Quantitative near-infrared spectroscopy discriminates between mitochondrial myopathies and normal muscle

Skeletal muscle mitochondrial capacity in patients with statin-associated muscle symptoms (SAMS)

OBJECTIVE

The objective of this article is to evaluate near-infrared spectroscopy (NIRS), a non-invasive technique to assess tissue oxygenation and mitochondrial function, as a diagnostic tool for statin-associated muscle symptoms (SAMS).

METHODS

We verified SAMS in 39 statin-treated patients (23 women) using a double-blind, placebo-controlled, cross-over protocol. Subjects with suspected SAMS were randomised to simvastatin 20 mg/day or placebo for 8 weeks, followed by a 4-week no treatment period and then assigned to the alternative treatment, either simvastatin or placebo. Tissue oxygenation was measured before and after each statin or placebo treatment using NIRS during handgrip exercise at increasing intensities of maximal voluntary contraction (MVC).

RESULTS

44% (n=17) of patients were confirmed as having SAMS (11 women) because they reported discomfort only during simvastatin treatment. There were no significant differences in percent change in tissue oxygenation in placebo versus statin at all % MVCs in all subjects. The percent change in tissue oxygenation also did not differ significantly between confirmed and unconfirmed SAMS subjects on statin (-2.4% vs -2.4%, respectively) or placebo treatment (-1.1% vs -9%, respectively). The percent change in tissue oxygenation was reduced after placebo therapy in unconfirmed SAMS subjects (-10.2%) (p≤0.01) suggesting potential measurement variability.

CONCLUSIONS

NIRS in the forearm cannot differentiate between confirmed and unconfirmed SAMS, but further research is needed to assess the usability of NIRS as a diagnostic tool for SAMS.

TRIAL REGISTRATION NUMBER

NCT03653663.

Exercise Testing, Physical Training and Fatigue in Patients with Mitochondrial Myopathy Related to mtDNA Mutations

Mutations in mitochondrial DNA (mtDNA) cause disruption of the oxidative phosphorylation chain and impair energy production in cells throughout the human body. Primary mitochondrial disorders due to mtDNA mutations can present with symptoms from adult-onset mono-organ affection to death in infancy due to multi-organ involvement. The heterogeneous phenotypes that patients with a mutation of mtDNA can present with are thought, at least to some extent, to be a result of differences in mtDNA mutation load among patients and even among tissues in the individual. The most common symptom in patients with mitochondrial myopathy (MM) is exercise intolerance. Since mitochondrial function can be assessed directly in skeletal muscle, exercise studies can be used to elucidate the physiological consequences of defective mitochondria due to mtDNA mutations. Moreover, exercise tests have been developed for diagnostic purposes for mitochondrial myopathy. In this review, we present the rationale for exercise testing of patients with MM due to mutations in mtDNA, evaluate the diagnostic yield of exercise tests for MM and touch upon how exercise tests can be used as tools for follow-up to assess disease course or effects of treatment interventions.

Measurement of muscle blood flow and O2 uptake via near-infrared spectroscopy using a novel occlusion protocol

We describe here a novel protocol that sequentially combines venous followed by arterial occlusions to determine muscle blood flow and O₂ uptake from a single measurement point using near-infrared spectroscopy (NIRS) during handgrip exercise. NIRS data were obtained from the flexor digitorum superficialis (FDS) muscle on the dominant arm of 15 young, healthy adults (3 women; 26 ± 7 years; 78.6 ± 9.1 kg). Participants completed a series of 15-s static handgrip contractions at 20, 40 and 60% of maximal voluntary contraction (MVC) immediately followed by either a: (i) venous occlusion (VO); (ii); arterial occlusion (AO); or venous then arterial occlusion (COMBO). Each condition was repeated 3 times for each exercise-intensity. The concordance correlation coefficient (CCC) and robust linear mixed effects modeling were used to determine measurement agreement between vascular occlusion conditions. FDS muscle blood flow ([Formula: see text]) and conductance ([Formula: see text]) demonstrated strong absolute agreement between VO and COMBO trials from rest up to 60%MVC, as evidenced by high values for CCC (> 0.82) and a linear relationship between conditions that closely approximated the line-of-identity (perfect agreement). Conversely, although FDS muscle O₂ uptake ([Formula: see text]) displayed "substantial" to "near perfect" agreement between methods across exercise intensities (i.e., CCC > 0.80), there was a tendency for COMBO trials to underestimate [Formula: see text] by up to 7%. These findings indicate that the COMBO method provides valid estimates of [Formula: see text] and, to a slightly lesser extent, [Formula: see text] at rest and during static handgrip exercise up to 60%MVC. Practical implications and suggested improvements of the method are discussed.

