Magnetic Resonance Assessment of Hypertrophic and Pseudo-Hypertrophic Changes in Lower Leg Muscles of Boys with Duchenne Muscular Dystrophy and Their Relationship to Functional Measurements

Acute microtubule changes linked to DMD pathology are insufficient to impair contractile function or enhance contraction-induced injury in healthy muscle

Duchenne muscular dystrophy (DMD) is marked by the genetic deficiency of the dystrophin protein in striated muscle whose consequence is a cascade of cellular changes that predispose the susceptibility to contraction injury central to DMD pathology. Recent evidence identified the proliferation of microtubules enriched in post-translationally modified tubulin as a consequence of dystrophins absence that increases the passive mechanics of the muscle fiber and the excess mechanotransduction elicited reactive oxygen species and calcium signals that promote contraction injury. Motivated by evidence that acutely normalizing the disease microtubule alterations reduced contraction injury in murine DMD muscle (mdx), here we sought the direct impact of these microtubule alterations independent of dystrophins absence and the multitude of other changes consequent to dystrophic disease. To this end we used acute pharmacologic (epithiolone-D, EpoD; 4 hours) or genetic (vashohibin-2 and small vasohibin binding protein overexpression via AAV9; 2 weeks) strategies to effectively model the proliferation of detyrosination enriched microtubules in the mdx muscle. Quantifying in vivo nerve evoked plantarflexor function we find no alteration in peak torque nor contraction kinetics in WT mice modeling these DMD relevant MT alterations. Quantifying the susceptibility to eccentric contraction injury we show EpoD treatment proffered a small but significant protection from contraction injury while VASH/SVBP had no discernable impact. We conclude that the disease dependent MT alterations act in concert with additional cellular changes to predispose contraction injury in DMD.

Draft Guidance for Industry Duchenne Muscular Dystrophy, Becker Muscular Dystrophy, and Related Dystrophinopathies - Developing Potential Treatments for the Entire Spectrum of Disease

Background

Duchenne muscular dystrophy (DMD) and related dystrophinopathies are neuromuscular conditions with great unmet medical needs that require the development of effective medical treatments.

Objective

To aid sponsors in clinical development of drugs and therapeutic biological products for treating DMD across the disease spectrum by integrating advancements, patient registries, natural history studies, and more into a comprehensive guidance.

Methods

This guidance emerged from collaboration between the FDA, the Duchenne community, and industry stakeholders. It entailed a structured approach, involving multiple committees and boards. From its inception in 2014, the guidance underwent revisions incorporating insights from gene therapy studies, cardiac function research, and innovative clinical trial designs.

Results

The guidance provides a deeper understanding of DMD and its variants, focusing on patient engagement, diagnostic criteria, natural history, biomarkers, and clinical trials. It underscores patient-focused drug development, the significance of dystrophin as a biomarker, and the pivotal role of magnetic resonance imaging in assessing disease progression. Additionally, the guidance addresses cardiomyopathy's prominence in DMD and the burgeoning field of gene therapy.

Conclusions

The updated guidance offers a comprehensive understanding of DMD, emphasizing patient-centric approaches, innovative trial designs, and the importance of biomarkers. The focus on cardiomyopathy and gene therapy signifies the evolving realm of DMD research. It acts as a crucial roadmap for sponsors, potentially leading to improved treatments for DMD.

Multivariate modeling of magnetic resonance biomarkers and clinical outcome measures for Duchenne muscular dystrophy clinical trials

Although regulatory agencies encourage inclusion of imaging biomarkers in clinical trials for Duchenne muscular dystrophy (DMD), industry receives minimal guidance on how to use these biomarkers most beneficially in trials. This study aims to identify the optimal use of muscle fat fraction biomarkers in DMD clinical trials through a quantitative disease-drug-trial modeling and simulation approach. We simultaneously developed two multivariate models quantifying the longitudinal associations between 6-minute walk distance (6MWD) and fat fraction measures from vastus lateralis and soleus muscles. We leveraged the longitudinal individual-level data collected for 10 years through the ImagingDMD study. Age of the individuals at assessment was chosen as the time metric. After the longitudinal dynamic of each measure was modeled separately, the selected univariate models were combined using correlation parameters. Covariates, including baseline scores of the measures and steroid use, were assessed using the full model approach. The nonlinear mixed-effects modeling was performed in Monolix. The final models showed reasonable precision of the parameter estimates. Simulation-based diagnostics and fivefold cross-validation further showed the model's adequacy. The multivariate models will guide drug developers on using fat fraction assessment most efficiently using available data, including the widely used 6MWD. The models will provide valuable information about how individual characteristics alter disease trajectories. We will extend the multivariate models to incorporate trial design parameters and hypothetical drug effects to inform better clinical trial designs through simulation, which will facilitate the design of clinical trials that are both more inclusive and more conclusive using fat fraction biomarkers.

Eccentric contraction-induced strength loss in dystrophin-deficient muscle: Preparations, protocols, and mechanisms

The absence of dystrophin hypersensitizes skeletal muscle of lower and higher vertebrates to eccentric contraction (ECC)-induced strength loss. Loss of strength can be accompanied by transient and reversible alterations to sarcolemmal excitability and disruption, triad dysfunction, and aberrations in calcium kinetics and reactive oxygen species production. The degree of ECC-induced strength loss, however, appears dependent on several extrinsic and intrinsic factors such as vertebrate model, skeletal muscle preparation (in vivo, in situ, or ex vivo), skeletal muscle hierarchy (single fiber versus whole muscle and permeabilized versus intact), strength production, fiber branching, age, and genetic background, among others. Consistent findings across research groups show that dystrophin-deficient fast(er)-twitch muscle is hypersensitive to ECCs relative to wildtype muscle, but because preparations are highly variable and sensitivity to ECCs are used repeatedly to determine efficacy of many preclinical treatments, it is critical to evaluate the impact of skeletal muscle preparations on sensitivity to ECC-induced strength loss in dystrophin-deficient skeletal muscle. Here, we review and discuss variations in skeletal muscle preparations to evaluate the factors responsible for variations and discrepancies between research groups. We further highlight that dystrophin-deficiency, or loss of the dystrophin-glycoprotein complex in skeletal muscle, is not a prerequisite for accelerated strength loss-induced by ECCs.

