Electrical impedance myography in spinal muscular atrophy: a longitudinal study

Electrical impedance myography in healthy volunteers

INTRODUCTION/AIMS

Electrical impedance myography (EIM) is a noninvasive technique being used in clinical studies to characterize muscle by phase, reactance, and resistance after application of a low-intensity current. The aim of this study was to obtain 50-kHz EIM data from healthy volunteers (HVs) for use in future clinical and research studies, perform reliability tests on EIM outcome measures, and compare findings with muscle ultrasound variables.

METHODS

Four arm and four leg muscles of HVs were evaluated using an EIM device with two sensors, P/N 20-0045 and P/N 014-009. Muscles were evaluated individually and eight-muscle average (8MU), four-muscle upper extremity average, and four-muscle lower extremity average. An intraclass correlation coefficient (ICC) was applied to assess interrater, intrarater, and intersensor reliability using a subset of HVs. Ultrasound studies on muscle thickness and elastography were also performed on a subset of HVs.

RESULTS

For the P/N 20-0045 sensor, the 8MU EIM mean and standard deviation (n = 41) was 14.54 ± 3.31 for phase, 7.04 ± 1.22 for reactance, and 28.91 ± 7.63 for resistance. Reliability for 8MU phase (n = 22) was good to excellent for both interrater (n = 22, ICC = 0.920, 95% CI 0.820 to 0.966) and intrarater (n = 22, ICC = 0.950, 95% CI 0.778 to 0.983). The P/N 014-009 sensor had similar reliability findings. Correlation analyses showed no association between EIM and muscle thickness.

DISCUSSION

EIM is a reproducible measure of muscle physiology. Obtaining EIM values from HVs allows us to gain a better understanding how EIM may be altered in diseased muscle.

Correlations between clinical motor scores and CMAP in patients with type 2 spinal muscular amyotrophy treated with nusinersen

BACKGROUND

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by the degeneration of the anterior horn cells of the spinal cord. Nusinersen for the treatment of SMA has been covered by public healthcare in France since May 2017.

OBJECTIVE

Our aim was to investigate whether there is a correlation between clinical and compound motor action potential (CMAP) measurements in SMA patients treated with nusinersen after 3  years' follow-up.

METHOD

Motor skills were evaluated regularly between M0 and M36 using the Motor Function Measure (MFM) score. CMAP measurements were collected regularly between M0 and M22.

RESULTS

Data for 10 patients with SMA type 2 were collected and divided into two age groups (< 5 years and > 5 years). Motor function improved, but not significantly, regarding distal motor skills (D3) in both groups, and in axial and proximal motor function (D2) in the younger group. CMAP measurements improved in all patients. CMAP increased significantly for the median nerve, and this improvement correlated significantly with global MFM and with axial and proximal tone (D2).

CONCLUSION

Our study shows gain in distal motor function with nusinersen, especially in younger patients with SMA type 2. These results encourage the screening of SMA patients and treatment as early as possible. CMAP measurements of the median nerve show clear improvement in patients treated with nusinersen and could be performed as routine follow-up.

Electrical Impedance Myography Correlates with Functional Measures of Disease Progression in D2-mdx Mice and Boys with Duchenne Muscular Dystrophy

BACKGROUND

Sensitive, objective, and longitudinal outcome measures applicable to both pre-clinical and clinical interventions are needed to assess muscle health in Duchenne muscular dystrophy (DMD). Electrical impedance myography (EIM) has the potential to non-invasively measure disease progression in mice and boys with DMD.

OBJECTIVE

We sought to evaluate how electrical impedance values (i.e., phase, reactance, and resistance) correlate to established measures of disease in both D2-mdx and wild type (WT) mice and boys with and without DMD.

METHODS

Histological, functional, and EIM data collected from previous studies of WT and D2-mdx mice at 6, 13, 21 and 43 weeks of age were reanalyzed. In parallel, previously collected functional outcome measures and EIM values were reanalyzed from boys with and without DMD at four different age groups from 2 to 14 years old.

RESULTS

In mice, disease progression as detected by histological, functional, and EIM measures, was appreciable over this time period and grip strength best correlated to longitudinal phase and reactance impedance values. In boys, disease progression quantified through commonly utilized functional outcome measures was significant and longitudinal phase demonstrated the strongest correlation with functional outcome measures.

CONCLUSION

Similar changes in EIM values, specifically in longitudinal reactance and phase, were found to show significant correlations to functional measures in both mice and boys. Thus, EIM demonstrates applicability in both pre-clinical and clinical settings and can be used as a safe, non-invasive, and longitudinal proxy biomarker to assess muscle health in DMD.

Identifying Biomarkers of Spinal Muscular Atrophy for Further Development

BACKGROUND

Spinal muscular atrophy (SMA) is caused by bi-allelic, recessive mutations of the survival motor neuron 1 (SMN1) gene and reduced expression levels of the survival motor neuron (SMN) protein. Degeneration of alpha motor neurons in the spinal cord causes progressive skeletal muscle weakness. The wide range of disease severities, variable rates of decline, and heterogenous clinical responses to approved disease-modifying treatment remain poorly understood and limit the ability to optimize treatment for patients. Validation of a reliable biomarker(s) with the potential to support early diagnosis, inform disease prognosis and therapeutic suitability, and/or confirm response to treatment(s) represents a significant unmet need in SMA.

OBJECTIVES

The SMA Multidisciplinary Biomarkers Working Group, comprising 11 experts in a variety of relevant fields, sought to determine the most promising candidate biomarker currently available, determine key knowledge gaps, and recommend next steps toward validating that biomarker for SMA.

METHODS

The Working Group engaged in a modified Delphi process to answer questions about candidate SMA biomarkers. Members participated in six rounds of reiterative surveys that were designed to build upon previous discussions.

RESULTS

The Working Group reached a consensus that neurofilament (NF) is the candidate biomarker best poised for further development. Several important knowledge gaps were identified, and the next steps toward filling these gaps were proposed.

CONCLUSIONS

NF is a promising SMA biomarker with the potential for prognostic, predictive, and pharmacodynamic capabilities. The Working Group has identified needed information to continue efforts toward the validation of NF as a biomarker for SMA.

Using machine learning algorithms to enhance the diagnostic performance of electrical impedance myography

INTRODUCTION/AIMS

We assessed the classification performance of machine learning (ML) using multifrequency electrical impedance myography (EIM) values to improve upon diagnostic outcomes as compared to those based on a single EIM value.

METHODS

EIM data was obtained from unilateral excised gastrocnemius in eighty diseased mice (26 D2-mdx, Duchenne muscular dystrophy model, 39 SOD1G93A ALS model, and 15 db/db, a model of obesity-induced muscle atrophy) and 33 wild-type (WT) animals. We assessed the classification performance of a ML random forest algorithm incorporating all the data (multifrequency resistance, reactance and phase values) comparing it to the 50 kHz phase value alone.