Cardiac Surgery-Associated Kidney Injury in Children and Renal Oximetry

OBJECTIVES

Cardiac surgery-associated acute kidney injury is common in children and associates with negative outcomes. Novel interventions to reduce cardiac surgery-associated acute kidney injury require knowledge of its pathophysiology. States of altered perfusion, oxygen delivery, and energy consumption occur during cardiopulmonary bypass and could protect against or contribute to renal cellular injury and recovery. Near-infrared spectroscopy is noninvasive technology for monitoring regional blood flow and tissue oxygenation. This study evaluated the relationship between renal regional oxygen saturation and cardiac surgery-associated acute kidney injury, using near-infrared spectroscopy monitoring before, during, and after cardiopulmonary bypass in children.

DESIGN

Prospective cohort study.

SETTING

Single-center, tertiary care pediatric hospital (Stollery Children's Hospital, Edmonton, AB, Canada).

PATIENTS

Children less than or equal to 10 kg undergoing congenital heart disease repair with cardiopulmonary bypass. Heart transplant, preoperative dialysis, sepsis, extracorporeal life support, congenital renal disease, and preoperative nephrotoxins were exclusions.

INTERVENTIONS

Renal regional near-infrared spectroscopy monitoring before, during, and after cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

Outcome measure was cardiac surgery-associated acute kidney injury (defined according to Kidney Disease: Improving Global Outcomes criteria). Regional oxygen saturation was measured continuously using near-infrared spectroscopy (INVOS 5100C Cerebral/Somatic Oximeter; Medronic, Troy, MI) from time of anesthesia to time of transfer to intensive care. Cardiac surgery-associated acute kidney injury occurred in 65%. Lower baseline (precardiopulmonary bypass) regional oxygen saturation was associated with decreased risk of cardiac surgery-associated acute kidney injury (p = 0.01); children with baseline regional oxygen saturation in the highest tertile were 7.14 times more likely to get cardiac surgery- associated acute kidney injury (vs lowest tertile). Area under the curve for ability of baseline regional oxygen saturation to predict cardiac surgery-associated acute kidney injury was 0.73 (95% CI, 0.60-0.85). Children with lower baseline glomerular filtration rate had lower mean renal regional oxygen saturation.

CONCLUSIONS

Findings demonstrate that preoperative oxygen supply/demand balance is an important predictor of cardiac surgery-associated acute kidney injury, suggesting lower preoperative (and intraoperative) renal blood flow may be protective. There is not yet a definite link between remote ischemic preconditioning and prevention of cardiac surgery-associated acute kidney injury; however, renal protective effects of sublethal ischemia should continue to be explored.

Near infrared spectroscopy with a vascular occlusion test as a biomarker in children with mitochondrial and other neuro-genetic disorders

Background

Mitochondrial and neurogenetic diseases can present diagnostic challenges. We investigated if near infrared spectroscopy with the vascular occlusion test is able to differentiate between children with mitochondrial disease and children with neurogenetic disease or healthy controls.

Methods

Prospective observational study conducted in a tertiary children’s hospital.

