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.     

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

Background Mitochondrial disorders are characterized by an accumulation of lactate and an insufficient oxygen extraction from blood during exercise. Therefore, both parameters (lactate and oxygen saturation) can be used as screening tests in mitochondrial disorders. However, conflicting results regarding sensitivities and specifities of both tests have been reported. Method We examined 27 patients with genetically defined mitochondrial disorders (single deletions n = 15,multiple deletions n = 5, A3243G mutation n = 7), patients with other neuromuscular disorders, and healthy controls. In the first test subjects performed intermittent isometric handgrip exercise (0.5 Hz) at 80 % (3 minutes) and 30 % (3 and 15 minutes) of maximal contraction force (MCF). Oxygen saturation and partial pressure in cubital venous blood from the exercising arm were measured. In the second test subjects underwent cycle ergometry at 30 W for 15 minutes. Venous lactate at rest, during and 15 minutes postexercise was determined. Result Both tests showed specificities of 92–96%. Sensitivities for changes of venous oxygen partial pressure and oxygen saturation ranged from 21–26% at 80% MCF for 3 minutes to 47–58% at 30% MCF for 15 minutes. Sensitivities for venous resting, peak, and post–exercise lactate was 33%, 58%, and 67%, respectively. The degree of deoxygenation, however,was independent of the intensity and duration of the applied forces. Oxygen desaturation and lactate increase in patients with mitochondrial disorders were not different in patients with and without clinical symptoms of myopathy. There were significant correlations between the heteroplasmy and both the degree of oxygen desaturation and lactate increase in patients with single deletions. In patients who performed both protocols (n = 16) a combination of both tests increased sensitivity up to 87%. Conclusion Oxygen desaturation in forearm exercise tests and lactate increase in cycle ergometry tests show a high specifity but only moderate sensitivity. Combination of the two screening test clearly increases the sensitivity.     

Stress lactate in mitochondrial myopathy under constant, unadjusted workload

As it is under debate if determination of lactate during cycle ergometry (lactate stress testing, LST) under a continuous, unadjusted, low workload is a valuable diagnostic tool for mitochondrial myopathy (MMP), the present study aimed to investigate how sensitive the LST is in a large cohort of patients with indications for MMP (MMP patients). Serum lactate was determined once before, three times during, and once after a 15-min, constant 30 W-workload on a bicycle ergometer in 115 healthy controls, 166 patients with neurological disorders other than MMP, and 291 MMP patients. Serum lactate's upper reference limit at rest, 5, 10, 15 min after starting, and 15 min after finishing the exercise was 2.0, 2.0, 2.1, 2.0 and 1.7 mmol/l, respectively. Resting lactate was increased in 75 MMP patients (26%). The specificity of resting lactate determination was 84%. The sensitivity of the LST was 66% and the specificity 84%. Among the 192 MMP patients with abnormal LST, 120 (63%) had a normal resting lactate. The LST is abnormal in two thirds of the MMP patients. The sensitivity of the LST is higher than that of resting lactate determination. The LST is a simple and cheap but effective and reliable screening method for detecting the impaired oxidative metabolism in MMP.     

Modified exercise test in screening for mitochondrial myopathies--adjustment of workload in relation to muscle strength

The aim of this study was to evaluate the usefulness of a modification of the bicycle ergometer test, the subanaerobic threshold exercise test (SATET), as a screening test for patients with mitochondrial myopathies. Since the original SATET is frequently found to be strenuous for weak patients, a new variable (relative muscle strength) was added to the workload formula. Plasma lactate levels were recorded at rest, then after 5 and 15 min of cycling on an ergometer, with constant workload. Nine patients with mitochondrial myopathy, 10 patients with other neuromuscular diseases and 9 healthy but sedentary volunteers undertook the test. An upper reference limit after exercise for plasma lactate was settled at 2.9 mmol/l. The modified SATET showed a sensitivity of 78% and a specificity compared to the healthy subjects of 100%. Compared to patients with other neuromuscular diseases, the specificity was lower (60%). All subjects completed the test without severe fatigue or pain.     