RESULTS

ML outperformed the 50 kHz analysis as based on receiver-operating characteristic curves and measurement of the area under the curve (AUC). For example, comparing all diseases together versus WT from the test set outputs, the AUC was 0.52 for 50 kHz phase, but was 0.94 for the ML model. Similarly, when comparing ALS versus WT, the AUCs were 0.79 for 50 kHz phase and 0.99 for ML.

DISCUSSION

Multifrequency EIM utilizing ML improves upon classification compared to that achieved with a single-frequency value. ML approaches should be considered in all future basic and clinical diagnostic applications of EIM.

Interrater and Intrarater Reliability of Electrical Impedance Myography: A Comparison between Large and Small Handheld Electrode Arrays

The objective of this study was to evaluate the interrater and intrarater reliability of electrical impedance myography (EIM) using handheld sensors of different sizes. Electrical impedance myography of the biceps brachii muscle of twenty healthy individuals was performed by two raters using both large and small sensors. The procedures were also repeated 5 to 8 days after the first recording session. The repeatability of the resistance, reactance, and phase angle at two different current frequencies (50 and 100 kHz) was assessed by the intraclass correlation coefficient (ICC). The ICCs of the large sensor were higher than those of the small sensor for both the intrarater and interrater reliabilities. High-frequency current tended to improve the ICC for the small sensor. These results indicate reasonable repeatability of the handheld electrode arrays for EIM measurements. The findings suggest that electrode array should be selected appropriately according to the size of the tested muscle.

Electrical Impedance Myography in Health and Physical Exercise: A Systematic Review and Future Perspectives

Background: Electrical impedance myography (EIM) is a non-invasive method that provides information about muscle health and changes that occur within it. EIM is based on the analysis of three impedance variables: resistance, reactance, and the phase angle. This systematic review of the literature provides a deeper insight into the scope and range of applications of EIM in health and physical exercise. The main goal of this work was to systematically review the studies on the applications of EIM in health and physical exercise in order to summarize the current knowledge on this method and outline future perspectives in this growing area, including a proposal for a research agenda. Furthermore, some basic assessment principles are provided. Methods: Systematic literature searches on PubMed, Scopus, SPORTDiscus and Web of Science up to September 2020 were conducted on any empirical investigations using localized bioimpedance devices to perform EIM within health and physical exercise contexts. The search included healthy individuals, elite soccer players with skeletal muscle injury, and subjects with primary sarcopenia. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist was used to develop the systematic review protocol. The quality and risk of bias of the studies included were assessed with the AQUA tool. Results: Nineteen eligible original articles were included in this review, which were separated into three tables according to the nature of the study. The first table includes six studies on the bioelectrical characterization of muscle. The second table includes five studies analyzing muscle changes in injured elite soccer players. The third table includes studies on the short-, medium-, and long-term bioelectrical adaptations to physical exercise. Conclusions: EIM has been used for the evaluation of the muscle condition in the clinical field over the last few years, especially in different neuromuscular diseases. It can also play an important role in other contexts as an alternative to complex and expensive methods such as magnetic resonance imaging. However, further research is needed. The main step in establishing EIM as a valid tool in the scientific field is to standardize the protocol for performing impedance assessments.

Update on Biomarkers in Spinal Muscular Atrophy

The availability of disease modifying therapies for spinal muscular atrophy (SMA) has created an urgent need to identify clinically meaningful biomarkers. Biomarkers present a means to measure and evaluate neurological disease across time. Changes in biomarkers provide insight into disease progression and may reveal biologic, physiologic, or pharmacologic phenomena occurring prior to clinical detection. Efforts to identify biomarkers for SMA, a genetic motor neuron disease characterized by motor neuron degeneration and weakness, have culminated in a number of putative molecular and physiologic markers that evaluate biological media (eg, blood and cerebrospinal fluid [CSF]) or nervous system function. Such biomarkers include SMN2 copy number, SMN mRNA and protein levels, neurofilament proteins (NFs), plasma protein analytes, creatine kinase (CK) and creatinine (Crn), and various electrophysiology and imaging measures. SMN2 copy number inversely correlates with disease severity and is the best predictor of clinical outcome in untreated individuals. SMN mRNA and protein are commonly measured in the blood or CSF of patients receiving SMA therapies, particularly those aimed at increasing SMN protein expression, and provide insight into current disease state. NFs have proven to be robust prognostic, disease progression, and pharmacodynamic markers for SMA infants undergoing treatment, but less so for adolescents and adults. Select plasma proteins are altered in SMA individuals and may track response to therapy. CK and Crn from blood correlate with motor function and disease severity status and are useful for predicting which individuals will respond to therapy. Electrophysiology measures comprise the most reliable means for monitoring motor function throughout disease course and are sensitive enough to detect neuromuscular changes before overt clinical manifestation, making them robust predictive and pharmacodynamic biomarkers. Finally, magnetic resonance imaging and muscle ultrasonography are non-invasive techniques for studying muscle structure and physiology and are useful diagnostic tools, but cannot reliably track disease progression. Importantly, biomarkers can provide information about the underlying mechanisms of disease as well as reveal subclinical disease progression, allowing for more appropriate timing and dosing of therapy for individuals with SMA. Recent therapeutic advancements in SMA have shown promising results, though there is still a great need to identify and understand the impact of biomarkers in modulating disease onset and progression.

Estimating myofiber cross-sectional area and connective tissue deposition with electrical impedance myography: A study in D2-mdx mice

INTRODUCTION

Surface electrical impedance myography (sEIM) has the potential for providing information on muscle composition and structure noninvasively. We sought to evaluate its use to predict myofiber size and connective tissue deposition in the D2-mdx model of Duchenne muscular dystrophy (DMD).

METHODS

We applied a prediction algorithm, the least absolute shrinkage and selection operator, to select specific EIM measurements obtained with surface and ex vivo EIM data from D2-mdx and wild-type (WT) mice (analyzed together or separately). We assessed myofiber cross-sectional area histologically and hydroxyproline (HP), a surrogate measure for connective tissue content, biochemically.

RESULTS

Using WT and D2-mdx impedance values together in the algorithm, sEIM gave average root-mean-square errors (RMSEs) of 26.6% for CSA and 45.8% for HP, which translate into mean errors of ±363 μm2 for a mean CSA of 1365 μm2 and of ±1.44 μg HP/mg muscle for a mean HP content of 3.15 μg HP/mg muscle. Stronger predictions were obtained by analyzing sEIM data from D2-mdx animals alone (RMSEs of 15.3% for CSA and 34.1% for HP content). Predictions made using ex vivo EIM data from D2-mdx animals alone were nearly equivalent to those obtained with sEIM data (RMSE of 16.59% for CSA), and slightly more accurate for HP (RMSE of 26.7%).