Results

Forty-three children with mitochondrial disease (including both genetically confirmed primary mitochondrial disease and cases with biochemical evidence of mitochondrial dysfunction), 19 children with non-mitochondrial neurogenetic disease and 13 healthy controls were recruited. The delta tissue oxygen index (ΔTOI) values showed greater variability amongst children with mitochondrial disease and neurogenetic disease than healthy controls despite the median ΔTOI being similar (median 14.1 and 18.8, t-test, p = 0.16). A low ΔTOI identifies cases with a higher probability of mitochondrial disease or neurogenetic disease compared to healthy controls (positive likelihood ratio: 3.67; 95%CI:1.01–13). A high ΔTOI with the near infrared spectroscopy with vascular occlusion test identifies cases with a lower probability of having a disease (negative likelihood ratio: 0.51; 95%CI:0.36–0.74).

Conclusion

Near infrared spectroscopy with vascular occlusion test might be able to discriminate children with mitochondrial disease and neurogenetic disease from healthy controls.

Enhanced Local Skeletal Muscle Oxidative Capacity and Microvascular Blood Flow Following 7-Day Ischemic Preconditioning in Healthy Humans

Ischemic preconditioning (IPC), which involves intermittent periods of ischemia followed by reperfusion, is an effective clinical intervention that reduces the risk of myocardial injury and confers ischemic tolerance to skeletal muscle. Repeated bouts of IPC have been shown to stimulate long-term changes vascular function, however, it is unclear what metabolic adaptations may occur locally in the muscle. Therefore, we investigated 7 days of bilateral lower limb IPC (4 × 5 min) above limb occlusion pressure (220 mmHg; n = 10), or sham (20 mmHg; n = 10), on local muscle oxidative capacity and microvascular blood flow. Oxidative capacity was measured using near-infrared spectroscopy (NIRS) during repeated short duration arterial occlusions (300 mmHg). Microvascular blood flow was assessed during the recovery from submaximal isometric plantar flexion exercises at 40 and 60% of maximal voluntary contraction (MVC). Following the intervention period, beyond the late phase of protection (72 h), muscle oxidative recovery kinetics were speeded by 13% (rate constant pre 2.89 ± 0.47 min^-1 vs. post 3.32 ± 0.69 min^-1; P < 0.05) and resting muscle oxygen consumption (mO₂) was reduced by 16.4% (pre 0.39 ± 0.16%.s^-1 vs. post 0.33 ± 0.14%.s^-1; P < 0.05). During exercise, changes in deoxygenated hemoglobin (HHb) from rest to steady state were reduced at 40 and 60% MVC (16 and 12%, respectively, P < 0.05) despite similar measures of total hemoglobin (tHb). At the cessation of exercise, the time constant for recovery in oxygenated hemoglobin (O₂Hb) was accelerated at 40 and 60% MVC (by 33 and 43%, respectively) suggesting enhanced reoxygenation in the muscle. No changes were reported for systemic measures of resting heart rate or blood pressure. In conclusion, repeated bouts of IPC over 7 consecutive days increased skeletal muscle oxidative capacity and microvascular muscle blood flow. These findings are consistent with enhanced mitochondrial and vascular function following repeated IPC and may be of clinical or sporting interest to enhance or offset reductions in muscle oxidative capacity.

Value of oxyneurography, based on near infrared spectroscopy, in the diagnosis of carpal tunnel syndrome in comparison to provocative clinical diagnostic tests and nerve conduction studies

OBJECTIVE

Evaluation of the diagnostic utility of the oxyneurography (ONG) in diagnosing carpal tunnel syndrome (CTS).

METHODS

ONG examination of the median nerve was performed in 260 patients. The results were compared with nerve conduction studies and clinical provocative tests.

RESULTS

ONG index greater than or equal to 62% was found in 95.18% of the patients with no or minimal Nerve Conduction Study (NCS) changes (1-2 according to the Padua classification) but only in 1.69% of the patients with advanced NCS changes (Padua 3-6). The sensitivity and specificity of the ONG study i.e. 95.18% and 98.31%, respectively, were compared with standard clinical tests: Tinel sign (61.45% and 14.69%), Phalen test (34.94% and 45.20%), reverse Phalen test (81.93% and 34.46%) and carpal compression test (91.57% and 72.32%).