Mechanisms of weakness in Mdx muscle following in vivo eccentric contractions

Skeletal muscle of the dystrophin-deficient mdx mouse is hypersensitive to eccentric (ECC) contraction-induced strength loss due to plasmalemmal electrical dysfunction. Despite plasmalemmal inexcitability being a logical mechanism responsible for weakness, it remains unclear if processes up- and/or down-stream remain functionally intact in injured mdx muscle. The purpose of this study was to analyze additional processes necessary for excitation-contraction coupling that are potentially disrupted by ECC contractions. Anterior crural muscles (tibialis anterior, extensor digitorum longus [EDL], and extensor hallucis muscles) of wildtype (WT) and mdx mice were injured in vivo with 50 ECC contractions and torque was measured immediately before and after the contraction bout. Following the in vivo assessment, EDL ex vivo isometric and caffeine forces were analyzed. In vivo isometric torque and ex vivo force in WT muscle were reduced 38 and 30% (p < 0.001), while caffeine force was also reduced (p = 0.021), albeit to a lesser degree (9%). In contrast, in vivo isometric torque, ex vivo isometric force and ex vivo caffeine-induced force were all reduced 56-67% (p < 0.001) in mdx muscle and did not differ from one another (p = 0.114). Disproportional reductions in isometric strength and caffeine-induced force confirm that ECC contractions uncoupled the plasmalemma from the ryanodine receptors (RyRs) in WT muscle. In mdx muscle, the proportional reductions in isometric strength and caffeine-induced force following ECC contractions reveal that dysfunction occurs at and/or distal to the RyRs immediately post-injury. Thus, weakness in injured mdx muscle cannot be isolated to one mechanism, rather several steps of muscle contraction are disrupted.

Step Activity Monitoring in Boys with Duchenne Muscular Dystrophy and its Correlation with Magnetic Resonance Measures and Functional Performance

BACKGROUND

Muscles of boys with Duchenne muscular dystrophy (DMD) are progressively replaced by fatty fibrous tissues, and weakness leads to loss of ambulation (LoA). Step activity (SA) monitoring is a quantitative measure of real-world ambulatory function. The relationship between quality of muscle health and SA is unknown in DMD.

OBJECTIVE

To determine SA in steroid treated boys with DMD across various age groups, and to evaluate the association of SA with quality of muscle health and ambulatory function.

METHODS

Quality of muscle health was measured by magnetic resonance (MR) imaging transverse magnetization relaxation time constant (MRI-T2) and MR spectroscopy fat fraction (MRS-FF). SA was assessed via accelerometry, and functional abilities were assessed through clinical walking tests. Correlations between SA, MR, and functional measures were determined. A threshold value of SA was determined to predict the future LoA.

RESULTS

The greatest reduction in SA was observed in the 9- < 11years age group. SA correlated with all functional and MR measures.10m walk/run test had the highest correlation with SA. An increase in muscle MRI-T2 and MRS-FF was associated with a decline in SA. Two years prior to LoA, SA in boys with DMD was 32% lower than age matched boys with DMD who maintained ambulation for more than two-year period. SA monitoring can predict subsequent LoA in Duchenne, as a daily step count of 3200 at baseline was associated with LoA over the next two-years.

CONCLUSION

SA monitoring is a feasible and accessible tool to measure functional capacity in the real-world environment.

Contraction-Induced Loss of Plasmalemmal Electrophysiological Function Is Dependent on the Dystrophin Glycoprotein Complex

Weakness and atrophy are key features of Duchenne muscular dystrophy (DMD). Dystrophin is one of the many proteins within the dystrophin glycoprotein complex (DGC) that maintains plasmalemmal integrity and cellular homeostasis. The dystrophin-deficient mdx mouse is also predisposed to weakness, particularly when subjected to eccentric (ECC) contractions due to electrophysiological dysfunction of the plasmalemma. Here, we determined if maintenance of plasmalemmal excitability during and after a bout of ECC contractions is dependent on intact and functional DGCs rather than, solely, dystrophin expression. Wild-type (WT) and dystrophic mice (mdx, mL172H and Sgcb-/- mimicking Duchenne, Becker and Limb-girdle Type 2E muscular dystrophies, respectively) with varying levels of dystrophin and DGC functionality performed 50 maximal ECC contractions with simultaneous torque and electromyographic measurements (M-wave root-mean-square, M-wave RMS). ECC contractions caused all mouse lines to lose torque (p<0.001); however, deficits were greater in dystrophic mouse lines compared to WT mice (p<0.001). Loss of ECC torque did not correspond to a reduction in M-wave RMS in WT mice (p=0.080), while deficits in M-wave RMS exceeded 50% in all dystrophic mouse lines (p≤0.007). Moreover, reductions in ECC torque and M-wave RMS were greater in mdx mice compared to mL172H mice (p≤0.042). No differences were observed between mdx and Sgcb-/- mice (p≥0.337). Regression analysis revealed ≥98% of the variance in ECC torque loss could be explained by the variance in M-wave RMS in dystrophic mouse lines (p<0.001) but not within WT mice (R 2=0.211; p=0.155). By comparing mouse lines that had varying amounts and functionality of dystrophin and other DGC proteins, we observed that (1) when all DGCs are intact, plasmalemmal action potential generation and conduction is maintained, (2) deficiency of the DGC protein β-sarcoglycan is as disruptive to plasmalemmal excitability as is dystrophin deficiency and, (3) some functionally intact DGCs are better than none. Our results highlight the significant role of the DGC plays in maintaining plasmalemmal excitability and that a collective synergism (via each DGC protein) is required for this complex to function properly during ECC contractions.

Lower Extremity Muscle Involvement in the Intermediate and Bethlem Myopathy Forms of COL6-Related Dystrophy and Duchenne Muscular Dystrophy: A Cross-Sectional Study

Collagen VI-related dystrophies (COL6-RDs) and Duchenne muscular dystrophy (DMD) cause progressive muscle weakness and disability. COL6-RDs are caused by mutations in the COL6 genes (COL6A1, COL6A2 and COL6A3) encoding the extracellular matrix protein collagen VI, and DMD is caused by mutations in the DMD gene encoding the cytoplasmic protein dystrophin. Both COL6-RDs and DMD are characterized by infiltration of the muscles by fatty and fibrotic tissue. This study examined the effect of disease pathology on skeletal muscles in lower extremity muscles of COL6-RDs using timed functional tests, strength measures and qualitative/ quantitative magnetic resonance imaging/spectroscopy measures (MRI/MRS) in comparison to unaffected (control) individuals. Patients with COL6-RD were also compared to age and gender matched patients with DMD.Patients with COL6-RD presented with a typical pattern of fatty infiltration of the muscle giving rise to an apparent halo effect around the muscle, while patients with DMD had evidence of fatty infiltration throughout the muscle areas imaged. Quantitatively, fat fraction, and transverse relaxation time (T2) were elevated in both COL6-RD and DMD patients compared to unaffected (control) individuals. Patients with COL6-RD had widespread muscle atrophy, likely contributing to weakness. In contrast, patients with DMD revealed force deficits even in muscle groups with increased contractile areas.

