Metabolic Myopathies and the Respiratory System

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.

Clinical Profile and Outcome of Pediatric Mitochondrial Myopathy in China

Introduction: Mitochondrial myopathy in children has notable clinical and genetic heterogeneity, but detailed data is lacking.

Patients and Methods: In this study, we retrospectively reviewed the clinical presentation, laboratory investigation, genetic and histopathological characteristics, and follow-ups of 21 pediatric mitochondrial myopathy cases from China.

Results: Twenty-four patients suspected with mitochondrial myopathy were enrolled initially and 21 were genetically identified. Fourteen patients were found to harbor mitochondrial DNA point mutations (14/21, 66.7%), including m.3243A>G (9/15, 60%), m.3303C>T (2/15, 13.3%), m.3302A>G (1/15, 6.7%), m.3250T>C (1/15, 6.7%), m.3251A>G (1/15, 6.7%), of whom 12 patients presented with progressive proximal mitochondrial myopathy (12/14, 85.7%). Three patients revealed large-scale deletion in blood or muscle tissue (3/21, 14.3%), presenting with Kearns-Sayer syndrome (1/3, 33.3%) or chronic progressive external ophthalmoplegia (2/3, 66.7%). Four patients were found to harbor pathogenic nuclear gene variants (4/21, 19.0%), including five variants in TK2 gene and two variants in SURF1 gene. During the follow-ups up to 7 years, 10 patients developed cardiomyopathy (10/21, 47.6%), 13 patients occurred at least once hypercapnic respiratory failure (13/21, 61.9%), six experienced recurrent respiratory failure and intubation (6/21, 28.6%), eight patients failed to survive (8/21, 38.1%). With nocturnal non-invasive ventilation of BiPAP, three patients recovered from respiratory failure, and led a relative stable and functional life (3/21, 14.3%).

Conclusion: Mitochondrial myopathy in children has great clinical, pathological, and genetical heterogeneity. Progressive proximal myopathy is most prevalent. Mitochondrial DNA point mutations are most common. And respiratory failure is a critical risk factor of poor prognosis.

Evidence of diaphragmatic dysfunction with severe alveolar hypoventilation syndrome in mitochondrial respiratory chain deficiency

Diaphragmatic dysfunction has been reported in congenital myopathies, muscular dystrophies, and occasionally, mitochondrial respiratory chain deficiency. Using a minimally invasive procedure in 3 young girls, 1 with a heteroplasmic MT-CYB mutation and 2 with biallelic pathogenic TK2 variants, we provided functional evidence of diaphragmatic dysfunction with global respiratory muscle weakness in mitochondrial respiratory chain deficiency. Analysis of respiratory muscle performance using esogastric pressures revealed paradoxical breathing and severe global inspiratory and expiratory muscle weakness with a sniff esophageal inspiratory pressure and a gastric pressure during cough averaging 50% and 40% of predicted values, respectively. This diaphragmatic dysfunction was responsible for severe undiagnosed nocturnal hypoventilation, requiring noninvasive ventilation. Our results underline the interest of this minimally invasive procedure for the evaluation of respiratory muscle performance and its potential value for the monitoring of future clinical trials in respiratory chain deficiency.

Construction and Analysis of Human Diseases and Metabolites Network

The relationship between aberrant metabolism and the initiation and progression of diseases has gained considerable attention in recent years. To gain insights into the global relationship between diseases and metabolites, here we constructed a human diseases-metabolites network (HDMN). Through analyses based on network biology, the metabolites associated with the same disorder tend to participate in the same metabolic pathway or cascade. In addition, the shortest distance between disease-related metabolites was shorter than that of all metabolites in the Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic network. Both disease and metabolite nodes in the HDMN displayed slight clustering phenomenon, resulting in functional modules. Furthermore, a significant positive correlation was observed between the degree of metabolites and the proportion of disease-related metabolites in the KEGG metabolic network. We also found that the average degree of disease metabolites is larger than that of all metabolites. Depicting a comprehensive characteristic of HDMN could provide great insights into understanding the global relationship between disease and metabolites.

Variants in HNRNPDL and SETX Not Necessarily Indicate Familial Amyotrophic Lateral Sclerosis or Limb Girdle Muscular Dystrophy 1G in Acute Muscular Respiratory Failure

Genetic work-up is useful for the identification of a primary myopathy. However, even sophisticated genetic methods may fail to detect the underlying cause of myopathy as in the following case. The patient is a 52-year-old female with a history of epilepsy, arterial hypertension, atrial flutter requiring cardioversion, ablation, and anticoagulation, coronary heart disease, hyperlipidemia, and hyper-CKemia. At age 52 years, she was referred for heart failure due to ischemic cardiomyopathy requiring appropriate medication and implantation of an ICD. During hospitalization she developed acute muscular respiratory failure requiring mechanical ventilation. Genetic panels for myopathy, neuropathy, and cardiomyopathy revealed variants of unknown significance in the HNRNPDL and SETX genes respectively. Clinical presentation and muscle biopsy, however, suggested metabolic myopathy. Acute muscular respiratory failure may require traditional diagnostic work-up for primary myopathy and long-term invasive and non-invasive ventilation. Panel investigations not necessarily lead to a conclusive diagnosis. The multisystem nature of the condition rather suggests a metabolic defect than LGMD-1G or fALS as genetic findings suggested.

Update Review about Metabolic Myopathies

The aim of this review is to summarize and discuss recent findings and new insights in the etiology and phenotype of metabolic myopathies. The review relies on a systematic literature review of recent publications. Metabolic myopathies are a heterogeneous group of disorders characterized by mostly inherited defects of enzymatic pathways involved in muscle cell metabolism. Metabolic myopathies present with either permanent (fixed) or episodic abnormalities, such as weakness, wasting, exercise-intolerance, myalgia, or an increase of muscle breakdown products (creatine-kinase, myoglobin) during exercise. Though limb and respiratory muscles are most frequently affected, facial, extra-ocular, and axial muscles may be occasionally also involved. Age at onset and prognosis vary considerably. There are multiple disease mechanisms and the pathophysiology is complex. Genes most recently related to metabolic myopathy include PGM1, GYG1, RBCK1, VMA21, MTO1, KARS, and ISCA2. The number of metabolic myopathies is steadily increasing. There is limited evidence from the literature that could guide diagnosis and treatment of metabolic myopathies. Treatment is limited to mainly non-invasive or invasive symptomatic measures. In conclusion, the field of metabolic myopathies is evolving with the more widespread availability and application of next generation sequencing technologies worldwide. This will broaden the knowledge about pathophysiology and putative therapeutic strategies for this group of neuromuscular disorders.

Long Term Non-invasive Ventilation in Children With Central Hypoventilation

Central hypoventilation (CH) is a quite rare disorder caused by some congenital or acquired conditions. It is featured by increased arterial concentration of serum carbon dioxide related to an impairment of respiratory drive. Patients affected by CH need to be treated by mechanical ventilation in order to achieve appropriate ventilation and oxygenation both in sleep and wakefulness. In fact, in severe form of Congenital Central Hypoventilation Syndrome (CCHS) hypercarbia can be present even during the day. Positive pressure ventilation via tracheostomy is the first therapeutic option in this clinical condition, especially in congenital forms. Non-Invasive ventilation is a an option that must be reserved for more stable clinical situations and that requires careful monitoring over time.