Lactate increase and oxygen desaturation in mitochondrial disorders – Evaluation of two diagnostic screening protocols

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.

Lowered oxidative capacity in spinal muscular atrophy, Jokela type; comparison with mitochondrial muscle disease

Introduction

Spinal muscular atrophy, Jokela type (SMAJ) is a rare autosomal dominantly hereditary form of spinal muscular atrophy caused by a point mutation c.197G>T in CHCHD10. CHCHD10 is known to be involved in the regulation of mitochondrial function even though patients with SMAJ do not present with multiorgan symptoms of mitochondrial disease. We aimed to characterize the cardiopulmonary oxidative capacity of subjects with SMAJ compared to healthy controls and patients with mitochondrial myopathy.

Methods

Eleven patients with genetically verified SMAJ, 26 subjects with mitochondrial myopathy (MM), and 28 healthy volunteers underwent a cardiopulmonary exercise test with lactate and ammonia sampling. The effect of the diagnosis group on the test results was analysed using a linear model.

Results

Adjusted for sex, age, and BMI, the SMAJ group had lower power output (p < 0.001), maximal oxygen consumption (VO2 max) (p < 0.001), and mechanical efficiency (p < 0.001) compared to the healthy controls but like that in MM. In the SMAJ group and healthy controls, plasma lactate was lower than in MM measured at rest, light exercise, and 30 min after exercise (p ≤ 0.001-0.030) and otherwise lactate in SMAJ was lower than controls and MM, in longitudinal analysis p = 0.018. In MM, the ventilatory equivalent for oxygen was higher (p = 0.040), and the fraction of end-tidal CO2 lower in maximal exercise compared to healthy controls (p = 0.023) and subjects with SMAJ.

Conclusion

In cardiopulmonary exercise test, subjects with SMAJ showed a similar decrease in power output and oxidative capacity as subjects with mitochondrial myopathy but did not exhibit findings typical of mitochondrial disease.

Insights on the role of L-lactate as a signaling molecule in skin aging

L-lactate is a catabolite from the anaerobic metabolism of glucose, which plays a paramount role as a signaling molecule in various steps of the cell survival. Its activity, as a master tuner of many mechanisms underlying the aging process, for example in the skin, is still presumptive, however its crucial position in the complex cross-talk between mitochondria and the process of cell survival, should suggest that L-lactate may be not a simple waste product but a fine regulator of the aging/survival machinery, probably via mito-hormesis. Actually, emerging evidence is highlighting that ROS are crucial in the signaling of skin health, including mechanisms underlying wound repair, renewal and aging. The ROS, including superoxide anion, hydrogen peroxide, and nitric oxide, play both beneficial and detrimental roles depending upon their levels and cellular microenvironment. Physiological ROS levels are essential for cutaneous health and the wound repair process. Aberrant redox signaling activity drives chronic skin disease in elderly. On the contrary, impaired redox modulation, due to enhanced ROS generation and/or reduced levels of antioxidant defense, suppresses wound healing via promoting lymphatic/vascular endothelial cell apoptosis and death. This review tries to elucidate this issue.

Bi-Allelic COQ4 Variants Cause Adult-Onset Ataxia-Spasticity Spectrum Disease

BACKGROUND

COQ4 codes for a mitochondrial protein required for coenzyme Q10 (CoQ10 ) biosynthesis. Autosomal recessive COQ4-associated CoQ10 deficiency leads to an early-onset mitochondrial multi-organ disorder.

METHODS

In-house exome and genome datasets (n = 14,303) were screened for patients with bi-allelic variants in COQ4. Work-up included clinical characterization and functional studies in patient-derived cell lines.

RESULTS

Six different COQ4 variants, three of them novel, were identified in six adult patients from four different families. Three patients had a phenotype of hereditary spastic paraparesis, two sisters showed a predominant cerebellar ataxia, and one patient had mild signs of both. Studies in patient-derived fibroblast lines revealed significantly reduced amounts of COQ4 protein, decreased CoQ10 concentrations, and elevated levels of the metabolic intermediate 6-demethoxyubiquinone.

