Diagnosis and treatment of mitochondrial myopathies

Effective and safe diet therapies for Lennox-Gastaut syndrome with mitochondrial dysfunction

Background:

Lennox-Gastaut syndrome (LGS) is a typical intractable form of epilepsy that most often occurs between the second and sixth year of life. This study aimed to evaluate the clinical efficacy and safety of ketogenic diet therapies (DTs) for LGS with mitochondrial dysfunction.

Methods:

This was a retrospective study involving 20 LGS patients with mitochondrial dysfunction who received several DTs from 2004 to 2014 at a single tertiary care center. Seizure reduction rate, cognitive function, retention rate, electroencephalography (EEG) changes, and adverse effects were examined before and after DTs.

Results:

The retention rates at 1 and 2 years after initiation of DTs were 45% and 40%, respectively. After 1-year follow up, we observed seizure freedom in two patients, 75% seizure reduction in two patients, 50% reduction in three patients, and 25% reduction in one patient. After 2-year follow up, the outcomes were seizure freedom in two patients, 90% seizure reduction in one patient, 75% reduction in two patients, and 50% reduction in two patients. EEG findings improved in nine patients. Nine patients were treated with DTs for 1 year; all patients demonstrated improved cognitive status. Eight patients were treated with DTs for 2 years, of whom seven had improved cognitive status. Poor tolerability of DTs was due to poor oral intake and gastrointestinal problems.

Conclusions:

We demonstrate that, in LGS with mitochondrial dysfunction, improvement of seizures and cognitive function are not inferior to those in other patients treated with DTs. This study showed that DTs are efficacious and feasible for LGS patients with mitochondrial dysfunction and can significantly improve their prognosis.

Educational Case: Mitochondrial Myopathy

The following fictional case is intended as a learning tool within the Pathology Competencies for Medical Education (PCME), a set of national standards for teaching pathology. These are divided into three basic competencies: Disease Mechanisms and Processes, Organ System Pathology, and Diagnostic Medicine and Therapeutic Pathology. For additional information, and a full list of learning objectives for all three competencies, see http://journals.sagepub.com/doi/10.1177/2374289517715040.¹

Acute Respiratory Failure Is the Initial Manifestation in the Adult-Onset A3243G tRNALeu mtDNA Mutation: A Case Report and the Literature Review

Isolated mitochondrial myopathy refers to the condition of mitochondrial disorders that primarily affect the skeletal muscle system. Here we report on a case of a patient who presented with acute respiratory failure as the initial and predominant clinical manifestation after using anesthetic drugs. The diagnosis of mitochondrial myopathy was made by histochemical findings of ragged red fibers with a modified Gomori trichrome Stain in the skeletal muscle biopsy and the genetic detection of an A3243G point mutation in the tRNA^Leu (UUR) gene of mitochondrial DNA (mtDNA) in a peripheral blood specimen. The patient revealed a benign clinical outcome with ventilator assistance and a cocktail treatment. Further, we performed a literature review on patients with respiratory failure as the early and predominant manifestation in adult-onset isolated mitochondrial myopathy. Eleven cases in nine studies (including our case) have been reported, and five of whom underwent DNA analysis all harbored the A3243G mutation in the tRNA^Leu gene of the mtDNA. Use of sedative drugs tends to induce acute respiratory failure in such cases.

Systematic review and meta-analysis of cardiac involvement in mitochondrial myopathy

Objective

Our goal was to perform a systematic review of the literature to demonstrate the prevalence of cardiac abnormalities identified using cardiac investigations in patients with mitochondrial myopathy (MM).

Methods

This systematic review surveys the available evidence for cardiac investigations in MM from a total of 21 studies including 825 participants. Data were stratified by genetic mutation and clinical syndrome.

Results

We identified echocardiogram and ECG as the principal screening modalities that identify cardiac structural (29%) and conduction abnormalities (39%) in various MM syndromes. ECG abnormalities were more prevalent in patients with m.3243A>G mutations than other gene defects, and patients with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) had a higher prevalence of ECG abnormalities than patients with other clinical syndromes. Echocardiogram abnormalities were significantly more prevalent in patients with m.3243A>G or m.8344A>G mutations compared with other genetic mutations. Similarly, MELAS and MERRF had a higher prevalence compared with other syndromes. We observed a descriptive finding of an increased prevalence of ECG abnormalities in pediatric patients compared with adults.

Conclusions

This analysis supports the presence of a more severe cardiac phenotype in MELAS and myoclonic epilepsy with ragged red fibres syndromes and with their commonly associated genetic mutations (m.3243A>G and m.8344A>G). This provides the first evidence basis on which to provide more intensive cardiac screening for patients with certain clinical syndromes and genetic mutations. However, the data are based on a small number of studies. We recommend further studies of natural history, therapeutic response, pediatric participants, and cardiac MRI as areas for future investigation.

Signaling and Regulation of the Mitochondrial Unfolded Protein Response

The mitochondrial proteome encompasses more than a thousand proteins, which are encoded by the mitochondrial and nuclear genomes. Mitochondrial biogenesis and network health relies on maintenance of protein import pathways and the protein-folding environment. Cell-extrinsic or -intrinsic stressors that challenge mitochondrial proteostasis negatively affect organellar function. During conditions of stress, cells use impaired protein import as a sensor for mitochondrial dysfunction to activate a stress response called the mitochondrial unfolded protein response (UPR^mt). UPR^mt activation leads to an adaptive transcriptional program that promotes mitochondrial recovery, metabolic adaptations, and innate immunity. In this review, we discuss the regulation of UPR^mt activation as well as its role in maintaining mitochondrial homeostasis in physiological and pathological scenarios.

Metabolic Myopathies and the Respiratory System

Metabolic myopathies are a heterogeneous group of disorders characterized by inherited defects of enzymatic pathways involved in muscle cellular energetics and adenosine triphosphate synthesis. Skeletal and respiratory muscles are most affected. There are multiple mechanisms of disease. The age of onset and prognosis vary. Metabolic myopathies cause exercise intolerance, myalgia, and increase in muscle breakdown products during exercise. Some affect smooth muscle like the diaphragm and cause respiratory failure. The pathophysiology is complex and the evidence in literature to guide diagnosis and management is sparse. Treatment is limited. This review discusses the pathophysiology and diagnostic evaluation of these disorders.

