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

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.

Increased blood lactate levels during exercise and mitochondrial DNA alterations converge on mitochondrial dysfunction in schizophrenia

BACKGROUND

Mitochondrial dysfunction and an elevation of lactate are observed in patients with schizophrenia (SZ). However, it is unknown whether mitochondrial dysfunction is associated with the presence of mitochondrial DNA (mtDNA) alterations and comorbid clinical conditions. We aimed to identify systemic mitochondrial abnormalities in blood samples of patients with SZ that may have a high impact on the brain due to its high bioenergetic requirements.

METHODS

Case/control study between 57 patients with SZ and 33 healthy controls (HCs). We measured lactate levels at baseline, during 15 min of exercise (at 5, 10 and 15 min) and at rest. We also evaluated the presence of clinical conditions associated with mitochondrial disorders (CAMDs), measured the neutrophil to lymphocyte ratio (NLR, a subclinical inflammatory marker), and analyzed mtDNA variation and copy number.

RESULTS

Linear models adjusting for covariates showed that patients with SZ exhibited higher elevation of lactate than HCs during exercise but not at baseline or at rest. In accordance, patients showed higher number of CAMDs and lower mtDNA copy number. Interestingly, CAMDs correlated with both lactate levels and mtDNA copy number, which in turn correlated with the NLR. Finally, we identified 13 putative pathogenic variants in the mtDNA of 11 participants with SZ not present in HCs, together with a lactate elevation during exercise that was significantly higher in these 11 carriers than in the noncarriers.

CONCLUSIONS

These results are consistent with systemic mitochondrial malfunctioning in SZ and pinpoint lactate metabolism and mtDNA as targets for potential therapeutic treatments.

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.

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.

Parkinson syndrome as a manifestation of mitochondriopathy

OBJECTIVES

Although there is growing evidence for a relation between Parkinson syndrome (PS) and mitochondriopathy (MCP), little is known about the frequency of PS in MCP.

MATERIAL AND METHODS

This study assessed the frequency of PS in patients with MCP, the phenotype of these patients, and their response to anti-Parkinson medication, during a 1-year period.

RESULTS

Between April 1999 and March 2000 PS was diagnosed in nine of 76 patients with MCP (12%). The frequency of MCP among 144 patients with PS attending the department during the investigational period was 6.3%. Systems most frequently affected by the MCP in the nine patients were the peripheral nervous system, central nervous system, endocrinium, heart, intestines, eyes, ears and kidneys. PS in MCPs responded well to amantadine, L-DOPA, dopamine agonists and catechole-o-methyl-transferase inhibitors.

CONCLUSION

Twelve per cent of the patients with MCP have phenotypic features of PS and 6% of the patients with PS have features of MCP. MCP patients with PS frequently show multisystem involvement. PS in MCP responds well to anti-Parkinson medication.

Phenotype variability in 130 adult patients with respiratory chain disorders

Despite continuously improving diagnostic facilities, respiratory chain disorders (RCDs) are easily overlooked or misdiagnosed. We thus studied phenotype variability and the diagnostic potential of clinical and laboratory investigations in patients with RCD. We retrospectively evaluated clinical and laboratory investigations in 130 patients with RCD: 63 women and 67 men, aged 17-87 years, diagnosed between January 1992 and December 1999. mtDNA mutations were found in 20 patients; a respiratory chain defect but no mutation in 4; an abnormal lactate stress test but no mutation or biochemical defect in 66; and ragged-red fibres or reduced oxidative enzyme staining but no mutation, biochemical defect or abnormal lactate stress test in 40 patients. The most frequent initial manifestation of RCD were limb weakness, muscle pain and sensory disturbances. The most frequent clinical findings at diagnosis were muscle pain, fatiguability, limb weakness, reduced tendon reflexes and muscle wasting, irrespective of the diagnostic evidence. Mean age at onset, disease duration and time until diagnosis were 39, 14 and 13 years, respectively, without sex differences. The family history was positive in 29% of the patients. Hyperlipidaemia was found in 45%, hyper-CK-aemia in 42%, short stature in 33%, thyroid dysfunction in 17%, diabetes in 12%, and epilepsy in 8% of the patients. Laboratory investigations that prove useful to support the diagnosis of RCD are muscle biopsy, electromyography, lactate stress testing, echocardiography and mtDNA analysis. Systems most often involved in RCDs were the PNS, CNS, endocrine system and heart. The diagnosis of RCD requires awareness of the great phenotypic heterogeneity and an individualized, integral, multidisciplinary approach.

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.