Prevalence and progression of mitochondrial diseases: a study of 50 patients

Clinical Spectrum of Biopsy Proven Mitochondrial Myopathy

Objectives

Clinical spectrum of mitochondrial myopathy extends beyond chronic progressive external ophthalmoplegia (CPEO). While information on encephalomyopathies is abundant, clinical data on predominant myopathic presentation of mitochondrial disorders are lacking.

Materials and Methods

Clinical, electrophysiological, biochemical, and follow-up data of patients with predominant myopathic presentation and muscle biopsy confirmed primary mitochondrial myopathy was obtained. We excluded known syndromes of mitochondrial cytopathies and encephalomyopathies.

Results

Among 16 patients, 7 had CPEO, 4 had CPEO with limb-girdle muscle weakness (LGMW), and 5 had isolated LGMW. Systemic features included seizures with photosensitivity (n = 3), diabetes (n = 1), cardiomyopathy (n = 1), and sensorineural hearing loss (n = 1) and were more common in isolated LGMW. Elevated serum creatine kinase (CK) and lactate levels and electromyography (EMG) myopathic potentials were more frequent with LGMW. During follow-up, LGMW had more severe progression of weakness.

Conclusion

We identified three subsets of mitochondrial myopathy with distinct clinical features and evolutionary patterns. Isolated LGMW was seen in 30% of patients and would represent severe end of the spectrum.

Estimation of the Number of Patients With Mitochondrial Diseases: A Descriptive Study Using a Nationwide Database in Japan

BACKGROUND

To provide a better healthcare system for patients with mitochondrial diseases, it is important to understand the basic epidemiology of these conditions, including the number of patients affected. However, little information about them has appeared in Japan to date.

METHODS

To gather data of patients with mitochondrial diseases, we estimated the number of patients with mitochondrial diseases from April 2018 through March 2019 using a national Japanese health care claims database, the National Database (NDB). Further, we calculated the prevalence of patients, and sex ratio, age class, and geographical distribution.

RESULTS

From April 2018 through March 2019, the number of patients with mitochondrial diseases was 3,629, and the prevalence was 2.9 (95% confidence interval [CI], 2.8-3.0) per 100,000 general population. The ratio of females and males was 53 to 47, and the most frequent age class was 40-49 years old. Tokyo had the greatest number of patients with mitochondrial diseases, at 477, whereas Yamanashi had the fewest, at 13. Kagoshima had the highest prevalence of patients with mitochondrial diseases, 8.4 (95% CI, 7.1-10.0) per 100,000 population, whereas Yamanashi had the lowest, 1.6 (95% CI, 0.8-2.7).

CONCLUSION

The number of patients with mitochondrial diseases estimated by this study, 3,269, was more than double that indicated by the Japanese government. This result may imply that about half of all patients are overlooked for reasons such as low severity of illness, suggesting that the Japanese healthcare system needs to provide additional support for these patients.

Prevalence and health care costs of mitochondrial disease in Ontario, Canada: A population-based cohort study

Background

Mitochondrial disease prevalence has been estimated at 1 in 4000 in the United States, and 1 in 5000 worldwide. Prevalence in Canada has not been established, though multi-linked health administrative data resources present a unique opportunity to establish robust population-based estimates in a single-payer health system. This study used administrative data for the Ontario, Canada population between April 1988 and March 2019 to measure mitochondrial disease prevalence and describe patient characteristics and health care costs.

Results

3069 unique individuals were hospitalized with mitochondrial disease in Ontario and eligible for the study cohort, representing a period prevalence of 2.51 per 10,000 or 1 in 3989. First hospitalization was most common between ages 0–9 or 50–69. The mitochondrial disease population experiences a high need for health care and incurred high costs (mean = CAD$24,023 in 12 months before first hospitalization) within the single-payer Ontario health care system.

Conclusions

This study provides needed insight into mitochondrial disease in Canada, and demonstrates the high health burden on patients. The methodology used can be adapted across jurisdictions with similar routine collection of health data, such as in other Canadian provinces. Future work should seek to validate this approach via record linkage of existing disease cohorts in Ontario, and identify specific comorbidities with mitochondrial disease that may contribute to high health resource utilization.

