Plasma carnitine insufficiency and effectiveness of L-carnitine therapy in patients with mitochondril myopathy

Plasma carnitine “insufficiency,” (plasma esterified carnitine to free carnitine ratio above 0.25) was found in 21 48 (43.8%) patients with mitochondrial myopathy, of whom 4 also showed both total and free carnitine deficiencies in plasma. In addition, plasma levels of SCAC and LCAC were higher in patients with mitochondrial myopathy than in controls (P < 0.001 and P <0.01, respectively). Patients diagnosed as having plasma carnitine insufficiency or deficiency were treated with L-carnitine (50–200 mg/kg per day in four daily doses). Muscle weakness improved in 19 of 20 patients, failure to thrive in 4 of 8, encephalopathy in 1 of 9, and cardiomyopathy in 8 of 8 patients. Plasma carnitine “insufficiency” provides an additional clue to the diagnosis of mitochondrial myopathy and an indication for L-carnitine therapy. © 1993 John Wiley & Sons, Inc.     

Zidovudine-induced mitochondrial myopathy is associated with muscle carnitine deficiency and lipid storage

The use of zidovudine (AZT) for the treatment of acquired immunodeficiency syndrome (AIDS) induces a DNA-depleting mitochondrial myopathy, which is histologically characterized by the presence of muscle fibers with “ragged-red”–llike features, red-rimmed or empty cracks, granular degeneration, and rods (AZT fibers). Because dysfunctioning muscle mitochondria may lead to defects of β-oxidation of fatty acids, we examined the degree of neutral fat accumulation and muscle carnitine levels in the muscle biopsy specimens from 21 patients with AZT-induced myopathic symptoms of varying severity. Six patients with no AZT fibers had normal endomyofibrillar lipid deposits and muscle carnitine levels; 7 patients with fewer than 5 AZT fibers per field had a mild (+) to moderate (++) increase in lipid droplets, and reduced muscle carnitine levels (3 patients); and 8 patients with more than 5 AZT fibers had severe muscle changes, a ++ to marked (+++) increase in lipid droplets, and reduced muscle carnitine levels (6 patients). Serial sections showed lipid globules often within “cracks” or vacuoles of the abnormal muscle fibers. We conclude that the muscle mitochondrial impairment caused by AZT results in (1) accumulation of lipid within the muscle fibers owing to poor utilization of long-chain fatty acids, (2) reduction of muscle carnitine levels probably due to decreased carnitine uptake by the muscle, and (3) depletion of energy stores within the muscle fibers. The findings may have potential therapeutic implications in the treatment of AZT-induced myopathic symptoms using oral carnitine supplementation.     

Isometric skeletal muscle force measurement in primary myopathies

Introduction: In myopathy patients, it is useful to measure skeletal muscle forces. Conventional methods require voluntary muscle activation, which can be unreliable. We evaluated a device for nonvoluntary force assessment. Methods: We tested 8 patients (unknown myopathy n = 2, inflammatory myopathy, facioscapulohumeral muscular dystrophy, mitochondrial myopathy, dysferlinopathy, multi‐minicore disease, Becker‐Kiener muscular dystrophy, n = 1 each). Isometric twitch torques of ankle dorsiflexors were measured after fibular nerve stimulation. Results: Six patients had decreased torques vs. 8 controls (men: median Newton‐meter 1.6 vs. 5.7, women: 0.2 vs. 3.9, both P < 0.0001). Values correlated with Manual Muscle Test results (r = 0.73; r² = 0.53; P < 0.0001). In weak dorsiflexors, torque could be measured despite lower signal‐to‐noise ratios. In 2 patients with hypertrophy, we measured increased torques. Conclusions: Nonvoluntary muscle force assessment can be used in patients with myopathies, and values correlate with voluntary forces determined by traditional methods. Muscle Nerve 53 : 913–917, 2016     

Changes in skeletal muscle histology and metabolism in patients undergoing exercise deconditioning: Effect of propionyl-L-carnitine

