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Introduction/aims Immune-mediated necrotizing myopathy (IMNM) is an immune-mediated myopathy typically presenting with progressive subacute weakness and characteristic, but nonspecific, myopathological findings. Atypical cases however can mimic other inherited or acquired myopathies, depriving patients of treatment. We describe a cohort of such patients. Methods We retrospectively identified IMNM patients who either previously carried a diagnosis of an inherited myopathy established on clinicopathological grounds or whose muscle biopsies displayed atypical features suggestive of a different myopathy. Results Among 131 IMNM patients, seven previously unreported patients (5%) met one of the above criteria. Three patients were diagnosed with limb-girdle muscular dystrophy on the basis of a chronic progressive course of weakness and family history of myopathy or cardiomyopathy. The other four patients displayed atypical histological features (two prominent mitochondrial abnormalities, one myofibrillar pathology, and one granulomatous inflammation). Immunostaining of biopsies from 12 additional IMNM patients did not identify myofibrillar pathology. The patient with granulomatous inflammation was known to have pulmonary sarcoidosis. Genetic testing for inherited myopathies was unrevealing. Antibodies against 3-hydroxy-3-methylglutaryl-CoA reductase or signal recognition particle were identified in 5 and 1 patients, respectively. Four patients presented with slowly progressive weakness over 3-13 y, while weakness was subacute over ≤6 mo in three patients. All patients responded to immunomodulatory therapy. Discussion Atypical clinical and histological features can occur in IMNM patients, causing delays in diagnosis and treatment. Clinicians should, therefore, consider IMNM in the differential diagnosis of unexplained proximal myopathies in spite of atypical clinical and myopathological findings.

Introduction Oxygen uptake efficiency slope (OUES) is a noninvasive cardiopulmonary exercise testing (CPET) measurement based on oxygen uptake (V˙O2 ) and minute ventilation (V˙E) and is a marker of the efficiency of oxygen utilization by the body. However, it has not been studied in mitochondrial disorders. We explored noninvasive CPET parameters, including OUES, as a way to reliably diagnose mitochondrial myopathy. Methods We performed cycle ergometer maximal exercise testing on definite and suspected mitochondrial myopathy subjects (MM-D and MM-S) and their age- and sex-matched controls. OUES was corrected for body surface area (OUES/BSA) to eliminate the effect of body size. Results A total of 40 participants, including 20 MM-D (n = 13; 6 males; aged 14-64 years) and 7 MM-S (5 males, aged 11-30 years) subjects and 20 controls, completed the study. MM-D subjects showed lower aerobic fitness than controls. OUES/BSA was lower in MM-D subjects, suggesting inefficient oxygen utilization. Area under the curve (AUC) and 95% confidence interval (CI) for OUES/BSA (AUC, 0.91; 95% CI, 0.80-1.00), peak V˙O2 percent predicted (AUC, 0.95; 95% CI, 0.86-1.00), and V˙O2 /work slope (AUC, 0.94; 95% CI, 0.85-1.00) showed excellent ability to diagnose mitochondrial myopathy in MM-D subjects. We applied a diagnostic approach based on the parameters just noted to MM-S subjects and their controls and were able to support or disprove the diagnosis of mitochondrial myopathy. Discussion We proposed and applied an approach based on the aformentioned three CPET parameters to diagnose mitochondrial myopathy reliably and found it to be clinically useful.

Introduction: Acyl-coenzyme A dehydrogenase 9 (ACAD9) has a role in mitochondrial complex I (CI) assembly. Only a few patients who carry ACAD9 mutations have been reported. They mainly present with severe hypertrophic cardiomyopathy, although a minority have only mild isolated myopathy. Although the secondary factors influencing disease severity have not been elucidated, conservation of CI assembly and residual enzymatic activity have been suggested as explanations for the mild phenotypes associated with ACAD9 mutations. Methods: We report a novel homozygous ACAD9 mutation (c.1240C>T; p.Arg414Cys) in a 34-year-old woman who presented with non-progressive myopathy. Results: We show that this ACAD9 mutation led to a severe defect in CI assembly in the patient's muscle. Furthermore, the impact of CI deficiency is confirmed by accumulation of mitochondrial DNA deletions. Conclusion: Our data suggest that a major defect of CI assembly is not responsible for a severe phenotype. Muscle Nerve 55: 919–922, 2017

