Predicting myofiber size with electrical impedance myography: A study in immature mice

Machine learning-enhanced electrical impedance myography to diagnose and track spinal muscular atrophy progression

Objective.To evaluate electrical impedance myography (EIM) in conjunction with machine learning (ML) to detect infantile spinal muscular atrophy (SMA) and disease progression.Approach. Twenty-six infants with SMA and twenty-seven healthy infants had been enrolled and assessed with EIM as part of the NeuroNEXT SMA biomarker study. We applied a variety of modern, supervised ML approaches to this data, first seeking to differentiate healthy from SMA muscle, and then, using the best method, to track SMA progression.Main Results.Several of the ML algorithms worked well, but linear discriminant analysis (LDA) achieved 88.6% accuracy on subject muscles studied. This contrasts with a maximum of 60% accuracy that could be achieved using the single or multifrequency assessment approaches available at the time. LDA scores were also able to track progression effectively, although a multifrequency reactance-based measure also performed very well in this context.Significance.EIM enhanced with ML promises to be effective for providing effective diagnosis and tracking children and adults with SMA treated with currently available therapies. The normative trends identified here may also inform future applications of the technology in very young children. The basic analyses applied here could also likely be applied to other neuromuscular disorders characterized by muscle atrophy.

Electrical impedance myography detects age-related skeletal muscle atrophy in adult zebrafish

Age-related deficits in skeletal muscle function, termed sarcopenia, are due to loss of muscle mass and changes in the intrinsic mechanisms underlying contraction. Sarcopenia is associated with falls, functional decline, and mortality. Electrical impedance myography (EIM)-a minimally invasive, rapid electrophysiological tool-can be applied to animals and humans to monitor muscle health, thereby serving as a biomarker in both preclinical and clinical studies. EIM has been successfully employed in several species; however, the application of EIM to the assessment of zebrafish-a model organism amenable to high-throughput experimentation-has not been reported. Here, we demonstrated differences in EIM measures between the skeletal muscles of young (6 months of age) and aged (33 months of age) zebrafish. For example, EIM phase angle and reactance at 2 kHz showed significantly decreased phase angle (5.3 ± 2.1 versus 10.7 ± 1.5°; p = 0.001) and reactance (89.0 ± 3.9 versus 172.2 ± 54.8 ohms; p = 0.007) in aged versus young animals. Total muscle area, in addition to other morphometric features, was also strongly correlated to EIM 2 kHz phase angle across both groups (r = 0.7133, p = 0.01). Moreover, there was a strong correlation between 2 kHz phase angle and established metrics of zebrafish swimming performance, including turn angle, angular velocity, and lateral motion (r = 0.7253, r = 0.7308, r = 0.7857, respectively, p < 0.01 for all). In addition, the technique was shown to have high reproducibility between repeated measurements with a mean percentage difference of 5.34 ± 1.17% for phase angle. These relationships were also confirmed in a separate replication cohort. Together, these findings establish EIM as a fast, sensitive method for quantifying zebrafish muscle function and quality. Moreover, identifying the abnormalities in the bioelectrical properties of sarcopenic zebrafish provides new opportunities to evaluate potential therapeutics for age-related neuromuscular disorders and to interrogate the disease mechanisms of muscle degeneration.

Electrical impedance myography combined with quantitative assessment techniques in paretic muscle of stroke survivors: Insights and challenges