CONCLUSIONS

ONG index lower than 62% was indicative of CTS. ONG has higher sensitivity and specificity then other clinical tests and it is an accurate and reliable method for the diagnosis of CTS.

SIGNIFICANCE

Oxyneurography is a non-invasive, fast and safe study which may play role in the diagnosis of carpal tunnel syndrome.

Evaluation of muscle oxygenation by near infrared spectroscopy in patients with facioscapulohumeral muscular dystrophy

The purpose of the study was to determine muscle metabolism adaptation to exercise in facioscapulohumeral muscular dystrophy patients (FSHD) and to study the correlation with clinical functional status (6-min walk test). 8 FSHD patients and 15 age-matched healthy controls (Controls) performed two isokinetic constant-load knee extension exercises: (1) at 20% of their maximal extensors' peak torque (i.e., the same relative workload) and (2) at (20N⋅m) (the same absolute workload) for up to 4 min. All exercises consisted of rhythmic, voluntary, isokinetic, concentric contractions of the quadriceps femoris at 90°/s, whereas the flexion was performed passively at the same speed. Muscle oxygenation in the vastus lateralis was evaluated using near-infrared spectroscopy (NIRS). The FSHD patients displayed a lower maximal peak torque than controls (-41%, p < 0.05). During the two-exercise modalities, deoxygenated haemoglobin (HHb) and total haemoglobin volume (tHb) were lower in the FSHD patients (p < 0.05). The initial muscle deoxygenation time delay was shorter in the control group (FSHD: 15.1 ± 4.1 s vs.

CONTROLS

10.4 ± 2.1 s, p < 0.05). Mean response time and maximal peak torque were both correlated with functional impairment (walking endurance). The results suggest that FSHD patients present an impairment in their capacity to deliver or to use oxygen.

The impact of venous occlusion per se on forearm muscle blood flow: implications for the near-infrared spectroscopy venous occlusion technique

The purpose of this study was to examine the effect of venous occlusion per se on forearm muscle blood flow, as determined by the near-infrared spectroscopy (NIRS) venous occlusion technique (NIRS-VOT). NIRS data were obtained from the flexor digitorum superficialis (FDS) muscle on the dominant arm of 16 young, ostensibly healthy participants (14 men and two women; 30 ± 6 year; 73 ± 7 kg). Participants completed a series of five venous occlusion trials while seated at rest, and a series of 12 venous occlusion trials during a reactive hyperaemia induced by 5 min of forearm arterial occlusion. The NIRS-VOT was used to assess FDS muscle blood flow (Q˙mus), beat-by-beat, over the first four cardiac beats during venous occlusions. Q˙mus was also reported as a cumulative value, wherein the first two, first three and first four cardiac beats were used to calculate muscle blood flow. We observed that Q˙mus was highest when calculated over the first cardiac beat during venous occlusions performed at rest and throughout reactive hyperaemia (P<0·05). Moreover, the inclusion of more than one cardiac beat in the calculation of Q˙mus underestimated muscle blood flows, irrespective of the prevailing level of arterial inflow. These findings support the idea that venous occlusion per se affects the measurement of Q˙mus via the NIRS-VOT. Accordingly, it is recommended that Q˙mus is determined over the first cardiac beat when using the NIRS-VOT to assess microvascular blood flow of human forearm muscles.

Increased nerve vascularization detected by color Doppler sonography in patients with ulnar neuropathy at the elbow indicates axonal damage

INTRODUCTION

The aim of this study was to establish the prevalence of increased intraneural vascularization detected by ultrasonography (IVUS) in patients with ulnar neuropathy at the elbow (UNE) and to determine its relationship to clinical, ultrasonographic, and electrodiagnostic findings.

METHODS

High-resolution ultrasonography and color Doppler imaging were performed in 137 patients with confirmed UNE, 24 patient controls, and 70 healthy controls (HCs).