Introduction Macrophagic myofasciitis (MMF) is a rare disorder, reported mainly in European adults, with occasional childhood cases. We report a series of 6 patients with pediatric MMF from the Indian subcontinent. Methods Clinical details, creatine kinase levels, and results of electromyography are described for patients diagnosed with MMF. Fresh-frozen and formalin-fixed muscle biopsies were evaluated by hematoxylin-eosin staining, histochemistry, immunohistochemistry, and electron microscopy. Results Six of 2,218 muscle biopsies were diagnosed as MMF; patient charts were reviewed. The 6 patients were all children; all presented with hypotonia and/or motor delay. Mean age at diagnosis was 16.2 months. There were 4 boys and 2 girls. All had a history of hepatitis B vaccination. Histopathology revealed infiltration by sheets of large periodic acid–Schiff stain-positive histiocytes. Ultrastructural examination demonstrated needle-shaped crystals within histiocytes. One patient had a co-existent neuromuscular disorder, merosin-deficient congenital muscular dystrophy. Conclusions MMF is a rare inflammatory myopathy that should be considered in the differential diagnosis of congenital myopathies in children. Muscle Nerve 56: 71–77, 2017.

Objective: We studied mitochondrial impairment as a factor in the pathologic equivalent of sarcopenia, muscle fiber atrophy associated with increased age. Methods: Mitochondrial oxidative enzyme activities and coenzyme Q10 levels were measured in frozen human proximal limb muscles with combined age and atrophy, age alone, atrophy alone, denervation, immune myopathies, and mitochondrial disorders with ophthalmoplegia. Results: Sarcopenia (age and atrophy) had reduced mean activities of mitochondrial Complexes I, II, and II+III, with severe reduction of Complex I activity in 54% of patients. Atrophy, and specific denervation atrophy, had similar patterns of changes. Age alone had moderately reduced Complex I activity. Mitochondrial myopathies had mildly lower Complex IV activity. Immune myopathies had unchanged enzyme activities. Interpretation: Mitochondrial oxidative enzyme activities, especially Complex I, but also Complexes II and II+III, are reduced in muscles with the pathologic equivalent of sarcopenia. Individually, atrophy and age have different patterns of oxidative enzyme changes. This article is protected by copyright. All rights reserved.

Introduction Neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP) is caused by m.8993T>G/C mutations in the mitochondrial adenosine triphosphate synthase subunit 6 gene (MT-ATP6). Traditionally, heteroplasmy levels between 70% and 90% lead to NARP, and >90% result in Leigh syndrome. Methods In this study we report a 30-year-old man with NARP and m.8993T>G in MT-ATP6. Results Although the patient carried the mutation in homoplasmic state in blood with similarly high levels in urine (94%) and buccal swab (92%), he presented with NARP and not the expected, more severe Leigh phenotype. The mutation could not be detected in any of the 3 analyzed tissues of the mother, indicating a large genetic shift between mother and offspring. Nerve biopsy revealed peculiar endoneurial Schwann cell nuclear accumulations, clusters of concentrically arranged Schwann cells devoid of myelinated axons, and degenerated mitochondria. Conclusions We emphasize the phenotypic variability of the m.8993T>G MT-ATP6 mutation and the need for caution in predictive counseling in such patients. Muscle Nerve 54: 328-333, 2016.

INTRODUCTION In myopathy patients, it is useful to measure skeletal muscle forces. Conventional methods require voluntary muscle activation, which can be unreliable. We evaluated a device for nonvoluntary force assessment. METHODS We tested 8 patients (unknown myopathy n = 2, inflammatory myopathy, facioscapulohumeral muscular dystrophy, mitochondrial myopathy, dysferlinopathy, multi-minicore disease, Becker-Kiener muscular dystrophy, n = 1 each). Isometric twitch torques of ankle dorsiflexors were measured after fibular nerve stimulation. RESULTS Six patients had decreased torques vs. 8 controls (men: median Newton-meter 1.6 vs. 5.7, women: 0.2 vs. 3.9, both P < 0.0001). Values correlated with Manual Muscle Test results (r = 0.73; r(2) = 0.53; P < 0.0001). In weak dorsiflexors, torque could be measured despite lower signal-to-noise ratios. In 2 patients with hypertrophy, we measured increased torques. CONCLUSIONS Nonvoluntary muscle force assessment can be used in patients with myopathies, and values correlate with voluntary forces determined by traditional methods. Muscle Nerve 53: 913-917, 2016.