Isokinetic strength and degeneration of lower extremity muscles in patients with myotonic dystrophy; an MRI study

Our aim was to determine isokinetic strength and degeneration of lower extremity muscles in patients with Myotonic Dystrophy (DM1). In 19 patients with DM1 and 19 matched controls, strength measured by isokinetic dynamometry was expressed as percentage of expected strength (ePct), adjusted for age, height, weight and gender. MRI of the hip, thigh and calf muscles were obtained. Fat fraction (FF), mean contractile cross-sectional area (cCSA) and specific strength (Nm/cm²) were calculated. Patients' ankle plantar flexors, knee flexors and extensors had higher FF (Δ: 0.08 - 0.42) and lower cCSA (Δ: 3.2 -17.1 cm²) compared to controls (p ≤ 0.005). EPct (Δ: 19.5 - 41.6%) and specific strength (Δ: 0.27 - 0.96 Nm/cm²) were lower in the majority of patients muscle groups (p˂0.05). Close correlations were found for patients when relating ePct to; FF for plantar flexors (R²=0.742, p<0.001) and knee extensors (R²=0.732, p<0.001), cCSA for plantar flexors (R²=0.696, p<0.001) and knee extensors (R²=0.633, p<0.001), and specific strength for dorsal flexors (ρ=0.855, p = 0.008). In conclusion, patients had weaker lower extremity muscles with higher FF, lower cCSA and specific strength compared to controls. Muscle degeneration determined by quantitative MRI strongly correlated to strength supporting its feasibility to quantify muscle dysfunction in DM1.

Using MRI to quantify skeletal muscle pathology in Duchenne muscular dystrophy: A systematic mapping review

There is a great demand for accurate non‐invasive measures to better define the natural history of disease progression or treatment outcome in Duchenne muscular dystrophy (DMD) and to facilitate the inclusion of a large range of participants in DMD clinical trials. This review aims to investigate which MRI sequences and analysis methods have been used and to identify future needs. Medline, Embase, Scopus, Web of Science, Inspec, and Compendex databases were searched up to 2 November 2019, using keywords “magnetic resonance imaging” and “Duchenne muscular dystrophy.” The review showed the trend of using T1w and T2w MRI images for semi‐qualitative inspection of structural alterations of DMD muscle using a diversity of grading scales, with increasing use of T2map, Dixon, and MR spectroscopy (MRS). High‐field (>3T) MRI dominated the studies with animal models. The quantitative MRI techniques have allowed a more precise estimation of local or generalized disease severity. Longitudinal studies assessing the effect of an intervention have also become more prominent, in both clinical and animal model subjects. Quality assessment of the included longitudinal studies was performed using the Newcastle‐Ottawa Quality Assessment Scale adapted to comprise bias in selection, comparability, exposure, and outcome. Additional large clinical trials are needed to consolidate research using MRI as a biomarker in DMD and to validate findings against established gold standards. This future work should use a multiparametric and quantitative MRI acquisition protocol, assess the repeatability of measurements, and correlate findings to histologic parameters.

Full wave 3D inverse scattering transmission ultrasound tomography in the presence of high contrast

We present here a quantitative ultrasound tomographic method yielding a sub-mm resolution, quantitative 3D representation of tissue characteristics in the presence of high contrast media. This result is a generalization of previous work where high impedance contrast was not present and may provide a clinically and laboratory relevant, relatively inexpensive, high resolution imaging method for imaging in the presence of bone. This allows tumor, muscle, tendon, ligament or cartilage disease monitoring for therapy and general laboratory or clinical settings. The method has proven useful in breast imaging and is generalized here to high-resolution quantitative imaging in the presence of bone. The laboratory data are acquired in ~ 12 min and the reconstruction in ~ 24 min-approximately 200 times faster than previously reported simulations in the literature. Such fast reconstructions with real data require careful calibration, adequate data redundancy from a 2D array of 2048 elements and a paraxial approximation. The imaging results show that tissue surrounding the high impedance region is artifact free and has correct speed of sound at sub-mm resolution.

12-Month changes of muscle strength, body composition and physical activity in adults with dystrophinopathies

PURPOSE

Muscular dystrophy (MD) is an umbrella term for muscle wasting conditions, for which longitudinal changes in function and body composition are well established in children with Duchenne (DMD), however, changes in adults with DMD and Beckers (BMD), respectively, remain poorly reported. This study aims to assess 12-month changes in lower-limb strength, muscle size, body composition and physical activity in adults with Muscular Dystrophy (MD).

METHODS

Adult males with Duchenne MD (DMD; N = 15) and Beckers MD (BMD; N = 12) were assessed at baseline and 12-months for body composition (Body fat and lean body mass (LBM)), Isometric maximal voluntary contraction (Knee-Extension (KEMVC) and Plantar-Flexion (PFMVC)) and physical activity (tri-axial accelerometry).

RESULTS

12-Month change in strength was found as -19% (PFMVC) and -14% (KEMVC) in DMD. 12-Month change in strength in BMD, although non-significant, was explained by physical activity (R²=0.532-0.585). Changes in LBM (DMD) and body fat (BMD) were both masked by non-significant changes in body mass.

DISCUSSION

12-Month changes in adults with DMD appear consistent with paediatric populations. Physical activity appears important for muscle function maintenance. Specific monitoring of body composition, and potential co-morbidities, within adults with MD is highlighted.Implications for rehabilitationQuantitative muscle strength assessment shows progressive muscle weakness in adults with Duchenne Muscular Dystrophy is comparable to paediatric reports (-14 to -19%).Physical activity should be encouraged in adults with Beckers Muscular Dystrophy, anything appears better than nothing.Body composition, rather than body mass, should be monitored closely to identify any increase in body fat.

Magnetic Resonance Imaging Studies in Duchenne Muscular Dystrophy: Linking Findings to the Physical Therapy Clinic

Duchenne muscular dystrophy (DMD) is a muscle degenerative disorder that manifests in early childhood and results in progressive muscle weakness. Physical therapists have long been an important component of the multidisciplinary team caring for people with DMD, providing expertise in areas of disease assessment, contracture management, assistive device prescription, and exercise prescription. Over the last decade, magnetic resonance imaging of muscles in people with DMD has led to an improved understanding of the muscle pathology underlying the clinical manifestations of DMD. Findings from magnetic resonance imaging (MRI) studies in DMD, paired with the clinical expertise of physical therapists, can help guide research that leads to improved physical therapist care for this unique patient population. The 2 main goals of this perspective article are to (1) summarize muscle pathology and disease progression findings from qualitative and quantitative muscle MRI studies in DMD and (2) link MRI findings of muscle pathology to the clinical manifestations observed by physical therapists with discussion of any potential implications of MRI findings on physical therapy management.