A retrospective cohort study exploring the association between different mitochondrial diseases and hearing loss

Some pathogenic variants in mtDNA and nuclear DNA, affecting mitochondrial function, are associated with hearing loss. Behavioral and electrophysiological auditory performance are obtained from 62 patients, clinically diagnosed with different mitochondrial diseases (MD) using tone/speech audiometry and Auditory Brainstem Responses (ABR). Audiological variables (hearing loss type, pure tone average (PTA), interaural asymmetry, speech perception and brainstem neural conductivity) were analyzed and related to Newcastle Mitochondrial Disease Scale for Adults (NMDAS). In 35% of MDs, a mild to severe symmetrical sensorineural hearing loss (SNHL) was found. Patients with Maternally Inherited Diabetes and Deafness (MIDD) show significantly higher PTAs compared to other MDs. For all MDs, speech recognition scores were in accordance with their individual age- and gender-corrected tone audiometry, but ABR peak latencies were prolonged in patients with MIDD, Mitochondrial Encephalopathy Lactate acidosis and Stroke-like episodes (MELAS), Chronic Progressive External Ophthalmoplegia (CPEO) and Subacute necrotizing encephalopathy (Leigh). Correlations between NMDAS and audiological variables were low.

Mitochondrial diseases in South Asia - A systematic review

BACKGROUND

Mitochondrial diseases are largely underdiagnosed due to their heterogeneity in clinical presentation and genotype. This is especially true for resource-constrained settings in South Asian countries such as Afghanistan, Bangladesh, Bhutan, India, Maldives, Pakistan, Nepal, Sri Lanka and Myanmar. This study aims to evaluate the current status of clinical presentations, diagnosis and treatment of Mitochondrial diseases in the South Asian region.

METHODS

We undertook a systematic review of the literature on mitochondrial diseases in the South Asian region. We searched Medline, Pubmed, Cochrane library, and Google scholar using the search terms, "Mitochondrial diseases" AND "Metabolic diseases" (Mesh terms) in the title or the abstract field for each South Asian Country (Afghanistan, Bangladesh, Bhutan, India, Maldives, Pakistan, Nepal, Sri Lanka and Myanmar).

RESULTS

We found 89 citations in Pubmed, 22 citations in Cochrane library and 68 in Google scholar respectively. A total of 25 non-duplicated studies met the inclusion and exclusion criteria. After assessing the quality of the published studies 18 were included. Which comprised of 17 case reports and one case-control study.

CONCLUSION

Studies that were published were case reports from India, Pakistan, and Sri Lanka. Due to the paucity of published data on mitochondrial diseases in the South Asian region, it is difficult to estimate its true burden.

Occurrence of Inborn Errors of Metabolism in Newborns, Diagnosis and Prophylaxis

Inborn errors of metabolism (IEM) are a heterogeneous group of rare genetic disorders that are generally transmitted as autosomal or X-linked recessive disorders. These defects arise due to mutations associated with specific gene(s), especially the ones associated with key metabolic enzymes. These enzymes or their product(s) are involved in various metabolic pathways, leading to the accumulation of intermediary metabolite(s), reflecting their toxic effects upon mutations. The diagnosis of these metabolic disorders is based on the biochemical analysis of the clinical manifestations produced and their molecular mechanism. Therefore, it is imperative to devise diagnostic tests with high sensitivity and specificity for early detection of IEM. Recent advances in biochemical and polymerase chain reaction-based genetic analysis along with pedigree and prenatal diagnosis can be life-saving in nature. The latest development in exome sequencing for rapid diagnosis and enzyme replacement therapy would facilitate the successful treatment of these metabolic disorders in the future. However, the longterm clinical implications of these genetic manipulations is still a matter of debate among intellectuals and requires further research.

Mitochondrial genome variant m.3250T>C as a possible risk factor for mitochondrial cardiomyopathy

The MT-TL1 gene codes for the mitochondrial leucine transfer RNA (tRNA^Leu(UUR) ) necessary for mitochondrial translation. Pathogenic variants in the MT-TL1 gene result in mitochondriopathy in humans. The m.3250T>C variant in the MT-TL1 gene has been previously associated with exercise intolerance and mitochondrial myopathy, yet disease classification for this variant has not been consistently reported. Molecular studies suggest the m.3250T>C variant does not alter tRNA^Leu(UUR) structure but may have a modest impact on aminoacylation capacity. However, functional studies are limited. Our study aimed to further define the clinical presentation, inheritance pattern, and molecular pathology of the m.3250T>C variant. Families with the m.3250T>C variant were recruited from the Mitochondrial Disease Clinic at Cincinnati Children's Hospital Medical Center and GeneDx laboratory database. Affected individuals most frequently presented with cardiac findings, exercise intolerance, and muscle weakness. Hypertrophic cardiomyopathy was the most frequent cardiac finding. Many asymptomatic individuals had homoplasmic or near homoplasmic levels of the m.3250T>C variant, suggesting the penetrance is incomplete. Patient-derived fibroblasts demonstrated lowered ATP production and increased levels of reactive oxygen species. Our results demonstrate that the m.3250T>C variant exhibits incomplete penetrance and may be a possible cause of cardiomyopathy by impacting cellular respiration in mitochondria.