Introduction: Mutations in the choline kinase beta (CHKB) gene are associated with a congenital muscular dystrophy with giant mitochondria at the periphery of muscle fibers. Methods: We describe a patient of Italian origin in whom whole-exome sequencing revealed a novel homozygous nonsense mutation, c.648C>A, p.(Tyr216*), in exon 5 of CHKB. Results: The patient presented with limb-girdle weakness and hypotonia from birth with mental retardation, and had sudden and transient deteriorations of muscle strength with acute intercurrent illnesses. Previously undescribed sarcolemmal overexpression of utrophin was noted in the muscle biopsy. Conclusions: Pathological features broaden the description of the entity and provide new insight in the pathogenic mechanisms. This case highlights the usefulness of next-generation sequencing in the diagnosis of rare and incompletely understood conditions. Muscle Nerve 51: 140–143, 2015

Introduction A 61-year-old woman with a five-year history of progressive muscle weakness and atrophy had a muscle biopsy characterized by a combination of dystrophic features (necrotic fibers and endomysial fibrosis) and mitochondrial alterations [ragged-red cytochrome c oxidase (COX)-negative fibers].

Introduction: Patients with mitochondrial myopathies may develop cardiac complications such as cardiomyopathy and/or cardiac conduction defects. To identify these potentially life-threatening and treatable conditions, it is common practice to screen patients intermittently with electrocardiography and echocardiography. The optimal time interval for such screening investigations is unknown. We developed this study to review our screening results in adult-onset patients with progressive external ophthalmoplegia (PEO). Methods: This study was a retrospective review of PEO patients with 5 years or more of cardiac screening investigations who did not have any cardiac symptoms. Results: Fifteen patients were included, and cardiomyopathy was identified on screening echocardiogram in 1 patient. Four patients had other abnormalities identified, which were unrelated to their mitochondrial myopathy. Conclusions: Only 1 patient in 15 developed cardiac complications related to mitochondrial disease during 5 years of follow-up. We suggest that a screening interval of 3–5 years is probably appropriate for adult-onset PEO patients who do not have cardiac symptoms. Muscle Nerve 60: 608–611, 2012

Case reports and open-label studies suggest that coenzyme Q(10) (CoQ(10)) treatment may have beneficial effects in mitochondrial disease patients; however, controlled trials are warranted to clinically prove its effectiveness. Thirty patients with mitochondrial cytopathy received 1200 mg/day CoQ(10) for 60 days in a randomized, double-blind, cross-over trial. Blood lactate, urinary markers of oxidative stress, body composition, activities of daily living, quality of life, forearm handgrip strength and oxygen desaturation, cycle exercise cardiorespiratory variables, and brain metabolites were measured. CoQ(10) treatment attenuated the rise in lactate after cycle ergometry, increased (∽1.93 ml) VO(2)/kg lean mass after 5 minutes of cycling (P < 0.005), and decreased gray matter choline-containing compounds (P < 0.05). Sixty days of moderate- to high-dose CoQ(10) treatment had minor effects on cycle exercise aerobic capacity and post-exercise lactate but did not affect other clinically relevant variables such as strength or resting lactate.

Cytochrome b mutations are rare causes of exercise intolerance. We report an 18-year-old man with exercise intolerance since childhood, resting lactic acidosis, cytochrome c oxidase (COX)-positive ragged-red fibers, and isolated muscle complex III deficiency due to a heteroplasmic m.14849T>C mutation in cytochrome b. We review previously described patients carrying mutations in the same gene. COX-positive ragged-red fibers together with exercise intolerance and lactic acidemia provide a clue for the diagnosis of this rare mitochondrial disorder.

Missense mutations in the gene for polymerase gamma 1 (POLG1) cause a number of phenotypically heterogeneous mitochondrial diseases, most commonly progressive external ophthalmoplegia, and are characterized by the accumulation of multiple, large-scale deletions of mitochondrial DNA. The triad of sensory ataxic neuropathy, dysarthria, and ophthalmoparesis (SANDO) has been demonstrated in a small subset of patients with POLG1 mutations. We report a sporadic case of an 80-year-old compound heterozygote man who presented with SANDO and was found to have three known pathogenic mutations in the POLG1 gene (p.T251I/p.P587L/p.G848S). To our knowledge, none of these mutations have been demonstrated previously in SANDO. This patient's late presentation illustrates that a mitochondrial disorder should be considered regardless of age if the clinical symptoms warrant.

We evaluated the diagnostic value of respiratory chain (RC) enzyme analysis of muscle in adult patients with mitochondrial myopathy (MM). RC enzyme activity was measured in muscle biopsies from 39 patients who carry either the 3243A>G mutation, other tRNA point mutations, or single, large-scale deletions of mtDNA. Findings were compared with those obtained from asymptomatic relatives with the 3243A>G mutation, myotonic dystrophy patients, and healthy subjects. Plasma lactate concentration, maximal oxygen uptake, and ragged-red fibers/cytochrome c-negative fibers in muscle were also determined. Only 10% of patients with the 3243A>G point mutation had decreased enzyme activity of one or more RC complexes, whereas this was the case for 83% of patients with other point mutations and 62% of patients with deletions. Abnormal muscle histochemistry was found in 65%, 100%, and 85% of patients, respectively, in these three groups. The results indicate that RC enzyme analysis in muscle is not a sensitive test for MM in adults. In these patients, abnormal muscle histochemistry appears to be a better predictor ofMM.