To define the skeletal muscle abnormalities in patients undergoing exercise deconditioning and evaluate the metabolic effect of propionyl-L-carnitine (PLC), muscle biopsies were obtained from 28 patients with effort angina and 31 control subjects. Coronary artery disease patients received either placebo (n = 12), PLC (1.5 g IV followed by infusion of 1 mg/kg/min for 30 min, n = 10), or L-carnitine (1 g IV followed by infusion of 0.65 mg/kg/min for 30 min, n = 6) for 2 days. Exercise deconditioned patients treated with placebo showed normal muscle content of total carnitine and glycogen, and decrease in percentage of type 1 fibers (P < 0.01) and in the activity of citrate synthase (P < 0.05), succinate dehydrogenase (P < 0.05), and cytochrome oxidase (P < 0.05), as compared to controls. Both PLC and L-carnitine did not modify muscle fiber composition or enzyme activities, but significantly increased muscle levels of total carnitine by 42% and 31%, respectively (P < 0.05). Moreover, PLC significantly increased glycogen muscle content (P < 0.01), while the equimolar dose of L-carnitine did not. This effect, probably due to the anaplerotic activity of the propionic group of PLC, suggests that this drug may be effective in improving energy metabolism of muscles with impaired oxidative capacity. © John Wiley & Sons, Inc. Muscle Nerve 20: 1115–1120, 1997     

Muscle fatigue,lactate, and pyruvate in mitochondrial myopathy with progressive external ophthalmoplegia

We studied muscle fatigue and serum lactate and pyruvate levels in 20 patients with mitochondrial myopathy with progressive external ophthalmoplegia (PEO). Fatigue was assessed in the adductor pollicis muscle (AP) using a low-intensity exercise protocol (20 min). Forces (TFs) and relaxation times of ulnar nerve evoked twitches, compound muscle action potentials (CMAPs), and maximal voluntary contractions (MVCs) were monitored. Serum lactate and pyruvate levels were independently measured at rest and after exercise on a bicycle (15 min, 30 W). Most patients showed abnormal fatigue of the AP with a reduction of TFs and MVCs and normal CMAPs. The reduced TFs were significantly correlated with lactate levels at rest (r = −0.60, P < 0.05) and less so with those after exercise (r = −0.47, P < 0.05). Pyruvate levels revealed a similar correlation although they were widely scattered. We conclude that abnormal fatigue in PEO is metabolic, is localized beyond the muscle fiber membrane, and involves the electromechanical coupling and the contractile apparatus. Serum lactate levels at rest are good predictors of fatigue in PEO. © 1996 John Wiley & Sons, Inc.     

脂质沉积性肌病30例临床资料分析

目的总结脂质沉积性肌病(LSM)的临床特点。方法总结30例LSM患者的临床资料并复习文献。结果LSM是脂肪酸代谢异常所致,90.0%以上患者有发作性近端肌无力,不耐受疲劳;73.3%有眼肌、颈肌受累;46.7%有消化道症状。60.0%～86.7%的患者肌酶升高;83.3%有肌电图异常,其中77.0%为肌源性损害;93.1%有脂质沉积;可有血浆肉毒碱降低。泼尼松或能量支持治疗有效率为95.2%。结论LSM是肌无力原因之一,可多系统受累或与其他代谢异常共存,多数预后较好。     

Mitochondria-lipid-glycogen myopathy, hyperlactacidemia, and carnitine deficiency

A 25-month-old girl had proximal myopathy, increased blood lactate and pyruvate concentrations, and transient ketoacidosis. Muscle biopsy revealed vacuolar myopathy with accumulation of both lipid and glycogen. Electronmicroscopy also showed abnormalities in the shape, size, and internal structure of muscle mitochondria. Carnitine content of skeletal muscle was reduced. Short-chain and long-chain acyl-carnitines were augmented in both plasma and skeletal muscle. Oral carnitine therapy improved muscle strength.     

Disorders of lipid metabolism in muscle

At rest and during sustained exercise, lipids are the main source of energy for muscle. Free fatty acids become available to muscle from plasma free fatty acids and triglycerides, and from intracellular triglyceride lipid droplets. Transport of long-chain fatty acyl groups into the mitochondria requires esterification and de-esterification with carnitine by the “twin” enzymes carnitine palmityltransferase (CPT) I and II, bound to the outer and inner faces of the inner mitochondrial membrane. Carnitine deficiency occurs in two clinical syndromes. (1) In the myopathic form, there is weakness; muscle biopsy shows excessive accumulation of lipid droplets; and the carnitine concentration is markedly decreased in muscle but normal in plasma. (2) In the systemic form, there are weakness and recurrent episodes of hepatic encephalopathy; muscle biopsy shows lipid storage; and the carnitine concentration is decreased in muscle, liver, and plasma. The etiology of carnitine deficiency is not known in either the myopathic or the systemic form, but administration of carnitine or corticosteroids has been beneficial in some patients. “Secondary” carnitine deficiency may occur in patients with malnutrition, liver disease, chronic hemodialysis, and, possibly, mitochondrial disorders. CPT deficiency causes recurrent myoglobinuria, usually precipitated by prolonged exercise or fasting. Muscle biopsy may be normal or show varying degrees of lipid storage. Genetic transmission is probably autosomal recessive, but the great male predominance (20/21) remains unexplained. In many cases, lipid storage myopathy is not accompanied by carnitine or CPT deficiency, and the biochemical error remains to be identified.     