Systems Biology Approaches Toward Understanding Primary Mitochondrial Diseases

Primary mitochondrial diseases form one of the most common and severe groups of genetic disease, with a birth prevalence of at least 1 in 5000. These disorders are multi-genic and multi-phenotypic (even within the same gene defect) and span the entire age range from prenatal to late adult onset. Mitochondrial disease typically affects one or multiple high-energy demanding organs, and is frequently fatal in early life. Unfortunately, to date there are no known curative therapies, mostly owing to the rarity and heterogeneity of individual mitochondrial diseases, leading to diagnostic odysseys and difficulties in clinical trial design. This review aims to discuss recent advances and challenges of systems approaches for the study of primary mitochondrial diseases. Although there has been an explosion in the generation of omics data, few studies have progressed toward the integration of multiple levels of omics. It is evident that the integration of different types of data to create a more complete representation of biology remains challenging, perhaps due to the scarcity of available integrative tools and the complexity inherent in their use. In addition, "bottom-up" systems approaches have been adopted for use in the iterative cycle of systems biology: from data generation to model prediction and validation. Primary mitochondrial diseases, owing to their complex nature, will most likely benefit from a multidisciplinary approach encompassing clinical, molecular and computational studies integrated together by systems biology to elucidate underlying pathomechanisms for better diagnostics and therapeutic discovery. Just as next generation sequencing has rapidly increased diagnostic rates from approximately 5% up to 60% over two decades, more recent advancing technologies are encouraging; the generation of multi-omics, the integration of multiple types of data, and the ability to predict perturbations will, ultimately, be translated into improved patient care.

RAGE in the pathophysiology of skeletal muscle

Emerging evidence suggests that the signalling of the Receptor for Advanced Glycation End products (RAGE) is critical for skeletal muscle physiology controlling both the activity of muscle precursors during skeletal muscle development and the correct time of muscle regeneration after acute injury. On the other hand, the aberrant re-expression/activity of RAGE in adult skeletal muscle is a hallmark of muscle wasting that occurs in response to ageing, genetic disorders, inflammatory conditions, cancer, and metabolic alterations. In this review, we discuss the mechanisms of action and the ligands of RAGE involved in myoblast differentiation, muscle regeneration, and muscle pathological conditions. We highlight potential therapeutic strategies for targeting RAGE to improve skeletal muscle function.

Mitochondrial genetic medicine

Inherited mitochondrial DNA (mtDNA) diseases were discovered 30 years ago, and their characterization has provided a new perspective on the etiology of the common metabolic and degenerative diseases, cancer, and aging. The maternally inherited mtDNA contains 37 critical bioenergetic genes that are present in hundreds of copies per cell, but the 'mitochondrial genome' encompasses an additional 1,000-2,000 nuclear DNA (nDNA) mitochondrial genes. The interaction between these two mitochondrial genetic systems provides explanations for phenomena such as the non-Mendelian transmission of the common 'complex' diseases, age-related disease risk and progression, variable penetrance and expressivity, and gene-environment interactions. Thus, mtDNA genetics contributes to the quantitative and environmental components of human genetics that cannot be explained by Mendelian genetics. Because mtDNA is maternally inherited and cytoplasmic, it has fostered the first germline gene therapy, nuclear transplantation. However, effective interventions are still lacking for existing patients with mitochondrial dysfunction.

Modulation of the de novo purine nucleotide pathway as a therapeutic strategy in mitochondrial myopathy

Mitochondrial myopathy (MM) is characterised by muscle weakness, exercise intolerance and various histopathological changes. Recently, a subset of MM has also been associated with aberrant activation of mammalian target of rapamycin complex 1 (mTORC1) in skeletal muscle. This aberrant mTORC1 activation promotes increased de novo nucleotide synthesis, which contributes to abnormal expansion and imbalance of skeletal muscle deoxyribonucleoside triphosphates (dNTP) pools. However, the exact mechanism via which mTORC1-stimulated de novo nucleotide biosynthesis ultimately disturbs muscle dNTP pools remains unclear. In this article, it is proposed that mTORC1-stimulated de novo nucleotide synthesis in skeletal muscle cells with respiratory chain dysfunction promotes an asymmetric increase of purine nucleotides, probably due to NAD⁺ deficiency. This in turn could disrupt purine nucleotide-dependent allosteric feedback regulatory mechanisms, ultimately leading to dNTP pools aberration. Pharmacological down-modulation of aminoimidazole carboxamide ribonucleotide transformylase/inosine monophosphate cyclohydrolase (ATIC) activity is also proposed as a potential therapeutic strategy in MM exhibiting mTORC1-driven abnormal metabolic reprogramming, including aberrant dNTPs pools.

Histopathologic Analysis in Sudden Infant and Child Deaths: A Practical Approach

The forensic pathologist responsible for sudden unexpected death (SUD) investigation in the pediatric setting faces many challenges. It usually takes many years to obtain reasonable experience and exposure to the wide variety of diseases that may present as SUD in a pediatric context, and to appreciate the differences in the etiology and clinical context between the pediatric and adult SUD setting. In pediatric SUD, it is necessary to conduct a systematic, pediatric-focused autopsy investigation including extensive histopathological assessment and ancillary testing. Postmortem histologic findings in the context of SUD in the pediatric population are often subtle and distinctly different from those seen in the adult population. The pathologist must have an understanding of both developmental and pathological processes in order to correctly interpret the findings during a pediatric autopsy. A system-based, histopathology-focused review of common entities, normal variants, and incidental findings that can prove challenging will be discussed. For the forensic pathologist tasked with pediatric SUD autopsies, development of a strong collaborative relationship with a pediatric pathologist and/or neuropathologist to assist with histopathological analysis is strongly endorsed.

Mitochondrial DNA Deletions With Low-Level Heteroplasmy in Adult-Onset Myopathy

We report the cases of 2 patients who presented to our Myositis Center with myalgias and elevated creatine kinase levels. Muscle biopsy showed pathological features consistent with mitochondrial myopathy. In both cases, a single large deletion in mitochondrial DNA at low-level heteroplasmy was identified by next-generation sequencing in muscle tissue. In 1 case, the deletion was identified in muscle tissue but not blood. In both cases, the deletion was only identified on next-generation sequencing of muscle mitochondrial DNA and missed on array comparative genome hybridization testing. These cases demonstrate that next-generation sequencing of mitochondrial DNA in muscle tissue is the most sensitive method of molecular diagnosis for mitochondrial myopathy due to mitochondrial DNA deletions.

