Screening for mitochondrial cytopathies: the sub-anaerobic threshold exercise test (SATET)

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.

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.

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.

Elimination Rate of Serum Lactate is Correlated with Amyotrophic Lateral Sclerosis Progression

Background:

Mitochondrial dysfunction plays an important role in the pathogenesis of amyotrophic lateral sclerosis (ALS). We aimed to demonstrate mitochondrial dysfunction in ALS using a lactate stress test and to examine the relationship between mitochondrial dysfunction with motor deterioration.

Methods:

We enrolled 116 patients and observed clinical variables, including the survival state.

Results:

Patients with a rapid slope of revised ALS functional rating scales (ALSFRS-r) (>20 U/year) exhibited the slowest elimination rate (median −4.67 × 10⁻³ mmol∙L⁻¹∙min⁻¹, coefficient of variation, 590.15%), the shortest duration (0.63 ± 0.28 years) and the worst ALSFRS-r (32.59 ± 4.93). Patients with a moderate slope of ALSFRS-r (10–20 U/year) showed a moderate elimination rate (median −11.33 × 10⁻³ mmol∙L⁻¹∙min⁻¹, coefficient of variation, 309.89%), duration (1.16 ± 0.45 years), and ALSFRS-r (34.83 ± 6.11). The slower progressing (<10 U/year group) patients exhibited a rapid elimination rate (median: −12.00 × 10⁻³ mmol∙L⁻¹∙min⁻¹, coefficient of variation: 143.08%), longer duration (median: 3 years, coefficient of variation: 193.33%), and adequate ALSFRS-r values (39.58 ± 9.44). Advanced-phase ALS patients also showed slower elimination rate (ER, quartiles −17.33, −5.67, 4.00) and worse ALSFRS-r (34.88 ± 9.27), while early-phase patients showed a more rapid ER (quartiles −25.17, −11.33, −3.50) and better ALSFRS-r (39.28 ± 7.59). These differences were statistically significant. Multiple linear regression analysis revealed strong direct associations among ER, ALSFRS-r slope (standard beta = 0.33, P = 0.007), and forced vital capacity (predict %) (standard beta = −0.458, P = 0.006, adjusted for ALSFRS-r, course and onset region). However, the data obtained from 3 years of follow-up showed no statistically significant difference in the survival rates between the most rapid and slowest ER groups.

Conclusion:

There is a potential linear relationship between ER and motor deterioration in ALS. Slower ER might be associated with faster disease progression.

Altered skeletal muscle (mitochondrial) properties in patients with mitochondrial DNA single deletion myopathy

Background

Mitochondrial myopathy severely affects skeletal muscle structure and function resulting in defective oxidative phosphorylation. However, the major pathomechanisms and therewith effective treatment approaches remain elusive. Therefore, the aim of the present study was to investigate disease-related impairments in skeletal muscle properties in patients with mitochondrial myopathy. Accordingly, skeletal muscle biopsies were obtained from six patients with moleculargenetically diagnosed mitochondrial myopathy (one male and five females, 53 ± 9 years) and eight age- and gender-matched healthy controls (two males and six females, 58 ± 14 years) to determine mitochondrial respiratory capacity of complex I-V, mitochondrial volume density and fiber type distribution.

Results

Mitochondrial volume density (4.0 ± 0.5 vs. 5.1 ± 0.8 %) as well as respiratory capacity of complex I-V were lower (P < 0.05) in mitochondrial myopathy and associated with a higher (P < 0.001) proportion of type II fibers (65.2 ± 3.6 vs. 44.3 ± 5.9 %). Additionally, mitochondrial volume density and maximal oxidative phosphorylation capacity correlated positively (P < 0.05) to peak oxygen uptake.

Conclusion

Mitochondrial myopathy leads to impaired mitochondrial quantity and quality and a shift towards a more glycolytic skeletal muscle phenotype.

Short-term effect of aerobic exercise on symptoms in multiple sclerosis and chronic fatigue syndrome: a pilot study

BACKGROUND

This pilot study was conducted to determine whether a 15-minute bout of moderate-intensity aerobic cycling exercise would affect symptoms (pain and fatigue) and function (Timed 25-Foot Walk test [T25FW] and Timed Up and Go test [TUG]) in people with multiple sclerosis (MS) or chronic fatigue syndrome (CFS), and to compare these results with those of a healthy control group.