Autosomal dominant progressive external ophthalmoplegia (adPEO) is a mitochondrial disorder caused by mutations in nuclear genes. Here we report the clinical and genetic features of adPEO in a Chinese family. All patients had gradual onset of ptosis, with or without ophthalmoplegia, around age 30. Thirteen patients had limb weakness around age 40. Eight patients developed dysphagia around age 50. Four patients died of cardiac abnormalities around age 60. Muscle biopsy of the proband indicated mitochondrial myopathy characterized by ragged-red fibers, cytochrome c oxidase-negative fibers, and multiple deletions of mitochondrial DNA. A heterozygous missense mutation of c.1342A>G in the C10orf2 gene resulting in the p.448N>D mutation in the protein was found in the proband and four other affected family members. In summary, we identified an adPEO family with a novel C10orf2 gene mutation that manifested an age-dependent phenotype. It suggests that greater attention must be paid to cardiac abnormalities in the late stages of this disease.

We report the clinical, histochemical, and molecular genetic findings in a patient with progressive mitochondrial cytopathy due to the m.8313G>A point mutation in the mitochondrial tRNA(Lys) (MTTK) gene. The clinical features in this case are severe, including short stature, myopathy, peripheral neuropathy, and osteoporosis, while extensive analysis of maternal relatives indicate that the mutation has arisen de novo and was not maternally inherited. This report of a second case, together with single muscle fiber mutation analysis that shows clear segregation of mutation load with cytochrome c oxidase deficiency, confirms that the mutation is pathologic.

The human mitochondrial genome encodes 13 proteins. All are subunits of the respiratory chain complexes involved in energy metabolism. These proteins are translated by a set of 22 mitochondrial transfer RNAs (tRNAs) that are required for codon reading. Human mitochondrial tRNA genes are hotspots for pathogenic mutations and have attracted interest over the last two decades with the rapid discovery of point mutations associated with a vast array of neuromuscular disorders and diverse clinical phenotypes. In this review, we use a scoring system to determine the pathogenicity of the mutations and summarize the current knowledge of structure-function relationships of these mutant tRNAs. We also provide readers with an overview of a large variety of mechanisms by which mutations may affect the mitochondrial translation machinery and cause disease.

Patients with mitochondrial myopathies (MM) or myophosphorylase deficiency (McArdle's disease, McA) show impaired capacity for O(2) extraction, low maximal aerobic power, and reduced exercise tolerance. Non-invasive tools are needed to quantify the metabolic impairment. Six patients with MM, 6 with McA, 25 with symptoms of metabolic myopathy but negative biopsy (patient-controls, P-CTRL) and 20 controls (CTRL) underwent an incremental cycloergometric test. Pulmonary O(2) uptake (VO(2)) and vastus lateralis oxygenation indices (by near-infrared spectroscopy, NIRS) were determined. Concentration changes of deoxygenated hemoglobin and myoglobin (Delta[deoxy(Hb + Mb)]) were considered an index of O(2) extraction. Delta[deoxy(Hb + Mb)] peak (percent limb ischemia) was lower in MM (25.3 +/- 12.0%) and McA (18.7 +/- 7.3) than in P-CTRL (62.4 +/- 3.9) and CTRL (71.3 +/- 3.9) subjects. VO(2) peak and Delta[deoxy(Hb + Mb)] peak were linearly related (r(2) = 0.83). In these patients, NIRS is a tool to detect and quantify non-invasively the metabolic impairment, which may be useful in the follow-up of patients and in the assessment of therapies and interventions.

Mitochondrial disorders share common cellular consequences: (1) decreased ATP production; (2) increased reliance on alternative anaerobic energy sources; and (3) increased production of reactive oxygen species. The purpose of the present study was to determine the effect of a combination therapy (creatine monohydrate, coenzyme Q(10), and lipoic acid to target the above-mentioned cellular consequences) on several outcome variables using a randomized, double-blind, placebo-controlled, crossover study design in patients with mitochondrial cytopathies. Three patients had mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), four had mitochondrial DNA deletions (three patients with chronic progressive external ophthalmoplegia and one with Kearns-Sayre syndrome), and nine had a variety of other mitochondrial diseases not falling into the two former groups. The combination therapy resulted in lower resting plasma lactate and urinary 8-isoprostanes, as well as attenuation of the decline in peak ankle dorsiflexion strength in all patient groups, whereas higher fat-free mass was observed only in the MELAS group. Together, these results suggest that combination therapies targeting multiple final common pathways of mitochondrial dysfunction favorably influence surrogate markers of cellular energy dysfunction. Future studies with larger sample sizes in relatively homogeneous groups will be required to determine whether such combination therapies influence function and quality of life.