Respiratory involvement in neuromuscular disorders

PURPOSE OF REVIEW

In numerous neuromuscular disorders (NMDs), respiratory muscle weakness is present, and acute or chronic respiratory failure may evolve. Very often, respiratory involvement substantially adds to the burden of disease, impairs quality of life, or reduces life expectancy. This article summarizes new aspects of both diagnosis and management of respiratory muscle weakness in patients with NMDs.

RECENT FINDINGS

Drugs like deflazacort, ataluren, eteplirsen, and nusinersen are now approved treatments for Duchenne Muscular Dystrophy and Spinal Muscular Atrophy, and others are on their way in NMDs. Although observing how innovative drugs will change the natural history of these diseases, including respiratory function over time, adequate symptomatic treatment remains meaningful and is strongly recommended. Physicians should systematically take respiratory involvement into account to improve patients' quality of life and prognosis.

SUMMARY

First, it is outlined in which subtypes of NMD respiratory muscle dysfunction is particularly relevant. Second, new developments regarding diagnostic procedures, including respiratory muscle strength testing, spirometry, and sleep studies, are covered. Third, this article gives an overview on current concepts of ventilatory support and management of secretions in patients with NMD.

Expanding the phenotype of SLC25A42-associated mitochondrial encephalomyopathy

SLC25A42 gene encodes an inner mitochondrial membrane protein that imports Coenzyme A into the mitochondrial matrix. A mutation in this gene was recently reported in a subject born to consanguineous parents who presented with mitochondrial myopathy with muscle weakness and lactic acidosis. In this report, we present 12 additional individuals with the same founder mutation who presented with variable manifestations ranging from asymptomatic lactic acidosis to a severe phenotype characterized by developmental regression and epilepsy. Our report confirms the link between SLC25A42 and mitochondrial disease in humans, and suggests that pathogenic variants in SLC25A42 should be interpreted with the understanding that the associated phenotype may be highly variable.

Mitochondrial genome variability: the effect on cellular functional activity

Mitochondria are the key players in cell metabolism, calcium homeostasis, and reactive oxygen species (ROS) production. Mitochondrial genome alterations are reported to be associated with numerous human disorders affecting nearly all tissues. In this review, we discuss the available information on the involvement of mitochondrial DNA (mtDNA) mutations in cell dysfunction.

Quantitative in vivo mapping of myocardial mitochondrial membrane potential

BACKGROUND

Mitochondrial membrane potential (ΔΨm) arises from normal function of the electron transport chain. Maintenance of ΔΨm within a narrow range is essential for mitochondrial function. Methods for in vivo measurement of ΔΨm do not exist. We use 18F-labeled tetraphenylphosphonium (18F-TPP+) to measure and map the total membrane potential, ΔΨT, as the sum of ΔΨm and cellular (ΔΨc) electrical potentials.

METHODS

Eight pigs, five controls and three with a scar-like injury, were studied. Pigs were studied with a dynamic PET scanning protocol to measure 18F-TPP+ volume of distribution, VT. Fractional extracellular space (fECS) was measured in 3 pigs. We derived equations expressing ΔΨT as a function of VT and the volume-fractions of mitochondria and fECS. Seventeen segment polar maps and parametric images of ΔΨT were calculated in millivolts (mV).

RESULTS

In controls, mean segmental ΔΨT = -129.4±1.4 mV (SEM). In pigs with segmental tissue injury, ΔΨT was clearly separated from control segments but variable, in the range -100 to 0 mV. The quality of ΔΨT maps was excellent, with low noise and good resolution. Measurements of ΔΨT in the left ventricle of pigs agree with previous in in-vitro measurements.

CONCLUSIONS

We have analyzed the factors affecting the uptake of voltage sensing tracers and developed a minimally invasive method for mapping ΔΨT in left ventricular myocardium of pigs. ΔΨT is computed in absolute units, allowing for visual and statistical comparison of individual values with normative data. These studies demonstrate the first in vivo application of quantitative mapping of total tissue membrane potential, ΔΨT.

Imaging of Left Ventricular Hypertrophy: a Practical Utility for Differential Diagnosis and Assessment of Disease Severity

PURPOSE OF REVIEW

Left ventricular hypertrophy (LVH) is often encountered in clinical practice, and it is a risk factor for cardiac mortality and morbidity. Determination of the etiology and disease severity is important for the management of patients with LVH. The aim of this review is to show the remarkable progress in cardiac imaging and its importance in clinical practice.

RECENT FINDINGS

This review focuses on clinical features and characteristic cardiac imaging in LVH caused by various diseases including hypertension, aortic valve stenosis, hypertrophic cardiomyopathy, and secondary cardiomyopathies. The usefulness of echocardiography as a tool of general versatility including hemodynamic evaluation and the usefulness of cardiac magnetic resonance imaging for assessment of cardiac morphology and myocardial tissue characteristics of relevance for LVH are described. Imaging modalities now have central roles in the differentiation and prognostic assessment of LVH.

Acute mitochondrial myopathy with respiratory insufficiency and motor axonal polyneuropathy

BACKGROUND

Mitochondrial myopathies (MMs) are mainly presented with chronic muscle weakness and accompanied with other syndromes. MM with acute respiratory insufficiency is rare.

AIMS

To reveal the clinical, pathological and molecular characteristics of a life-threatening MM.

METHODS

Muscle biopsy and enzyme staining were performed in skeletal muscles. Mitochondrial DNA (mtDNA) sequencing was analyzed and heteroplasmy were quantified by pyrosequencing.

RESULTS

All three patients had tachycardia, acute lactic acidosis, dyspnea and sudden severe muscle weakness. Two patients had calf edema and abdominal pain, and one had a heart attack. Electromyography in two patients showed dramatically decreased axonal amplitudes of motor nerves. Muscle biopsies showed ragged red fibers and dramatic mitochondrial abnormality. A mtDNA m.3243A>G mutation was identified in Patient 1 (mutation load: 29% in blood and 73% in muscle) and Patient 3 (79% in blood and 89% in muscle). A mtDNA m.8344A>G mutation was found in Patient 2 (mutation load 80.4% in blood).

CONCLUSION

MM characterized by lactic acidosis, respiratory failure and acute motor axonal neuropathy is life threatening.