METHODS

Eight people with MS (Expanded Disability Status Scale score 5-6; Karnofsky score 50-80), eight people with CFS (Karnofsky score 50-80), and eight healthy volunteers participated in the study. Pain and fatigue levels and results of the T25FW and TUG were established at baseline as well as at 30 minutes, 2 hours, and 24 hours following a 15-minute stationary cycling aerobic exercise test. Repeated-measures analysis of variance (ANOVA) and covariance (ANCOVA) were used to analyze the findings over time.

RESULTS

At baseline there were statistically significant differences between groups in fatigue (P = .039), T25FW (P = .034), and TUG (P = .010). A significant group/time interaction emerged for fatigue levels (P= .005). We found no significant group/time interaction for pain levels or function.

CONCLUSIONS

Undertaking 15 minutes of moderate-intensity aerobic cycling exercise had no significant adverse effects on pain or function in people with MS and CFS (with a Karnofsky score of 50-80) within a 24-hour time period. These initial results suggest that people with MS or CFS may undertake 15 minutes of cycling as moderate aerobic exercise with no expected negative impact on pain or function.

Effects of aerobic training on exercise-related oxidative stress in mitochondrial myopathies

In mitochondrial myopathies with respiratory chain deficiency impairment of energy cell production may lead to in excess reactive oxygen species generation with consequent oxidative stress and cell damage. Aerobic training has been showed to increase muscle performance in patients with mitochondrial myopathies. Aim of this study has been to evaluate, in 7 patients (6F e 1 M, mean age 44.9 ± 12.1 years) affected by mitochondrial disease, concomitantly to lactate exercise curve, the occurrence of oxidative stress, as indicated by circulating levels of lipoperoxides, in rest condition and as effect of exercise, and also, to verify if an aerobic training program is able to modify, in these patients, ox-redox balance efficiency. At rest and before training blood level of lipoperoxides was 382.4 ± 37.8 AU, compared to controls (318.7 ± 63.8; P < 0.05), this corresponding to a moderate oxidative stress degree according to the adopted scale. During incremental exercise blood level of lipoperoxides did not increase, but maintained significantly higher compared to controls. After an aerobic training of 10 weeks the blood level of lipoperoxides decreased by 13.7% at rest (P < 0.01) and 10.4%, 8.6% and 8.5% respectively at the corresponding times during the exercise test (P = 0.06). These data indicate that, in mitochondrial patients, oxidative stress occurs and that an aerobic training is useful in partially reverting this condition.

No effect of combined coenzyme Q10 and selenium supplementation on atorvastatin-induced myopathy

OBJECTIVE

The aim of the present study was to evaluate the possible effects of Q10 and selenium supplementation on statin-induced myopathy (SIM), both for subjective symptoms and muscle function.

DESIGN

Patients (N = 43) who had experienced previous or ongoing SIM on atorvastatin therapy were recruited. Following a 6-week washout period during which no statins were administered, the patients were re-challenged with 10 mg of atorvastatin. Patients (N = 41) who experienced SIM continued the atorvastatin treatment and were in addition randomized to receive 12 weeks supplement of 400 mg Q10 and 200 μg selenium per day or a matching double placebo. SIM was assessed using 3 validated symptom questionnaires, and a muscle function test was performed at the beginning and at the end of the study.

RESULTS

The patients receiving the active supplement experienced significant increases in their serum Q10 and selenium concentrations compared with the group receiving placebo. No statistically significant differences in symptom questionnaire scores or muscle function tests were revealed between the groups.

CONCLUSIONS

Despite substantial increases in the serum Q10 and selenium levels following the oral supplementation, this study revealed no significant effects on SIM compared with the placebo.