Exercise therapy improves mitochondrial function in patients with mitochondrial myopathy (MM). We undertook this study to determine the metabolic abnormalities that are improved by exercise therapy. This study identified metabolic pathology using 31P-magnetic resonance spectroscopy and magnetic resonance imaging (MRI) in a group of patients with MM compared to a control group matched for age, gender, and physical activity. We also observed the effect of exercise therapy for 12 weeks on muscle metabolism and physical function in the MM group. During muscle activity, there was impaired responsiveness of the mitochondria to changes in cytosolic adenosine diphosphate concentration, increased dependence on anaerobic energy pathways, and an adaptive increase in proton efflux in patients with MM. Following exercise therapy, mitochondrial function and muscle mass improved without any change in proton efflux rate. These metabolic findings were accompanied by improvements in functional ability. We conclude that there are significant metabolic differences between patients with MM and a control population, independent of age, gender, and physical activity. Exercise therapy can assist in improving mitochondrial function in MM patients. Muscle Nerve, 2005

Mutations in the mitochondrial genome contribute to the pathophysiology of many neuromuscular diseases. Recently there has been an increased appreciation of the role of mitochondrial DNA (mtDNA) mutations in the etiology of exercise intolerance. Using TTGE (temporal temperature-gradient gel electrophoresis) and sequence analyses of the entire mitochondrial genome, we identified a novel heteroplasmic mutation (8300T>C) in the tRNAlys gene (MTTK) from a patient with unexplained exercise intolerance. The mutation was present in blood, hair, and muscle, with the highest percentage of heteroplasmy found in muscle. The results of muscle respiratory chain enzyme analysis are consistent with tRNA mutation. These data suggest that this novel mutation is yet another mtDNA mutation associated with muscle disease and should be considered in patients with similar symptoms. Muscle Nerve, 2006

Patients with mitochondrial myopathies (MM) usually suffer from exercise intolerance due to their impaired oxidative capacity and physical deconditioning. We evaluated the effects of a 12-week supervised randomized rehabilitation program involving endurance training in patients with MM. Twenty MM patients were assigned to a training or control group. For three nonconsecutive days each week, patients combined cycle exercise at 70% of their peak work rate with three upper-body weight-lifting exercises performed at 50% of maximum capacity. Training increased maximal oxygen uptake (28.5%), work output (15.5%), and minute ventilation (40%), endurance performance (62%), walking distance in shuttle walking test (+95 m), and peripheral muscle strength (32%-62%), and improved Nottingham Health Profile scores (21.47%) and clinical symptoms. Control MM patients did not change from baseline. Results show that our exercise program is an adequate training strategy for patients with mitochondrial myopathy.

We report five patients with mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) who had demyelinating peripheral neuropathy. The MNGIE neuropathy had clinical and electrodiagnostic features typical of acquired, rather than inherited, etiologies. In fact, three patients were actually treated for chronic inflammatory demyelinating polyneuropathy (CIDP). We discuss findings that may help distinguish patients with MNGIE from those with CIDP.

We report 50 patients with various clinical phenotypes of mitochondrial disease studied over the past 10 years in a large urban area (Madrid Health Area 5). The clinical phenotypes showed a large variety of abnormalities in molecular biology and biochemistry. The prevalence of mitochondrial diseases was found to be 5.7 per 100,000 in the population over 14 years of age. Clinical and electrophysiological assessment reveal signs of neuropathy in 10 patients. Electromyographic findings consistent with myopathy were obtained in 37 cases. Six patients died of medical complications. Disease phenotype influenced survival to some degree (P < 0.01). Age of onset and gender were not associated with differences in survival. Mitochondrial disease is thus far more common than expected and a common cause of chronic morbidity.

The syndrome of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episode (MELAS) is typically associated with a single point mutation in the mitochondrial genome (mtDNA). Because mtDNA is known to have a higher mutation rate than nuclear DNA, we speculate that some patients with MELAS syndrome may harbor more than one mutation in mtDNA. For this purpose, mtDNA extracted from muscle containing dysmorphic mitochondria from a 32-year-old man with MELAS was sequenced in its entirety to identify all possible mutations. The result showed a homoplasmic A14693G and a heteroplasmic A3243G. The A14693G transition was not present in 205 unrelated control individuals, was not seen in 76 species randomly selected from GenBank, and appears to disrupt the base pairing within the T-loop of mtDNA tRNA(Glu). His asymptomatic siblings' blood showed wild-type at these positions, whereas the blood of the patient's oligosymptomatic diabetic mother had a heteroplasmic A14693G and an apparent homoplasmic wild-type A3243, suggesting an association of A14693G with diabetes mellitus. This case demonstrates the importance of sequencing the mtDNA in its entirety to evaluate the molecular basis of mitochondriopathy.