Myopathology of Adult and Paediatric Mitochondrial Diseases

Mitochondria are dynamic organelles ubiquitously present in nucleated eukaryotic cells, subserving multiple metabolic functions, including cellular ATP generation by oxidative phosphorylation (OXPHOS). The OXPHOS machinery comprises five transmembrane respiratory chain enzyme complexes (RC). Defective OXPHOS gives rise to mitochondrial diseases (mtD). The incredible phenotypic and genetic diversity of mtD can be attributed at least in part to the RC dual genetic control (nuclear DNA (nDNA) and mitochondrial DNA (mtDNA)) and the complex interaction between the two genomes. Despite the increasing use of next-generation-sequencing (NGS) and various omics platforms in unravelling novel mtD genes and pathomechanisms, current clinical practice for investigating mtD essentially involves a multipronged approach including clinical assessment, metabolic screening, imaging, pathological, biochemical and functional testing to guide molecular genetic analysis. This review addresses the broad muscle pathology landscape including genotype–phenotype correlations in adult and paediatric mtD, the role of immunodiagnostics in understanding some of the pathomechanisms underpinning the canonical features of mtD, and recent diagnostic advances in the field.

McArdle disease: a "pediatric" disorder presenting in an adult with acute kidney injury

Rhabdomyolysis is characterized by the acute breakdown of skeletal muscle, resulting in the release of muscle cell contents, subsequent myoglobinuria, and in severe cases, acute renal failure. A number of etiologies have been identified in acute rhabdomyolysis, in which drugs and trauma account for the majority of cases. One etiological category that is commonly overlooked in the adult population is an underlying genetic defect. This may be challenging to diagnose due to its rarity in the adult demographic and the marked heterogeneity, often requiring a high level of clinical suspicion before investigation is pursued. Once diagnosed, however, appropriate steps can be taken to reduce future episodes of rhabdomyolysis, further renal injury, and other systemic complications. Here, we report a case of an adult patient presenting with acute rhabdomyolysis secondary to McArdle disease, a genetic disease causing defective glycogenolysis. The case highlights the importance of recognizing the potential of undiagnosed "pediatric" disorders in adulthood and particularly for underlying genetic causes of rhabdomyolysis.

Dietary nitrate does not reduce oxygen cost of exercise or improve muscle mitochondrial function in patients with mitochondrial myopathy

Muscle weakness and exercise intolerance negatively affect the quality of life of patients with mitochondrial myopathy. Short-term dietary nitrate supplementation has been shown to improve exercise performance and reduce oxygen cost of exercise in healthy humans and trained athletes. We investigated whether 1 wk of dietary inorganic nitrate supplementation decreases the oxygen cost of exercise and improves mitochondrial function in patients with mitochondrial myopathy. Ten patients with mitochondrial myopathy (40 ± 5 yr, maximal whole body oxygen uptake = 21.2 ± 3.2 ml·min−1·kg body wt−1, maximal work load = 122 ± 26 W) received 8.5 mg·kg body wt−1·day−1 inorganic nitrate (~7 mmol) for 8 days. Whole body oxygen consumption at 50% of the maximal work load, in vivo skeletal muscle oxidative capacity (evaluated from postexercise phosphocreatine recovery using 31P-magnetic resonance spectroscopy), and ex vivo mitochondrial oxidative capacity in permeabilized skinned muscle fibers (measured with high-resolution respirometry) were determined before and after nitrate supplementation. Despite a sixfold increase in plasma nitrate levels, nitrate supplementation did not affect whole body oxygen cost during submaximal exercise. Additionally, no beneficial effects of nitrate were found on in vivo or ex vivo muscle mitochondrial oxidative capacity. This is the first time that the therapeutic potential of dietary nitrate for patients with mitochondrial myopathy was evaluated. We conclude that 1 wk of dietary nitrate supplementation does not reduce oxygen cost of exercise or improve mitochondrial function in the group of patients tested.

Kennedy disease with difficulty in differential diagnosis: A case report

RATIONALE

Kennedy disease (KD) is also known as spinal bulbar muscular dystrophy. As KD has similar symptoms with most neuromuscular diseases, so it is difficult to make a rapid diagnosis clinically.

PATIENT CONCERNS

We report a case of a 43-year-old male with progressive limb proximal weakness without family history. Physical examination showed gynecomastia, erectile dysfunction, bilateral tendon reflex and quadriceps weakness, and tongue muscle atrophy.

DIAGNOSES

Laboratory examination found increased creatine kinase, impaired glucose tolerance, and abnormal lactic acid values. There was no mutation or copy number variant in SMN1 gene and related mitochondrion genes tested, even with the use of multiplex ligation probe- dependent amplification technique. Diagnosis was confirmed with genetic analysis which displayed trinucleotide CAG (glutamine)- repeat expansion in the androgen-receptor gene.

INTERVENTIONS AND OUTCOMES

The patient achieved good prognosis with symptomatic treatment after diagnosis.

LESSONS

To diagnose KD, clinicians should pay more attention to differentiate KD and myasthenia gravis, mitochondrial myopathy, and amyotrophic lateral sclerosis. Gene analysis was the key in detecting this rare confusing disease in the patient.

A preliminary study in the alterations of mitochondrial respiration in patients with carbon monoxide poisoning measured in blood cells

OBJECTIVES

Carbon monoxide (CO) is a colorless and odorless gas responsible for poisoning mortality and morbidity in the United States. At this time, there is no reliable method to predict the severity of poisoning or clinical prognosis following CO exposure. Whole blood cells, such as peripheral blood mononuclear cells (PBMCs) and platelets, have been explored for their potential use to act as sensitive biomarkers for mitochondrial dysfunction which may have a role in CO poisoning.

DESIGN

The objective of this study was to measure mitochondrial respiration using intact cells obtained from patients exposed to CO as a potential biomarker for mitochondrial inhibition with results that can be obtained in a time frame useful for guiding clinical care. This was a prospective, observational pilot study performed from July 2015 to July 2016 at a single academic tertiary care center that is the location of the region's only multi chamber hyperbaric.

MEASUREMENTS

Clinical characteristics, patient demographics, mitochondrial respiration and outcomes were recorded.

MAIN RESULTS

There were 7 patients enrolled with a mean COHb level 26.8 ± 10 and with a mean lactate of 1.1 ± 0.4 mmol/L. All 7 CO exposures were related to heat generators used during winter months with two deaths. There was a positive correlation between maximal respiration and COHb levels with both high maximal respiration and high spare respiratory capacity correlating with a high COHb level. There was a subset of PBMCs (n = 4) that were analyzed for Complex IV (cytochrome c oxidase) activity.

CONCLUSIONS

In this pilot study, measurements can be performed in an appropriate timeline for clinical care with potential to serve as a prognostic marker. Further work is necessary to develop high-resolution respirometry as a clinical tool for assessing the severity of illness and guiding therapy.