Muscle Metabolites: Functional MR Spectroscopy during Exercise Imposed by Tetanic Electrical Nerve Stimulation

Permission from the ethics committee and informed consent were obtained. The purpose of this study was to prospectively evaluate a method developed for the noninvasive assessment of muscle metabolites during exercise. Hydrogen 1 magnetic resonance (MR) spectroscopy peaks were measured during tetanic isometric muscle contraction imposed by supramaximal repetitive nerve stimulation. The kinetics of creatine-phosphocreatine and acetylcarnitine signal changes (P < .001) could be assessed continuously before, during, and after exercise. The control peak (trimethylammonium compounds), which served as an internal reference, did not change. This technique-that is, functional MR spectroscopy-opens the possibility for noninvasive diagnostic muscle metabolite testing in a clinical setting.

Statin-associated myopathy with normal creatine kinase levels. Case report from a Norwegian family

Trøseid M, Henriksen OA, Lindal S. Statin-associated myopathy with normal creatine kinase levels - case report from a Norwegian family. APMIS 2005;113:635-7. Recent reports suggest that statins may cause myopathy with normal creatine kinase levels. We describe four related patients with statin-associated muscle symptoms and normal creatine kinase levels. In two out of the four patients (mother and son), pathological findings on EMG suggested myopathy, and muscle biopsies showed evidence of mitochondrial pathology. A third patient (daughter) had slight myopathic findings on EMG and muscle biopsy, but not enough to be classified as pathological. In a fourth patient, there were no pathological findings. Creatine kinase levels were normal and symptoms diminished after discontinuation of drugs in all four patients. Our findings are consistent with other reports of statin-associated myopathy with normal creatine kinase levels. An inherited vulnerability, possibly a mitochondrial pathology, might cause or aggravate symptoms in some patients.

Abnormal blood lactate accumulation after exercise in patients with multiple mitochondrial DNA deletions and minor muscular symptoms

STUDY OBJECTIVES

Muscle is one of the most commonly affected organs in mitochondrial disorders, and the symptoms are often exercise related. The cardiopulmonary exercise test with the determination of lactic acid formation could give supplementary information about the exercise-induced metabolic stress and compensatory mechanisms used in these disorders. The aim of this study was to evaluate the exercise capacity and lactate kinetics related to exercise in subjects with two genetically characterized mitochondrial disorders (multiple mitochondrial DNA deletions with PEO, MELAS) compared with lactate kinetics in subjects with metabolic myopathy (McArdle's disease) and in the healthy controls.

DESIGN

The subjects were consecutive, co-operative patients of Department of Neurology of Helsinki University Hospital. Molecular genetic analyses were used for group classification of the mitochondrial myopathy.

STUDY SUBJECTS

The study groups consisted of 11 patients with multiple deletions (PEO) and five patients with a point mutation in the mitochondrial DNA (MELAS), four patients with a muscle phosphorylase enzyme deficiency (McArdle's disease) and 13 healthy controls. The clinical disease of the patients was relatively mild.

MEASUREMENTS AND RESULTS

A graded exercise test with ventilatory gas analyses and venous blood lactic acid analyses was performed. The main finding was the prolonged accumulation of blood lactate after the exercise in the PEO and MELAS groups compared with the controls. An overcompensation in ventilation was found in the MELAS and PEO group.

CONCLUSIONS

The blood lactate accumulation after exercise occurs in patients with multiple mitochondrial DNA deletions or MELAS even in patients with only mild exercise intolerance. Cardiopulmonary exercise can be used in the diagnostic process of patients with mitochondrial myopathies.

A novel mutation in the mitochondrial tRNAPhe gene associated with mitochondrial myopathy

We report a novel heteroplasmic T-->C mutation at nt position 582 within the mitochondrial tRNA(Phe) gene of a 70-year-old woman with mitochondrial myopathy. No other family members were affected, suggesting that our patient was a sporadic case. The muscle showed frequent ragged red fibers and 43% cytochrome c oxidase deficient fibers. The mutation alters a conserved base pairing in the aminoacyl acceptor stem. The mutation load was 70% in muscle homogenate and varied from 0 to 95% in individual muscle fiber segments. Cytochrome c oxidase-negative fibers showed significantly higher levels of mutated mtDNA (>75%) than Cytochrome c oxidase-positive fibers (<55%). This mutation adds to the previously described four pathogenic mutations in the tRNA(Phe) gene.

