Stress lactate in mitochondrial myopathy under constant, unadjusted workload

Diagnostic accuracy of the lactate stress test for detecting mitochondrial disorders: Systematic review and meta-analysis

Due to their variable phenotypes, mitochondrial disorders (MDs) can be difficult to diagnose. The absolute load lactate stress test (LSTA) and the relative load lactate stress test (LSTR) have been shown to be useful screening tools for the detection of MDs. In this study, we aimed to perform a meta-analysis to evaluate the diagnostic accuracy of these tests in detecting MDs. The study protocol was registered with PROSPERO (no. CRD42022331710). We performed a comprehensive search of PubMed, Web of Science and Scopus from January 10th, 2022 to July 27th, 2022 and included case-control and cohort diagnostic studies that targeted participants with MDs and used LSTA and/or LSTR as index tests. Two reviewers worked separately to compile information from selected articles. Risk of bias and applicability were assessed using the QUADAS-2 tool. Sensitivity and specificity, as well as diagnostic odds ratios (DORs) and area under the curve (AUC) were calculated using Meta-DiSc 2.0 and Stata software. The analysis included 14 studies with a total of 1064 participants, divided into six studies with 793 participants for LSTA and eight studies with 271 participants for LSTR. For LSTA the meta-analysis gave a pooled sensitivity of 0.67 (95 % CI 0.62, 0.72), a specificity of 0.93 (95 % CI 0.85, 0.97), DOR of 26.63 (95 % CI 10.99, 64.52), and AUC of 0.70 (95 % CI 0.66, 0.74). For LSTR, the pooled sensitivity was 0.52 (95 % CI 0.33, 0.70), specificity 0.94 (95 % CI 0.79, 0.99), DOR 18.14 (95 % CI 2.99, 109.85), and the AUC 0.80 (95 % CI 0.76, 0.83). LSTA and LSTR showed as screening tests moderate sensitivity and high specificity for MD diagnosis, particularly for LSTR. The choice of test may depend on the patient's individual aerobic capacity and motor skills and the availability of equipment.

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.

Letter to the Editor: Comments on "Obesity associated with a novel mitochondrial tRNACys m.5802A>G mutation in a Chinese family"

In a recent article Wang et al. reported about a 9-year-old Chinese male with obesity, insulin resistance, acanthosis nigricans, hyperuricemia, and non-alcoholic fatty liver disease (NALFD) being attributed to the variant m.5802A>G in tRNA(Cys) (Bioscience Reports (2020) 40(1), BSR20192153). Pathogenicity of the variant was substantiated by documentation of perturbed stability and mobility of the tRNA(Cys). The interesting study has a number of shortcomings, which do not allow drawing conclusions as provided before they are solved. Obesity is multifactorial and many differential causes of mtDNA variants were not discussed or excluded.

Biomarkers for Detecting Mitochondrial Disorders

(1) Objectives: Mitochondrial disorders (MIDs) are a genetically and phenotypically heterogeneous group of slowly or rapidly progressive disorders with onset from birth to senescence. Because of their variegated clinical presentation, MIDs are difficult to diagnose and are frequently missed in their early and late stages. This is why there is a need to provide biomarkers, which can be easily obtained in the case of suspecting a MID to initiate the further diagnostic work-up. (2) Methods: Literature review. (3) Results: Biomarkers for diagnostic purposes are used to confirm a suspected diagnosis and to facilitate and speed up the diagnostic work-up. For diagnosing MIDs, a number of dry and wet biomarkers have been proposed. Dry biomarkers for MIDs include the history and clinical neurological exam and structural and functional imaging studies of the brain, muscle, or myocardium by ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), MR-spectroscopy (MRS), positron emission tomography (PET), or functional MRI. Wet biomarkers from blood, urine, saliva, or cerebrospinal fluid (CSF) for diagnosing MIDs include lactate, creatine-kinase, pyruvate, organic acids, amino acids, carnitines, oxidative stress markers, and circulating cytokines. The role of microRNAs, cutaneous respirometry, biopsy, exercise tests, and small molecule reporters as possible biomarkers is unsolved. (4) Conclusions: The disadvantages of most putative biomarkers for MIDs are that they hardly meet the criteria for being acceptable as a biomarker (missing longitudinal studies, not validated, not easily feasible, not cheap, not ubiquitously available) and that not all MIDs manifest in the brain, muscle, or myocardium. There is currently a lack of validated biomarkers for diagnosing MIDs.