CONCLUSION

We report bi-allelic variants in COQ4 causing an adult-onset ataxia-spasticity spectrum phenotype and a disease course much milder than previously reported. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Diagnostic Values of Venous Peak Lactate, Lactate-to-pyruvate Ratio, and Fold Increase in Lactate from Baseline in Aerobic Exercise Tests in Patients with Mitochondrial Diseases

Objective Although aerobic exercise tests on cycle ergometry have long been used for initial assessments of cases of suspected mitochondrial disease, the test parameters in patients with final diagnoses of other diseases via the widely used 15 W for 15 minutes exercise protocol have not been fully characterized. Methods We retrospectively reviewed all patients who underwent the test at our institution. We classified the patients with genetic diagnoses or those who met previously reported clinical criteria as having mitochondrial diseases and those with a final diagnosis of another disease as having other diseases. Results were available from 6 patients with mitochondrial disease and 15 with other diseases. Results During the test, elevated venous peak lactate above the upper normal limit of healthy controls at rest [19.2 mg/dL (2.13 mM)] was observed in 3 patients with mitochondrial diseases (50.0%) and 5 with other diseases (33.3%). In the group of patients with elevated venous peak lactate, a lactate-to-pyruvate ratio of >20 was observed in all 3 patients with mitochondrial disease but in only 1 of the 5 with other diseases. More than a 2-fold increase in venous lactate from baseline was observed in 4 patients with mitochondrial disease (66.7%) and 1 with another disease (6.7%). Conclusion Elevated venous peak lactate levels were observed in patients with final diagnoses of other diseases, even under a low 15-minute workload at 15 W. The lactate-to-pyruvate ratio and increase in lactate level from baseline may add diagnostic value to venous peak lactate levels alone.

3D localized lactate detection in muscle tissue using double-quantum filtered 1 H MRS with adiabatic refocusing pulses at 7 T

PURPOSE

Lactate is a key metabolite in skeletal muscle and whole-body physiology. Its MR visibility in muscle is affected by overlapping lipid signals and fiber orientation. Double-quantum filtered (DQF) 1 H MRS selectively detects lactate at 1.3 ppm, but at ultra-high field the efficiency of slice-selective 3D-localization with conventional RF pulses is limited by bandwidth. This novel 3D-localized 1 H DQF MRS sequence uses adiabatic refocusing pulses to unambiguously detect lactate in skeletal muscle at 7 T.

METHODS

Lactate double-quantum coherences were 3D-localized using slice-selective Shinnar-Le Roux optimized excitation and adiabatic refocusing pulses (similar to semi-LASER). DQF MR spectra were acquired at 7 T from lactate phantoms, meat specimens with injected lactate (exploring multiple TEs and fiber orientations), and human gastrocnemius in vivo during and after exercise (without cuff ischemia).

RESULTS

Lactate was readily detected, achieving the full potential of 50% signal with a DQF, in solution. The effects of fiber orientation and TE on the lactate doublet (peak splitting, amplitude, and phase) were in good agreement with theory and literature. Exercise-induced lactate accumulation was detected with 30 s time resolution.

CONCLUSION

This novel 3D-localized 1 H DQF MRS sequence can dynamically detect glycolytically generated lactate in muscle during exercise and recovery at 7 T.

The Role of Lactate Exercise Test and Fasting Plasma C-Peptide Levels in the Diagnosis of Mitochondrial Diabetes: Analysis of Clinical Characteristics of 12 Patients With Mitochondrial Diabetes in a Single Center With Long-Term Follow-Up

Objective

The aim of this study was to analyze the clinical characteristics and the pattern of long-term changes of fasting plasma C-peptide in patients with mitochondrial diabetes (MD), and to provide guidance for the diagnosis and treatment of MD.

Methods

We retrieved MD patients with long-term follow-up at Peking Union Medical College Hospital from January 2005 to July 2021 through the medical record retrieval system and retrospectively analyzed their clinical data, biochemical parameters, fasting plasma C-peptide, glycosylated hemoglobin and treatment regimens. Non-parametric receiver operating characteristic (ROC) curves were used to assess the relationship between exercise test-related plasma lactate levels and suffering from MD.

Results

A total of 12 MD patients were included, with clinical characteristics of early-onset, normal or low body weight, hearing loss, maternal inheritance, and negative islet-related autoantibodies. In addition, patients with MD exhibited significantly higher mean plasma lactate levels immediately after exercise compared to patients with type 1 diabetes mellitus (T1DM) (8.39 ± 2.75 vs. 3.53 ± 1.47 mmol/L, p=0.003) and delayed recovery time after exercise (6.02 ± 2.65 vs. 2.17 ± 1.27 mmol/L, p=0.011). The optimal cut-off points identified were 5.5 and 3.4 mmol/L for plasma lactate levels immediately after and 30 minutes after exercise, respectively. The fasting plasma C-peptide levels decreased as a negative exponential function with disease progression (Y= 1.343*e-0.07776X, R2 = 0.4154). Treatment regimens in MD patients were varied, with no metformin users and a weak correlation between insulin dosage and body weight.