Enterovirus related metabolic myopathy: a postviral fatigue syndrome

OBJECTIVE

To detect and characterise enterovirus RNA in skeletal muscle from patients with chronic fatigue syndrome (CFS) and to compare efficiency of muscle energy metabolism in enterovirus positive and negative CFS patients.

METHODS

Quadriceps muscle biopsy samples from 48 patients with CFS were processed to detect enterovirus RNA by two stage, reverse transcription, nested polymerase chain reaction (RT-NPCR), using enterovirus group specific primer sets. Direct nucleotide sequencing of PCR products was used to characterise the enterovirus. Controls were 29 subjects with normal muscles. On the day of biopsy, each CFS patient undertook a subanaerobic threshold exercise test (SATET). Venous plasma lactate was measured immediately before and after exercise, and 30 minutes after testing. An abnormal lactate response to exercise (SATET+) was defined as an exercise test in which plasma lactate exceeded the upper 99% confidence limits for normal sedentary controls at two or more time points.

RESULTS

Muscle biopsy samples from 20.8% of the CFS patients were positive for enterovirus sequences by RT-NPCR, while all the 29 control samples were negative; 58.3% of the CFS patients had a SATET+ response. Nine of the 10 enterovirus positive cases were among the 28 SATET+ patients (32.1%), compared with only one (5%) of the 20 SATET- patients. PCR products were most closely related to coxsackie B virus.

CONCLUSIONS

There is an association between abnormal lactate response to exercise, reflecting impaired muscle energy metabolism, and the presence of enterovirus sequences in muscle in a proportion of CFS patients.

Decreased platelet cytochrome c oxidase activity is accompanied by increased blood lactate concentration during exercise in patients with Alzheimer disease

Increasing evidence indicates that mitochondrial dysfunction occurs in the central nervous system as well as in the peripheral tissues from Alzheimer's disease (AD) patients. We have recently shown that mitochondrial cytochrome c oxidase (COX) activity is significantly reduced in brain and platelets from AD patients compared to controls. In the present study we investigated whether impaired COX activity could have functional consequences on energy metabolism. Blood lactate concentration was monitored at rest and during incremental exercise in 22 AD patients in whom COX activity in platelets was decreased compared to controls (35.7 +/- 11.4 vs 48.4 +/- 1.4 nmol/min/mg, P < 0.01). In both resting and exercising conditions, blood lactate was significantly higher in AD patients than in controls. Although the magnitude of exercise-related lactate accumulation was not different between the two groups, an anticipated anaerobic lactate threshold during the incremental forearm exercise was found in AD patients (50% of maximal voluntary contraction MVC compared to 60% in controls). COX activity was inversely related to lactate at a significant level for resting condition (r = -0.65) and borderline for anaerobic threshold exercise level. These results support the hypothesis of a systemic impairment of the mitochondrial function in AD and indicate that decreased COX activity could have functional consequences on metabolism.

Reproducibility of the lactate stress test

Although the lactate stress test has been shown to be a powerful tool to screen for respiratory chain disorders, little is known about the reproducibility of this test. The aim of the present study was thus to look for the reliability of the lactate stress test in disease controls and patients with primary respiratory chain disorders. In 23 patients with respiratory chain disorder, 16 women and 7 men, aged 36-81 years, and 18 disease controls, 5 women and 18 men, aged 37-70 years, the lactate stress test was carried out when respiratory chain disorder was suspected for the first time and at the first follow-up. The sensitivity of the first lactate stress test was 81% and the specificity 72%. The sensitivity of the second lactate stress test was 74% and the specificity 61%. The reproducibility of the lactate stress test was 65% in patients with respiratory chain disorders and 78% in disease controls. In conclusion, this study shows that the sensitivity, specificity, and reproducibility of the lactate stress test is high enough to warrant its application as a reliable, supplementary method to search for respiratory chain disorders.