Multiple mitochondrial DNA deletions in hereditary inclusion body myopathy

We have recently described an autosomal dominant hereditary inclusion body myopathy (h-IBM). Clinically it is is characterized by congenital joint contractures and slowly progressive, proximal muscle weakness and ophthalmoplegia. There is deterioration of muscle function between 30 and 50 years of age. While young patients show minor pathological changes in muscle, the middle-aged and old patients show rimmed vacuoles and inclusions of filaments measuring 15–18 nm in diameter. Except for the absence of significant inflammation the histopathology is similar to that found in sporadic inclusion body myositis (s-IBM). In s-IBM mitochondrial alterations including cytochrome c oxidase (COX) -deficient muscle fibers are common. These are due to multiple mitochondrial DNA (mtDNA) deletions. In this study we investigated the occurrence of mitochondrial alterations in autosomal dominant h-IBM. Young affected individuals showed no mitochondrial changes but three patients aged 38, 51 and 59 years, respectively, showed ragged red fibers and COX-deficient muscle fibers. Polymerase chain reaction analysis showed multiple mtDNA deletions. By in situ hybridization clonal expansions of mtDNA with deletions were demonstrated in COX-deficient muscle fibers. Most of the analyzed deletion breakpoints showed nucleotide repeats flanking the deletions. The results show that COX-deficient muscle fibers and somatic mtDNA deletions are present in this family with h-IBM. The same factors may be involved in the development of mtDNA deletions in s-IBM and this family with h-IBM. Received: 13 July 1999 / Revised: 6 October 1999 · Accepted: 12 October 1999     

A non-ischemic forearm exercise test for the screening of patients with exercise intolerance.

BACKGROUND: The forearm exercise test is a common investigation that allows detection of some metabolic myopathies. It is not completely standardized and, when performed in ischemic conditions, may induce rhabdomyolysis in patients with glycogenosis. OBJECTIVE: To develop a standardized non-ischemic exercise test for a safe screening of patients with exercise intolerance. METHODS: Twenty-six healthy subjects and 32 patients with exercise intolerance performed an isometric exercise at 70% of the maximal voluntary contraction during 30 seconds in non-ischemic conditions. Blood concentrations of creatine kinase, lactate, and ammonia were analyzed. RESULTS: A nearly fourfold lactate rise was induced by exercise in healthy subjects. All patients with normal muscle biopsy showed values similar to those of healthy subjects. No significant lactate increase was observed in six patients with a myophosphorylase defect and one with a debrancher defect. Disparate lactate responses were observed in 14 patients with a mitochondrial myopathy. The blood lactate level at rest was abnormally high in four of these patients. The lactate surface normalized by the mechanical energy production was above the normal range in eight patients. CONCLUSIONS: The authors propose a standardized non-ischemic grip test that overcomes the main drawbacks of the classic ischemic forearm exercise test. It provides a specific, efficient, and safe screening test for patients with exercise intolerance. Its sensitivity was very good for patients with a glycogenolysis defect but remains partial in patients with a mitochondrial disorder.     

Cycle ergometry is not a sensitive diagnostic test for mitochondrial myopathy

Cycle exercise has repeatedly been used to diagnose patients suspected of having mitochondrial myopathy (MM), in whom exercise intolerance and lactic acidosis are common. No standardized test, however, has been established. We evaluated the diagnostic value of incremental and constant workload (20 min at 65 % VO(2max)) cycle tests for the diagnosis of MM. Plasma lactate and oxidative capacity (VO(2) and workload) were measured in 15 well-characterized MM patients during cycling. Findings were compared with those in 10 myotonic dystrophy patients and 18 sedentary, healthy subjects.All MM patients had ragged red or COX-negative fibers on muscle biopsy. VO(2max) and maximal workload were lower in MM than in control groups (P < 0.02). Resting plasma lactate was higher in MM than in control groups (P < 0.005; sensitivity = 93 %; specificity = 85 %), while exercise-induced increases in plasma lactate were only higher during the constant workload protocol in MM patients vs. control groups (P < 0.05; sensitivity = 27 %; specificity = 86 %). The findings indicate that the diagnostic value of a constant workload protocol is superior to an incremental cycle test, but that the test is less sensitive for MM than simple testing of resting lactate and muscle morphology. Cycle testing of MM patients remains an important research tool, but should not be a standard diagnostic procedure for MM.     

Mitochondrial myopathy and familial thiamine deficiency

We studied two siblings with a mitochondrial myopathy, familial thiamine deficiency, and an A3243G mutation of the mitochondrial DNA (mtDNA). The elder brother (patient 1, now 36 years old) developed myopathy and beriberi heart at 20 years of age. Thiamine therapy resolved the cardiac symptoms and hyperpyruvicemia and improved the myopathy. The younger brother presented aged 19 years with a myopathy (patient 2, now 35 years old). Thiamine deficiency was present in the siblings and parents, and ragged-red fibers (RRFs) were noted in muscle biopsies from the siblings. Analysis 17 years later demonstrated thiamine malabsorption and an A3243G mutation of the mtDNA in both siblings and their mother, progressive myopathy, and an increased number of RRFs and elevated serum CKMB activity in patient 1. Thiamine treatment decreased the serum concentrations of lactate and pyruvate in patient 2, but not patient 1. The role of thiamine in mitochondrial dysfunction caused by an electron transfer disorder in the setting of A3243G mtDNA mutation is discussed.     