Conclusions

The increased level of plasma lactate during exercise or its delayed recovery after exercise would contribute to the diagnosis of MD. Changes of fasting plasma C-peptide in MD patients over the course of the disease indicated a rapid decline in the early stages, followed by a gradual slowing rate of decline.

Plasma lactate responses during and after submaximal handgrip exercise are not diagnostically helpful in mitochondrial myopathy

INTRODUCTION/BACKGROUND

Mitochondrial myopathy (MM) encompasses a clinical heterogenous group of patients that can be difficult to diagnose. The aim of this study was to investigate if changes in plasma lactate concentration during a 6-minute submaximal handgrip test (6MHGT) and a 20-minute post-exercise recovery period can be used as a diagnostic test for MM.

METHODS

Twenty-nine patients with MM and nineteen healthy controls (HC) performed an intermittent handgrip exercise test at ½ Hz for 6 min at 50% of maximal voluntary contraction force. We calculated the area under the curve (AUC) of change in plasma lactate during exercise and recovery and compared AUC between groups (MM vs. HC, and between MM subgroups based on disease severity).

RESULTS

The change in plasma lactate during exercise and recovery was similar in MM and HC (p = 0.65 and p = 0.57) and similar between MM subgroups (p ≥ 0.24).

CONCLUSION

Plasma lactate measured during and after a submaximal 6MHGT cannot be used as a diagnostic variable for MM.

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.

Mitochondrial multiorgan disorder syndrome score generated from definite mitochondrial disorders

OBJECTIVES

Mitochondrial disorders (MIDs) frequently present as mitochondrial multiorgan disorder syndrome (MIMODS) at onset or evolve into MIMODS during the course. This study aimed to find which organs and/or tissues are most frequently affected by MIMODS, which are the most frequent abnormalities within an affected organ, whether there are typical MIMODS patterns, and to generate an MIMODS score to assess the diagnostic probability for an MID.

METHODS

This is a retrospective evaluation of clinical, biochemical, and genetic investigations of adult patients with definite MIDs. A total of 36 definite MID patients, 19 men and 17 women, aged 29-82 years were included in this study. The diagnosis was based on genetic testing (n=21), on biochemical investigations (n=17), or on both (n=2).

RESULTS

The number of organs most frequently affected was 4 ranging from 1 to 9. MIMODS was diagnosed in 97% of patients. The organs most frequently affected were the muscle (97%), central nervous system (CNS; 72%), endocrine glands (69%), heart (58%), intestines (55%), and peripheral nerves (50%). The most frequent CNS abnormalities were leukoencephalopathy, prolonged visually evoked potentials, and atrophy. The most frequent endocrine abnormalities included thyroid dysfunction, short stature, and diabetes. The most frequent cardiac abnormalities included arrhythmias, systolic dysfunction, and hypertrophic cardiomyopathy. The most frequent MIMODS patterns were encephalomyopathy, encephalo-myo-endocrinopathy, and encepalo-myo-endocrino-cardiopathy. The mean ± 2SD MIMODS score was 35.97±27.6 (range =11-71). An MIMODS score >10 was regarded as indicative of an MID.

CONCLUSION

Adult MIDs manifest as MIMODS in the vast majority of the cases. The organs most frequently affected in MIMODS are muscles, CNS, endocrine glands, and heart. An MIMODS score >10 suggests an MID.

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.

SANDO syndrome in a cohort of 107 patients with CPEO and mitochondrial DNA deletions

OBJECTIVE

The sensory ataxic neuropathy with dysarthria and ophthalmoparesis (SANDO) syndrome is a subgroup of mitochondrial chronic progressive external ophthalmoplegia (CPEO)-plus disorders associated with multiple mitochondrial DNA (mtDNA) deletions. There is no systematic survey on SANDO in patients with CPEO with either single or multiple large-scale mtDNA deletions.

METHODS

In this retrospective analysis, we characterised the frequency, the genetic and clinical phenotype of 107 index patients with mitochondrial CPEO (n=66 patients with single and n=41 patients with multiple mtDNA deletions) and assessed these for clinical evidence of a SANDO phenotype. Patients with multiple mtDNA deletions were additionally screened for mutations in the nuclear-encoded POLG, SLC25A4, PEO1 and RRM2B genes. The clinical, histological and genetic data of 11 patients with SANDO were further analysed.