Mitochondrial DNA--related mitochondrial dysfunction in neurodegenerative diseases

Mitochondrial dysfunction occurs in several late-onset neurodegenerative diseases. Determining its origin and significance may provide insight into the pathogeneses of these disorders. Regarding origin, one hypothesis proposes mitochondrial dysfunction is driven by mitochondrial DNA (mtDNA) aberration. This hypothesis is primarily supported by data from studies of cytoplasmic hybrid (cybrid) cell lines, which facilitate the study of mitochondrial genotype-phenotype relationships. In cybrid cell lines in which mtDNA from persons with certain neurodegenerative diseases is assessed, mitochondrial physiology is altered in ways that are potentially relevant to programmed cell death pathways. Connecting mtDNA-related mitochondrial dysfunction with programmed cell death underscores the crucial if not central role for these organelles in neurodegenerative pathophysiology. This review discusses the cybrid technique and summarizes cybrid data implicating mtDNA-related mitochondrial dysfunction in certain neurodegenerative diseases.

Lactate stress testing in 155 patients with mitochondriopathy

OBJECTIVE

Few data are available about the diagnostic yield of the lactate stress test (LST) in a large group of patients with mitochondriopathy (MCP).

METHODS

Serum lactate was determined once before, three times during, and once after a 15-minute, constant 30W workload on a bicycle in 62 controls, aged 17 to 84 years, 155 patients with MCP, aged 17 to 87 years, and 31 patients with neurological disorders other than MCP.

RESULTS

Lactate's upper reference limits at rest, 5, 10, 15 minutes after starting, and 15 minutes after finishing the exercise were 2.0, 2.1, 2.1, 2.1 and 1.8 mmol/l respectively. The test was regarded abnormal if more than two of the five lactate values exceeded the cut-off levels. Among the 103 patients with abnormal LST, 64 (62 %) had normal resting lactate. The sensitivity of the test was 67% and the specificity 94%.

CONCLUSION

The LST proved to have a high sensitivity and specificity in the detection of patients with MCP, being thus a simple but powerful tool to assess the impaired oxidative metabolism in MCP patients.

Laboratory approach to mitochondrial diseases

Dysfunction in mitochondrial processes has been related to several pathologies. In these disorders, the cell suffers oxidative imbalance that is mostly due to defects in pyruvate metabolism, mitochondrial fatty acids oxidation, the citric acid cycle or electron transport by the mitochondrial respiratory chain. These metabolic alterations produce mitochondrial diseases that have been related to inherited syndromes, such as MERRF or MELAS. The main affected organs are brain, skeletal muscle, kidney, heart and liver, because of the high energetic demand and the oxidative metabolism. Moreover, the relationship between mitochondrial dysfunction and neurodegenerative processes, such as Parkinson disease or Alzheimer disease, as well as ageing, has been shown. Because mitochondrias are the target of several xenobiotics, such as aspirin, AZT or alcohol consumption, mitochondrial impairment has also been proposed as a mechanism of toxicity. Most laboratory tests that are available in the diagnosis of mitochondrial illness are assayed in tissue biopsies and are usually difficult to interpret. Recently, it has been shown that non-invasive techniques, such as nuclear magnetic resonance or the 2-keto[1-(13)C]isocaproic acid breath test, may be useful to assess mitochondrial function. This article attempts to show the laboratory approach to mitochondrial diseases, reviewing new techniques that could be of great value in the research of mitochondrial function, such as the 2-keto[1-(13)C]isocaproic breath test.

Diagnostic yield of the lactate stress test in respiratory chain disorders under absolute and relative workload

Usually, the lactate stress test is carried under a constant, low workload maintained for 15 min, although there are indications that the lactate response to exercise is a function of the relative workload, and that a workload for only 5 min does not decrease its sensitivity. Thus, this study compared the diagnostic yield of the lactate stress test when carried out under a constant workload of 30 W (LSTA) and under a workload of 30% of maximum (LSTR), and when the workload was maintained for 5 and 15 min. In 16 patients with respiratory chain disorders, 12 women, four men, aged 27--88 years, the LSTA and LSTR were carried out on 2 different days, within 48 h. For both tests serum lactate was determined before, during and after a 15-min workload on a bicycle. Upper reference limits at rest, 5, 10, 15 min after starting, and 15 min after finishing the exercise were 1.9, 2.0, 2.0, 2.0, and 1.6 mmol/l for the LSTA, and 2.3, 3.0, 3.2, 3.4 and 2.7 mmol/l for the LSTR. The sensitivity was 88% for the LSTA and 63% for the LSTR. The specificity of the LSTA was 94%. The diagnostic yield of both tests was similar when the workload was maintained for 5 or 15 min. In conclusion, the LSTA should be preferred to the LSTR. Three lactate determinations during 15 min have no advantage over a single lactate determination after a 5 min workload of 30 W.