Detection of Mutations in Mitochondrial DNA by Droplet Digital PCR

Mutations in mitochondrial DNA (mtDNA) may result in various pathological processes. Detection of mutant mtDNAs is a problem for diagnostic practice that is complicated by heteroplasmy - a phenomenon of the inferring presence of at least two allelic variants of the mitochondrial genome. Also, the level of heteroplasmy largely determines the profile and severity of clinical manifestations. Here we discuss detection of mutations in heteroplasmic mtDNA using up-to-date methods that have not yet been introduced as routine clinical assays. These methods can be used for detecting mutations in mtDNA to verify diagnosis of "mitochondrial disease", studying dynamics of mutant mtDNA in body tissues of patients, as well as investigating structural features of mtDNAs. Original data on allele-specific discrimination of m.11778G>A mutation by droplet digital PCR are presented, which demonstrate an opportunity for simultaneous detection and quantitative assessment of mutations in mtDNAs.

Future Directions for Pharmacotherapies for Treatment-resistant Bipolar Disorder

Current pharmacological treatments for bipolar disorder (BD) are limited and efficacy has historically been discovered through serendipity. There is now scope for new drug development, focused on the underlying biology of BD that is not targeted by current therapies. The need for novel treatments is urgent when considering treatment resistant BD, where current therapies have failed. While established drugs targeting the monoamine systems continue to be worthwhile, new biological targets including inflammatory and oxidative an nitrosative pathways, apoptotic and neurotrophic pathways, mitochondrial pathways, the N-methyl-Daspartate (NMDA)-receptor complex, the purinergic system, neuropeptide system, cholinergic system and melatonin pathways are all being identified as potential anchors for the discovery of new agents. Many agents are experimental and efficacy data is limited, however further investigation may provide a new line for drug discovery, previously stalled by lack of corporate interest.

Anesthetic management of 877 pediatric patients undergoing muscle biopsy for neuromuscular disorders: a 20-year review

BACKGROUND

The objectives are to review the anesthetic management and anesthetic-related adverse events in patients undergoing muscle biopsy for a broad spectrum of neuromuscular disorders (NMD).

AIM

The study aims to assess the hypothesis that perceived awareness of potential anesthesia-induced hyperkalemia and MH in patients with NMD reduces the frequency of such events.

METHODS

A 20-year retrospective review of 877 consecutive patients undergoing muscle biopsy to establish diagnoses of NMD has been performed. Patients were categorized prebiopsy into six groups: M (myopathy and muscular dystrophy), MM (mitochondrial or metabolic myopathy), N (neurodegenerative, peripheral neuropathy or spinal muscular atrophy disorder), D (dermatomyositis), C (cardiomyopathy), or S (seizure disorder). Data were collected for demographics, anesthetic management, pre- and postoperative anesthesia-induced muscle injury, postbiopsy histopathologic diagnosis, and concordance comparisons between pre- and postbiopsy diagnoses.

RESULTS

There were 513 males (58.5%) and 364 females (41.5%) (1.4:1) with 137 individuals (15.6%) operated on under 1 year of age and two-thirds by 6 years of age. NMD diagnosis was reached in 409 (46.6%) while 468 (53.4%) had no specific pathology. No patients exhibited signs of anesthesia-induced muscle injury (malignant hyperthermia, rhabdomyolysis, cardiac arrest, or postoperative deterioration of weakness). MM was the largest group pre biopsy (367, 41.8%). Anesthetic agents were: nitrous oxide in 657 (74.9%); volatile agents in 139 (15.8%); intravenous agents in 836 (95.3%) (primarily propofol, midazolam, and fentanyl); nondepolarizing muscle relaxants in 404 (46.1%); and regional anesthesia in 112 (12.8%) [most commonly spinal anesthesia in 80 (71.4%)]. Comparing preoperative diagnostic category with postoperative diagnosis, there was a concordance of 78% (319/409) between the two for cases with a definitive diagnosis and 89.7% (787/877) for all cases.

CONCLUSIONS

In this retrospective study, no patient exhibited signs or symptoms of hyperkalemia or MH probably because the incidence is very low and becomes even less likely due to the selection of the various anesthetic agents and strategies administered.

Faster and stronger manifestation of mitochondrial diseases in skeletal muscle than in heart related to cytosolic inorganic phosphate (Pi) accumulation

A model of the cell bioenergetic system was used to compare the effect of oxidative phosphorylation (OXPHOS) deficiencies in a broad range of moderate ATP demand in skeletal muscle and heart. Computer simulations revealed that kinetic properties of the system are similar in both cases despite the much higher mitochondria content and "basic" OXPHOS activity in heart than in skeletal muscle, because of a much higher each-step activation (ESA) of OXPHOS in skeletal muscle than in heart. Large OXPHOS deficiencies lead in both tissues to a significant decrease in oxygen consumption (V̇o2) and phosphocreatine (PCr) and increase in cytosolic ADP, Pi, and H(+) The main difference between skeletal muscle and heart is a much higher cytosolic Pi concentration in healthy tissue and much higher cytosolic Pi accumulation (level) at low OXPHOS activities in the former, caused by a higher PCr level in healthy tissue (and higher total phosphate pool) and smaller Pi redistribution between cytosol and mitochondria at OXPHOS deficiency. This difference does not depend on ATP demand in a broad range. A much greater Pi increase and PCr decrease during rest-to-moderate work transition in skeletal muscle at OXPHOS deficiencies than at normal OXPHOS activity significantly slows down the V̇o2 on-kinetics. Because high cytosolic Pi concentrations cause fatigue in skeletal muscle and can compromise force generation in skeletal muscle and heart, this system property can contribute to the faster and stronger manifestation of mitochondrial diseases in skeletal muscle than in heart. Shortly, skeletal muscle with large OXPHOS deficiencies becomes fatigued already during low/moderate exercise.

Confirmed versus suspected: The social significance of a genetic or non-genetic diagnosis of mitochondrial disease

This study assessed attitudes and beliefs regarding the importance of a genetic versus non-genetic diagnosis within the mitochondrial disease community. Survey respondents were categorized into two groups - those with a genetic diagnosis, and those with a non-genetic diagnosis of mitochondrial disease. We found that while both groups perceive problems with the support available to adult mitochondrial disease patients, the non-genetic group experiences less medical and social support due to lack of a definitive diagnosis. Understanding the efficacy of existing resources for mitochondrial disease sub-groups will allow for the development or improvement of resources designed to meet patient needs.