Adenylosuccinic acid therapy ameliorates murine Duchenne Muscular Dystrophy

Arising from the ablation of the cytoskeletal protein dystrophin, Duchenne Muscular Dystrophy (DMD) is a debilitating and fatal skeletal muscle wasting disease underpinned by metabolic insufficiency. The inability to facilitate adequate energy production may impede calcium (Ca²⁺) buffering within, and the regenerative capacity of, dystrophic muscle. Therefore, increasing the metabogenic potential could represent an effective treatment avenue. The aim of our study was to determine the efficacy of adenylosuccinic acid (ASA), a purine nucleotide cycle metabolite, to stimulate metabolism and buffer skeletal muscle damage in the mdx mouse model of DMD. Dystrophin-positive control (C57BL/10) and dystrophin-deficient mdx mice were treated with ASA (3000 µg.mL⁻¹) in drinking water. Following the 8-week treatment period, metabolism, mitochondrial density, viability and superoxide (O₂⁻) production, as well as skeletal muscle histopathology, were assessed. ASA treatment significantly improved the histopathological features of murine DMD by reducing damage area, the number of centronucleated fibres, lipid accumulation, connective tissue infiltration and Ca²⁺ content of mdx tibialis anterior. These effects were independent of upregulated utrophin expression in the tibialis anterior. ASA treatment also increased mitochondrial viability in mdx flexor digitorum brevis fibres and concomitantly reduced O₂⁻ production, an effect that was also observed in cultured immortalised human DMD myoblasts. Our data indicates that ASA has a protective effect on mdx skeletal muscles.

Muscle MRI: A biomarker of disease severity in Duchenne muscular dystrophy? A systematic review

OBJECTIVE

To assess the evidence of a relationship between muscle MRI and disease severity in Duchenne muscular dystrophy (DMD).

METHODS

We conducted a systematic review of studies that analyzed correlations between MRI measurements and motor function in patients with DMD. PubMed, Cochrane, Scopus, and Web of Science were searched using relevant keywords and inclusion/exclusion criteria (January 1, 1990-January 31, 2019). We evaluated article quality using the Joanna Briggs Institute scale. Information regarding the samples included, muscles evaluated, MRI protocols and motor function tests used was collected from each article. Correlations between MRI measurements and motor function were reported exhaustively.

RESULTS

Seventeen of 1,629 studies identified were included. Most patients included were ambulant with a mean age of 8.9 years. Most studies evaluated lower limb muscles. Moderate to excellent correlations were found between MRI measurements and motor function. The strongest correlations were found for quantitative MRI measurements such as fat fraction or mean T2. Correlations were stronger for lower leg muscles such as soleus. One longitudinal study reported that changes in soleus mean T2 were highly correlated with changes in motor function.

CONCLUSION

The findings of this systematic review showed that MRI measurements can be used as biomarkers of disease severity in ambulant patients with DMD. Guidelines are proposed to help clinicians choose the most appropriate MRI measurements and muscles to evaluate. Studies exploring upper limb muscles, other stages of the disease, and sensitivity of measurements to change are needed.

Two-Year Longitudinal Changes in Lower Limb Strength and Its Relation to Loss in Function in a Large Cohort of Patients With Duchenne Muscular Dystrophy

OBJECTIVE

The main objective of this study was to examine the effect of disease on strength in two functionally important lower limb muscles for a period of 2 yrs in children with Duchene muscular dystrophy.

DESIGN

Seventy-seven Duchene muscular dystrophy children participated in this study. Plantar flexors, knee extensors, strength, and performance on timed tests (6-min walk, 4-stairs, 10-m walk, supine-up) were assessed yearly for 2 yrs. Multivariate normal regression was used to assess changes in strength over time in the Duchene muscular dystrophy group. Spearman correlations were computed to examine relationship between strength and function.

RESULTS

Normalized plantar flexor and knee extensor strength showed a significant decrease (P < 0.05) over 2 yrs, with larger declines in knee extensor. At baseline, knee extensor strongly correlated with performance on timed tests. However, plantar flexor strength was found to be a stronger predictor of loss in ambulatory function. Modest correlations (r = 0.19-0.34) were found between the decline in strength and functional performance over 2 yrs.

CONCLUSIONS

This study describes the loss of lower limb strength in a large cohort of Duchene muscular dystrophy children for 2 yrs. The findings support that lower limb strength alone cannot account for the decline in performance on functional tests, and the role of other contributing factors, such as compensatory strategies, should be considered.

Imaging respiratory muscle quality and function in Duchenne muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is characterized by damage to muscles including the muscles involved in respiration. Dystrophic muscles become weak and infiltrated with fatty tissue, resulting in progressive respiratory impairment. The objective of this study was to assess respiratory muscle quality and function in DMD using magnetic resonance imaging and to determine the relationship to clinical respiratory function.

METHODS

Individuals with DMD (n = 36) and unaffected controls (n = 12) participated in this cross sectional magnetic resonance imaging study. Participants underwent dynamic imaging of the thorax to assess diaphragm and chest wall mobility and chemical shift-encoded imaging of the chest and abdomen to determine fatty infiltration of the accessory respiratory muscles. Additionally, clinical pulmonary function measures were obtained.

RESULTS

Thoracic cavity area was decreased in individuals with DMD compared to controls during tidal and maximal breathing. Individuals with DMD had reduced chest wall movement in the anterior-posterior direction during maximal inspirations and expirations, but diaphragm descent during maximal inspirations (normalized to height) was only decreased in a subset of individuals with maximal inspiratory pressures less than 60% predicted. Muscle fat fraction was elevated in all three expiratory muscles assessed (p < 0.001), and the degree of fatty infiltration correlated with percent predicted maximal expiratory pressures (r =  - 0.70, p < 0.001). The intercostal muscles demonstrated minimal visible fatty infiltration; however, this analysis was qualitative and resolution limited.

INTERPRETATION

This magnetic resonance imaging investigation of diaphragm movement, chest wall movement, and accessory respiratory muscle fatty infiltration provides new insights into the relationship between disease progression and clinical respiratory function.