The effect of exercise on gait and balance in patients with chronic fatigue syndrome

This study investigated anecdotal reports of gait and balance abnormalities in subjects with Chronic Fatigue Syndrome (CFS) by examining the effects of a light exercise test on postural sway and various gait parameters. Tests were performed on 11 CFS patients and 11 age- and sex-matched sedentary controls. Results demonstrated that postural sway was not significantly different in both groups before or after the exercise test. There were, however, significant differences in gait parameters between the two groups confirming anecdotal evidence, but these differences were not exacerbated by the exercise test. Heart rate responses demonstrated that both groups were exercising at similar loads, although this was perceived to be higher by the CFS group.

Diagnostic yield of the lactate stress test in 160 patients with suspected respiratory chain disorder

Aim of the present study was to investigate the diagnostic yield of the lactate-stress-test in unselected patients with suspected respiratory-chain disorder (RCD). Serum lactate was determined before, during and after a 15-minute, constant 30W workload on a bicycle in 48 controls and 160 patients with suspected RCD. 122 had definite RCD, 38 neurological disorders other than RCD. The sensitivity, specificity, positive predictive value, negative predictive value and pre-test likelihood of the test was 69, 74, 89, 42 and 76% respectively. In conclusion, the lactate-stress-test proved a powerful tool to assess impaired oxidative metabolism in RCDs.

Oxygen consumption measurement in lymphocytes for the diagnosis of pediatric patients with oxidative phosphorylation diseases

OBJECTIVES

To evaluate the results of oxygen consumption measurement in lymphocytes for the diagnosis and treatment monitoring of pediatric patients with oxidative phosphorylation diseases.

DESIGN AND METHODS

Twenty-four children with an oxidative phosphorylation disease were studied. Results were compared with those of 87 healthy children. Oxygen consumption measurements in digitonine-permeabilized lymphocytes incubated with pyruvate plus malate and succinate were performed in a Clark-type oxygen electrode.

RESULTS

A total of 58% of patients showed a decreased oxygen consumption in lymphocytes incubated with pyruvate. In 4 patients, this analysis was the unique initial biochemical test, which revealed an impaired mitochondrial energy metabolism. Significant differences were observed in lymphocytes incubated with pyruvate between patients and reference values (p<0.00005), and in lymphocytes incubated with pyruvate before and after treatment (p<0.05).

CONCLUSIONS

This test is useful for diagnosing oxidative phosphorylation diseases in patients who did not have other biochemical alterations, although false-negative results can be found. It is not useful for treatment monitoring.

A novel heteroplasmic point mutation in the mitochondrial tRNA(Lys) gene in a sporadic case of mitochondrial encephalomyopathy: de novo mutation and no transmission to the offspring

We have identified a new mutation in the tRNA(Lys) gene of mtDNA, in a 49-year-old patient with mitochondrial encephalomyopathy. The mutation is a heteroplasmic G-->A transition at position 8328, which affects the anticodon stem loop at a conserved site. The mutation was neither found in 100 controls nor in the maternal relatives of the patient. The level of mutated mtDNA was 57% in muscle, 13% in fibroblasts, and 10% in lymphocytes. Histochemistry of muscle tissue revealed cytochrome c oxidase-deficient fibers with abnormal accumulation of mitochondria. Biochemistry of muscle mitochondria showed slight cytochrome c oxidase deficiency. The mean ratio of mutant mtDNA to normal mtDNA in cytochrome c oxidase-positive muscle fibers was 59%, whereas a mean ratio of 95% was found in cytochrome c oxidase-negative fibers. The difference between cytochrome c oxidase-positive and cytochrome c oxidase-negative fibers was highly significant (P < 0.001). The mutation was not found in muscle or lymphocytes of the mother and daughter of the proband. This is the first report of a de novo point mutation in the tRNA(Lys) gene in an individual expressing disease and the first report of lack of transmission of the mutation to the offspring of a patient expressing a mitochondrial encephalomyopathy caused by a point mutation in mtDNA.