DISCUSSION

Surface EIM combined with a predictive algorithm can provide estimates of muscle pathology comparable to values obtained using ex vivo EIM, and can be used as a surrogate measure of disease severity and progression and response to therapy.

Intra- and inter-rater reliability of electrical impedance myography using adhesive electrodes in healthy volunteers

In spite of the growing use of the electrical impedance myography (EIM) measures for clinical assessment and follow-up of diseased muscle tissue, reliability studies are scarce. We evaluate the reliability of the (EIM) technique using four adhesive electrodes over the muscle of interest. Intra- and inter-rater reliability was studied within the same session and between sessions. Thirty-one healthy and volunteer subjects aged between 20 and 26 years were recruited. Phase angle, reactance and resistance were assessed for each EIM measurement. Intraclass correlation coefficient (ICC) was used to determine the relative reliability. Absolute reliability was expressed as the standard error of measurement and the minimum detectable change. Relative reliability within the same session and between sessions for the EIM technique was excellent (ICCs > 0.9) concerning both intra- and inter-rater reliability, except for the component reactance. The absolute reliability was very high for the three EIM components. EIM measures using four adhesive electrodes over the area of interest is a reliable technique to assess muscle tissue status. This study confirms that these measurement results barely vary depending on the examiner and the moment. The present study also confirms phase angle as the least affected EIM component by examiner and evaluation moment.

Electrical impedance myography as a biomarker of inclusion body myositis: A cross-sectional study

OBJECTIVE

To assess the value of electrical impedance myography (EIM) in inclusion body myositis (IBM).

METHODS

Patients with clinically defined IBM and healthy controls (HC) of similar age group were recruited. Each participant underwent manual muscle testing (MMT), 6-min walk test (6MWT), handgrip dynamometry, and IBM-functional rating scale assessment (IBM-FRS). EIM measurements were obtained from bilateral deltoid, biceps, forearm-flexors, quadriceps, tibialis anterior, and medial gastrocnemius.

RESULTS

Fourteen IBM patients and 12 HCs with mean age 68.6 ± 6 and 67.4 ± 5.4 years were included in the final analysis. Averaged phase value at 50 kHz (EIM50) and ratio of phase value at 50 kHz/200 kHz (EIMPR) from six-muscles were significantly lower in IBM patients when compared to HC (5.23 ± 1.34 vs 7.88 ± 1.9, p-value 0.002, and 0.55 ± 0.09 vs. 0.68 ± 0.09, p-value 0.004, respectively). A strong correlation was noted between IBM-FRS, 6MWT, disease-duration and the averaged value of EIM50 and EIMPR in the IBM patients (Spearman |rho|>0.7, p-values < 0.01).

CONCLUSIONS

EIM can differentiate between IBM patients and HCs and EIM parameters correlate with clinical outcome measures.

SIGNIFICANCE

EIM may be a potential objective biomarker for IBM. A longitudinal validation study is warranted.

Electrical Impedance Methods in Neuromuscular Assessment: An Overview

Electrical impedance methods have been used as evaluation tools in biological and medical science for well over 100 years. However, only recently have these techniques been applied specifically to the evaluation of conditions affecting nerve and muscle. This specific application, termed electrical impedance myography (EIM), is finding wide application as it can provide a quantitative index of muscle condition that can assist with diagnosis, track disease progression, and assess the beneficial impact of therapy. Using noninvasive surface methods, EIM has been studied in a number of conditions ranging from amyotrophic lateral sclerosis to muscular dystrophy to disuse atrophy. Data support that the technique is sensitive to disease status and can offer the possibility of performing clinical trials with fewer subjects than would otherwise be possible. Recent advances in the field include improved approaches for using EIM as a "virtual biopsy" and the development of combined needle impedance-electromyography technology.

Biomarkers and the Development of a Personalized Medicine Approach in Spinal Muscular Atrophy

Recent unprecedented advances in treatment for spinal muscular atrophy (SMA) enabled patients to access the first approved disease modifying therapy for the condition. There are however many uncertainties, regarding timing of treatment initiation, response to intervention, treatment effects and long-term outcomes, which are complicated by the evolving phenotypes seen in the post-treatment era for patients with SMA. Biomarkers of disease, with diagnostic, prognostic, predictive, and pharmacodynamic value are thus urgently required, to facilitate a wider understanding in this dynamic landscape. A spectrum of these candidate biomarkers, will be evaluated in this review, including genetic, epigenetic, proteomic, electrophysiological, and imaging measures. Of these, SMN2 appears to be the most significant modifier of phenotype to date, and its use in prognostication shows considerable clinical utility. Longitudinal studies in patients with SMA highlight an emerging role of circulatory markers such as neurofilament, in tracking disease progression and response to treatment. Furthermore, neurophysiological biomarkers such as CMAP and MUNE values show considerable promise in the real word setting, in following the dynamic response and output of the motor unit to therapeutic intervention. The specific value for these possible biomarkers across diagnosis, prognosis, prediction of treatment response, efficacy, and safety will be central to guide future patient-targeted treatments, the design of clinical trials, and understanding of the pathophysiological mechanisms of disease and intervention.

Time-course bicep tissue bio-impedance changes throughout a fatiguing exercise protocol

This study investigated the localized electrical-impedance changes in the biceps tissues throughout a fatiguing exercise protocol. During the protocol, 17 subjects performed 10 sets of bicep curl repetitions at either 60% or 75% of their one-repetition maximum weight until task failure. The localized tissue impedance (resistance, reactance, phase angle) was measured at 10 kHz, 50 kHz, and 100 kHz immediately after each of 10 sets for comparison against the baseline pre-fatigue measures. A trend of decreasing resistance and reactance magnitude were observed, with greater changes occurring as the protocol progressed. Statistical testing demonstrated statistically significant changes in resistance, reactance, and phase angle for both groups of participants. The significant changes in resistance were observed at earlier time-points than the reactance and phase angle changes for both groups.

Functional Biomarkers for Amyotrophic Lateral Sclerosis

The clinical diagnosis of amyotrophic lateral sclerosis (ALS) relies on determination of progressive dysfunction of both cortical as well as spinal and bulbar motor neurons. However, the variable mix of upper and lower motor neuron signs result in the clinical heterogeneity of patients with ALS, resulting frequently in delay of diagnosis as well as difficulty in monitoring disease progression and treatment outcomes particularly in a clinical trial setting. As such, the present review provides an overview of recently developed novel non-invasive electrophysiological techniques that may serve as biomarkers to assess UMN and LMN dysfunction in ALS patients.