Muscle carnitine levels in neuromuscular disease

The production of energy in muscle from long-chain fatty acid oxidation is dependent upon the presence of carnitine. An abnormally low level of muscle carnitine, as seen in patients with the carnitine deficiency syndrome, results in marked muscle weakness. Muscle from 83 consecutive patients undergoing diagnostic muscle biopsy was assayed for carnitine. Carnitine levels (mean ± SEM, expressed as nmoles carnitine per mg noncollagen protein) in muscle from patients with Duchenne dystrophy (8.1 ± 1.7) and possible Becker dystrophy (10.6 ± 3.0) were significantly (P < 0.001) different from histologically normal muscle (24.0 ± 1.4). Carnitine levels in patients with limb-girdle dystrophy (16.1 ± 3.1) and polymyositis/dermatomyositis (16.6 ± 3.2) were also low, although not as low as in Duchenne dystrophy. Carnitine levels from patients with denervation atrophy (22.1 = 3.6), nonspecific fiber atrophy (21.3 ± 1.3), and a group of miscellaneous neuromuscular diseases (20.4 ± 1.4) were not significantly different from histologically normal muscle. The low values of carnitine seen in Duchenne dystrophy and a group of possible Becker dystrophy patients may be a nonspecific effect, related to severe muscle damage.     

Muscle carnitine deficiency: adult onset lipid storage myopathy with sensory neuropathy

Muscle carnitine deficiency usually results in a lipid storage myopathy, but more rarely, neuropathy occurs in this condition. We report herein a 29-year-old man with muscle carnitine deficiency who developed not only a lipid storage myopathy, but also a severe sensory neuropathy. Oral therapy with levo-carnitine (3 g per day) for 3 months produced a remarkable improvement of the myopathy and sensory neuropathy. Six months later, he remained in good condition under strict dietary control. This report emphasizes that severe neuropathy may occur in some patients with muscle carnitine deficiency, and highlights the need for the neurologist’s familiarity with those afflicted to achieve optimal clinical management.     

脂质沉积性肌病的临床、神经电生理和病理学特征

目的研究脂质沉积性肌病(LSM)的临床、神经电生理及病理学特点。方法回顾性分析5例LSM患者的临床、神经电生理及病理学资料。结果本组发病年龄平均为25.6岁;均为亚急性或慢性起病,病情缓慢进展或出现缓解复发;2例有家族史;主要表现为运动耐受差和不同程度的肌无力(5/5)、腱反射减弱或消失(5/5)、肌痛或肌压痛(4/5)、肌萎缩(3/5)、末梢型感觉障碍(2/5);肌酶轻至中度升高(4/5),肌电图表现周围神经源性损害合并肌源性损害(2/5),或者单纯肌源性损害(1/5);肌肉病理学显示纤维间、肌膜下、胞核周围大量排列成串的或成团的脂滴,无炎性细胞浸润。高肉碱、低脂肪饮食、以及糖皮质激素治疗有效。结论LSM临床以肌无力和运动不耐受为主要症状,神经电生理表现为肌源性损害和/或周围神经损害,肌肉活检为诊断脂质沉积性肌病所必需。     

Reduced brain stem excitability in mitochondrial myopathy: Evidence for early detection with blink reflex habituation studies

Blink reflex (BR) was studied in 17 patients with histochemically and genetically confirmed mitochondrial myopathy (MM). Fourteen patients had chronic progressive external ophthalmoplegia (CPEO) associated with a mild to moderate craniosomatic myopathy without any symptoms or signs of central nervous system (CNS) involvement, 2 myoclonic epilepsy with ragged red fibers syndrome, and 1 Kearns-Sayre syndrome. The mean latencies of the early (R1) and late (R2) responses were prolonged (P < 0.01 and P < 0.001, respectively), and the corresponding amplitudes decreased (P < 0.001). Increased habituation of the reflex was clearly observed in 10 out of 14 patients tested (71.4%), 9 of whom presented CPEO. These findings suggest that the brain stem reticular network is in a state of basal inhibition which is presumably due to a subclinical impairment of the cerebral cellular metabolism. Multimodal evoked potentials revealed abnormalities suggestive of CNS involvement in 7 out of 17 patients (41.2%), 4 of whom had CPEO. These observations document the validity of BR in detecting clinically silent brain stem impairment in patients with apparently pure MM and provide important clues for a further understanding of the underlying pathophysiology. © 1996 John Wiley & Sons, Inc.     