Metabolic changes in patients with mitochondrial myopathies and effects of coenzyme Q10 therapy

We used a standardized bicycle ergometry protocol with a stepwise increasing workload (30–100 W) to evaluate various metabolic factors for the diagnosis and metabolic monitoring of mitochondrial encephalomyopathies. All patients (n = 9) showed pathological venous lactate/pyruvate (L/P) ratios, which normalized in three patients after 6 months of coenzyme Q₁₀ (CoQ) therapy. Thus, the L/P ratio proved to be the clinically most useful parameter in the evaluation and monitoring of mitochondrial diseases, showing higher sensitivity than lactate measurements only. CoQ may exert a favourable effect in some patients with mitochondrial diseases. Received: 15 October 1997 Received in revised form: 6 February 1998 Accepted: 20 March 1998     

A forearm exercise screening test for mitochondrial myopathy

BACKGROUND: The authors hypothesized that impaired oxygen extraction in mitochondrial myopathy (MM) results in a high oxygen saturation in venous effluent blood from working muscle and that this phenomenon can be used as a diagnostic tool for MM. METHODS: Twelve patients with MM, 10 patients with muscular dystrophy, and 12 healthy subjects were studied. All subjects performed intermittent static handgrip exercise (1/2 Hz) at 40% of maximal voluntary contraction (MVC) for 3 minutes. Cubital venous oxygen saturation and brachial artery flow were measured in the exercised arm. RESULTS: Exercise-induced venous oxygen desaturation was smaller in patients with MM (Delta - 7 +/- 5%) than in subjects with muscular dystrophy (Delta - 38 +/- 2%; p = 0.00001) and healthy subjects (Delta - 43 +/- 2%; p = 0.0000002). MVC and exercise blood flow were similar in patients with MM (18 +/- 3 kg; 436 +/- 65 mL/min) and patients with muscular dystrophy (15 +/- 3 kg; 460 +/- 85 mL/min), but were higher in healthy subjects (32 +/- 4 kg; 630 +/- 58 mL/min; p < 0.03). In seven patients with MM and seven patients with McArdle disease, studied with a slightly different protocol, exercise-induced oxygen desaturation was also impaired in MM (Delta - +/- 5%) compared with McArdle disease (Delta - 26 +/- 3%; p = 0.007). CONCLUSION: Oxygen desaturation in venous blood from exercising muscle is markedly lower in patients with mitochondrial myopathy than in subjects with other muscle diseases and healthy subjects, suggesting that a forearm exercise test can be a diagnostic screening tool for mitochondrial myopathy.     

Oral glucose lactate stimulation test in mitochondrial diseases

We selected 23 patients with neurologic diseases, including 15 boys and 8 girls from 1 month to 10 years of age, who were divided into mitochondrial and nonmitochondrial disease groups. All patients were required to fast and rest for at least 4–10 hours before the test. Glucose was administered orally using a 50% glucose: water solution at a dose of 1.75 mg/kg. Blood samples then were drawn from a retained heparinized venous line at 0, 30, 60, 120, and 180 min and tested for lactate and glucose levels. Of the mitochondrial disease group, 10 of 11 patients had an upward sloping curve of lactate metabolis; the one who had a flat line response was a patient who suffered from a free-fatty-acid metabolic defect presenting with a recurrent Reye-like syndrome. There was a relatively flat line response in the nonmitochondrial disease group except in a patient with alternating hemiplegia whose symptoms responded well to flunarizine therapy. A significant increase in blood lactate content at 60 min after glucose loading occurred in the mitochondrial disease group, in contrast to that of the nonmitochondrial group. Sensitivity and specificity of a 5 mg/dl increase in blood lactate concentration at 60 min was 72.7% vs. 91.7%. Moreover, all 4 patients whose blood lactate increased by 13 mg/dl at 60 min were in the mitochondrial disease group.     

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.     