[The use of cardiopulmonary exercise test in patients with mitochondrial myopathies]

BACKGROUND

Exercise tests has been used in the diagnosis of metabolic myopathies. If there is an abnormal response pattern in mild mitochondrial myopathy (MM) and her role in the initial suspect diagnostic is unknown.

SUBJECTS AND METHODS

Prospective study of 26 patients with mitochondrial myopathy (15 men, 11 women) and a control group of 14 sedentary volunteers (9 men, 5 woman) with similar antropometric characteristics. We have made pulmonary function tests and treadmill exercise with serial venous sampling of blood lactate, piruvate, ketone bodies, free fatty acids and creatinkinase.

RESULTS

Patients with MM showed exercise limitation with lower maximal power (MM = 143 [47] vatts, C = 187 [40] vatts, p = 0.006), maximal oxygen uptake (MM = 27 [8] ml/min/kg, C = 40 [7] ml/min/kg, p = 0.001) and maximal oxygen pulse (MM = 11 [3] ml/beat, C = 14 [3] ml/beat, p = 0.006). For the same oxygen uptake the heart rate was higher and the anaerobic threshold was earlier in MM patients (MM = 48% [14], C = 62% [12], p = 0.01). We found a considerable slower maximal oxygen uptake in patients with lipid increase in muscle biopsy (n = 15). Acetoacetate curves, beta-hydroxybutyrate, free fatty acids and creatinkinase were similar in both groups. The exercise test was pathologic in 17/26 of the patients. The sensitivity of the exercise test for maximal oxygen uptake was 0.65.

CONCLUSIONS

The cardiopulmonary exercise test is a useful test in the suspect initial diagnostic of patients with mild mitochondrial myopathy because the majority of patients show a pathologic test.

Effects of aerobic training on lactate and catecholaminergic exercise responses in mitochondrial myopathies

The aim of this study was to evaluate the effects of an aerobic training program on the metabolic and sympathetic responses to exercise in 12 patients with mitochondrial myopathies. A 10-week course of aerobic training, consisting of supervised exercise every other day on an electrically braked pedal-rate bicycle ergometer was prescribed to each patient and four healthy controls. Venous lactate, epinephrine (EP) and norepinephrine (NEP) levels were assessed at baseline and after the aerobic training by means of constant-workload exercise performed at near lactate threshold (LT). In patients, a decrease in exercise peak values, significant for lactate (-38.6%, P < 0.01) but not for catecholamines (EP: -26.0%, NEP: -22.1%) was observed after training, findings confirmed by the lactate/EP and lactate/NEP area ratios. The results show that lactate accumulation during exercise is decreased after aerobic training in mitochondrial myopathies and that the effect is partially dissociated from the catecholaminergic response. This in turn suggests that the lactate decrease can be explained, at least in part, by the improved muscle oxidative metabolism consequent to the proposed training program.

Heterogeneity in chronic fatigue syndrome: evidence from magnetic resonance spectroscopy of muscle

It has been shown previously that some patients with chronic fatigue syndrome show an abnormal increase in plasma lactate following a short period of moderate exercise, in the sub-anaerobic threshold exercise test (SATET). This cannot be explained satisfactorily by the effects of 'inactivity' or 'deconditioning', and patients with abnormal lactate responses to exercise (SATET +ve) have been found to have significantly fewer Type 1 muscle fibres in quadriceps biopsies than SATET -ve patients. We performed phosphorus magnetic resonance spectroscopy on forearm muscles of 10 SATET +ve patients, 9 SATET -ve patients and 13 sedentary volunteers. There were no differences in resting spectra between these groups but at the end of exercise, intracellular pH in the SATET +ve patients was significantly lower than in both the SATET -ve cases and controls (P < 0.03), and the SATET +ve patients also showed a significantly lower ATP synthesis rate during recovery (P < 0.01), indicating impaired mitochondrial oxidative phosphorylation. These observations support other evidence which indicates that chronic fatigue syndrome is a heterogeneous disorder, and confirms the view that some chronic fatigue syndrome patients have a peripheral component to their fatigue.