Aging is a non-modifiable risk factor for stroke and the global burden of stroke is continuing to increase due to the aging society. Muscle dysfunction, common sequela of stroke, has long been of research interests. Therefore, how to accurately assess muscle function is particularly important. Electrical impedance myography (EIM) has proven to be feasible to assess muscle impairment in patients with stroke in terms of micro structures, such as muscle membrane integrity, extracellular and intracellular fluids. However, EIM alone is not sufficient to assess muscle function comprehensively given the complex contributors to paretic muscle after an insult. This article discusses the potential to combine EIM and other common quantitative methods as ways to improve the assessment of muscle function in stroke survivors. Clinically, these combined assessments provide not only a distinct advantage for greater accuracy of muscle assessment through cross-validation, but also the physiological explanation on muscle dysfunction at the micro level. Different combinations of assessments are discussed with insights for different purposes. The assessments of morphological, mechanical and contractile properties combined with EIM are focused since changes in muscle structures, tone and strength directly reflect the muscle function of stroke survivors. With advances in computational technology, finite element model and machine learning model that incorporate multi-modal evaluation parameters to enable the establishment of predictive or diagnostic model will be the next step forward to assess muscle function for individual with stroke.

Altered electrical properties in skeletal muscle of mice with glycogen storage disease type II

Electrical impedance methods, including electrical impedance myography, are increasingly being used as biomarkers of muscle health since they measure passive electrical properties of muscle that alter in disease. One disorder, Pompe Disease (Glycogen storage disease type II (GSDII)), remains relatively unstudied. This disease is marked by dramatic accumulation of intracellular myofiber glycogen. Here we assessed the electrical properties of skeletal muscle in a model of GSDII, the Pompe^6neo/6neo (Pompe) mouse. Ex vivo impedance measurements of gastrocnemius (GA) were obtained using a dielectric measuring cell in 30-week-old female Pompe (N = 10) and WT (N = 10) mice. Longitudinal and transverse conductivity, σ, and the relative permittivity, ε_r, and Cole–Cole complex resistivity parameters at 0 Hz and infinite frequency, ρ_o and ρ_∞, respectively, and the intracellular resistivity, ρ_{intracellular} were determined from the impedance data. Glycogen content (GC) was visualized histologically and quantified biochemically. At frequencies > 1 MHz, Pompe mice demonstrated significantly decreased longitudinal and transverse conductivity, increased Cole–Cole parameters, ρ_o and ρ_o-ρ_∞, and decreased ρ_{intracellular}. Changes in longitudinal conductivity and ρ_{intracellular} correlated with increased GC in Pompe animals. Ex vivo high frequency impedance measures are sensitive to alterations in intracellular myofiber features considered characteristic of GSDII, making them potentially useful measures of disease status.

Relationships between in vivo surface and ex vivo electrical impedance myography measurements in three different neuromuscular disorder mouse models

Electrical impedance myography (EIM) using surface techniques has shown promise as a means of diagnosing and tracking disorders affecting muscle and assessing treatment efficacy. However, the relationship between such surface-obtained impedance values and pure muscle impedance values has not been established. Here we studied three groups of diseased and wild-type (WT) animals, including a Duchenne muscular dystrophy model (the D2-mdx mouse), an amyotrophic lateral sclerosis (ALS) model (the SOD1 G93A mouse), and a model of fat-related atrophy (the db/db diabetic obese mouse), performing hind limb measurements using a standard surface array and ex vivo measurements on freshly excised gastrocnemius muscle. A total of 101 animals (23 D2-mdx, 43 ALS mice, 12 db/db mice, and corresponding 30 WT mice) were studied with EIM across a frequency range of 8 kHz to 1 MHz. For both D2-mdx and ALS models, moderate strength correlations (Spearman rho values generally ranging from 0.3-0.7, depending on the impedance parameter (i.e., resistance, reactance and phase) were obtained. In these groups of animals, there was an offset in frequency with impedance values obtained at higher surface frequencies correlating more strongly to impedance values obtained at lower ex vivo frequencies. For the db/db model, correlations were comparatively weaker and strongest at very high and very low frequencies. When combining impedance data from all three disease models together, moderate correlations persisted (with maximal Spearman rho values of 0.45). These data support that surface EIM data reflect ex vivo muscle tissue EIM values to a moderate degree across several different diseases, with the highest correlations occurring in the 10-200 kHz frequency range. Understanding these relationships will prove useful for future applications of the technique of EIM in the assessment of neuromuscular disorders.

