the natural sciences, the main body of this paper reviews potential properties of structure based surface EMG (sEMG) models. The specific peculiarities of the categories (i) source description, (ii) motor unit structure, (iii) volume conduction, (iv) recording configurations and (v) recruitment and firing behaviour are discussed. For a specific goal, not all aspects conceivable have to be part of a model description. Therefore, finally an attempt is made to integrate the 'question level' and the 'model property level' in a matrix providing direction to the development and application of sEMG models with different characteristics and varying complexity. From this overview it appears that the least complex are models describing how the morphological muscle features are reflected in multi-channel EMG measurements. The most challenging questions in terms of model complexity are related to supporting the diagnosis of neuromuscular disorders.

Magnetic stimulation-induced modulations of motor unit firings extracted from multi-channel surface EMG.

The noninvasive assessment of motor unit (MU) firing patterns on the basis of topographical information from 128-channel high-density surface electromyography (SEMG) is reported. First, multi-channel MU action potential (MUAP) templates are obtained by clustering detected firing events according to the surface topography of the MUAP. Second, a template-matching algorithm is used to find all firings of a MU, including the superimpositions of MUAPs. From a single recording, the firing pattern of up to five MUs could be derived. The modulation of MU firing by transcranial magnetic stimulation was analyzed in peri-stimulus time histograms. The results are similar to previous results of transcranial magnetic stimulation (TMS) obtained by needle electromyographic (EMG) recordings. The method can be used to investigate MU firing patterns in patients with central motor disorders. An additional advantage of the technique, apart from its noninvasiveness, is the structural and functional information that it provides on the MUs, which is not obtained by needle EMG.

Volume conduction models for surface EMG; confrontation with measurements.

Volume conduction models are used to describe and explain recorded motor unit potentials (MUPs). So far it has remained unclear which factors have to be taken into account in a volume conduction model. In the present study, five different models are confronted with measured MUP distributions over the skin surface above the m. biceps brachii generated by MUs at different depths and recorded by small surface electrodes. All model simulations include fibres of finite length. The models differ in the size of the volume conductor (finite/infinite), the number of different layers (1, 2 or 3) and the conductivities of these layers (representing muscle, subcutaneous fat and skin). All measured and simulated MUPs contain a mainly negative propagating wave followed by a positive wave simultaneously present at all electrode positions. The magnitude of the different MUP components relative to each other and as a function of motor unit (MU) and electrode position differ between the models studied and the measurements. All simulated MUPs changed faster with observation distance than the measured MUPs. The three-layer model, in which muscle tissue was surrounded by a subcutaneous fat layer and by a layer of skin resulted in MUPs closest to the measured MUPs.

The incidence of sudden infant death syndrome in North Carolina's cities and counties: 1972--1974.

Between January 1, 1972 and December 31, 1974, 534 Sudden Infant Death Syndrome cases were reported in North Carolina. All but the out-of-state cases were mapped by county and city locations to determine if urban or rural cases predominated. The mapping was also undertaken to see if significant spatial variabilities could be detected between the county and city populations of infants at risk. The state had an overall SIDS rate of 2.06 per thousand live births. The mapping revealed that counties had a range from zero to a high of 6.6 and that cities with populations of over 10,000 had SIDS rates which ranged from zero to a high of 10.6. The proportions of SIDS cases occurring in either urban or rural locations roughly approximated the distribution of the state's population, with neither location accounting for disproportionately more cases. The larger cities, however, reported more cases than did their suburbs and the immediately surrounding rural areas. The largest and smallest cities, when grouped accordingly, had the lowest urban SIDS rates. The summary SIDS rates for whites was 1.23 per thousant live births, for blacks it was 3.75, and for Indians it was 6.56 per thousand live births.