Electrical impedance myography in facioscapulohumeral muscular dystrophy: A 1-year follow-up study

INTRODUCTION

Electrical impedance myography (EIM) is a noninvasive technique for measuring muscle composition and a potential physiological biomarker for facioscapulohumeral muscular dystrophy (FSHD).

METHODS

Thirty-two participants with genetically confirmed and clinically affected FSHD underwent EIM in 7 muscles bilaterally. Correlations between EIM and baseline clinical measures were used to select EIM variables of interest in FSHD, and EIM and clinical measures were followed for 1 year.

RESULTS

There were no significant changes in the EIM variables. Although 50-kHZ reactance correlated the strongest with clinical measures at baseline, the 50-211-kHZ phase ratio demonstrated lower within-subject 12-month variability, potentially offering sample size savings for FSHD clinical trial planning.

DISCUSSION

EIM did not identify significant disease progression over 12 months. It is currently unclear whether this is because of limitations of the technology or the slow rate of disease progression in this cohort of FSHD patients over this period of time. Muscle Nerve 58: 213-218, 2018.

Electrical impedance myography as a biomarker of myostatin inhibition with ActRIIB-mFc: a study in wild-type mice

Aim:

We sought to determine the sensitivity of electrical impedance myography (EIM) to myofiber hypertrophy induced by treatment with various doses of ActRIIB-mFc, an inhibitor of myostatin signaling.

Methods:

Wild-type C57BL/6 J mice (n = 40, male) were treated with three different doses of ActRIIB-mFc (i.e., RAP-031) or vehicle twice weekly for 5 weeks. End point assessments included gastrocnemius EIM, force measurements, muscle mass and myofiber size quantification.

Results:

ActRIIB-mFc increased body mass, muscle mass and myofiber size across all doses. Alterations in EIM 50 kHz phase and center frequency (fc) were also present, with trends in a dose-dependent fashion. Significant correlations between EIM parameters and myofiber/functional data were identified.

Conclusion:

EIM outcomes can serve as effective biomarkers of myostatin signaling inhibition, demonstrating a dose sensitivity and correlation to standard assessments.

We were interested in studying the sensitivity of a technique, called electrical impedance myography (EIM), to noninvasively assess the size of muscle fibers. In this technique a minute electrical current is used to probe the tissue. To do so, we gave a drug (ActRIIB-mFc) to mice that enlarges muscle fibers at three different doses. We were able to show that the EIM technique was able to detect this differential effect and functional changes induced by the drug correlated to the EIM data. This work suggests that EIM will be useful as a noninvasive marker muscle health.

Electrical impedance myography detects age-related muscle change in mice

Loss of muscle mass and strength represents one of the most significant contributors to impaired function in older adults. Convenient and non-invasive biomarkers are needed that can readily identify and track age-related muscle change. Previous data has suggested electrical impedance myography (EIM) has the potential to serve in this capacity. In this study we investigated how changes in EIM compared with other standard measures of muscle structure and function in aged compared with young mice. A total of 19 male mice aged approximately 25 months and 19 male mice aged 3 months underwent surface multifrequency EIM of the right gastrocnemius muscle using standard methods. Fore and hind limb grip strength, sciatic compound muscle action potential amplitude, and in-situ force of the gastrocnemius were also measured; after sacrifice, gastrocnemius myofiber size was assessed using standard histology. Spearman correlation coefficients were calculated to investigate the association between EIM and muscle characteristics. EIM in aged animals demonstrated significantly lower 50 kHz impedance phase (p<0.001) and reactance (p<0.01) values as well as reduced multifrequency parameters. In contrast, absolute gastrocnemius muscle mass was no different between young and aged mice (p = 0.58) but was reduced in aged mice after normalization to body mass (p<0.001). Median myofiber size in the aged mice was not different from that of young mice (p = 0.72). Aged mice showed reduced muscle function on the basis of normalized fore limb (p<0.001) and normalized hind limb (p<0.001) grip strength, as well as normalized gastrocnemius twitch (p<0.001) and normalized maximal isometric force (p<0.001). Sciatic compound muscle action potential amplitude was reduced in aged mice (p<0.05). EIM parameters showed good correlation with reduced standard physiological and electrophysiological measures of muscle health. Our study suggests that EIM is sensitive to aged-related muscle change and may represent a convenient and valuable method of quantifying loss of muscle health.

Alterations in Localized Electrical Impedance Myography of Biceps Brachii Muscles Paralyzed by Spinal Cord Injury

This study assessed electrical impedance myography (EIM) changes after spinal cord injury (SCI) with a localized multifrequency technology. The EIM measurement was performed on the biceps brachii muscle at rest condition of 17 cervical SCI subjects, and 23 neurologically intact subjects as control group. The results showed that there was a significant decrease in muscle reactance (X) and phase angle (θ) at selected frequencies (i.e., 50 and 100 kHz) in SCI compared to control. There was no significant difference in muscle resistance (R) between the two groups. The anisotropy examination revealed that SCI group had a decreased anisotropy ratio in resistance. In addition, the multifrequency spectrum analysis showed a decreased slope of the log(freq)-resistance regression in SCI group when compared to healthy control. Findings of the EIM changes are related to inherit muscle changes after the injury. Since EIM requires no patient effort and is quick and convenient to conduct, it may provide a useful tool for examination of paralyzed muscle changes after SCI.

Electrical impedance myography for assessment of Duchenne muscular dystrophy

OBJECTIVE

Sensitive, objective, and easily applied methods for evaluating disease progression and response to therapy are needed for clinical trials in Duchenne muscular dystrophy (DMD). In this study, we evaluated whether electrical impedance myography (EIM) could serve this purpose.

METHODS

In this nonblinded study, 36 boys with DMD and 29 age-similar healthy boys underwent multifrequency EIM measurements for up to 2 years on 6 muscles unilaterally along with functional assessments. A linear mixed-effects model with random intercept and slope terms was used for the analysis of multifrequency EIM values and functional measures. Seven DMD boys were initiated on corticosteroids; these data were analyzed using a piecewise linear mixed-effects model.

RESULTS

In boys > 7.0 years old, a significant difference in the slope of EIM phase ratio trajectories in the upper extremity was observed by 6 months of -0.074/month, p = 0.023, 95% confidence interval (CI) = -0.013, -0.14; at 2 years, this difference was -0.048/month, p < 0.0001, 95% CI = -0.028, -0.068. In boys ≤ 7.0 years old, differences appeared at 6 months in gastrocnemius (EIM phase slope = -0.83 °/kHz/mo, p = 0.007, 95% CI = -0.26, -1.40). EIM outcomes showed significant differences earlier than functional tests. Initiation of corticosteroids significantly improved the slope of EIM phase ratio (0.057/mo, p = 0.00019, 95% CI = 0.028, 0.086) and EIM phase slope (0.14 °/kHz/mo, p = 0.013, 95% CI = 0.028, 0.25), consistent with corticosteroids' known clinical benefit.