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.

Potential Utility of Electrical Impedance Myography in Evaluating Age-Related Skeletal Muscle Function Deficits

Skeletal muscle function deficits associated with advancing age are due to several physiological and morphological changes including loss of muscle size and quality (conceptualized as a reduction in the intrinsic force-generating capacity of a muscle when adjusted for muscle size). Several factors can contribute to loss of muscle quality, including denervation, excitation-contraction uncoupling, increased fibrosis, and myosteatosis (excessive levels of inter- and intramuscular adipose tissue and intramyocellular lipids). These factors also adversely affect metabolic function. There is a major unmet need for tools to rapidly and easily assess muscle mass and quality in clinical settings with minimal patient and provider burden. Herein, we discuss the potential for electrical impedance myography (EIM) as a tool to evaluate muscle mass and quality in older adults. EIM applies weak, non-detectible (e.g., 400 μA), mutifrequency (e.g., 1 kHz–1 MHz) electrical currents to a muscle (or muscle group) through two excitation electrodes, and resulting voltages are measured via two sense electrodes. Measurements are fast (~5 s/muscle), simple to perform, and unaffected by factors such as hydration that may affect other simple measures of muscle status. After nearly 2 decades of study, EIM has been shown to reflect muscle health status, including the presence of atrophy, fibrosis, and fatty infiltration, in a variety of conditions (e.g., developmental growth and maturation, conditioning/deconditioning, and obesity) and neuromuscular diseases states [e.g., amyotrophic lateral sclerosis (ALS) and muscular dystrophies]. In this article, we describe prior work and current evidence of EIM’s potential utility as a measure of muscle health in aging and geriatric medicine.

Predicting myofiber cross-sectional area and triglyceride content with electrical impedance myography: A study in db/db mice

BACKGROUND

Electrical impedance myography (EIM) provides insight into muscle composition and structure. We sought to evaluate its use in a mouse obesity model characterized by myofiber atrophy.

METHODS

We applied a prediction algorithm, ie, the least absolute shrinkage and selection operator (LASSO), to surface, needle array, and ex vivo EIM data from db/db and wild-type mice and assessed myofiber cross-sectional area (CSA) histologically and triglyceride (TG) content biochemically.

RESULTS

EIM data from all three modalities provided acceptable predictions of myofiber CSA with average root mean square error (RMSE) of 15% in CSA (ie, ±209 μm2 for a mean CSA of 1439 μm2 ) and TG content with RMSE of 30% in TG content (ie, ±7.3 nmol TG/mg muscle for a mean TG content of 25.4 nmol TG/mg muscle).

CONCLUSIONS

EIM combined with a predictive algorithm provides reasonable estimates of myofiber CSA and TG content without the need for biopsy.

Electrical impedance myography: A critical review and outlook

Electrical impedance myography (EIM) technology is finding application in neuromuscular disease research as a tool to assess muscle health. Correlations between EIM outcomes, functional, imaging and histological data have been established in a variety of neuromuscular disorders; however, an analytical discussion of EIM is lacking. This review presents an explanation for clinicians and others who are applying EIM and interpreting impedance outcomes. The background of EIM is presented, including the relation between EIM, volume conduction properties, tissue structure, electrode configuration and conductor volume. Also discussed are technical considerations to guide the reader to critically evaluate EIM and understand its limitations and strengths.