Fulminant lipid storage myopathy due to multiple acyl‐coenzyme a dehydrogenase deficiency

Introduction: The lipid storage myopathies, primary carnitine deficiency, neutral lipid storage disease, and multiple acyl coenzyme A dehydrogenase deficiency (MADD), are progressive disorders that cause permanent weakness. These disorders of fatty acid metabolism and intracellular triglyceride degradation cause marked fat deposition and damage to muscle cells. Methods: We describe a rapidly progressive myopathy in a previously healthy 33‐year‐old woman. Over 4 months, she developed a proximal and axial myopathy associated with diffuse myalgia and dysphagia, ultimately leading to respiratory failure and death. Results: Muscle biopsy showed massive accumulation of lipid. Plasma acylcarnitine and urine organic acid analysis was consistent with MADD. This was confirmed by molecular genetic testing, which revealed 2 pathogenic mutations in the ETFDH gene. Conclusions: This report illustrates a late‐onset case of MADD and reviews the differential diagnosis and evaluation of patients with proximal myopathy and excessive accumulation of lipid on muscle biopsy. Muscle Nerve 52 : 289–293, 2015     

Contractile properties are impaired in congenital myopathies

The ratio between muscle strength and muscle cross-sectional area is called the specific force. Fatty replacement of muscles is seen in many myopathies, affecting the specific force, without necessarily affecting the ability of the remaining muscle fibers to contract. This ability is called the contractility and is the ratio between muscle strength and the lean muscle cross-sectional area, i.e. the contractile cross-sectional area. We hypothesized that contractility is disrupted in patients with congenital myopathy, because of defects in contractile proteins of the sarcomere. Peak torque across ankle and knee joints was measured by isokinetic dynamometry in 16 patients with congenital myopathy and 13 healthy controls. Five patients only participated partially in the dynamometer measurements due to severe muscle weakness. Dixon MRI technique was used to quantify muscle fat fractions and calculate cross-sectional area. Patients with congenital myopathy had lower cross-sectional area in all muscle groups (P<0.01), higher fat fraction (P<0.01) and less strength (P<0.005) in all studied muscle groups. Their fat content was more than doubled and peak torque lower than half that in healthy controls. Muscle contractility was reduced (P<0.01) in three of four patient muscle groups. In conclusion, muscle contractility was reduced in patients with congenital myopathy, across different diagnoses, and was independent of the level of muscle fat fraction, suggesting that intrinsic defects of the myocyte are responsible for reduced contractility.     

Muscle function in A canine model of X‐linked myotubular myopathy

Introduction: We established a colony of dogs that harbor an X‐linked MTM1 missense mutation.Muscle from affected male dogs exhibits reduction and altered localization of the MTM1 gene product, myotubularin, and provides a model analogous to X‐linked myotubular myopathy (XLMTM). Methods: We studied hindlimb muscle function in age‐matched canine XLMTM genotypes between ages 9 and 18 weeks. Results: By the end of the study, affected dogs produce only ～15% of the torque generated by normals or carriers (0.023 ± 0.005 vs. 0.152 ± 0.007 and 0.154 ± 0.003 N‐m/kg body mass, respectively, P < 0.05) and are too weak to stand unassisted. At this age, XLMTM dogs also demonstrate an abnormally low twitch:tetanus ratio, a right‐shifted torque‐frequency relationship and an increase in torque during repetitive stimulation (P < 0.05). Conclusions: We hypothesize that muscle weakness results from impaired excitation‐contraction (E‐C) coupling. Interventions that improve E‐C coupling might be translated from the XLMTM dog model to patients. Muscle Nerve 46: 588–591, 2012     