INTERPRETATION

EIM detects deterioration in muscles of both younger and older boys by 6 months; it also identifies the therapeutic effect of corticosteroid initiation. Because EIM is rapid to apply, painless, and requires minimal operator training, the technique deserves to be further evaluated as a biomarker in DMD clinical therapeutic trials. Ann Neurol 2017;81:622-632.

Electrical impedance myography in individuals with collagen 6 and laminin α-2 congenital muscular dystrophy: a cross-sectional and 2-year analysis

INTRODUCTION

Electrical impedance myography (EIM) is a noninvasive electrophysiological technique that characterizes muscle properties through bioimpedance. We compared EIM measurements to function, strength, and disease severity in a population with congenital muscular dystrophy (CMD).

METHODS

Forty-one patients with CMD, either collagen 6 related disorders (COL6-RD; n = 21) or laminin α-2-related disorders (LAMA2-RD; n = 20), and 21 healthy pediatric controls underwent 2 yearly EIM exams. In the CMD cohorts, EIM was compared with functional and strength measurements.

RESULTS

Both CMD cohorts exhibited change over time and had correlation with disease severity. The 50-kHZ phase correlated well with function and strength in the COL6-RD cohort but not in the LAMA2-RD cohort.

DISCUSSION

EIM is a potentially useful measure in clinical studies with CMD because of its sensitivity to change over a 1-year period and correlation with disease severity. For COL6-RD, there were also functional and strength correlations. Muscle Nerve 57: 54-60, 2018.

Non-invasive evaluation of muscle disease in the canine model of Duchenne muscular dystrophy by electrical impedance myography

Dystrophin-deficient dogs are by far the best available large animal models for Duchenne muscular dystrophy (DMD), the most common lethal childhood muscle degenerative disease. The use of the canine DMD model in basic disease mechanism research and translational studies will be greatly enhanced with the development of reliable outcome measures. Electrical impedance myography (EIM) is a non-invasive painless procedure that provides quantitative data relating to muscle composition and histology. EIM has been extensively used in neuromuscular disease research in both human patients and rodent models. Recent studies suggest that EIM may represent a highly reliable and convenient outcome measure in DMD patients and the mdx mouse model of DMD. To determine whether EIM can be used as a biomarker of disease severity in the canine model, we performed the assay in fourteen young (~6.6-m-old; 6 normal and 8 affected) and ten mature (~16.9-m-old; 4 normal and 6 affected) dogs of mixed background breeds. EIM was well tolerated with good inter-rater reliability. Affected dogs showed higher resistance, lower reactance and phase. The difference became more straightforward in mature dogs. Importantly, we observed a statistically significant correlation between the EIM data and muscle fibrosis. Our results suggest that EIM is a valuable objective measurement in the canine DMD model.

Electrical Impedance Myography and Its Applications in Neuromuscular Disorders

Electrical impedance myography (EIM) refers to the specific application of electrical bioimpedance techniques for the assessment of neuromuscular disorders. In EIM, a weak, high-frequency electrical current is applied to a muscle or muscle group of interest and the resulting voltages measured. Among its advantages, the technique can be used noninvasively across a variety of disorders and requires limited subject cooperation and evaluator training to obtain accurate and repeatable data. Studies in both animals and human subjects support its potential utility as a primary diagnostic tool, as well as a biomarker for clinical trial or individual patient use. This review begins by providing an overview of the current state and technological advances in electrical impedance myography and its specific application to the study of muscle. We then provide a summary of the clinical and preclinical applications of EIM for neuromuscular conditions, and conclude with an evaluation of ongoing research efforts and future developments.

The Effect of Subcutaneous Fat on Electrical Impedance Myography: Electrode Configuration and Multi-Frequency Analyses

This study investigates the impact of the subcutaneous fat layer (SFL) thickness on localized electrical impedance myography (EIM), as well as the effects of different current electrodes, varying in distance and direction, on EIM output. Twenty-three healthy subjects underwent localized multi-frequency EIM on their biceps brachii muscles with a hand-held electrode array. The EIM measurements were recorded under three different configurations: wide (or outer) longitudinal configuration 6.8 cm, narrow (or inner) longitudinal configuration 4.5 cm, and narrow transverse configuration 4.5 cm. Ultrasound was applied to measure the SFL thickness. Coefficients of determination (R²) of three EIM variables (resistance, reactance, and phase) and SFL thickness were calculated. For the longitudinal configuration, the wide distance could reduce the effects of the subcutaneous fat when compared with the narrow distance, but a significant correlation still remained for all three EIM parameters. However, there was no significant correlation between SFL thickness and reactance in the transverse configuration (R² = 0.0294, p = 0.434). Utilizing a ratio of 50kHz/100kHz phase was found to be able to help reduce the correlation with SFL thickness for all the three configurations. The findings indicate that the appropriate selection of the current electrode distance, direction and the multi-frequency phase ratio can reduce the impact of subcutaneous fat on EIM. These settings should be evaluated for future clinical studies using hand-held localized arrays to perform EIM.

Electrical impedance myography in facioscapulohumeral muscular dystrophy

INTRODUCTION

In this study we determined the reliability and validity of electrical impedance myography (EIM) in facioscapulohumeral muscular dystrophy (FSHD).

METHODS

We performed a prospective study of EIM on 16 bilateral limb and trunk muscles in 35 genetically defined and clinically affected FSHD patients (reliability testing on 18 patients). Summary scores based on body region were derived. Reactance and phase (50 and 100 kHz) were compared with measures of strength, FSHD disease severity, and functional outcomes.

RESULTS

Participants were mostly men, mean age 53.0 years, and included a full range of severity. Limb and trunk muscles showed good to excellent reliability [intraclass correlation coefficients (ICC) 0.72-0.99]. Summary scores for the arm, leg, and trunk showed excellent reliability (ICC 0.89-0.98). Reactance was the most sensitive EIM parameter to a broad range of FSHD disease metrics.

CONCLUSIONS

EIM is a reliable measure of muscle composition in FSHD that offers the possibility to serially evaluate affected muscles. Muscle Nerve 54: 696-701, 2016.

Localized Electrical Impedance Myography of the Biceps Brachii Muscle during Different Levels of Isometric Contraction and Fatigue

This study assessed changes in electrical impedance myography (EIM) at different levels of isometric muscle contraction as well as during exhaustive exercise at 60% maximum voluntary contraction (MVC) until task failure. The EIM was performed on the biceps brachii muscle of 19 healthy subjects. The results showed that there was a significant difference between the muscle resistance (R) measured during the isometric contraction and when the muscle was completely relaxed. Post hoc analysis shows that the resistance increased at higher contractions (both 60% MVC and MVC), however, there were no significant changes in muscle reactance (X) during the isometric contractions. The resistance also changed during different stages of the fatigue task and there were significant decreases from the beginning of the contraction to task failure as well as between task failure and post fatigue rest. Although our results demonstrated an increase in resistance during isometric contraction, the changes were within 10% of the baseline value. These changes might be related to the modest alterations in muscle architecture during a contraction. The decrease in resistance seen with muscle fatigue may be explained by an accumulation of metabolites in the muscle tissue.