Chronic corticosteroid administration causes mitochondrial dysfunction in skeletal muscle

Corticosteroid myopathy is a major clinical problem in patients undergoing chronic corticosteroid treatment and shows insidious and progressive muscle atrophy in proximal limbs. Although several mechanisms underlying the pathophysiology of muscle injury have been postulated, precise pathogenesis is still not clear. We evaluated the mitochondrial functions in patients receiving corticosteroids compared with those in healthy controls or patients not receiving corticosteroids. The serum levels and total production of lactate were investigated by an aerobic exercise test using a bicycle ergometer. Mitochondrial respiratory activities and oxidative damage in biopsied skeletal muscles were also studied. The results of aerobic exercise tests revealed a significant overproduction of lactate in patients treated with corticosteroids (p < 0.005), which was positively correlated with total corticosteroid doses administered (p < 0.0001). In these patients, mitochondrial enzyme activity in complex I was significantly decreased (p < 0.05) and oxidative damage of biopsied skeletal muscle was remarkable both in mitochondrial and nuclear DNAs (p < 0.001). The results suggest that chronic corticosteroid administration induces mitochondrial dysfunction and oxidative damage in skeletal muscles, which may be the pathogenesis, at least in part, of corticosteroid-induced myopathy. Received: 5 November 2001 Received in revised form: 4 February 2002 Accepted: 7 February 2002     

Concentric-needle versus macro EMG. II. Detection of neuromuscular disorders.

OBJECTIVE: Little is known about the relation and sensitivity of macro-EMG (MA-EMG) compared with concentric-needle EMG (CN-EMG) in the detection of neuromuscular disorders. METHODS: CN-EMGs and MA-EMGs were recorded from the right brachial biceps muscle of 40 healthy subjects, aged 17-83 years, 20 patients with neurogenic disorders, aged 25-75 years, and 20 patients with myopathy, aged 18-76 years. Motor unit action potentials (MUAPs) were examined. RESULTS: In patients with neurogenic disorders CN-MUAP duration, CN-MUAP amplitude, percent polyphasia, MA-MUAP amplitude, MA-MUAP area and fibre density were significantly increased. In patients with myopathy, only fibre density was significantly increased. In patients with neurogenic disorders, the sensitivity of CN-EMG was 80%, and that of MA-EMG 85%. In myopathies, the sensitivity was 50% for each technique. Pooling the results of both EMG techniques, the sensitivity increased to 90% in patients with neurogenic disorders, and to 65% in myogenic disease. CONCLUSIONS: MA-EMG has a similar sensitivity in the detection of neuromuscular disorders as CN-EMG. Particularly when myopathy is suspected, both techniques should be applied if one is unrevealing.     

<Superscript>31</Superscript>P-MRS of skeletal muscle is not a sensitive diagnostic test for mitochondrial myopathy

Clinical phenotypes of persons with mitochondrial DNA (mtDNA) mutations vary considerably. Therefore, diagnosing mitochondrial myopathy (MM) patients can be challenging and warrants diagnostic guidelines. (31)phosphorous magnetic resonance spectroscopy ((31)P-MRS) have been included as a minor diagnostic criterion for MM but the diagnostic strength of this test has not been compared with that of other commonly used diagnostic procedures for MM. To investigate this, we studied seven patients with single, large-scale deletions-, nine with point mutations of mtDNA and 14 healthy subjects, who were investigated for the following: 1) (31)P-MRS of lower arm and leg muscles before and after exercise, 2) resting and peak-exercise induced increases of plasma lactate, 3) muscle morphology and -mitochondrial enzyme activity, 4) maximal oxygen uptake (VO(2max)), 5) venous oxygen desaturation during handgrip exercise and 6) a neurological examination. All MM patients had clinical symptoms of MM, > 2% ragged red fibers in muscle, and impaired oxygen desaturation during handgrip. Fourteen of 16 patients had impaired VO(2max), 10/16 had elevated resting plasma lactate, and 10/11 that were investigated had impaired citrate synthase-corrected complex I activity. Resting PCr/P(i) ratio and leg P(i) recovery were lower in MM patients vs. healthy subjects. PCr and ATP production after exercise were similar in patients and healthy subjects. Although the specificity for MM of some (31)P-MRS variables was as high as 100%, the sensitivity was low (0-63%) and the diagnostic strength of (31)P-MRS was inferior to the other diagnostic tests for MM. Thus, (31)P-MRS should not be a routine test for MM, but may be an important research tool.     

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.     

A mitochondrial myopathy in an infant with lactic acidosis

We describe a girl with mitochondrial myopathy, who presented with general muscle weakness, muscle hypotonia and motor retardation. The level of blood lactate and pyruvate was consistently increased. Enzymatic studies showed impairment of NADH-dehydrogenase activity (complex I of the respiratory chain) in skeletal muscle. Electron-microscopy of a muscle biopsy showed abnormalities of a mitochondrial myopathy. The girl, now aged 30 months, has been treated with riboflavine (vitamin B2) since the age of 14 months, and lactate and pyruvate levels have decreased to normal. The patient still shows mild muscle hypotonia and weakness, but good motor progress and normal cognitive development.     