Effectiveness of High-Speed T2-Corrected Multiecho MR Spectroscopic Method for Quantifying Thigh Muscle Fat Content in Boys With Duchenne Muscular Dystrophy

OBJECTIVE. The purpose of this study was to investigate the potential of high-speed T2-corrected multiecho (HISTO) MR spectroscopy (MRS) for rapidly quantifying the fat content of thigh muscles in children with Duchenne muscular dystrophy (DMD). SUBJECTS AND METHODS. This study prospectively enrolled 58 boys with DMD (mean age, 7.5 years; range, 4-11 years) and 30 age-matched healthy boys (mean age, 7.2 years; range, 4-11 years) at one institution over a 1-year period. T1- and T2-weighted, multiecho Dixon, and HISTO sequences were performed on the right adductor magnus and vastus lateralis muscles. The fat fractions of these muscles were acquired from HISTO and multiecho Dixon images. An experienced radiologist graded the degree of fat infiltration of the adductor magnus and vastus lateralis muscles on axial T1-weighted images. The Bland-Altman method was used to assess the consistency and repeatability of the HISTO sequence. Pearson linear correlation analysis was used to determine the correlation coefficient relating HISTO fat fraction to multiecho Dixon fat fraction values. Spearman rank correlation analysis was used to assess the relation between the HISTO fat fraction values and T1-weighted image fat infiltration grades. The independent t test was used to compare the HISTO fat fraction values of the boys with DMD with those of the healthy control subjects. RESULTS. Bland-Altman analysis showed that 95.5% of the HISTO fat fraction values of the adductor magnus were within the 95% CI. HISTO fat fraction and multiecho Dixon fat fraction values of the adductor magnus and vastus lateralis muscles were highly positively correlated (adductor magnus, r = 0.983; vastus lateralis, r = 0.967; p < 0.0001). HISTO fat fraction values were also highly positively correlated with the grades of fat infiltration on T1-weighted images (adductor magnus, r = 0.911; vastus lateralis, r = 0.937; p < 0.0001). The HISTO fat fraction of the adductor magnus muscle was 33.3% ± 22.6% and of the vastus lateralis muscle was 25.6% ± 20.3% in patients with DMD. The corresponding values were 2.9% ± 2.1% and 2.3% ± 1.9% in the control group. The differences were statistically significant (p < 0.0001). CONCLUSION. The HISTO sequence is a rapid and feasible noninvasive MRS technique for quantifying the fat infiltration of thigh muscles in children with known or suspected DMD. It is useful for diagnosis and for assessment of disease activity and prognosis.

Advancements in magnetic resonance imaging-based biomarkers for muscular dystrophy

Recent years have seen steady progress in the identification of genetic muscle diseases as well as efforts to develop treatment for these diseases. Consequently, sensitive and objective new methods are required to identify and monitor muscle pathology. Magnetic resonance imaging offers multiple potential biomarkers of disease severity in the muscular dystrophies. This Review uses a pathology-based approach to examine the ways in which MRI and spectroscopy have been used to study muscular dystrophies. Methods that have been used to quantitate intramuscular fat, edema, fiber orientation, metabolism, fibrosis, and vascular perfusion are examined, and this Review describes how MRI can help diagnose these conditions and improve upon existing muscle biomarkers by detecting small increments of disease-related change. Important challenges in the implementation of imaging biomarkers, such as standardization of protocols and validating imaging measurements with respect to clinical outcomes, are also described.

Muscle contractility in spinobulbar muscular atrophy

Spinobulbar muscular atrophy (SBMA) is caused by a trinucleotide repeat expansion in the androgen receptor gene on the X chromosome. There is a toxic effect of the mutant receptor on muscle and neurons resulting in muscle weakness and atrophy. The weakness can be explained by wasting due to loss of muscle cells, but it is unknown whether weakness also relates to poor muscle contractility of the remaining musculature. In this study, we investigated the muscle contractility in SBMA. We used stationary dynamometry and quantitative MRI to assess muscle strength and absolute and fat-free, cross-sectional areas. Specific muscle force (strength per cross-sectional area) and contractility (strength per fat-free cross-sectional area) were compared with healthy controls and their relation to walking distance and disease severity was investigated. Specific force was reduced by 14-49% in SBMA patients compared to healthy controls. Contractility was reduced by 22-39% in elbow flexion, knee extension, ankle dorsi- and plantarflexion in SBMA patients. The contractility decreased with increasing muscle fat content in muscles with affected contractility in SBMA. The decreased muscle contractility in SBMA may relate to motor neuron degeneration and changed fibre type distribution and muscle architecture.

Variable rescue of microtubule and physiological phenotypes in mdx muscle expressing different miniaturized dystrophins

Delivery of miniaturized dystrophin genes via adeno-associated viral vectors is one leading approach in development to treat Duchenne muscular dystrophy. Here we directly compared the functionality of five mini- and micro-dystrophins via skeletal muscle-specific transgenic expression in dystrophin-deficient mdx mice. We evaluated their ability to rescue defects in the microtubule network, passive stiffness and contractility of skeletal muscle. Transgenic mdx mice expressing the short dystrophin isoform Dp116 served as a negative control. All mini- and micro-dystrophins restored elevated detyrosinated α-tubulin and microtubule density of mdx muscle to values not different from C57BL/10, however, only mini-dystrophins restored the transverse component of the microtubule lattice back to C57BL/10. Passive stiffness values in mdx muscles expressing mini- or micro-dystrophins were not different from C57BL/10. While all mini- and micro-dystrophins conferred significant protection from eccentric contraction-induced force loss in vivo and ex vivo compared to mdx, removal of repeats two and three resulted in less protection from force drop caused by eccentric contraction ex vivo. Our data reveal subtle yet significant differences in the relative functionalities for different therapeutic constructs of miniaturized dystrophin in terms of protection from ex vivo eccentric contraction-induced force loss and restoration of an organized microtubule lattice.

Texture as an imaging biomarker for disease severity in golden retriever muscular dystrophy

INTRODUCTION

Golden retriever muscular dystrophy (GRMD), an X-linked recessive disorder, causes similar phenotypic features to Duchenne muscular dystrophy (DMD). There is currently a need for a quantitative and reproducible monitoring of disease progression for GRMD and DMD.

METHODS

To assess severity in the GRMD, we analyzed texture features extracted from multi-parametric MRI (T1w, T2w, T1m, T2m, and Dixon images) using 5 feature extraction methods and classified using support vector machines.

RESULTS

A single feature from qualitative images can provide 89% maximal accuracy. Furthermore, 2 features from T1w, T2m, or Dixon images provided highest accuracy. When considering a tradeoff between scan-time and computational complexity, T2m images provided good accuracy at a lower acquisition and processing time and effort.

CONCLUSIONS

The combination of MRI texture features improved the classification accuracy for assessment of disease progression in GRMD with evaluation of the heterogenous nature of skeletal muscles as reflection of the histopathological changes. Muscle Nerve 59:380-386, 2019.