Muscle fatigue: general understanding and treatment

Muscle fatigue is a common complaint in clinical practice. In humans, muscle fatigue can be defined as exercise-induced decrease in the ability to produce force. Here, to provide a general understanding and describe potential therapies for muscle fatigue, we summarize studies on muscle fatigue, including topics such as the sequence of events observed during force production, in vivo fatigue-site evaluation techniques, diagnostic markers and non-specific but effective treatments.

Diagnostic Algorithm for Glycogenoses and Myoadenylate Deaminase Deficiency Based on Exercise Testing Parameters: A Prospective Study

Aim

Our aim was to evaluate the accuracy of aerobic exercise testing to diagnose metabolic myopathies.

Methods

From December 2008 to September 2012, all the consecutive patients that underwent both metabolic exercise testing and a muscle biopsy were prospectively enrolled. Subjects performed an incremental and maximal exercise testing on a cycle ergometer. Lactate, pyruvate, and ammonia concentrations were determined from venous blood samples drawn at rest, during exercise (50% predicted maximal power, peak exercise), and recovery (2, 5, 10, and 15 min). Biopsies from vastus lateralis or deltoid muscles were analysed using standard techniques (reference test). Myoadenylate deaminase (MAD) activity was determined using p-nitro blue tetrazolium staining in muscle cryostat sections. Glycogen storage was assessed using periodic acid-Schiff staining. The diagnostic accuracy of plasma metabolite levels to identify absent and decreased MAD activity was assessed using Receiver Operating Characteristic (ROC) curve analysis.

Results

The study involved 51 patients. Omitting patients with glycogenoses (n = 3), MAD staining was absent in 5, decreased in 6, and normal in 37 subjects. Lactate/pyruvate at the 10th minute of recovery provided the greatest area under the ROC curves (AUC, 0.893 ± 0.067) to differentiate Abnormal from Normal MAD activity. The lactate/rest ratio at the 10th minute of recovery from exercise displayed the best AUC (1.0) for discriminating between Decreased and Absent MAD activities. The resulting decision tree achieved a diagnostic accuracy of 86.3%.

Conclusion

The present algorithm provides a non-invasive test to accurately predict absent and decreased MAD activity, facilitating the selection of patients for muscle biopsy and target appropriate histochemical analysis.

Abnormal Lactate Levels Affect Motor Performance in Myotonic Dystrophy Type 1

Myotonic Dystrophy type 1 (DM1) is a dominantly inherited disease comprehending multiple features. Fatigue and exhaustion during exercise often represent significant factors able to negatively influence their compliance to rehabilitation programs. Mitochondrial abnormalities and a significant increase in oxidative markers, previously reported, suggest the hypothesis of a mitochondrial functional impairment. The study aims at evaluating oxidative metabolism efficiency in 18 DM1 patients and in 15 healthy subjects, through analysis of lactate levels at rest and after an incremental exercise test. The exercise protocol consisted of a submaximal incremental exercise performed on an electronically calibrated treadmill, maintained in predominantly aerobic condition. Lactate levels were assessed at rest and at 5, 10 and 30 minutes after the end of the exercise. The results showed early exercise-related fatigue in DM1 patients, as they performed a mean number of 9 steps, while controls completed the whole exercise. Moreover, while resting values of lactate were comparable between the patients and the control group (p=0.69), after the exercise protocol, dystrophic subjects reached higher values of lactate, at any recovery time (p<0,05). These observations suggest an early activation of anaerobic metabolism, thus evidencing an alteration in oxidative metabolism of such dystrophic patients. As far as intense aerobic training could be performed in DM1 patients, in order to improve maximal muscle oxidative capacity and blood lactate removal ability, then, this safe and validate method could be used to evaluate muscle oxidative metabolism and provide an efficient help on rehabilitation programs to be prescribed in such patients.