RESULTS

None of the 66 patients with single, large-scale mtDNA deletions fulfilled the clinical criteria of SANDO syndrome. In contrast, 9 of 41 patients (22%) with multiple mtDNA deletions and two additional family members fulfilled the clinical criteria for SANDO. Within this subgroup, multiple mtDNA deletions were associated with the following nuclear mutations: POLG (n=6), PEO1 (n=2), unidentified (n=2). The combination of sensory ataxic neuropathy with ophthalmoparesis (SANO) was observed in 70% of patients with multiple mtDNA deletions but only in 4% with single deletions. The combination of CPEO and sensory ataxic neuropathy (SANO, incomplete SANDO) was found in 43% of patients with multiple mtDNA deletions but not in patients with single deletions.

CONCLUSION

The SANDO syndrome seems to indicate a cluster of symptoms within the wide range of multisystemic symptoms associated with mitochondrial CPEO. SANO seems to be the most frequent phenotype associated with multiple mtDNA deletions in our cohort but not or is rarely associated with single, large-scale mtDNA deletions.

Forearm deoxyhemoglobin and deoxymyoglobin (deoxy[Hb + Mb]) measured by near-infrared spectroscopy (NIRS) using a handgrip test in mitochondrial myopathy

The purpose of this paper is to test whether peripheral oxygenation responses measured with near-infrared spectroscopy (NIRS) would differ between patients suffering from mitochondrial myopathy (MM) and healthy controls during an incremental handgrip exercise test. Two groups of subjects were studied: 11 patients with MM and 11 age- and gender-matched untrained healthy controls. A handgrip exercise until exhaustion protocol was used consisting of 2 min periods of work (½ Hz) at different intensities, separated by a 60 s rest period. The changes in deoxyhemoglobin and deoxymyoglobin (deoxy[Hb + Mb]) during each work step were expressed in percent to the maximum deoxy[Hb + Mb]-value measured during arterial occlusion in forearm muscles. A repeated measures analysis of variance was used to compare the increase in deoxy[Hb + Mb] between MM patients and controls with increasing intensity. Statistical analysis revealed a significant difference between both populations (P < 0.001) indicating that the increase in deoxy[Hb + Mb] showed a significantly different pattern in the two populations. In the post hoc analysis significant lower deoxy[Hb + Mb] -values were found for MM patients at every intensity. The results of this paper show significantly different skeletal muscle oxygenation responses, measured with an optical method as NIRS, between MM patients and age- and gender-matched healthy subjects at submaximal and maximal level during an incremental handgrip exercise. This optical method is thus a valuable tool to assess differences in peripheral oxygenation. Moreover, this method could be used as an evaluation tool for follow up in interventional pharmacological studies and rehabilitation programs.

Functional relevance of mitochondrial abnormalities in sporadic inclusion body myositis

Cytochrome c oxidase (COX)-deficient fibers and multiple mitochondrial DNA (mtDNA) deletions are frequent findings in sporadic inclusion body myositis (s-IBM). However, the functional impact of these defects is not known. We investigated oxygen desaturation during exercise using the forearm exercise test, accumulation of lactate during exercise using a cycle ergometry test and mitochondrial changes (COX-deficient fibers, biochemical activities of respiratory chain complexes, multiple mtDNA deletions by long-range polymerase chain reaction) in 10 patients with s-IBM and compared the findings with age and sex-matched normal and diseased controls (without mitochondrial disorders) as well as patients with mitochondrial disorder due to nuclear gene defects resulting in multiple mtDNA deletions (MITO group). The mean age of the s-IBM patients was 68.2 ± 5.7 years (range: 56-75). Patients with s-IBM had statistically significantly reduced oxygen desaturation (ΔsO2) during the handgrip exercise (p<0.05) and elevated peak serum lactate levels during cycle ergometry compared to normal controls (p<0.05). The percentage of COX-deficient fibers in s-IBM and MITO patients was significantly increased compared to normal controls (p<0.01). Five out of nine s-IBM patients had multiple mtDNA deletions. Thirty-three percent of s-IBM patients showed an increased citrate synthase content and decreased activities of complex IV (COX). The biochemical pattern of respiratory chain complexes in patients with s-IBM and MITO was similar. Histopathological analysis showed similar changes in s-IBM and MITO due to nuclear gene defects. Functional tests reflecting mitochondrial impairment suggest a contribution of mitochondrial defects to disease-related symptoms such as fatigue and exertion-induced symptoms.

Treatable inborn errors of metabolism causing neurological symptoms in adults

BACKGROUND

The identification of inborn errors of metabolism (IEM) in adults presenting with a wide range of neurological symptoms is a relatively new field in medicine. We sought to identify which treatable IEM have been diagnosed for the first time in adults and generate a protocol for metabolic screening targeting those treatable disorders.