RESULTS

IVUS was found in 21 (15%) of 137 patients with UNE, in 1 (4%) of 24 patient controls, and in 0 of 70 HCs (P = 0.001). Patients with IVUS were more likely to have severe weakness (P = 0.01), severe atrophy of ulnar-innervated muscles (P = 0.008), axonal damage (P = 0.001), and more pronounced nerve enlargement (P = 0.03) than those without IVUS.

CONCLUSIONS

IVUS in the ulnar nerve can be detected in patients with UNE and is associated with nerve enlargement and clinical and electrodiagnostic severity. In addition, IVUS is associated with axonal damage.

Shoulder and forearm oxygenation and myoelectric activity in patients with work-related muscle pain and healthy subjects

We tested hypotheses of (a) reduced oxygen usage, oxygen recovery, blood flow and oxygen consumption; and (b) increased muscle activity for patients diagnosed with work-related muscle pain (WRMP) in comparison to healthy controls. Oxygenation was measured with near infrared spectroscopy (NIRS), and muscle activity with EMG for the extensor carpi radialis (ECR) and trapezius descendens (TD) muscles. Eighteen patients with diffuse neck-shoulder-arm pain and 17 controls (matched in age and sex) were equipped with NIRS and EMG probes. After determining an individual's maximum voluntary contraction (MVC) force, short-term (20 s) isometric contractions for the ECR and TD of 10, 30, 50 and 70 % MVC generated ∆StO₂ and StO₂% recovery (Rslope) from NIRS, and RMS%max from EMG signals. In addition, upper arm venous (VO) and arterial (AO) occlusions generated slopes of total hemoglobin (HbTslope) and deoxyhemoglobin (HHbslope) for the resting ECR as surrogates of blood flow and oxygen consumption, respectively. Mixed model analyses, t tests, and Mann-Whitney test were used to assess differences between groups. There was no significant difference in MVC between groups for either muscle. Also, ∆StO₂%, Rslope for either muscle, and ECR-HbTslope were not different between groups, thus our hypotheses of reduced oxygen use, recovery, and blood flow for patients were not confirmed. However, patients had a significantly lower ECR-HHbslope confirming our hypothesis of reduced consumption. Further, there was no difference in RMS%max during contractions meaning that the hypothesis of increased activity for patients was not confirmed. When taking into account the number of NIRS variables studied, differences we found between our patient group and healthy controls (i.e., in forearm oxygen consumption and shoulder oxygen saturation level) may be considered modest. Overall our findings may have been impacted by the fact that our patients and controls were similar in muscle strength, which is in contrast to previous studies.

Skeletal muscle reoxygenation after high-intensity exercise in mitochondrial myopathy

This study addressed whether O(2) delivery during recovery from high-intensity, supra-gas exchange threshold exercise would be matched to O(2) utilization at the microvascular level in patients with mitochondrial myopathy (MM). Off-exercise kinetics of (1) pulmonary O(2) uptake VO(2P) (2) an index of fractional O(2) extraction by near-infrared spectroscopy (Δ[deoxy-Hb + Mb]) in the vastus lateralis and (3) cardiac output (Q'(T)) by impedance cardiography were assessed in 12 patients with biopsy-proven MM (chronic progressive external ophthalmoplegia) and 12 age- and gender-matched controls. Kinetics of VO(2P) were significantly slower in patients than controls (τ = 53.8 ± 16.5 vs. 38.8 ± 7.6 s, respectively; p < 0.05). Q'(T), however, declined at similar rates (τ = 64.7 ± 18.8 vs. 73.0 ± 21.6 s; p > 0.05) being typically slower than [Formula: see text] in both groups. Importantly, Δ[deoxy-Hb + Mb] dynamics (MRT) were equal to, or faster than, τVO(2P) in patients and controls, respectively. In fact, there were no between-group differences in τVO(2P)MRTΔ[deoxy-Hb + Mb] (1.1 ± 0.4 vs. 1.0 ± 0.2, p > 0.05) thereby indicating similar rates of microvascular O(2) delivery. These data indicate that the slower rate of recovery of muscle metabolism after high-intensity exercise is not related to impaired microvascular O(2) delivery in patients with MM. This phenomenon, therefore, seems to reflect the intra-myocyte abnormalities that characterize this patient population.