Muscle fibre characteristics and lactate responses to exercise in chronic fatigue syndrome

OBJECTIVES

To examine the proportions of type 1 and type 2 muscle fibres and the degree of muscle fibre atrophy and hypertrophy in patients with chronic fatigue syndrome in relation to lactate responses to exercise, and to determine to what extent any abnormalities found might be due to inactivity.

METHODS

Quadriceps needle muscle biopsies were obtained from 105 patients with chronic fatigue syndrome and the proportions of type 1 and 2 fibres and fibre atrophy and hypertrophy factors were determined from histochemical preparations, using a semiautomated image analysis system. Forty one randomly selected biopsies were also examined by electron microscopy. Lactate responses to exercise were measured in the subanaerobic threshold exercise test (SATET).

RESULTS

Inactivity would be expected to result in a shift to type 2 fibre predominance and fibre atrophy, but type 1 predominance (23%) was more common than type 2 predominance (3%), and fibre atrophy was found in only 10.4% of cases. Patients with increased lactate responses to exercise did have significantly fewer type 1 muscle fibres (p<0.043 males, p<0.0003 females), but there was no evidence that this group was less active than the patients with normal lactate responses. No significant ultrastructural abnormalities were found.

CONCLUSION

Muscle histometry in patients with chronic fatigue syndrome generally did not show the changes expected as a result of inactivity. However, patients with abnormal lactate responses to exercise had a significantly lower proportion of mitochondria rich type 1 muscle fibres.

Mitochondrial disease. Pulmonary function, exercise performance, and blood lactate levels

Mitochondrial diseases are a heterogeneous group of disorders in which it has been suggested that genetic defects in oxidative phosphorylation lead to specific alterations in exercise performance and lactate metabolism during exercise. To investigate this possibility, we evaluated pulmonary function tests, incremental exercise testing, and serial blood lactate levels in a group of subjects with mitochondrial disease (M) and compared them with a group of patients with nonmitochondrial (N) myopathies and healthy subjects (H). The two groups were demographically comparable and had no significant differences in pulmonary function. Both groups showed similar degrees of reduced exercise tolerance compared with a group of healthy subjects (M: 61.08% predicted VO2max +/- 19.58 SD, n = 13; N: 62.14 +/- 28.89, n = 7; H: 115.17 +/- 19.35, n = 12; p < 0.001). The mitochondrial disease group more frequently showed abnormalities in cardiac response to exercise than did the nonmitochondrial myopathy subjects (M: 12/13, N: 3/7, H: 3/12, p = 0.002). Minute ventilation greater than predicted occurred with similar frequency in both groups. Although resting lactate level was increased in some subjects with mitochondrial myopathy compared with disease controls, there were no differences between groups for peak venous lactate level normalized for oxygen uptake or the rate of lactate clearance. These findings, while confirming the presence of some specific abnormalities in mitochondrial disease, are against the notion that exercise limitation in this condition directly results from specific abnormalities in oxidative metabolism.

Hyperventilation in normal subjects. A clinical, gas-analytic and EMG study

Clinical (paresthesiae and Trousseau's sign), EMG, blood biochemical (lactic acid and total Ca2+) and gas-analytic (pH, PCO2, PO2, HCO3) changes were studied during and after 5 min voluntary hyperventilation (HV) in 15 normal subjects. Paresthesiae and spontaneous motor activity were common manifestations (87% and 67% respectively) in our test. They were significantly related only to changes in pH, PCO2 and PO2, with paresthesiae arising earlier and at milder gas-analytic changes than motor activity. Paresthesiae and motor activity induced by HV cannot be used as reliable indicators of pathology. Their presence simply reflects the degree of the blood gas-analytic changes (pH, PCO2 and PO2) which therefore should be carefully monitored before drawing any conclusion from an HV test.