Peak-ratio interference pattern analysis in the detection of neuromuscular disorders

Peak-ratio interference pattern analysis (peak-ratio method) is said to have a high sensitivity and to be independent of sex and age. This study was carried out to prove or disprove these findings. The peak-ratio method and qualitative motor unit action potential (MUAP) analysis were applied to the right brachial biceps and anterior tibial muscles of 44 healthy subjects, aged 23–87 years, 25 neuropathy patients, aged 21–83 years, and 29 myopathy patients, aged 19–70 years. Peak-ratio parameters were independent of sex and age. They tended to be lower in the anterior tibial muscle than in the brachial biceps muscle. Neuropathy patients typically showed decreased peak-ratio, short time intervals and increased amplitude/turn. Myopathy patients typically showed increased peak-ratio, turns/s and short time intervals. The sensitivity of the peak-ratio method was 72% for neuropathy patients and 59% for myopathy patients. The sensitivity of the peak-ratio method was similar to that of the MUAP analysis in neuropathy patients and higher than that of the MUAP analysis in myopathy patients. The specificity of the peak-ratio method was 80%. The peak-ratio method proved to be a valuable, supplementary electromyographic tool for the detection of neuromuscular disorders.     

Misdiagnosis of mitochondrial myopathies: a study of 12 thymectomized patients

INTRODUCTION: Myasthenia gravis and mitochondrial myopathies have common symptoms (fatigability, ophthalmoplegia) that could lead to diagnosis confusion. METHODS: We systematically reviewed medical history and ancillary investigations regarding 12 patients (7F/5M, mean age 47+/-14 years) having a mitochondrial myopathy but who were previously misdiagnosed as autoimmune myasthenia gravis and in whom a thymectomy was performed. RESULTS: Ocular palsy, ptosis and bulbar palsy were present in all patients. Limb fatigability was present in 9 cases. Symptoms were fluctuant but without remission. The misdiagnosis of myasthenia was based on the following arguments: 1) decremental EMG response (2 cases); 2) positive injectable anticholinesterase drugs test (3 cases); 3) partial response to oral anticholinesterase medications (2 cases); 4) AChR antibodies titer of 0.6 nM considered as positive (1 case). A multisystemic involvement was present in 5 patients: peripheral neuropathy (2 cases), deafness (2 cases), cardiopathy (3 cases), cerebellar involvement (2 cases) and myoclonia (1 case). The diagnosis of mitochondrial myopathy (at a mean age of 38+/-12 years) has been certified on the results of muscle biopsy showing mitochondrial proliferation (12 cases) and deleted mitochondrial DNA (8 cases). CONCLUSIONS: In a patient presenting with oculomotor symptoms and muscle fatigability, progressive course and multisystemic involvement are major arguments for a mitochondrial myopathy. In the absence of relevant criteria arguing for Myasthenia Gravis (significant variability of muscle weakness, positive titer of anti-AChR or anti-MuSK antibodies, decremental EMG response), a muscle biopsy is required before indication of thymectomy to exclude a mitochondrial disease.     

Cytochrome c oxidase deficiency in the muscle of patients with zidovudine myopathy is segmental and affects both mitochondrial DNA- and nuclear DNA-encoded subunits

Zidovudine (AZT) can induce a mitochondrial disorder associated with mitochondrial (mt) DNA depletion affecting skeletal muscle, heart, and liver. Zidovudine myopathy is characterized by ragged-red fibers and partial cytochrome c oxidase (COX) deficiency. We evaluated at a single fiber level the expression of COX II (mtDNA-encoded) and COX IV (nuclear DNA-encoded) subunits in 12 HIV-infected patients with zidovudine myopathy. We also evaluated COX activity on longitudinal muscle sections in one patient. In all patients, evaluation of the expression of COX II and COX IV subunits showed focal deficiency. All fibers negative for COX II or COX IV were negative by COX histochemistry; 32–92% (median 61%) of COX-negative fibers were negative for COX II antigens, and 7–58% (median 28%) were negative for COX IV antigens. One hundred and thirty-nine of 317 COX-negative fibers 139 (43.8%) were selectively negative for COX II; 28 of 317 (8.8%) COX-negative fibers were selectively negative for COX IV. A study of longitudinal distribution of COX activity demonstrated that COX deficiency was segmental with blurred borders, as previously observed in patients with myoclonus epilepsy with ragged-red fibers. We conclude that proteins encoded by mtDNA are predominantly, but not exclusively, involved in zidovudine myopathy. Our results confirm the value of single muscle fiber evaluation in the assessment of mitochondrial abnormalities related to zidovudine. Received: 8 July 1999 / Revised: 6 October 1999 / Accepted: 12 October 1999