DNAJB6 myopathy: A vacuolar myopathy with childhood onset

Introduction: DNAJB6 mutations cause an autosomal dominant myopathy that can manifest as limb‐girdle muscular dystrophy (LGMD1D/1E) or distal‐predominant myopathy. In the majority of patients this myopathy manifests in adulthood and shows vacuolar changes on muscle biopsy. Methods: Clinical, electrophysiological, pathological, and molecular findings are reported. Results: We report a 56‐year‐old woman, who, like 3 other family members, became symptomatic in childhood with slowly progressive limb‐girdle muscle weakness, normal serum creatine kinase (CK) values, and myopathic electromyographic findings. Muscle biopsy showed vacuolar changes and congophilic inclusions, and molecular analysis revealed a pathogenic mutation in the DNAJB6 gene. Differences and similarities with previously described cases are assessed. Conclusions: Childhood‐onset of DNAJB6 myopathy is more frequent than previously believed; congophilic inclusions may be present in the muscle of these patients. Muscle Nerve 49:607–610, 2014     

Lipid storage myopathy in Kearns-Sayre syndrome

A 7-year-old girl had external ophthalmoplegia, limb weakness, short stature, hearing loss, pigmentary degeneration of the retina, and increased CSF protein content. Muscle biopsy revealed vacuolar myopathy with accumulation of lipids. Electronmicroscopy showed abnormalities of shape, size, and internal structure of muscle mitochondria. Muscle activity of palmitoyl-CoA synthetase was decreased, and the content of lipids was increased. Serum and muscle carnitine levels were normal, as were muscle carnitine palmitoyltransferase and carnitine acetyltransferase.     

Inflammatory myopathy with cytochrome oxidase negative muscle fibers: Methotrexate treatment

Inflammatory myopathy with cytochrome oxidase negative muscle fibers (IM/COX−) is characterized by slowly progressive weakness, most prominent in the quadriceps, muscle fibers with reduced COX staining and mitochondrial DNA mutations, and a poor response to corticosteroid treatment. We reviewed records of quantitative measurements of muscle strength in 7 IM/COX− patients to evaluate the outcomes after treatment with oral, once weekly, methotrexate for an average of 15 months. We compared the results to 6 patients with IM/COX− who received no long-term immunosuppression, and to 4 with inclusion body myositis (IBM) who received methotrexate during the same period. Methotrexate treatment of IM/COX− was followed by improved muscle strength in 5 of 7 patients, averaging 17 ± 5%. In contrast, there was no improvement in the strength of 6 untreated IM/COX− patients (−6 ± 4%; P = 0.003), or 4 methotrexate-treated IBM patients (1 ± 2%; P = 0.03). We conclude that, despite clinical similarities to inclusion body myositis, which is usually refractory to immunosuppressive therapy, strength in IM/COX− appears to improve with methotrexate treatment. Biopsy studies of inflammatory myopathies with evaluation of muscle for mitochondrial changes and vacuoles can help to direct the choice of appropriate immunomodulating treatments. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1724–1728, 1998     

Mitochondrial myopathy with progressive decrease in mitochondrial tRNALeu(UUR) mutant genomes

A female patient with mitochondrial myopathy had a mitochondrial DNA mutation at nucleotide pair 3243, commonly seen in patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS), but unlike MELAS patients, she had no central nervous system symptoms. Muscle weakness, which was most severe when she was 7 years old, improved gradually with age. Comparison of two muscle biopsies obtained at an interval of 12.5 years (7 and 20 years of age, respectively), revealed that the number of ragged-red fibers was markedly decreased and histochemical cytochrome c oxidase activity increased in parallel with the decrease in population of mutant genomes.     

Palmitate oxidation in human muscle: Comparison to CPT and carnitine

The evaluation of palmitate oxidation in muscle tissue may be a useful screening test for detecting defects in fatty acid metabolism in human neuromuscular disease. If the test is to be useful, it is necessary to obtain data on a wide variety of muscle illnesses for comparative purposes. We report our experience with palmitate oxidation, muscle carnitine, and carnitine palmityl transferase (CPT) activity in 148 muscle biopsies from a variety of illnesses. The efficacy of using total protein, citrate synthase, and (1-¹⁴C) pyruvate oxidation as internal references was investigated. Palmitate oxidation was significantly less than normal (P ≤ 0.01) in Duchenne muscular dystrophy, congenital nonprogressive myopathy, congenital muscular dystrophy, malignant hyperpyrexia, and denervation, depending on the internal reference used. Muscle carnitine levels followed a similar pattern, however, CPT activity did not. The possibility of these findings being secondary to inactivity is discussed.