Progress in Diagnosing Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis, and Stroke-like Episodes

Objective:

Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) is a progressive, multisystem affected mitochondrial disease associated with a number of disease-related defective genes. MELAS has unpredictable presentations and clinical course, and it can be commonly misdiagnosed as encephalitis, cerebral infarction, or brain neoplasms. This review aimed to update the diagnosis progress in MELAS, which may provide better understanding of the disease nature and help make the right diagnosis as well.

Data Sources:

The data used in this review came from published peer review articles from October 1984 to October 2014, which were obtained from PubMed. The search term is “MELAS”.

Study Selection:

Information selected from those reported studies is mainly based on the progress on clinical features, blood biochemistry, neuroimaging, muscle biopsy, and genetics in diagnosing MELAS.

Results:

MELAS has a wide heterogeneity in genetics and clinical manifestations. The relationship between mutations and phenotypes remains unclear. Advanced serial functional magnetic resonance imaging (MRI) can provide directional information on this disease. Muscle biopsy has meaningful value in diagnosing MELAS, which shows the presence of ragged red fibers and mosaic appearance of cytochrome oxidase negative fibers. Genetic studies have reported that approximately 80% of MELAS cases are caused by the mutation m.3243A>G of the mitochondrial transfer RNA (Leu (UUR)) gene (MT-TL1).

Conclusions:

MELAS involves multiple systems with variable clinical symptoms and recurrent episodes. The prognosis of MELAS patients depends on timely diagnosis. Therefore, overall diagnosis of MELAS should be based on the maternal inheritance family history, clinical manifestation, and findings from serial MRI, muscle biopsy, and genetics.

Characteristic cardiac phenotypes are detected by cardiovascular magnetic resonance in patients with different clinical phenotypes and genotypes of mitochondrial myopathy

BACKGROUND

Mitochondrial myopathies (MM) are a heterogeneous group of inherited conditions resulting from a primary defect in the mitochondrial respiratory chain with consecutively impaired cellular energy metabolism. Small sized studies using mainly electrocardiography (ECG) and echocardiography have revealed cardiac abnormalities ranging from conduction abnormalities and arrhythmias to hypertrophic or dilated cardiomyopathy in these patients. Recently, characteristic patterns of cardiac involvement were documented by cardiovascular magnetic resonance (CMR) in patients with chronic progressive external ophthalmoplegia (CPEO)/Kearns-Sayre syndrome (KSS) and with mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS). The present study aimed to characterize the prevalence and pattern of cardiac abnormalities and to test the additional diagnostic value of CMR in this patient population. The hypothesis that different neuromuscular MM syndromes present with different cardiac disease phenotypes was evaluated.

METHODS

Sixty-four MM patients (50 ± 15 years, 44% male) and 25 matched controls (52 ± 14 years, 36% male) prospectively underwent cardiac evaluations including CMR (comprising cine- and late-gadolinium-enhancement (LGE) imaging). Based on the neuromuscular phenotype and genotype, the patients were grouped: (a) CPEO/KSS (N = 33); (b) MELAS/-like (N = 11); c) myoclonic epilepsy with ragged-red fibers (MERRF) (N = 3) and d) other non-specific MM forms (N = 17).

RESULTS

Among the 64 MM patients, 34 (53%) had at least one abnormal CMR finding: 18 (28%) demonstrated an impaired left ventricular ejection-fraction (LV-EF <60%), 14 (22%) had unexplained LV hypertrophy and 21 (33%) were LGE-positive. Compared to controls, MM patients showed significantly higher maximal wall thickness (10 ± 3 vs. 8 ± 2 mm, p = 0.005) and concentricity (LV mass to end-diastolic volume: 0.84 ± 0.27 vs. 0.67 ± 0.11, p < 0.0001) with frequent presence of non-ischemic LGE (30% vs. 0%, p = 0.001). CPEO/KSS showed a predominantly intramural pattern of LGE mostly confined to the basal LV inferolateral wall (8/10; 80%) in addition to a tendency toward concentric remodelling. MELAS/-like patients showed the highest frequency of cardiac disease (in 10/11 (91%)), a mostly concentric LV hypertrophy (6/9; 67%) with or without LV systolic dysfunction and a predominantly focal, patchy LGE equally distributed among LV segments (8/11; 73%). Patients with MERRF and non-specific MM had no particular findings. Pathological CMR findings indicating cardiac involvement were detected significantly more often than pathological ECG results or elevated cardiac serum biomarkers (34 (53%) vs. 18 (28%) vs. 21 (33%); p = 0.008).

CONCLUSION

Cardiac involvement is a frequent finding in MM patients - and particularly present in KSS/CPEO as well as MELAS/-like patients. Despite a high variability in clinical presentation, CPEO/KSS patients typically show an intramural pattern of LGE in the basal inferolateral wall whereas MELAS patients are characterized by overt concentric hypertrophy and a rather unique, focally accentuated and diffusely distributed LGE.

Therapeutic strategies for mitochondrial disorders

OBJECTIVES

There is currently no curative therapy for mitochondrial disorders, although symptomatic measures can be highly effective and greatly improve the quality of life and outcome of these patients. This review highlights potential strategies for the therapeutic management of mitochondrial disorders.

METHODS

Data for this review were identified by searches of MEDLINE, Current Contents, using various relevant search terms.

RESULTS

Strategies to establish a therapeutic regimen aim to enhance respiratory chain function, eliminate noxious compounds, shift the heteroplasmy rate, alter mitochondrial dynamics, transfer cytoplasm, and promote gene therapy. Symptomatic measures rely on drugs (e.g., antiepileptics), avoidance of mitochondrion-toxic agents, substitution of blood cells, hemodialysis, invasive measures (such as a pacemaker), surgery (e.g., ptosis correction), physiotherapy, speech therapy, occupational therapy, dietary measures (e.g., ketogenic diet, anaplerotic diet), and the avoidance of mitochondrion-toxic agents (e.g., ozone). With the increasing awareness of mitochondrial disorders, the number of treatment studies is growing and its quality is improving. If high quality studies (high Jadad score) yield statistical significance for end points, a treatment is more reliable than with lower quality studies.

CONCLUSIONS

Despite the lack of a proven treatment for mitochondrial disorders, a nihilistic attitude toward treatment is not justified. A number of studies are seeking targeted therapies, and highly effective symptomatic measures are available.