Impaired Glucose Tolerance in Adults with Duchenne and Becker Muscular Dystrophy

The aim of this study was to determine the response to an oral glucose tolerance test (OGTT) in adult males with Becker muscular dystrophy (BMD) and Duchenne muscular dystrophy (DMD), and to investigate whether body composition contributes to any variance in the glucose response. Twenty-eight adult males with dystrophinopathy (BMD, n = 13; DMD, n = 15) and 12 non-dystrophic controls, ingested 75 g oral anhydrous glucose solution. Fingertip capillary samples were assessed for glucose at 30-min intervals over 2-h post glucose ingestion. Fat free mass relative to body mass (FFM/BM) and body fat (BF%) was assessed using bioelectrical impedance. Vastus lateralis muscle anatomical cross sectional area (VL ACSA) was measured using B-mode ultrasonography. Blood glucose was higher in MD groups than control at 60, 90 and 120 min post ingestion of glucose. Compared to controls, FFM/BM and VL ACSA were lower in MD groups compared to controls (p < 0.001). Glucose tolerance values at 120 min were correlated with FFM/BM and BF% in the BMD group only. Our results suggest that glucose tolerance is impaired following OGTT in adult males with BMD and DMD. It is recommended that adults with BMD and DMD undertake routine glucose tolerance assessments to allow early detection of impaired glucose tolerance.

Relationships between muscle size, strength, and physical activity in adults with muscular dystrophy

BACKGROUND

Muscular dystrophy (MD) is characterized by progressive muscle wasting and weakness, yet few comparisons to non-MD controls (CTRL) of muscle strength and size in this adult population exist. Physical activity (PA) is promoted to maintain health and muscle strength within MD; however, PA reporting in adults with MD is limited to recall data, and its impact on muscle strength is seldom explored.

METHODS

This study included 76 participants: 16 non-MD (CTRL, mean age 35.4), 15 Duchenne MD (DMD, mean age 24.2), 18 Becker's MD (BMD, mean age 42.4), 13 limb-girdle MD (LGMD, mean age 43.1), and 14 facioscapulohumeral MD (mean age 47.7). Body fat (%) and lean body mass (LBM) were measured using bioelectrical-impedance. Gastrocnemius medialis (GM) anatomical cross-sectional area (ACSA) was determined using B-mode ultrasound. Isometric maximal voluntary contraction (MVC) was assessed during plantar flexion (PFMVC) and knee extension (KEMVC). PA was measured for seven continuous days using triaxial accelerometry and was expressed as daily average minutes being physically active (TPAmins ) or average daily percentage of waking hours being sedentary (sedentary behaviour). Additionally, 10 m walk time was assessed.

RESULTS

Muscular dystrophy groups had 34-46% higher body fat (%) than CTRL. DMD showed differences in LBM with 21-28% less LBM than all other groups. PFMVC and KEMVC were 36-75% and 24-92% lower, respectively, in MD groups than CTRL. GM ACSA was 47% and 39% larger in BMD and LGMD, respectively, compared with CTRL. PFMVC was associated with GM ACSA in DMD (P = 0.026, R = 0.429) and CTRL (P = 0.015, R = 0.553). MD groups were 14-38% more sedentary than CTRL groups, while DMD were more sedentary than BMD (14%), LGMD (8%), and facioscapulohumeral MD (14%). Sedentary behaviour was associated with LBM in DMD participants (P = 0.021, R = -0.446). TPAmins was associated with KEMVC (P = 0.020, R = 0.540) in BMD participants, while TPAmins was also the best predictor of 10 m walk time (P < 0.001, R2  = 0.540) in ambulant MD, revealed by multiple linear regression.

CONCLUSIONS

Quantified muscle weakness and impaired 10 m walking time is reported in adults with MD. Muscle weakness and 10 m walk time were associated with lower levels of TPA in adults with MD. Higher levels of sedentary behaviour were associated with reduced LBM in DMD. These findings suggest a need for investigations into patterns of PA behaviour, and relevant interventions to reduce sedentary behaviour and encourage PA in adults with MD regardless of impairment severity.

Non-invasive tracking of disease progression in young dystrophic muscles using multi-parametric MRI at 14T

In this study, multi-parametric magnetic resonance imaging (MRI) was conducted to monitor skeletal muscle changes in dystrophic (mdx^4cv) and age-matched control (C57BL/6J) mice starting at 3 weeks of age. The objective of this study was to evaluate and characterize changes in muscle tissue characteristics of hind limbs in young, dystrophic mice using MRI. Mdx^4cv (n = 25) and age-matched C57BL/6J (n = 5) were imaged at 3, 5, 7, 9, and 11 weeks of age. Multiple MR measurements were taken from the tibialis anterior, gastrocnemius, and soleus muscles. There were significant differences between dystrophic and control groups for all three muscle types when comparing transverse relaxation times (T₂) in lower hind limb muscles. Additionally, fractional anisotropy, radial diffusivity, and eigenvalue analysis of diffusion tensor imaging also demonstrated significant differences between groups. Longitudinal relaxation times (T₁) displayed no significant differences between groups. The earliest time points in the magnetization transfer ratio measurements displayed a significant difference. Histological analysis revealed significant differences in the tibialis anterior and gastrocnemius muscles between groups with the mdx mice displaying greater variability in muscle fiber size in later time points. The multi-parametric MRI approach offers a promising alternative for future development of a noninvasive avenue for tracking both disease progression and treatment response.

The cardiorespiratory response and physiological determinants of the assisted 6-minute handbike cycle test in adult males with muscular dystrophy

INTRODUCTION

The assisted 6-minute cycle test (A6MCT) distance was assessed in adults with muscular dystrophy (MD).

METHODS

Forty-eight males, including those with Duchenne MD (DMD), limb-girdle MD (LGMD), fascioscapulohumeral MD (FSHD), and Becker MD (BMD), as well as a group without MD (CTRL), completed handgrip strength (HGS), lung function [forced expiratory volume in 1 second (FEV1 ) and forced vital capacity (FVC)], body fat, and biceps thickness assessments. During the A6MCT, ventilation (VE), oxygen uptake (VO2 ), carbon dioxide (VCO2 ), and heart rate (HR) were recorded.

RESULTS

A6MCT and HGS were lower in MD than CTRL subjects. FEV1 , FVC, and biceps thickness were lower in MD than CTRL; lower in DMD than BMD, LGMD, and FSHD; but were not different between BMD, LGMD, and FSHD. A6MCT correlated with HGS, FEV1 , FVC, body fat, VO2 , VCO2 , HR, and VE (r = 0.455-0.708) in pooled BMD, LGMD, and FSHD participants.

DISCUSSION

A shorter A6MCT distance in adult males with MD was attributable to HGS and lung function. The A6MCT is appropriate for assessment of physical function in adults with MD. Muscle Nerve 58: 427-433, 2018.