Effectiveness of impedance parameters for muscle quality evaluation in healthy men

We investigated the relationship between impedance parameters and skeletal muscle function in the lower extremities, as well as the effectiveness of impedance parameters in evaluating muscle quality. Lower extremity impedance of 19 healthy men (aged 23-31 years) measured using the direct segmental multi-frequency bioelectrical impedance analysis were arc-optimized using the Cole-Cole model, following which phase angle (PA), [Formula: see text], and β were estimated. Skeletal muscle function was assessed by muscle thickness, muscle intensity, and isometric knee extension force (IKEF). IKEF was positively correlated with PA (r = 0.58, p < 0.01) and β (r = 0.34, p < 0.05) was negatively correlated with [Formula: see text] (r = - 0.43, p < 0.01). Stepwise multiple regression analysis results revealed that PA, β, and [Formula: see text] were correlated with IKEF independently of muscle thickness. This study suggests that arc-optimized impedance parameters are effective for evaluating muscle quality and prediction of muscle strength.

Using Electrical Impedance Myography as a Biomarker of Muscle Deconditioning in Rats Exposed to Micro- and Partial-Gravity Analogs

As astronauts prepare to undertake new extra-terrestrial missions, innovative diagnostic tools are needed to better assess muscle deconditioning during periods of weightlessness and partial gravity. Electrical impedance myography (EIM) has been used to detect muscle deconditioning in rodents exposed to microgravity during spaceflight or using the standard ground-based model of hindlimb unloading via tail suspension (HU). Here, we used EIM to assess muscle changes in animals exposed to two new models: hindlimb suspension using a pelvic harness (HLS) and a partial weight-bearing (PWB) model that mimics partial gravity (including Lunar and Martian gravities). We also used a simple needle array electrode in lieu of surface or ex vivo EIM approaches previously employed. Our HLS results confirmed earlier findings obtained after spaceflight and tail suspension. Indeed, one EIM measure (i.e., phase-slope) that was previously reported as highly sensitive, was significantly decreased after HLS (day 0: 14.60 ± 0.97, day 7: 11.03 ± 0.81, and day 14: 10.13 ± 0.55 | Deg/MHz|, p < 0.0001), and was associated with a significant decrease in muscle grip force. Although EIM parameters such as 50 kHz phase, reactance, and resistance remained variable over 14 days in PWB animals, we identified major PWB-dependent effects at 7 days. Moreover, the data at both 7 and 14 days correlated to previously observed changes in rear paw grip force using the same PWB model. In conclusion, our data suggest that EIM has the potential to serve as biomarker of muscle deconditioning during exposure to both micro- and partial- gravity during future human space exploration.

Partial Weight-Bearing in Female Rats: Proof of Concept in a Martian-Gravity Analog

Many studies have investigated the physiological response to microgravity in both astronauts and animals. However, while space agencies have sought to deploy more women on their missions; animal models rarely include females studies or comparisons between males and females. Therefore, we exposed adult female rats to 2 weeks of partial weight-bearing at either 100% of their normal loading (PWB100) or 40% of their normal loading (PWB40), corresponding to Martian gravity-analog, and assess muscle function, force and histomorphometry. Females exposed to PWB showed an 11.62% decline in hindlimb grip force associated with an 11.84% decrease in soleus myofiber size after 14 days of exposure, while maintaining normal blood oxygenation and stress levels. This pilot study represents the first experiment designed to understand the muscular disuse associated with a partial reduction in mechanical loading in female rats, and the first step needed to develop successful mitigating strategies.

NEW AND NOTEWORTHY

This research article describes the first use of quadrupedal partial weight-bearing in female rats. This study demonstrates the feasibility of partial gravity analogs in females and allows for future investigations about the impact of sex on muscle deconditioning due to reduced mechanical loading.

Altered muscle electrical tissue properties in a mouse model of premature aging

INTRODUCTION

Improved methods are needed to detect and quantify age-related muscle change. In this study we assessed the electrical properties of muscle impacted by acquired mitochondrial DNA mutations via the PolG mouse, which exhibits typical age-associated features, and the impact of a potential therapy, nicotinamide mononucleotide (NMN).

METHODS

The gastrocnemii of 24 PolG and 30 wild-type (WT) mice (8 PolG and 17 WT treated with NMN) were studied in an electrical impedance-measuring cell. Conductivity and relative permittivity were determined from the impedance data. Myofiber cross-sectional area (CSA) was quantified histologically.