Cervical Spinal Cord Atrophy Profile in Adult SMN1-Linked SMA

PURPOSE

The mechanisms underlying the topography of motor deficits in spinal muscular atrophy (SMA) remain unknown. We investigated the profile of spinal cord atrophy (SCA) in SMN1-linked SMA, and its correlation with the topography of muscle weakness.

MATERIALS AND METHODS

Eighteen SMN1-linked SMA patients type III/V and 18 age/gender-matched healthy volunteers were included. Patients were scored on manual muscle testing and functional scales. Spinal cord was imaged using 3T MRI system. Radial distance (RD) and cord cross-sectional area (CSA) measurements in SMA patients were compared to those in controls and correlated with strength and disability scores.

RESULTS

CSA measurements revealed a significant cord atrophy gradient mainly located between C3 and C6 vertebral levels with a SCA rate ranging from 5.4% to 23% in SMA patients compared to controls. RD was significantly lower in SMA patients compared to controls in the anterior-posterior direction with a maximum along C4 and C5 vertebral levels (p-values < 10-5). There were no correlations between atrophy measurements, strength and disability scores.

CONCLUSIONS

Spinal cord atrophy in adult SMN1-linked SMA predominates in the segments innervating the proximal muscles. Additional factors such as neuromuscular junction or intrinsic skeletal muscle defects may play a role in more complex mechanisms underlying weakness in these patients.

Baseline results of the NeuroNEXT spinal muscular atrophy infant biomarker study

OBJECTIVE

This study prospectively assessed putative promising biomarkers for use in assessing infants with spinal muscular atrophy (SMA).

METHODS

This prospective, multi-center natural history study targeted the enrollment of SMA infants and healthy control infants less than 6 months of age. Recruitment occurred at 14 centers within the NINDS National Network for Excellence in Neuroscience Clinical Trials (NeuroNEXT) Network. Infant motor function scales and putative electrophysiological, protein and molecular biomarkers were assessed at baseline and subsequent visits.

RESULTS

Enrollment began November, 2012 and ended September, 2014 with 26 SMA infants and 27 healthy infants enrolled. Baseline demographic characteristics of the SMA and control infant cohorts aligned well. Motor function as assessed by the Test for Infant Motor Performance Items (TIMPSI) and the Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) revealed significant differences between the SMA and control infants at baseline. Ulnar compound muscle action potential amplitude (CMAP) in SMA infants (1.4 ± 2.2 mV) was significantly reduced compared to controls (5.5 ± 2.0 mV). Electrical impedance myography (EIM) high-frequency reactance slope (Ohms/MHz) was significantly higher in SMA infants than controls SMA infants had lower survival motor neuron (SMN) mRNA levels in blood than controls, and several serum protein analytes were altered between cohorts.

INTERPRETATION

By the time infants were recruited and presented for the baseline visit, SMA infants had reduced motor function compared to controls. Ulnar CMAP, EIM, blood SMN mRNA levels, and serum protein analytes were able to distinguish between cohorts at the enrollment visit.

The neuromuscular impact of symptomatic SMN restoration in a mouse model of spinal muscular atrophy

BACKGROUND

Significant advances in the development of SMN-restoring therapeutics have occurred since 2010 when very effective biological treatments were reported in mouse models of spinal muscular atrophy. As these treatments are applied in human clinical trials, there is pressing need to define quantitative assessments of disease progression, treatment stratification, and therapeutic efficacy. The electrophysiological measures Compound Muscle Action Potential and Motor Unit Number Estimation are reliable measures of nerve function. In both the SMN∆7 mouse and a pig model of spinal muscular atrophy, early SMN restoration results in preservation of electrophysiological measures. Currently, clinical trials are underway in patients at post-symptomatic stages of disease progression. In this study, we present results from both early and delayed SMN restoration using clinically-relevant measures including electrical impedance myography, compound muscle action potential, and motor unit number estimation to quantify the efficacy and time-sensitivity of SMN-restoring therapy.

METHODS

SMA∆7 mice were treated via intracerebroventricular injection with antisense oligonucleotides targeting ISS-N1 to increase SMN protein from the SMN2 gene on postnatal day 2, 4, or 6 and compared with sham-treated spinal muscular atrophy and control mice. Compound muscle action potential and motor unit number estimation of the triceps surae muscles were performed at day 12, 21, and 30 by a single evaluator blinded to genotype and treatment. Similarly, electrical impedance myography was measured on the biceps femoris muscle at 12days for comparison.

RESULTS

Electrophysiological measures and electrical impedance myography detected significant differences at 12days between control and late-treated (4 or 6days) and sham-treated spinal muscular atrophy mice, but not in mice treated at 2days (p<0.01). EIM findings paralleled and correlated with compound muscle action potential and motor unit number estimation (r=0.61 and r=0.50, respectively, p<0.01). Longitudinal measures at 21 and 30days show that symptomatic therapy results in reduced motor unit number estimation associated with delayed normalization of compound muscle action potential.

CONCLUSIONS

The incomplete effect of symptomatic treatment is accurately identified by both electrophysiological measures and electrical impedance myography. There is strong correlation between these measures and with weight and righting reflex. This study predicts that measures of compound muscle action potential, motor unit number estimation, and electrical impedance myography are promising biomarkers of treatment stratification and effect for future spinal muscular atrophy trials. The ease of application and simplicity of electrical impedance myography compared with standard electrophysiological measures may be particularly valuable in future pediatric clinical trials.

Assessment of aged mdx mice by electrical impedance myography and magnetic resonance imaging

INTRODUCTION

Similar to magnetic resonance imaging (MRI), electrical impedance myography (EIM) is dependent on the presence and location of water in muscle to assess neuromuscular diseases. We compared the 2 technologies in mdx mice to better understand their relationship.

METHODS

EIM and MRI, using T2 relaxation and diffusion-weighted imaging (DWI), were performed on the gastrocnemius of 10 mdx and 10 wild-type mice. Muscle function and tissue composition measurements were compared with the EIM and MRI data.

RESULTS

EIM reactance and T2 relaxation mapping can discriminate healthy from diseased mice (P <  .001 for both), but DWI could not. Both T2 relaxation and EIM reactance also correlated closely with muscle function/composition and with each other.

CONCLUSION

Given the low cost of EIM and the simplicity of application, it may be a valuable alternative to muscle MRI in Duchenne muscular dystrophy, where simple cumulative indices of muscle health are being sought.