Myopathology in times of modern imaging

Over the last two decades, muscle (magnetic resonance) imaging has become an important complementary tool in the diagnosis and differential diagnosis of inherited neuromuscular disorders, particularly in conditions where the pattern of selective muscle involvement is often more predictive of the underlying genetic background than associated clinical and histopathological features. Following an overview of different imaging modalities, the present review will give a concise introduction to systematic image analysis and interpretation in genetic neuromuscular disorders. The pattern of selective muscle involvement will be presented in detail in conditions such as the congenital or myofibrillar myopathies where muscle imaging is particularly useful to inform the (differential) diagnosis, and in disorders such as Duchenne or fascioscapulohumeral muscular dystrophy where the diagnosis is usually made on clinical grounds but where detailed knowledge of disease progression on the muscle imaging level may inform better understanding of the natural history. Utilizing the group of the congenital myopathies as an example, selected case studies will illustrate how muscle MRI can be used to inform the diagnostic process in the clinico-pathological context. Future developments, in particular, concerning the increasing use of whole-body MRI protocols and novel quantitative fat assessments techniques potentially relevant as an outcome measure, will be briefly outlined.

Skeletal Muscle Quantitative Nuclear Magnetic Resonance Imaging and Spectroscopy as an Outcome Measure for Clinical Trials

Recent years have seen tremendous progress towards therapy of many previously incurable neuromuscular diseases. This new context has acted as a driving force for the development of novel non-invasive outcome measures. These can be organized in three main categories: functional tools, fluid biomarkers and imagery. In the latest category, nuclear magnetic resonance imaging (NMRI) offers a considerable range of possibilities for the characterization of skeletal muscle composition, function and metabolism. Nowadays, three NMR outcome measures are frequently integrated in clinical research protocols. They are: 1/ the muscle cross sectional area or volume, 2/ the percentage of intramuscular fat and 3/ the muscle water T2, which quantity muscle trophicity, chronic fatty degenerative changes and oedema (or more broadly, “disease activity”), respectively. A fourth biomarker, the contractile tissue volume is easily derived from the first two ones. The fat fraction maps most often acquired with Dixon sequences have proven their capability to detect small changes in muscle composition and have repeatedly shown superior sensitivity over standard functional evaluation. This outcome measure will more than likely be the first of the series to be validated as an endpoint by regulatory agencies. The versatility of contrast generated by NMR has opened many additional possibilities for characterization of the skeletal muscle and will result in the proposal of more NMR biomarkers. Ultra-short TE (UTE) sequences, late gadolinium enhancement and NMR elastography are being investigated as candidates to evaluate skeletal muscle interstitial fibrosis. Many options exist to measure muscle perfusion and oxygenation by NMR. Diffusion NMR as well as texture analysis algorithms could generate complementary information on muscle organization at microscopic and mesoscopic scales, respectively. ³¹P NMR spectroscopy is the reference technique to assess muscle energetics non-invasively during and after exercise. In dystrophic muscle, ³¹P NMR spectrum at rest is profoundly perturbed, and several resonances inform on cell membrane integrity. Considerable efforts are being directed towards acceleration of image acquisitions using a variety of approaches, from the extraction of fat content and water T2 maps from one single acquisition to partial matrices acquisition schemes. Spectacular decreases in examination time are expected in the near future. They will reinforce the attractiveness of NMR outcome measures and will further facilitate their integration in clinical research trials.

Muscle MRI and functional outcome measures in Becker muscular dystrophy

Becker muscular dystrophy (BMD) is a neuromuscular disorder allelic to Duchenne muscular dystrophy (DMD), caused by in-frame mutations in the dystrophin gene, and characterized by a clinical progression that is both milder and more heterogeneous than DMD. Muscle magnetic resonance imaging (MRI) has been proposed as biomarker of disease progression in dystrophinopathies. Correlation with clinically meaningful outcome measures such as North Star Ambulatory Assessment (NSAA) and 6 minute walk test (6MWT) is paramount for biomarker qualification. In this study, 51 molecularly confirmed BMD patients (aged 7-69 years) underwent muscle MRI and were evaluated with functional measures (NSAA and 6MWT) at the time of the MRI, and subsequently after one year. We confirmed a pattern of fatty substitution involving mainly the hip extensors and most thigh muscles. Severity of muscle fatty substitution was significantly correlated with specific DMD mutations: in particular, patients with an isolated deletion of exon 48, or deletions bordering exon 51, showed milder involvement. Fat infiltration scores correlated with baseline functional measures, and predicted changes after 1 year. We conclude that in BMD, skeletal muscle MRI not only strongly correlates with motor function, but also helps in predicting functional deterioration within a 12-month time frame.

Muscle MRI findings in a one-year-old girl with merosin-deficient congenital muscular dystrophy type 1A due to LAMA2 mutation: A case report

The objective of the present study was to characterize the muscle magnetic resonance imaging (MRI) features of a 1-year-old girl with merosin-deficient congenital muscular dystrophy type 1A (MDC1A). Beginning as an infant, this patient exhibited severe hypotonia and proximal weakness, as well as delays in developmental milestones. Her serum creatine kinase levels at 3 months, 8 months and 1 year were 2,959, 1,621 and 1,659 U/l, respectively. Brain MRI indicated symmetric, mild T1WI low, mild T2WI and FLAIR high radial patterns in the white matter of the Cornu posterius of the ventricular lateral. Gene sequencing demonstrated a heterozygous frame-shift mutation in the LAMA2 gene, consisting of an AG deletion at nucleotides 2049-2050 (LAMA2 c.2049_2050delAG). Lower limb muscle MRI presented obvious fatty infiltration of the muscles and muscle atrophy during the early stage of the disease. The gluteus maximus, erector spinae, vastus intermedius, vastus lateralis, adductor magnus, soleus and gastrocnemius muscles were involved, whereas the piriformis, obturator internus, pectineus, adductor longus, adductor brevis and sartorius muscles presented mild or no involvement. Fatty infiltration of the erector spinae was observed during the early stage of the disease. As an additional tool in the differential diagnosis of muscle disorders, muscle MRI can delay the need for muscle biopsy.

Skeletal muscle water T2 as a biomarker of disease status and exercise effects in patients with Duchenne muscular dystrophy

The purpose of this study was to examine exercise effects on muscle water T₂ in patients with Duchenne muscular dystrophy (DMD). In 12 DMD subjects and 19 controls, lower leg muscle fat (%) was measured by Dixon and muscle water T₂ and R₂ (1/T₂) by the tri-exponential model. Muscle water R₂ was measured again at 3 hours after an ankle dorsiflexion exercise. The muscle fat fraction was higher in DMD participants than in controls (p < .001) except in the tibialis posterior muscle. Muscle water T₂ was measured independent of the degree of fatty degeneration in DMD muscle. At baseline, muscle water T₂ was higher in all but the extensor digitorum longus muscles of DMD participants than controls (p < .001). DMD participants had a lower muscle torque (p < .001) and exerted less power (p < .01) during exercise than controls. Nevertheless, muscle water R₂ decreased (T₂ increased) after exercise from baseline in DMD subjects and controls with greater changes in the target muscles of the exercise than in ankle plantarflexor muscles. Skeletal muscle water T₂ is a sensitive biomarker of the disease status in DMD and of the exercise response in DMD patients and controls.