Effect of L-carnitine on exercise performance in patients with mitochondrial myopathy

Exercise intolerance due to impaired oxidative metabolism is a prominent symptom in patients with mitochondrial myopathy (MM), but it is still uncertain whether L-carnitine supplementation is beneficial for patients with MM. The aim of our study was to investigate the effects of L-carnitine on exercise performance in MM. Twelve MM subjects (mean age±SD=35.4±10.8 years) with chronic progressive external ophthalmoplegia (CPEO) were first compared to 10 healthy controls (mean age±SD=29±7.8 years) before they were randomly assigned to receive L-carnitine supplementation (3 g/daily) or placebo in a double-blind crossover design. Clinical status, body composition, respiratory function tests, peripheral muscle strength (isokinetic and isometric torque) and cardiopulmonary exercise tests (incremental to peak exercise and at 70% of maximal), constant work rate (CWR) exercise test, to the limit of tolerance [Tlim]) were assessed after 2 months of L-carnitine/placebo administration. Patients with MM presented with lower mean height, total body weight, fat-free mass, and peripheral muscle strength compared to controls in the pre-test evaluation. After L-carnitine supplementation, the patients with MM significantly improved their Tlim (14±1.9 vs 11±1.4 min) and oxygen consumption (V˙O2) at CWR exercise, both at isotime (1151±115 vs 1049±104 mL/min) and at Tlim (1223±114 vs 1060±108 mL/min). These results indicate that L-carnitine supplementation may improve aerobic capacity and exercise tolerance during high-intensity CWRs in MM patients with CPEO.

Scoliosis in mitochondrial myopathy: case report and review of the literature

The mitochondrial myopathies include a diverse group of disorders characterized by morphological abnormalities of muscle mitochondria. Little is reported about spinal deformity associated with this syndrome.This study presents a case of scoliosis occurring in the setting of mitochondrial myopathies and explores the possible mechanisms between the 2 diseases.A previously unreported scoliosis in mitochondrial myopathies is described. The patient was a 16-year-old Chinese adolescent boy undergoing a posterior correction at thoracic 2-lumbar 3 (T2-L3) levels using the Moss-SI spinal system. At 48-month follow-up, the patient was clinically pain free and well balanced. Plain radiographs showed solid spine fusion with no loss of deformity correction. After evaluating 60 mitochondrial myopathies, patients referred to Peking Union Medical College Hospital from February 2009 to May 2013, the prevalence of scoliosis among patients with mitochondrial myopathies was 5% (3/60), much higher than that among general population (2%).The scoliosis in mitochondrial myopathies is usually extensive and progressively aggravated and the correction of the associated spinal deformities is generally difficult. Therefore, the exact role of mitochondrial myopathy in the development of scoliosis requires further study for a better understanding of the disease, as well as adequate and effective patient care.

Turn up the power - pharmacological activation of mitochondrial biogenesis in mouse models

The oxidative phosphorylation (OXPHOS) system in mitochondria is responsible for the generation of the majority of cellular energy in the form of ATP. Patients with genetic OXPHOS disorders form the largest group of inborn errors of metabolism. Unfortunately, there is still a lack of efficient therapies for these disorders other than management of symptoms. Developing therapies has been complicated because, although the total group of OXPHOS patients is relatively large, there is enormous clinical and genetic heterogeneity within this patient population. Thus there has been a lot of interest in generating relevant mouse models for the different kinds of OXPHOS disorders. The most common treatment strategies tested in these mouse models have aimed to up-regulate mitochondrial biogenesis, in order to increase the residual OXPHOS activity present in affected animals and thereby to ameliorate the energy deficiency. Drugs such as bezafibrate, resveratrol and AICAR target the master regulator of mitochondrial biogenesis PGC-1α either directly or indirectly to manipulate mitochondrial metabolism. This review will summarize the outcome of preclinical treatment trials with these drugs in mouse models of OXPHOS disorders and discuss similar treatments in a number of mouse models of common diseases in which pathology is closely linked to mitochondrial dysfunction. In the majority of these studies the pharmacological activation of the PGC-1α axis shows true potential as therapy; however, other effects besides mitochondrial biogenesis may be contributing to this as well.

Neuronal and astrocyte dysfunction diverges from embryonic fibroblasts in the Ndufs4fky/fky mouse

Mitochondrial dysfunction causes a range of early-onset neurological diseases and contributes to neurodegenerative conditions. The mechanisms of neurological damage however are poorly understood, as accessing relevant tissue from patients is difficult, and appropriate models are limited. Hence, we assessed mitochondrial function in neurologically relevant primary cell lines from a CI (complex I) deficient Ndufs4 KO (knockout) mouse (Ndufs4fky/fky) modelling aspects of the mitochondrial disease LS (Leigh syndrome), as well as MEFs (mouse embryonic fibroblasts). Although CI structure and function were compromised in all Ndufs4fky/fky cell types, the mitochondrial membrane potential was selectively impaired in the MEFs, correlating with decreased CI-dependent ATP synthesis. In addition, increased ROS (reactive oxygen species) generation and altered sensitivity to cell death were only observed in Ndufs4fky/fky primary MEFs. In contrast, Ndufs4fky/fky primary isocortical neurons and primary isocortical astrocytes displayed only impaired ATP generation without mitochondrial membrane potential changes. Therefore the neurological dysfunction in the Ndufs4fky/fky mouse may partly originate from a more severe ATP depletion in neurons and astrocytes, even at the expense of maintaining the mitochondrial membrane potential. This may provide protection from cell death, but would ultimately compromise cell functionality in neurons and astrocytes. Furthermore, RET (reverse electron transfer) from complex II to CI appears more prominent in neurons than MEFs or astrocytes, and is attenuated in Ndufs4fky/fky cells.

Ultrastructural mitochondrial alterations in equine myopathies of unknown origin

BACKGROUND

Very few mitochondrial myopathies have been described in horses.

OBJECTIVE

To examine the ultrastructure of muscle mitochondria in equine cases of myopathy of unknown origin.

MATERIALS & METHODS

Biopsies of vastus lateralis of the Musculus quadriceps femoris were taken predominantly immediately post mortem and processed for transmission electron microscopy. As a result, electron micrographs of 90 horses in total were available for analysis comprising 4 control horses, 16 horses suffering from myopathy and 70 otherwise diseased horses.

RESULTS

Following a thorough clinical and laboratory work-up, four out of five patients that did not fit into the usual algorithm to detect known causes of myopathy showed ultrastructural mitochondrial alterations. Small mitochondria with zones with complete disruption of cristae associated with lactic acidemia were detected in a 17-year-old pony mare, extremely long and slender mitochondria with longitudinal cristae in a 5-year-old Quarter horse stallion, a mixture of irregular extremely large mitochondria (measuring 2500 by 800 nm) next to smaller ones in an 8-year-old Hanoverian mare and round mitochondria with only few cristae in a 11-year-old pony gelding. It remains uncertain whether the subsarcolemmal mitochondrial accumulations observed in the fifth patient have any pathological significance.