A comparison of three electrophysiological methods for the assessment of disease status in a mild spinal muscular atrophy mouse model

Objectives

There is a need for better, noninvasive quantitative biomarkers for assessing the rate of progression and possible response to therapy in spinal muscular atrophy (SMA). In this study, we compared three electrophysiological measures: compound muscle action potential (CMAP) amplitude, motor unit number estimate (MUNE), and electrical impedance myography (EIM) 50 kHz phase values in a mild mouse model of spinal muscular atrophy, the Smn1^c/c mouse.

Methods

Smn1^c/c mice (N = 11) and wild type (WT) animals (−/−, N = 13) were measured on average triweekly until approximately 1 year of age. Measurements included CMAP, EIM, and MUNE of the gastrocnemius muscle as well as weight and front paw grip strength. At the time of sacrifice at one year, additional analyses were performed on the animals including serum survival motor neuron (SMN) protein levels and muscle fiber size.

Results

Both EIM 50 kHz phase and CMAP showed strong differences between WT and SMA animals (repeated measures 2-way ANOVA, P<0.0001 for both) whereas MUNE did not. Both body weight and EIM showed differences in the trajectory over time (p<0.001 and p = 0.005, respectively). At the time of sacrifice at one year, EIM values correlated to motor neuron counts in the spinal cord and SMN levels across both groups of animals (r = 0.41, p = 0.047 and r = 0.57, p  = 0.003, respectively), while CMAP did not. Motor neuron number in Smn1^c/c mice was not significantly reduced compared to WT animals.

Conclusions

EIM appears sensitive to muscle status in this mild animal model of SMA. The lack of a reduction in MUNE or motor neuron number but reduced EIM and CMAP values support that much of the pathology in these animals is distal to the cell body, likely at the neuromuscular junction or the muscle itself.

Electrical impedance myography for the in vivo and ex vivo assessment of muscular dystrophy (mdx) mouse muscle

INTRODUCTION

Sensitive, non-invasive techniques are needed that can provide biomarkers of disease status and the effects of therapy in muscular dystrophy.

METHODS

We evaluated electrical impedance myography (EIM) to serve in this role by studying 2-month-old and 18-month-old mdx and wild-type (WT) animals (10 animals in each of 4 groups).

RESULTS

Marked differences were observed in EIM values between mdx and WT animals; the differences were more pronounced between the older age groups (e.g., reactance of 92.6 ± 4.3 Ω for mdx animals vs. 130 ± 4.1 Ω for WT animals, P<0.001). In addition, in vivo EIM parameters correlated significantly with the extent of connective tissue deposition in the mdx animals.

CONCLUSIONS

EIM has the potential to serve as a valuable non-invasive method for evaluating muscular dystrophy. It can be a useful biomarker to assist with therapeutic testing in both pre-clinical and clinical studies.

Cross-sectional evaluation of electrical impedance myography and quantitative ultrasound for the assessment of Duchenne muscular dystrophy in a clinical trial setting

BACKGROUND

Electrical impedance myography and quantitative ultrasound are two noninvasive, painless, and effort-independent approaches for assessing neuromuscular disease. Both techniques have potential to serve as useful biomarkers in clinical trials in Duchenne muscular dystrophy. However, their comparative sensitivity to disease status and how they relate to one another are unknown.

METHODS

We performed a cross-sectional analysis of electrical impedance myography and quantitative ultrasound in 24 healthy boys and 24 with Duchenne muscular dystrophy, aged 2 to 14 years with trained research assistants performing all measurements. Three upper and three lower extremity muscles were studied unilaterally in each child, and the data averaged for each individual.

RESULTS

Both electrical impedance myography and quantitative ultrasound differentiated healthy boys from those with Duchenne muscular dystrophy (P < 0.001 for both). Quantitative ultrasound values correlated with age in Duchenne muscular dystrophy boys (rho = 0.45; P = 0.029), whereas electrical impedance myography did not (rho = -0.31; P = 0.14). However, electrical impedance myography phase correlated with age in healthy boys (rho = 0.51; P = 0.012), whereas quantitative ultrasound did not (rho = -0.021; P = 0.92). In Duchenne muscular dystrophy boys, electrical impedance myography phase correlated with the North Star Ambulatory Assessment (rho = 0.65; P = 0.022); quantitative ultrasound revealed a near-significant association (rho = -0.56; P = 0.060). The two technologies trended toward a moderate correlation with one another in the Duchenne muscular dystrophy cohort but not in the healthy group (rho = -0.40; P = 0.054 and rho = -0.32; P = 0.13, respectively).

CONCLUSIONS

Electrical impedance myography and quantitative ultrasound are complementary modalities for the assessment of boys with Duchenne muscular dystrophy; further study and application of these two modalities alone or in combination in a longitudinal fashion are warranted.

Optimizing electrical impedance myography measurements by using a multifrequency ratio: a study in Duchenne muscular dystrophy

OBJECTIVE

Electrical impedance myography (EIM) is an electrophysiological technique for neuromuscular evaluation that is impacted by subcutaneous fat (SF). Exploiting the differing frequency dependences of muscle and fat, we assessed a 2-frequency EIM phase ratio in Duchenne muscular dystrophy (DMD) boys.

METHODS

Twenty-eight DMD boys aged 2-13years underwent EIM and the 6-minute walk test (6MWT). For each subject, 50kHz phase data was input into the numerator while 20-500kHz phase values were input into the denominator. We then performed correlation analyses seeking to identify the denominator frequency that simultaneously optimized SF and 6MWT correlations. This optimized ratio was then tested in 24 healthy boys.

RESULTS

50kHz phase correlated to 6MWT in DMD boys with R=0.52, p=0.0066, and to SF thickness with R=-0.67, p<0.001. An optimized ratio of 50/200kHz phase reduced the correlation of SF thickness to R=-0.075, p=0.45 while improving the relationship to the 6MWT (R=0.60, p=0.001). In normal subjects, the optimization decreased SF correlation from R=0.61 from R=0.16 with 6MWT correlation remaining unchanged.

CONCLUSIONS

The 50/200kHz EIM phase ratio removes the impact of SF while maintaining EIM's association with function.

SIGNIFICANCE

The use of a phase ratio may enhance EIM's application for evaluation of neuromuscular disease.