Consecutive 60-min microdialysis samples were taken from the tibial anterior muscle in 11 healthy subjects and 4 patients with mitochondrial myopathy before (2-3 samples) and after (3-4 samples, 2 controls and 1 patient excluded) sustained isometric foot dorsiflexions. Before exercise, mean concentrations of lactate, pyruvate, hypoxanthine, urate, aspartate, and glutamate did not significantly differ between controls and patients. After exercise, the controls showed significantly increased concentrations of lactate, pyruvate, and urate, decreased hypoxanthine, and no change in aspartate and glutamate. Similar findings were observed in the patients. Plasma lactate was unchanged. Exercise-induced increase in integrated electromyogram amplitude and rated subjective fatigue were correlated to increased post-exercise lactate concentrations, with no obvious difference between the groups. Microdialysis of skeletal muscle allows the detection and monitoring of biochemical changes in the interstitial space. With the exercise protocol used, however, it was not possible to demonstrate any biochemical difference between healthy controls and patients with mitochondrial myopathy.

The metabolic myopathies are a heterogeneous group of disorders inherited by a variety of modes that include gene defects in both the nuclear and mitochondrial genomes. Many factors impact on the expression of the pathogenic mutations that cause these disorders including genetic background, environmental factors, and coexisting disorders. Molecular technology has greatly improved the ability to make definitive diagnoses in many of the metabolic myopathies in the last decade and particularly has demonstrated that the wide diversity in the severity of mutations contributes to understanding genotype-phenotype correlations. In some cases, molecular testing obviates the necessity to perform an invasive muscle biopsy. However, it is also clear that the diagnostic yield from molecular testing is incomplete and particularly low among the mitochondrial myopathies as a group, ranging from approximately 6% to 19% in well-classified high-risk groups. Therefore, it is often essential to combine clinical, biochemical, histopathologic, and molecular data for each patient in order to arrive at a definitive diagnosis. The approach to the laboratory diagnosis of metabolic myopathies is described emphasizing both noninvasive and invasive testing, highlighting the molecular methodologies with the benefits and disadvantages of each technology, and documenting how to determine whether patients have coexisting disorders.

1H magnetic resonance spectroscopy (MRS) of the brain and (31)P MRS and saturation transfer of resting skeletal muscle were used to investigate intracellular metabolites and fluxes through the creatine kinase (CK) reaction in a patient with the syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). Acute cortical lesions were characterized by severely elevated lactate levels and reduced concentrations of N-acetylaspartyl compounds, glutamate, and myo-inositol. Similar but less extreme alterations were also observed in gray matter regions that appeared normal on magnetic resonance images. Investigation of the gastrocnemius muscle at rest demonstrated a reduced phosphocreatine level, elevated concentrations of inorganic phosphate and free adenosine 5'-diphosphate, and an abnormally low phosphorylation potential. Besides a moderately increased muscular phosphocreatine concentration, none of the metabolic disturbances detected on MRS improved with oral creatine supplementation. Forward and reverse fluxes through the CK reaction did not significantly change upon creatine treatment. Follow-up MRS investigations may thus provide objective markers of treatment response in vivo without the hazards or inconvenience of biopsy.

We report a patient with progressive external ophthalmoplegia (PEO), exercise intolerance, and deafness after aminoglycoside exposure, harboring two pathogenic mutations in her mtDNA: an A1555G in the 12S rRNA gene and a G4309A in the tRNA(Ile) gene. Muscle histochemistry showed abundant ragged-red fibers, and biochemistry revealed normal respiratory chain function. The A1555G mutation was homoplasmic in blood from the proband and from all maternal relatives. The G4309A mutation was abundant in the proband's muscle, less abundant in her blood, still less abundant in the mother's blood, and absent in blood from other maternal relatives. Family members were asymptomatic. Our data suggest that the former mutation resulted in aminoglycoside-induced deafness and the latter caused PEO plus exercise intolerance.