Biomarkers of peripheral muscle fatigue during exercise

BACKGROUND

Biomarkers of peripheral muscle fatigue (BPMFs) are used to offer insights into mechanisms of exhaustion during exercise in order to detect abnormal fatigue or to detect defective metabolic pathways. This review aims at describing recent advances and future perspectives concerning the most important biomarkers of muscle fatigue during exercise.

RESULTS

BPMFs are classified according to the mechanism of fatigue related to adenosine-triphosphate-metabolism, acidosis, or oxidative-metabolism. Muscle fatigue is also related to an immunological response. impaired calcium handling, disturbances in bioenergetic pathways, and genetic responses. The immunological and genetic response may make the muscle susceptible to fatigue but may not directly cause muscle fatigue. Production of BPMFs is predominantly dependent on the type of exercise. BPMFs need to change as a function of the process being monitored, be stable without appreciable diurnal variations, correlate well with exercise intensity, and be present in detectable amounts in easily accessible biological fluids. The most well-known BPMFs are serum lactate and interleukin-6. The most widely applied clinical application is screening for defective oxidative metabolism in mitochondrial disorders by means of the lactate stress test. The clinical relevance of most other BPMFs, however, is under debate, since they often depend on age, gender, physical fitness, the energy supply during exercise, the type of exercise needed to produce the BPMF, and whether healthy or diseased subjects are investigated.

CONCLUSIONS

Though the role of BPMFs during fatigue is poorly understood, measuring BPMFs under specific, standardised conditions appears to be helpful for assessing biological states or processes during exercise and fatigue.

Exercise testing in metabolic myopathies

Metabolic myopathies are a group of genetic disorders specifically affecting glucose/glycogen, lipid, and mitochondrial metabolism. The main metabolic myopathies that are evaluated in this article are the mitochondrial myopathies, fatty acid oxidation defects, and glycogen storage disease. This article focuses on the usefulness of exercise in the evaluation of genetic metabolic myopathies.

Metabolic myopathies: functional evaluation by different exercise testing approaches

Metabolic myopathies are a clinically and etiologically heterogeneous group of disorders due to defects in muscular energy metabolism. They include glycogen storage diseases, fatty acid oxidation defects, and mitochondrial disorders. The typical manifestations of a metabolic myopathy are exercise-induced myalgias, exercise intolerance, and cramps. Evaluating subjects with such symptoms is not easy because of the frequent lack of clinical features. Exercise tests are, therefore, reliable screening tools. Here, we discuss the possible role of such exercise testing techniques in the diagnostic approach of a patient with suspected metabolic myopathy.

LACTATE STRESS TESTING IN SPORADIC AMYOTROPHIC LATERAL SCLEROSIS

Mitochondrial dysfunction is frequently observed in ALS. Mitochondrial dysfunction may result in increased serum lactate at rest or low levels of exercise, being used for diagnostic purposes. The study investigated if resting-lactate-determination is superior to lactate-stress-testing (LST) in demonstrating mitochondrial dysfunction in ALS. Included were 15 ALS patients, 4 women, 11 men, aged 37-72. Severity of the disease was assessed by the Norris-score. The control group comprised 66 healthy subjects, 40 women, 26 men, aged 36-76. Serum lactate was determined before, three times during, and once after a constant workload with 30 W on a bicycle ergometer. According to the EIEscorial criteria 8 patients had definite, 4 probable, 3 possible ALS. Resting lactate was increased in 2 patients, 1 with definite and 1 with possible ALS. The LST was abnormal in 5 patients with definite, 1 with probable and 1 with possible ALS. The mean Norris-score was 67.8 in patients with abnormal LST and 74.6 in patients with normal LST. In conclusion, the LST is more suitable than resting-lactate-determination in demonstrating mitochondrial dysfunction in ALS. The LST suggests mitochondrial dysfunction in half of the ALS patients. Mitochondrial dysfunction in ALS is related to the clinical severity of the disease.