CONCLUSIONS

Ultrastructural alterations in mitochondria were detected in at least four horses. To conclude that these are due to mitochondrial dysfuntions, biochemical tests should be performed.

PRACTICAL APPLICATIONS

The possibility of a mitochondrial myopathy should be included in the differential diagnosis of muscle weakness.

Selected case from the Arkadi M. Rywlin International Pathology Slide Series: Mitochondrial myopathy presenting with chronic progressive external ophthalmoplegia (CPEO): a case report

A 43-year-old female patient diagnosed with chronic progressive external ophthalmoplegia (CPEO) because of mitochondrial myopathy documented by muscle biopsy is presented. The chief complaints were represented by blepharoptosis and ophthalmoplegia. The muscle biopsy was evaluated by histology, using the appropriate histochemical and histoenzimological stains. Ragged red fibers with Gomori trichrome stain were seen, which showed cytochrome c oxydase deficiency and abnormal succinate dehydrogenase staining in around 20% of muscle fibres. Electron microscopy was also performed which demonstrated abnormal, hyperplastic, pleomorphic, and hypertrophic mitochondria, characterized by paracrystalline inclusions arranged in parallel rows ("parking-lot" inclusions), consisting of rectangular arrays of mitochondrial membranes in a linear or grid-like pattern. In conclusion, mitochondrial myopathy was definitely diagnosed. Although molecular analysis, which was subsequently carried out, failed to reveal mutations in the mitochondrial DNA or in selected nuclear genes, the pathologic diagnosis was not changed. The differential diagnosis of CPEO with other forms of ocular myopathies as well as the possible association of CPEO with systemic syndromes is discussed. Ophtalmologists and medical internists should always suspect CPEO when dealing with patients affected by ocular myopathy, either in its pure form or in association with other myopathic or systemic signs.

Late-Emerging Lethal Exacerbation of Cardiomyopathy in a Patient with Mitochondrial Myopathy After Total Hip Arthroplasty: A Case Report

CASE

Mitochondrial myopathy comprises heterogeneous neuromuscular disorders caused by mitochondrial dysfunction. Acute exacerbation of mitochondrial myopathy infrequently occurs sequentially after perioperative stress. We present an unusual case of a late-emerging and lethal exacerbation of mitochondrial myopathy after total hip arthroplasty. Despite special attention paid to perioperative control, the patient's condition drastically deteriorated on postoperative day thirteen, and she died later as a result of cardiomyopathy.

CONCLUSION

When performing surgery on a patient with mitochondrial myopathy, the merits of surgery must always surpass the possible grave risk of this condition.

Physiological functions of peroxisome proliferator-activated receptor β

The peroxisome proliferator-activated receptors, PPARα, PPARβ, and PPARγ, are a family of transcription factors activated by a diversity of molecules including fatty acids and fatty acid metabolites. PPARs regulate the transcription of a large variety of genes implicated in metabolism, inflammation, proliferation, and differentiation in different cell types. These transcriptional regulations involve both direct transactivation and interaction with other transcriptional regulatory pathways. The functions of PPARα and PPARγ have been extensively documented mainly because these isoforms are activated by molecules clinically used as hypolipidemic and antidiabetic compounds. The physiological functions of PPARβ remained for a while less investigated, but the finding that specific synthetic agonists exert beneficial actions in obese subjects uplifted the studies aimed to elucidate the roles of this PPAR isoform. Intensive work based on pharmacological and genetic approaches and on the use of both in vitro and in vivo models has considerably improved our knowledge on the physiological roles of PPARβ in various cell types. This review will summarize the accumulated evidence for the implication of PPARβ in the regulation of development, metabolism, and inflammation in several tissues, including skeletal muscle, heart, skin, and intestine. Some of these findings indicate that pharmacological activation of PPARβ could be envisioned as a therapeutic option for the correction of metabolic disorders and a variety of inflammatory conditions. However, other experimental data suggesting that activation of PPARβ could result in serious adverse effects, such as carcinogenesis and psoriasis, raise concerns about the clinical use of potent PPARβ agonists.

Differentiation of the intracellular structure of slow- versus fast-twitch muscle fibers through evaluation of the dielectric properties of tissue

Slow-twitch (type 1) skeletal muscle fibers have markedly greater mitochondrial content than fast-twitch (type 2) fibers. Accordingly, we sought to determine whether the dielectric properties of these two fiber types differed, consistent with their distinct intracellular morphologies. The longitudinal and transverse dielectric spectrum of the ex vivo rat soleus (a predominantly type 1 muscle) and the superficial layers of rat gastrocnemius (predominantly type 2) (n = 15) were measured in the 1 kHz-10 MHz frequency range and modeled to a resistivity Cole-Cole function. Major differences were especially apparent in the dielectric spectrum in the 1 to 10 MHz range. Specifically, the gastrocnemius demonstrated a well-defined, higher center frequency than the soleus muscle, whereas the soleus muscle showed a greater difference in the modeled zero and infinite resistivities than the gastrocnemius. These findings are consistent with the fact that soleus tissue has larger and more numerous mitochondria than gastrocnemius. Evaluation of tissue at high frequency could provide a novel approach for assessing intracellular structure in health and disease.

Metabolic myopathies

PURPOSE OF REVIEW

The metabolic myopathies result from inborn errors of metabolism affecting intracellular energy production due to defects in glycogen, lipid, adenine nucleotides, and mitochondrial metabolism. This article provides an overview of the most common metabolic myopathies.

RECENT FINDINGS

Our knowledge of metabolic myopathies has expanded rapidly in recent years, providing us with major advances in the detection of genetic and biochemical defects. New and improved diagnostic tools are now available for some of these disorders, and targeted therapies for specific biochemical deficits have been developed (ie, enzyme replacement therapy for acid maltase deficiency).

SUMMARY

The diagnostic approach for patients with suspected metabolic myopathy should start with the recognition of a static or dynamic pattern (fixed versus exercise-induced weakness). Individual presentations vary according to age of onset and the severity of each particular biochemical dysfunction. Additional clinical clues include the presence of multisystem disease, family history, and laboratory characteristics. Appropriate investigations, timely treatment, and genetic counseling are discussed for the most common conditions.