METHODS

Medline/Pubmed searches of English language literature limited to the adult age group were performed. Diseases identified through this search were then compared to previously published lists of treatable IEM in both adults and children.

RESULTS

85% of the treatable conditions known to cause global developmental delay or intellectual disability in children had reports where the diagnosis of that IEM was made in one or more adult patients with neurological symptoms. Screening tests in blood, urine, CSF and MRI can detect most of these treatable conditions but the diagnostic accuracy of these screening tests in adults is not clear.

CONCLUSION

Treatable IEM need to be considered in the differential diagnosis of neurological symptoms in patients of any age.

[Metabolic and mitochondrial myopathies]

Metabolic myopathies include a broad group of diseases involving inherited enzyme defects in the various metabolic pathways and skeletal musculature. They show an extensive phenotypic variability of symptoms and different ages of manifestation. Symptoms often included intolerance to duress or permanent paresis. Some forms of metabolic myopathy, in particular mitochondriopathy, are associated with multsystemic organ participation. The diagnostics must be adjusted to individual cases and carried out in stages. Primary investigations should include blood parameters (e.g. creatine kinase measurement, muscle load tests and determination of the acylcarnitine spectrum) and a second step includes muscle biopsy for histological and enzyme investigations and special molecular genetic tests although the causative enzyme defect cannot be clarified in every case. On the other hand by means of a thorough investigation it is particularly important in patients with load intolerance to differentiate between other causes, in particular psychosomatic diseases. If this is not done there is a danger of classifying the symptoms of a metabolic myopathy as a somatoform disorder. Therapy is mostly symptom-oriented as Pompe disease is the only one which can be treated with enzyme replacement therapy.

Blood metabolite data in response to maximal exercise in healthy subjects

Maximal exercise test with gas exchange measurement evaluates exercise capacities with maximal oxygen uptake (VO(2) max) measurement. Measurements of lactate (L), lactate/pyruvate ratio (L/P) and ammonium (A) during rest, exercise and recovery enhance interpretative power of maximal exercise by incorporating muscular metabolism exploration. Maximal exercise test with gas exchange measurement is standardized in cardiopulmonary evaluations but, no reference data of blood muscular metabolites are available to evaluate the muscular metabolism. We determined normal values of L, L/P and A during a standardized maximal exercise and recovery in 48 healthy sedentary volunteers and compared with results obtained in four patients with exercise intolerance and a mitochondrial disease. In healthy subjects, L, L/P and A rose during exercise. In 98% of them L, L/P or A decreased between the fifth and the fifteenth minutes of recovery. In mitochondrial patients, VO(2) max was normal or low, and L, L/P and A had the same evolution as normal subjects or showed no decrease during recovery. We gave normal L, L/P and A values, which establish references for a maximal exercise test with muscular metabolism exploration. This test is helpful for clinicians in functional evaluation, management and treatment of metabolic myopathy and would be a useful tool in diagnosis of metabolic myopathy.

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 production upon short-term non-ischemic forearm exercise in mitochondrial disorders and other myopathies

BACKGROUND

The nonischemic forearm exercise test (NIFET) has been shown to be as effective as the classic ischemic forearm exercise test (IFET) in the diagnosis of patients with McArdle disease. Recently, the lactate increase normalized to the mechanical energy production in NIFET was suggested to have a intermediate sensitivity and satisfactory specifity for the screening of mitochondrial disorders.

METHODS

NIFET at 80% maximal contraction force (MCF) was performed in normal controls (n = 41), patients with mitochondrial disorders (n = 15) and other myopathies (diseased controls, n = 20). 26 healthy volunteers also underwent IFET at 80% MCF. The ratio of lactate increase and workload was defined as specific lactate production (mmol x s/N x l).

RESULTS

In normal controls there was no significant different lactate increase during NIFET and IFET. The workload performed showed only a weak significant positive correlation with the lactate increase in the NIFET in normal controls (r(2) = 0.20) but not in IFET and NIFET with patients. A moderate negative correlation of specific lactate production and the absolute workload was found in all groups and in both protocols (r(2) = 0.22-0.34). The specific lactate production was highest in patients with other myopathies, intermediate in patients with mitochondrial disorders and lowest in normal controls. NIFET showed a sensitivity of only 20 % and a specifity of 95% for normal controls, but only 75 % for diseased controls.

CONCLUSION

The specific lactate production during NIFET is neither sufficiently specific nor sensitive for the diagnosis of mitochondrial disorders. Increased specific lactate production during rest-to-work transition period might be caused by increased acetyl group deficits.