Influence of posture and frequency modes in total body water estimation using bioelectrical impedance spectroscopy in boys and adult males

The aim of the study was to examine differences in total body water (TBW) measured using single-frequency (SF) and multi-frequency (MF) modes of bioelectrical impedance spectroscopy (BIS) in children and adults measured in different postures using the deuterium (²H) dilution technique as the reference. Twenty-three boys and 26 adult males underwent assessment of TBW using the dilution technique and BIS measured in supine and standing positions using two frequencies of the SF mode (50 kHz and 100 kHz) and the MF mode. While TBW estimated from the MF mode was comparable, extra-cellular fluid (ECF) and intra-cellular fluid (ICF) values differed significantly (p < 0.01) between the different postures in both groups. In addition, while estimated TBW in adult males using the MF mode was significantly (p < 0.01) greater than the result from the dilution technique, TBW estimated using the SF mode and prediction equation was significantly (p < 0.01) lower in boys. Measurement posture may not affect estimation of TBW in boys and adult males, however, body fluid shifts may still occur. In addition, technical factors, including selection of prediction equation, may be important when TBW is estimated from measured impedance.

Spinal muscular atrophy: development and implementation of potential treatments

In neurodegenerative disorders, effective treatments are urgently needed, along with methods to determine whether treatment worked. In this review, we discuss the rapid progress in the understanding of recessive proximal spinal muscular atrophy and how this is leading to exciting potential treatments of the disease. Spinal muscular atrophy is caused by loss of the survival motor neuron 1 (SMN1) gene and reduced levels of SMN protein. The critical downstream targets of SMN deficiency that result in motor neuron loss are not known. However, increasing SMN levels has a marked impact in mouse models, and these therapeutics are rapidly moving toward clinical trials. Promising preclinical therapies, the varying degree of impact on the mouse models, and potential measures of treatment effect are reviewed. One key issue discussed is the variable outcome of increasing SMN at different stages of disease progression.

Update in electromyography

PURPOSE OF REVIEW

It is easy to forget the contribution of electromyography (EMG) to the investigation of paediatric peripheral neuromuscular disease, and this review highlights its continued importance.

RECENT FINDINGS

The discovery that Brown-Vialetto-van Leare disease, when associated with disorder of riboflavin metabolism, may be treatable has raised awareness of the importance of EMG for its early detection. Unexpected discovery of motor neuronopathy, which may be useful for the definition of the phenotype of several conditions, now has an added significance. The investigation of disorders of peripheral nerve cannot proceed without nerve conduction studies but particular interest has been shown in its role in the management of obstetric brachial plexus palsy, with investigation within 1 month now recommended. The key role of neurophysiology in identifying abnormalities of the neuromuscular junction, and therefore leading investigators to a diagnosis of myasthenia, is once again highlighted. EMG in muscle disease continues to have a role, particularly when identifying myotonia.

SUMMARY

Paediatric EMG, while a daunting technical challenge to some practitioners, remains a valuable investigative tool for the specialists in paediatric neuromuscular disorders and will continue to deliver important diagnostic information, often as quickly and accurately as other more recent innovations.

Electrical impedance myography for the assessment of children with muscular dystrophy: a preliminary study

Electrical impedance myography (EIM) provides a non-invasive approach for quantifying the severity of neuromuscular disease. Here we determine how well EIM data correlates to functional and ultrasound (US) measures of disease in children with Duchenne muscular dystrophy (DMD) and healthy subjects. Thirteen healthy boys, aged 2-12 years and 14 boys with DMD aged 4-12 years underwent both EIM and US measurements of deltoid, biceps, wrist flexors, quadriceps, tibialis anterior, and medial gastrocnemius. EIM measurements were performed with a custom-designed probe using a commercial multifrequency bioimpedance device. US luminosity data were quantified using a gray-scale analysis approach. Children also underwent the 6-minute walk test, timed tests and strength measurements. EIM and US data were combined across muscles. EIM 50 kHz phase was able to discriminate DMD children from healthy subjects with 98% accuracy. In the DMD patients, average EIM phase measurements also correlated well with standard functional measures. For example the 50 kHz phase correlated with the Northstar Ambulatory Assessment test (R = 0.83, p = 0.02). EIM 50 kHz phase and US correlated as well, with R = -0.79 (p < 0.001). These results show that EIM provides valuable objective measures Duchenne muscular dystrophy severity.

The effect of subcutaneous fat on electrical impedance myography when using a handheld electrode array: the case for measuring reactance

OBJECTIVE

Recent developments in electrical impedance myography (EIM) have led to the use of handheld electrode arrays (HEAs) for data acquisition. Although preferable for several reasons, this approach tends to be more affected by subcutaneous fat (SF) than the original approach in which the impedance-measuring electrodes are widely spaced. In this study, we seek to identify the EIM parameter least impacted by subcutaneous fat (SF) when using an HEA.

METHODS

18 normal subjects underwent 50 kHz EIM and ultrasound of the medial gastrocnemius muscles on the dominant side. Coefficients of determination (R(2) values) were calculated for each of the three major EIM variables (reactance, resistance, and phase) and SF thickness.

RESULTS

For both resistance and phase, a strong relationship to SF thickness was observed (R(2) = 0.64 and R(2) = 0.70, respectively, p < 0.001 for both). In contrast, for reactance, the relationship was non-significant, with R(2) = 0.07, p = 0.30.

CONCLUSIONS

Unlike resistance and phase, both of which are highly impacted by SF thickness, the reactance shows no significant relationship.

SIGNIFICANCE

Future clinical studies employing HEA's to perform EIM should evaluate alterations in reactance in addition to those in resistance and phase.

Machine learning algorithms to classify spinal muscular atrophy subtypes

OBJECTIVES

The development of better biomarkers for disease assessment remains an ongoing effort across the spectrum of neurologic illnesses. One approach for refining biomarkers is based on the concept of machine learning, in which individual, unrelated biomarkers are simultaneously evaluated. In this cross-sectional study, we assess the possibility of using machine learning, incorporating both quantitative muscle ultrasound (QMU) and electrical impedance myography (EIM) data, for classification of muscles affected by spinal muscular atrophy (SMA).

METHODS

Twenty-one normal subjects, 15 subjects with SMA type 2, and 10 subjects with SMA type 3 underwent EIM and QMU measurements of unilateral biceps, wrist extensors, quadriceps, and tibialis anterior. EIM and QMU parameters were then applied in combination using a support vector machine (SVM), a type of machine learning, in an attempt to accurately categorize 165 individual muscles.

RESULTS

For all 3 classification problems, normal vs SMA, normal vs SMA 3, and SMA 2 vs SMA 3, use of SVM provided the greatest accuracy in discrimination, surpassing both EIM and QMU individually. For example, the accuracy, as measured by the receiver operating characteristic area under the curve (ROC-AUC) for the SVM discriminating SMA 2 muscles from SMA 3 muscles was 0.928; in comparison, the ROC-AUCs for EIM and QMU parameters alone were only 0.877 (p < 0.05) and 0.627 (p < 0.05), respectively.

CONCLUSIONS

Combining EIM and QMU data categorizes individual SMA-affected muscles with very high accuracy. Further investigation of this approach for classifying and for following the progression of neuromuscular illness is warranted.