CONSEQUENCES OF MISDIAGNOSING MITOCHONDRIAL DISORDER

Diagnosing mitochondrial disorder remains a challenge. In a 75-year-old women, with short stature, muscle cramps, ptosis, fasciculations and progressive, proximal limb weakness and wasting, hyponatriemia, abnormal lactate-stress-test, and slightly abnormal electromyography, muscle biopsy suggested granulomatous myositis. Corticosteroids and azathioprin were ineffective. After a second work-up amyotrophic-lateral-sclerosis was diagnosed. Riluzole was started, without effect. She developed respiratory insufficiency, requiring mechanical ventilation. Apical ballooning was found. After switching to non-invasive positive pressure ventilation and physiotherapy, she markedly improved. After a third diagnostic work-up, mitochondrial disorder was suspected. Unfortunately, she died suddenly from a cardiac arrhythmia at home. Mitochondrial disorder may mimic motor neuron disease, muscle biopsy may mimic myositis, and may show only little evidence for respiratory chain disorder.

Apical ballooning (Takotsubo syndrome) in mitochondrial disorder during mechanical ventilation

Takotsubo syndrome may be associated with neuromuscular disorders, but has never been described in a patient with mitochondrial disorder. A 75-year-old woman developed muscle cramps, ptosis, fasciculations and slowly progressive weakness and wasting of all four limbs, starting 2.5 years earlier. After exclusion of various differential diagnoses, including non-specific granulomatous myositis, inclusion body myositis, and motor neuron disease, mitochondrial disorder was assumed. Muscle weakness progressed to respiratory insufficiency, requiring mechanical ventilation. Five days after intubation, she developed hypotension, torsades de pointes, ST-segment elevation, and negative T waves. Echocardiography revealed apical ballooning with akinesia of the left ventricular anteroseptal, apical, apicolateral and inferior segments. Coronary angiography was normal, and ventriculography confirmed apical hypokinesia and ballooning. Takotsubo syndrome was diagnosed, resolving completely within 7 weeks under bisoprolol. This case shows that Takotsubo syndrome occurs also in mitochondrial disorder and under mechanical ventilation, and may be triggered by stress from respiratory insufficiency, intubation, pain from tracheostomy, stress from mechanical ventilation, medication, or from the uncertain prognosis.

Successful heart failure therapy in mitochondrial disorder with noncompaction cardiomyopathy

OBJECTIVE

Effective heart failure therapy with angiotensin-converting enzyme inhibitors and beta-blockers in a patient with mitochondrial disorder and asymptomatic previously stable hypertrophic cardiomyopathy and left ventricular noncompaction (NCCMP) has not been reported.

CASE REPORT

In a 58-year-old male with juvenile seizures, impaired hearing, recurrent pancreatitis, diabetes, recurrent emesis and diarrhea, discrete weakness for hip flexion, general wasting, and reduced tendon reflexes, elevated muscle-enzymes, abnormal lactate-stress-test, and mitochondrial dysfunction on muscle biopsy, mitochondrial disorder was diagnosed at age 51 year. Echocardiography revealed myocardial thickening and NCCMP. Cardiac abnormalities did not progress upon repeated follow-ups. At age 57 year he developed acute heart failure during respiratory infection. Echocardiography additionally revealed reduced left ventricular systolic function, and a restrictive filling pattern. Within seven weeks of therapy with ramipril (2.5 mg/day) and bisoprolol (1.25 mg/day) the restrictive filling pattern disappeared and fractional shortening normalized.

CONCLUSION

This case shows that heart failure in a patient with mitochondrial disorder and previously stable hypertrophic cardiomyopathy and NCCMP promptly resolves under therapy with angiotensin-converting enzyme inhibitors and beta-blockers.