Duchenne Regulatory Science Consortium Meeting on Disease Progression Modeling for Duchenne Muscular Dystrophy

INTRODUCTION

The Duchenne Regulatory Science Consortium (D-RSC) was established to develop tools to accelerate drug development for DMD.  The resulting tools are anticipated to meet validity requirements outlined by qualification/endorsement pathways at both the U.S. Food and Drug Administration (FDA) and European Medicines Administration (EMA), and will be made available to the drug development community. The initial goals of the consortium include the development of a disease progression model, with the goal of creating a model that would be used to forecast changes in clinically meaningful endpoints, which would inform clinical trial protocol development and data analysis.  Methods: In April of 2016 the consortium and other experts met to formulate plans for the development of the model.  Conclusions: Here we report the results of the meeting, and discussion as to the form of the model that we plan to move forward to develop, after input from the regulatory authorities.

Magnetic resonance imaging patterns of muscle involvement in genetic muscle diseases: a systematic review

A growing body of the literature supports the use of magnetic resonance imaging as a potential biomarker for disease severity in the hereditary myopathies. We performed a systematic review of the medical literature to evaluate patterns of fat infiltration observed in magnetic resonance imaging studies of muscular dystrophy and congenital myopathy. Searches were performed using MEDLINE, EMBASE, and grey literature databases. Studies that described fat infiltration of muscles in patients with muscular dystrophy or congenital myopathy were selected for full-length review. Data on preferentially involved or spared muscles were extracted for analysis. A total of 2172 titles and abstracts were screened, and 70 publications met our criteria for inclusion in the systematic review. There were 23 distinct genetic disorders represented in this analysis. In most studies, preferential involvement and sparing of specific muscles were reported. We conclude that magnetic resonance imaging studies can be used to identify distinct patterns of muscle involvement in the hereditary myopathies. However, larger studies and standardized methods of reporting are needed to develop imaging as a diagnostic tool in these diseases.

Upper Limb Evaluation in Duchenne Muscular Dystrophy: Fat-Water Quantification by MRI, Muscle Force and Function Define Endpoints for Clinical Trials

Objective

A number of promising experimental therapies for Duchenne muscular dystrophy (DMD) are emerging. Clinical trials currently rely on invasive biopsies or motivation-dependent functional tests to assess outcome. Quantitative muscle magnetic resonance imaging (MRI) could offer a valuable alternative and permit inclusion of non-ambulant DMD subjects. The aims of our study were to explore the responsiveness of upper-limb MRI muscle-fat measurement as a non-invasive objective endpoint for clinical trials in non-ambulant DMD, and to investigate the relationship of these MRI measures to those of muscle force and function.

Methods

15 non-ambulant DMD boys (mean age 13.3 y) and 10 age-gender matched healthy controls (mean age 14.6 y) were recruited. 3-Tesla MRI fat-water quantification was used to measure forearm muscle fat transformation in non-ambulant DMD boys compared with healthy controls. DMD boys were assessed at 4 time-points over 12 months, using 3-point Dixon MRI to measure muscle fat-fraction (f.f.). Images from ten forearm muscles were segmented and mean f.f. and cross-sectional area recorded. DMD subjects also underwent comprehensive upper limb function and force evaluation.

Results

Overall mean baseline forearm f.f. was higher in DMD than in healthy controls (p<0.001). A progressive f.f. increase was observed in DMD over 12 months, reaching significance from 6 months (p<0.001, n = 7), accompanied by a significant loss in pinch strength at 6 months (p<0.001, n = 9) and a loss of upper limb function and grip force observed over 12 months (p<0.001, n = 8).

Conclusions

These results support the use of MRI muscle f.f. as a biomarker to monitor disease progression in the upper limb in non-ambulant DMD, with sensitivity adequate to detect group-level change over time intervals practical for use in clinical trials. Clinical validity is supported by the association of the progressive fat transformation of muscle with loss of muscle force and function.

Contractile properties are disrupted in Becker muscular dystrophy, but not in limb girdle type 2I

We investigated whether a linear relationship between muscle strength and cross-sectional area (CSA) is preserved in calf muscles of patients with Becker muscular dystrophy (BMD, n = 14) and limb-girdle type 2I muscular dystrophy (LGMD2I, n = 11), before and after correcting for muscle fat infiltration. The Dixon magnetic resonance imaging technique was used to quantify fat and calculate a fat-free contractile CSA. Strength was assessed by dynamometry. Muscle strength/CSA relationships were significantly lower in patients versus controls. The strength/contractile-CSA relationship was still severely lowered in BMD, but was almost normalized in LGMD2I. Our findings suggest close to intact contractile properties in LGMD2I, which are severely disrupted in BMD. Ann Neurol 2016;80:466-471.

Current and emerging treatment strategies for Duchenne muscular dystrophy

Duchenne muscular dystrophy (DMD) is the most common form of muscular dystrophy in childhood. It is caused by mutations of the DMD gene, leading to progressive muscle weakness, loss of independent ambulation by early teens, and premature death due to cardiorespiratory complications. The diagnosis can usually be made after careful review of the history and examination of affected boys presenting with developmental delay, proximal weakness, and elevated serum creatine kinase, plus confirmation by muscle biopsy or genetic testing. Precise characterization of the DMD mutation is important for genetic counseling and individualized treatment. Current standard of care includes the use of corticosteroids to prolong ambulation and to delay the onset of secondary complications. Early use of cardioprotective agents, noninvasive positive pressure ventilation, and other supportive strategies has improved the life expectancy and health-related quality of life for many young adults with DMD. New emerging treatment includes viral-mediated microdystrophin gene replacement, exon skipping to restore the reading frame, and nonsense suppression therapy to allow translation and production of a modified dystrophin protein. Other potential therapeutic targets involve upregulation of compensatory proteins, reduction of the inflammatory cascade, and enhancement of muscle regeneration. So far, data from DMD clinical trials have shown limited success in delaying disease progression; unforeseen obstacles included immune response against the generated mini-dystrophin, inconsistent evidence of dystrophin production in muscle biopsies, and failure to demonstrate a significant improvement in the primary outcome measure, as defined by the 6-minute walk test in some studies. The long-term safety and efficacy of emerging treatments will depend on the selection of appropriate clinical end points and sensitive biomarkers to detect meaningful changes in disease progression. Correction of the underlying mutations using new gene-editing technologies and corticosteroid analogs with better safety profiles offers renewed hope for many individuals with DMD and their families.