A randomized trial of coenzyme Q10 in mitochondrial disorders

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.

Resting Muscle Oxygen Consumption by Near-Infrared Spectroscopy in Peripheral Arterial Disease: A Parameter to be Considered in a Clinical Setting?

Resting muscle VO 2 consumption (rmVO2) as measured by near-infrared spectroscopy (NIRS) has been poorly studied in peripheral arterial disease (PAD). We studied the feasibility of its assessment in a clinical setting, compared values from PAD and healthy participants, and identified factors affecting rmVO 2 in PAD. A total of 119 PAD patients with claudication and 30 healthy participants were enrolled. Ankle brachial index (ABI), adipose tissue thickness, and rmVO2 in the gastrocnemius after venous (rmVO2ven) or arterial (rmVO2art) occlusion were measured with NIRS. Compared to rmVO2art, rmVO2ven determination was less painful (P = .001), with higher values (P < .0001). rmVO2ven of PAD patients was not significantly different from healthy participants and was inversely correlated with the corresponding ABI (P = .018). rmVO2ven from severely diseased legs was higher than values from borderline/moderately diseased legs (P = .003). The determination of rmVO2ven by NIRS is suitable for the clinical setting and allows noninvasive quantification of a compensatory peripheral adaptation in patients with PAD.

Energy metabolism in human calf muscle performing isometric plantar flexion superimposed by 20-Hz vibration

Vibration training is commonly expected to induce an active muscle contraction via a complex reflex mechanism. In calf muscles of 20 untrained subjects, the additional energy consumption in response to vibration superimposed on an isometric contraction was examined by (31)P magnetic resonance spectroscopy and by near infrared spectroscopy. Subjects performed 3 min of isometric plantar flexion exercise at 40% MVC under four conditions: with (VIB) and without (CON) superimposed 20 Hz vibration at +/-2 mm amplitude, both combined with or without arterial occlusion (AO). After contraction under all conditions, the decreases in oxygenated haemoglobin were not significantly different. After VIB + AO consumption of ATP was increased by 60% over CON + AO, visible by significant decreases in [PCr] and intracellular pH (P < 0.05). The additional energy consumption by vibration was not detectable under natural perfusion. Probably without AO the additional energy consumption by vibration was compensated by oxidative phosphorylation enabled by additional perfusion.

Muscle deoxygenation of upper-limb muscles during progressive arm-cranking exercise

The purpose of this study was to determine which upper-limb muscle exhibits the greatest change in muscle deoxygenation during arm-cranking exercise (ACE). We hypothesized that the biceps brachii (BB) would show the greatest change in muscle deoxygenation during progressive ACE to exhaustion relative to triceps brachii (TR), brachioradialis (BR), and anterior deltoid (AD). Healthy young men (n = 11; age = 27 +/- 1 y; mean +/- SEM) performed an incremental ACE test to exhaustion. Near-infrared spectroscopy (NIRS) was used to monitor the relative concentration changes in oxy- (O2Hb), deoxy- (HHb), and total hemoglobin (Hbtot), as well as tissue oxygenation index (TOI) in each of the 4 muscles. During submaximal arm exercise, we found that changes to NIRS-derived measurements were not different between the 4 muscles studied (p > 0.05). At maximal exercise HHb was significantly higher in the BB compared with AD (p < 0.05). Relative to the other 3 muscles, BB exhibited the greatest decrease in O2Hb and TOI (p < 0.05). Our investigation provides two new and important findings: (i) during submaximal ACE the BB, TR, BR, and AD exhibit similar changes in muscle deoxygenation and (ii) during maximal ACE the BB exhibits the greatest change in intramuscular O2 status.