Mitochondrial myopathies (MM) are characterized by alterations in oxidative phosphorylation. The resultant increase in glycolytic flux produces a variable lactic acidosis. Intracellular acidification can induce both metabolic and, in the case of skeletal muscle, contractile dysfunction. Skeletal muscle lactate transporters have recently been identified which include both monocarboxylate transporter 1 (MCT1) and 4 (MCT4). Lactate import into oxidative skeletal muscle appears to be catalyzed by MCT1, whereas its extrusion from glycolytic fibers may be mediated by MCT4. We describe the expression of these isoforms in a patient with MM as compared to controls (n = 5). MCT4 content was 86% (>3 SD) higher in the patient with MM, whereas MCT1 content was less markedly elevated (47%), as compared to controls. These findings support previous work suggesting that the major role of MCT4 is to defend intracellular pH by extruding lactate and H+ to the interstitium. The role of MCT1 in MM is less clear. © 2001 John Wiley & Sons, Inc. Muscle Nerve 24: 394–398, 2001

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

We studied two siblings with a mitochondrial myopathy, familial thiamine deficiency, and an A3243G mutation of the mitochondrial DNA (mtDNA). The elder brother (patient 1, now 36 years old) developed myopathy and beriberi heart at 20 years of age. Thiamine therapy resolved the cardiac symptoms and hyperpyruvicemia and improved the myopathy. The younger brother presented aged 19 years with a myopathy (patient 2, now 35 years old). Thiamine deficiency was present in the siblings and parents, and ragged-red fibers (RRFs) were noted in muscle biopsies from the siblings. Analysis 17 years later demonstrated thiamine malabsorption and an A3243G mutation of the mtDNA in both siblings and their mother, progressive myopathy, and an increased number of RRFs and elevated serum CKMB activity in patient 1. Thiamine treatment decreased the serum concentrations of lactate and pyruvate in patient 2, but not patient 1. The role of thiamine in mitochondrial dysfunction caused by an electron transfer disorder in the setting of A3243G mtDNA mutation is discussed.

Phosphorus magnetic resonance spectroscopy (P-MRS) has now been used in the investigation of muscle energy metabolism in health and disease for over 15 years. The present review describes the basics of the metabolic observations made by P-MRS including the assumptions and problems associated with the use of this technique. Extramuscular factors, which may affect the P-MRS results, are detailed. The important P-MRS observations in patients with mitochondrial myopathies, including the monitoring of experimental therapies, are emphasized. The findings in other metabolic myopathies (those associated with glycolytic defects or endocrine disturbances) and in the destructive myopathies (the dystrophies and the inflammatory myopathies) are also described. Observations made in normal and abnormal fatigue, fibromyalgia, and malignant hyperthermia are considered. Finally, a summary of the possible diagnostic use of P-MRS in exercise intolerance is provided.

We found six patients with AMPD deficiency in muscle who were homozygous for the most common mutation, Q12X in the AMPD gene (AMPD1), associated with this disease. Three patients had AMPD deficiency alone, showing a mild clinical phenotype. Two patients showed a defect of PPL in muscle, and were homozygous for the most common mutation associated with McArdle's disease, R49X in the muscle PPL gene (PYGM). In one of these patients, the clinical phenotype was more severe than usually seen in patients with McArdle's disease. The remaining patient harbored the mtDNA A3243G mutation, showing one of the usual clinical patterns associated with this mutation. We conclude that the Q12X mutation in AMPD1 may result in a mild clinical effect; that it is frequent in the Spanish population, and therefore frequently associated with other metabolic diseases; and that the effect of the association of AMPD and PPL deficiencies seems to be neutral.

We have previously demonstrated that patients with mitochondrial myopathies can benefit from short-term aerobic exercise training. In this study, we compared the responses to short-term aerobic training of patients with mitochondrial myopathies, patients with nonmetabolic myopathies, and sedentary normal subjects. Training consisted of 8 weeks of treadmill exercise at 70% to 85% of estimated maximum heart rate reserve. All groups showed significant improvements in estimated aerobic capacity as well as heart rate and blood lactate at submaximal exercise intensities. The increase in estimated aerobic capacity was greater in the mitochondrial myopathy patients than in the other two groups. Phosphorus magnetic resonance spectroscopy demonstrated increased oxidative capacity of muscle in patients with mitochondrial myopathies in response to this training but not in patients with other, nonmetabolic myopathies or sedentary control subjects. A self-assessed measurement of functional status (SF-36) suggested improved quality of life associated with the training. This study demonstrates that short-term aerobic training at low intensity can benefit patients with nonmetabolic myopathies but to a lesser extent than patients with mitochondrial myopathies.