RESULTS

Untreated PolG mice demonstrated alterations in several impedance features, including 50-kHz relative permittivity and center frequency. A potential effect of NMN was also observed in these parameters in PolG but not WT animals. Impedance values correlated with myofiber CSA.

DISCUSSION

Electrical impedance is sensitive to myofiber features considered characteristic of aging and to the impact of a potential therapy.

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.

Exploring the relationship between electrical impedance myography and quantitative ultrasound parameters in Duchenne muscular dystrophy

OBJECTIVES

Quantitative ultrasound (QUS), including grayscale level analysis (GLA) and quantitative backscatter analysis (QBA), and electrical impedance myography (EIM) have been proposed as biomarkers in Duchenne muscular dystrophy (DMD). However, the relationship between these methods has not been assessed.

METHODS

QUS values (including GLA and QBA) and several EIM measures were recorded from six muscles in 36 DMD and 29 healthy boys between ages 5 and 13 years at baseline, 6-months, and 12-months.

RESULTS

In the DMD boys, a moderate correlation was noted between QUS and EIM parameters, with the strongest correlations being identified for averaged muscle values. Of the individual muscles, biceps brachii and deltoid showed the strongest correlations. For example, in biceps, the QBA/EIM correlation coefficient (Spearman rho) was ≥0.70 (p < 0.01). Importantly, changes in QUS values over 12 months also correlated moderately with changes in EIM parameters and EIM/QBA rho values mostly varied between -0.53 and -0.70 (p ≤ 0.02). No significant correlations were identified in the healthy boys.

CONCLUSIONS

A moderate correlation of QUS with EIM in DMD boys suggests that the two technologies provide related data but are sensitive to different pathological features of muscle.

SIGNIFICANCE

The use of both technologies jointly in assessing DMD progression and response to therapy should be considered.

Fatigue-Induced Cole Electrical Impedance Model Changes of Biceps Tissue Bioimpedance

Bioimpedance, or the electrical impedance of biological tissues, describes the passive electrical properties of these materials. To simplify bioimpedance datasets, fractional-order equivalent circuit presentations are often used, with the Cole-impedance model being one of the most widely used fractional-order circuits for this purpose. In this work, bioimpedance measurements from 10 kHz to 100 kHz were collected from participants biceps tissues immediately prior and immediately post completion of a fatiguing exercise protocol. The Cole-impedance parameters that best fit these datasets were determined using numerical optimization procedures, with relative errors of within approximately ± 0.5 % and ± 2 % for the simulated resistance and reactance compared to the experimental data. Comparison between the pre and post fatigue Cole-impedance parameters shows that the R ∞ , R 1 , and f p components exhibited statistically significant mean differences as a result of the fatigue induced changes in the study participants.

Estimating Myofiber Size With Electrical Impedance Myography: a Study In Amyotrophic Lateral Sclerosis MICE

INTRODUCTION

A method for quantifying myofiber size noninvasively would find wide use, including primary diagnosis and evaluating response to therapy.

METHODS

Using prediction algorithms, including the least absolute shrinkage and selection operator, we applied multifrequency electrical impedance myography (EIM) to amyotrophic lateral sclerosis superoxide dismutase 1 G93A mice of different ages and assessed myofiber size histologically.

RESULTS

Multifrequency EIM data provided highly accurate predictions of myofiber size, with a root mean squared error (RMSE) of only 14% in mean myofiber area (corresponding to ± 207 µm² for a mean area of 1,488 µm² ) and an RMSE of only 8.8% in predicting the coefficient of variation in fiber size distribution.

DISCUSSION

This impedance-based approach provides predictive variables to assess myofiber size and distribution with good accuracy, particularly in diseases in which myofiber atrophy is the predominant histological feature, without the requirement for biopsy or burdensome quantification. Muscle Nerve 58: 713-717, 2018.