Supplemental oxygen and muscle metabolism in mitochondrial myopathy patients

Patients with mitochondrial myopathy (MM) have a reduced capacity to perform exercise due to a reduced oxidative capacity. We undertook this study to determine whether skeletal muscle metabolism could be improved with oxygen therapy in patients with MM. Six patients with MM and six controls, matched for age, gender and physical activity, underwent (31)P-magnetic resonance spectroscopy ((31)P-MRS) examination. (31)P-MR spectra were collected at rest and in series during exercise and recovery whilst breathing normoxic (0.21 O(2)) or hyperoxic (1.0 O(2)) air. At rest, MM showed an elevated [ADP] (18 +/- 3 micromol/l) and pH (7.03 +/- 0.01) in comparison to the control group (12 +/- 1 micromol/l, 7.01 +/- 0.01) (P < 0.05) consistent with mitochondrial dysfunction. Oxygen supplementation did not change resting metabolites in either MM or the control group (P > 0.05). Inferred maximal ATP synthesis rate improved by 33% with oxygen in MM (21 +/- 3 vs. 28 +/- 5 mmol/(l min), P < 0.05) but only improved by 5% in controls (40 +/- 3 vs. 42 +/- 3 mmol/(l min), P > 0.05). We conclude that oxygen therapy is associated with significant improvements in muscle metabolism in patients with MM. These data suggest that patients with MM could benefit from therapies which improve the provision of oxygen.

Impaired oxygen extraction in metabolic myopathies: Detection and quantification by near-infrared spectroscopy

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 myopathies: diagnosis, exercise intolerance, and treatment options

Mitochondrial myopathies are caused by genetic mutations that directly influence the functioning of the electron transport chain (ETC). It is estimated that 1 of 8,000 people have pathology inducing mutations affecting mitochondrial function. Diagnosis often requires a multifaceted approach with measurements of serum lactate and pyruvate, urine organic acids, magnetic resonance spectroscopy (MRS), muscle histology and ultrastructure, enzymology, genetic analysis, and exercise testing. The ubiquitous distribution of the mitochondria in the human body explains the multiple organ involvement. Exercise intolerance is a common but often an overlooked hallmark of mitochondrial myopathies. The muscle consequences of ETC dysfunction include increased reliance on anaerobic metabolism (lactate generation, phosphocreatine degradation), enhanced free radical production, reduced oxygen extraction and electron flux through ETC, and mitochondrial proliferation or biogenesis (see article by Hood in current issue). Treatments have included antioxidants (vitamin E, alpha lipoic acid), electron donors and acceptors (coenzyme Q10, riboflavin), alternative energy sources (creatine monohydrate), lactate reduction strategies (dichloroacetate) and exercise training. Exercise is a particularly important modality in diagnosis as well as therapy (see article by Taivassalo in current issue). Increased awareness of these disorders by exercise physiologists and sports medicine practitioners should lead to more accurate and more rapid diagnosis and the opportunity for therapy and genetic counseling.

A mitochondrial tRNA aspartate mutation causing isolated mitochondrial myopathy

Several mutations in mitochondrial transfer RNA (tRNA) genes can cause mitochondrial myopathy. We describe a young girl who presented with pronounced exercise intolerance. The anaerobic threshold and the maximal oxygen consumption were decreased. She had decreased complex I and IV enzyme activity and ragged red fibers on muscle biopsy. An A to G transition at nucleotide position 7526 in tRNA Aspartate (tRNA(Asp)) gene was heteroplasmic in several of the patient's tissues. We were unable to detect the mutation in muscle tissue from the patient's mother. This case adds a new genetic etiology for mitochondrial myopathy. It also illustrates for patients with combined deficiency of the complex I and IV enzyme activity the value of sequencing in the affected tissue muscle, and not only in blood, all mitochondrial tRNA genes including those not commonly affected, such as in this case mt tRNA(Asp).

Chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) is a descriptive term for a heterogenous group of disorders characterized by chronic, progressive, bilateral, and usually symmetric ocular motility deficit and ptosis. Significant pain, proptosis, or pupil involvement are not features of CPEO and should prompt evaluation for alternative etiologies. Mitochondrial DNA mutations are increasingly being recognized as the etiology for CPEO syndromes. Clinicians should recognize the specific syndromes associated with CPEO, characterized by variable systemic, neurologic, or other findings. Treatment is limited, but newer therapies are being investigated.

Near infrared muscle spectroscopy in patients with Friedreich's ataxia

Friedreich's ataxia is a progressive neurodegenerative disorder of the afferent cerebellar pathways associated with mitochondrial dysfunction at the cellular level. We have used noninvasive continuous near infrared muscle spectroscopy (NIRS) to investigate the delivery and utilization of oxygen in response to exercise in this disorder. Patients performed an incremental treadmill walking protocol in which levels of muscle deoxygenation or oxygenation were continuously measured in the medial calf muscle. The kinetics of recovery from exercise-induced deoxygenation, called the half-time of recovery (t(1/2)) were determined. The t(1/2) was prolonged in patients with Friedreich's ataxia compared with controls, and the degree of prolongation correlated with the length of the shorter GAA repeat, a genetic measure that correlates with the age of onset of disease. The t(1/2) also correlated inversely with patient age and with the maximum treadmill speed attained. Several patients also displayed features consistent with inadequate oxygen utilization by muscle. These results suggest that NIRS may be an effective tool for monitoring the biochemical and functional features of Friedreich's ataxia in parallel.

Development of lipophilic cations as therapies for disorders due to mitochondrial dysfunction

Mitochondrial dysfunction causes or exacerbates a number of diseases. These include genetic disorders such as Friedreich's ataxia where the primary lesion is a defect in a nuclear gene and those diseases caused by mutations to mitochondrial DNA. Mitochondrial damage also contributes to neurodegenerative diseases, diabetes and ischaemia-reperfusion injury. Drug therapies to prevent or alleviate mitochondrial dysfunction use redox active compounds, anti-oxidants or mitochondrial co-factors, however, their effectiveness is limited. A promising approach to increase the selectivity and potency of these compounds is to modify them so that they concentrate within mitochondria. This can be done by incorporating a lipophilic cation which causes the molecules to concentrate several hundred-fold in mitochondria, driven by the membrane potential across the inner membrane. As lipophilic cations cross biological membranes easily, they can be delivered to mitochondria of the heart, brain and skeletal muscle, the organs most affected by mitochondrial damage. Mitochondria-targeted lipophilic cations may lead to improved therapies for diseases involving mitochondrial dysfunction.

Performance of near-infrared spectroscopy in measuring local O(2) consumption and blood flow in skeletal muscle

The aim of this study was to investigate local muscle O(2) consumption (muscV(O(2))) and forearm blood flow (FBF) in resting and exercising muscle by use of near-infrared spectroscopy (NIRS) and to compare the results with the global muscV(O(2)) and FBF derived from the well-established Fick method and plethysmography. muscV(O(2)) was derived from 1) NIRS using venous occlusion, 2) NIRS using arterial occlusion, and 3) the Fick method [muscV(O(2(Fick)))]. FBF was derived from 1) NIRS and 2) strain-gauge plethysmography. Twenty-six healthy subjects were tested at rest and during sustained isometric handgrip exercise. Local variations were investigated with two independent and simultaneously operating NIRS systems at two different muscles and two measurement depths. muscV(O(2)) increased more than fivefold in the active flexor digitorum superficialis muscle, and it increased 1.6 times in the brachioradialis muscle. The average increase in muscV(O(2(Fick))) was twofold. FBF increased 1.4 times independent of the muscle or the method. It is concluded that NIRS is an appropriate tool to provide information about local muscV(O(2)) and local FBF because both place and depth of the NIRS measurements reveal local differences that are not detectable by the more established, but also more global, Fick method.