We have evaluated needle muscle biopsies in 220 patients with various neuromuscular disorders, using a method developed previously at Tuft's University. The method uses a 14-gauge needle propelled by an automatic device. An average of 3.5 samplings were taken per patient. Muscle samples were used for histological and molecular genetic analysis, and for the isolation of muscle satellite cells for in vitro cultures. The biopsy is well tolerated by the patients who never declined multiple samplings. Complications were few and minor, with no sequelae. In most cases the small size of the muscle specimen (ca. 15 mg per sampling) was sufficient to perform the various procedures and to yield a diagnosis. Specimens were considered insufficient for histological results in 9 patients (4%), due to technical artifacts or insufficient material. We now routinely use this method, which has several advantages over the surgical technique for most muscle biopsies.

The majority of patients with MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) have an AG mutation at nucleotide 3243 in mitochondrial transfer (t)RNA. To date there have only been 10 reported cases of MELAS syndrome in patients with a TC mutation at position 3271 of mitochondrial tRNA. Although many of the clinical features are similar between patients with these different mutations, it appears that the age at onset is later for the 3271 mutation. This report provides information from a North American kindred with the 3271 mutation (n = 6 proven; n = 2 probable; n = 3 possible) that adds clinical, physiological, histological, and molecular information to the pool of information on this rare disorder. Many of these features were similar to previous reports of both 3243 and 3271 patients. We conclude that the phenotypic expression of these different mutations are similar, but the age of onset for 3271 patients is later than for 3243 patients. © 1998 John Wiley & Sons, Inc. Muscle Nerve,21: 25–33, 1998.

Fatigue in patients with mitochondrial cytopathies is associ- ated with decreased basal and postactivity muscle phosphocreatine (PCr). Creatine monohydrate supplementation has been shown to increase muscle PCr and high-intensity power output in healthy subjects. We studied the effects of creatine monohydrate administration (5 g PO b.i.d. ◊ 14 days → 2 g PO b.i.d. ◊ 7 days) in 7 mitochondrial cytopathy patients using a random- ized, crossover design. Measurements included: activities of daily living (vi- sual analog scale); ischemic isometric handgrip strength (1 min); basal and postischemic exercise lactate; evoked and voluntary contraction strength of the dorsiflexors; nonischemic, isometric, dorsiflexion torque (NIDFT, 2 min); and aerobic cycle ergometry with pre- and post-lactate measurements. Cre- atine treatment resulted in significantly (P < 0.05) increased handgrip strength, NIDFT, and postexercise lactate, with no changes in the other measured variables. We concluded that creatine monohydrate increased the strength of high-intensity anaerobic and aerobic type activities in patients with mitochondrial cytopathies but had no apparent effects upon lower in- tensity aerobic activities. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20: 1502-1509, 1997

31P-magnetic resonance spectroscopy (31P-MRS) provides new biochemical information on mitochondrial disorders affecting brain and muscle. To elucidate the mechanisms of mitochondrial abnormalities, however, animal models are needed. We assessed the movbr (mottled viable brindled) mouse for its value in studying (1) energetics of a mitochondrial disorder and (2) 31P-MRS changes associated with mitochondrial abnormalities in vivo. The maximal activity of succinate-cytochrome c reductase was significantly reduced in movbr muscle compared to controls, whereas cytochrome oxidase activity was only reduced in movbr brain. 31P-MRS of movbr brain showed an increased pH, but no changes in any metabolite ratios. The phosphocreatine (PCr) recovery rate after exercise was reduced in muscles from movbr mice, indicating impairment of oxidative metabolism. We conclude that movbr brain and muscle tissue have biochemical abnormalities consistent with mitochondrial impairment. The PCr recovery rate, measured by 31P-MRS, was sensitive to the muscle abnormality. This strain is best described as having chronic mitochondrial dysfunction. © 1997 John Wiley & Sons, Inc. Muscle Nerve20: 1352–1359, 1997

We present a family with severe exercise intolerance, progressive proximal weakness, and lactic acidemia. Fifteen of 24 family members in five generations were affected. Since the affected males do not have offspring at this time, the family pedigree is consistent with either maternal or autosomal dominant inheritance. Muscle histochemistry showed ragged-red fibers and electron microscopy showed globular mitochondrial inclusions. Biochemical analysis showed reduced muscle activities of mitochondrial NADH-cytochrome c reductase (1 of 2 patients), succinate-cytochrome c reductase (2 patients), and cytochrome c oxidase (2 patients). For 1 patient, sequence analysis of 44% of the muscle mitochondrial DNA including all 22 transfer RNA regions showed no point mutation with pathogenic significance. Southern blot analysis showed no deletion. Six affected members of the family were treated with methylprednisolone (0.25 mg/kg) for 3 months. Muscle strength, serum lactate, and energy metabolism at rest (measured by 31P magnetic resonance spectroscopy) significantly improved with treatment. © 1997 John Wiley & Sons, Inc. Muscle Nerve20: 1219–1224, 1997