Codon optimization is an essential parameter for the efficient allotopic expression of mtDNA genes

Mutations in mitochondrial DNA can be inherited or occur de novo leading to several debilitating myopathies with no curative option and few or no effective treatments. Allotopic expression of recoded mitochondrial genes from the nucleus has potential as a gene therapy strategy for such conditions, however progress in this field has been hampered by technical challenges. Here we employed codon optimization as a tool to re-engineer the protein-coding genes of the human mitochondrial genome for robust, efficient expression from the nucleus. All 13 codon-optimized constructs exhibited substantially higher protein expression than minimally-recoded genes when expressed transiently, and steady-state mRNA levels for optimized gene constructs were 5-180 fold enriched over recoded versions in stably-selected wildtype cells. Eight of thirteen mitochondria-encoded oxidative phosphorylation (OxPhos) proteins maintained protein expression following stable selection, with mitochondrial localization of expression products. We also assessed the utility of this strategy in rescuing mitochondrial disease cell models and found the rescue capacity of allotopic expression constructs to be gene specific. Allotopic expression of codon optimized ATP8 in disease models could restore protein levels and respiratory function, however, rescue of the pathogenic phenotype for another gene, ND1 was only partially successful. These results imply that though codon-optimization alone is not sufficient for functional allotopic expression of most mitochondrial genes, it is an essential consideration in their design.

Epidemiological study and genetic characterization of inherited muscle diseases in a northern Spanish region

Background

Inherited muscle diseases are a group of rare heterogeneous muscle conditions with great impact on quality of life, for which variable prevalence has previously been reported, probably due to case selection bias. The aim of this study is to estimate the overall and selective prevalence rates of inherited muscle diseases in a northern Spanish region and to describe their demographic and genetic features. Retrospective identification of patients with inherited muscle diseases between 2000 and 2015 from multiple data sources. Demographic and molecular data were registered.

Results

On January 1, 2016, the overall prevalence of inherited muscle diseases was 59.00/ 100,000 inhabitants (CI 95%; 53.35–65.26). Prevalence was significantly greater in men (67.33/100,000) in comparison to women (50.80/100,000) (p = 0.006). The highest value was seen in the age range between 45 and 54 (91.32/100,000) years. Myotonic dystrophy type 1 was the most common condition (35.90/100,000), followed by facioscapulohumeral muscular dystrophy (5.15/100,000) and limb-girdle muscular dystrophy type 2A (2.5/100,000).

Conclusions

Prevalence of inherited muscle diseases in Navarre is high in comparison with the data reported for other geographical regions. Standard procedures and analyses of multiple data sources are needed for epidemiological studies of this heterogeneous group of diseases.

Lethal neonatal mitochondrial phenotype caused by a novel polymerase subunit gamma mutation: A case report

RATIONALE

Polymerase subunit gamma (POLG) is a gene that codes for the catalytic subunit of the mitochondrial DNA polymerase, which is involved in the replication of mitochondrial DNA. Mutations in these genes are associated with a range of clinical syndromes characterized by secondary mtDNA defect including mtDNA mutation and mtDNA depletion which may culminate in complete failure of energy production (respiratory changes complex 1 defect) as in this case.

PATIENT CONCERNS

We herein report a full term Saudi female neonate born to consanguineous parents, who was noticed immediately after birth to have severe hypotonia, poor respiratory effort, and dysmorphic features. She had 3 siblings who died with same clinical scenario in neonatal period.

DIAGNOSES

Molecular genetic testing revealed a novel compound heterozygous mutation of POLG gene c.680G>A (p.Arg227Gin) and c.3098C>T (p.Ala1033Val).

INTERVENTIONS

The patient remained in neonatal intensive care unit with multidisciplinary team management and was ventilator dependent until she passed away.

OUTCOMES

The detected mutation had led to complete failure of energy production (respiratory changes complex 1 defect) until she died at the age of 5 months.

LESSONS

Mitochondrial respiratory chain defect should be considered in patients with severe neonatal hypotonia,encephalopathy, and respiratory failure especially in highly consanguineous population.

Mitochondrial mutations associated with hearing and balance disorders

Hearing and balance disorders are related to the inner ear and are among the major cause of falls in older adults. Hearing loss that commonly occurs with aging (aka presbyacusis) can result from noise exposure, smoking, ototoxic drugs and genetic factors such as mutations in nuclear and mitochondrial genes. Mutations in mitochondrial DNA (mtDNA) have been reported to play an important role in cell function by providing energy, as well as, cell death (apoptosis). This study aims to systematically review mitochondrial mutations associated with presbyacusis and suggests preventive measurements to improve the quality of life in older adults.

Cardiopulmonary Exercise Testing and Metabolic Myopathies

Skeletal muscle requires a large increase in its ATP production to meet the energy needs of exercise. Normally, most of this increase in ATP is supplied by the aerobic process of oxidative phosphorylation. The main defects in muscle metabolism that interfere with production of ATP are (1) disorders of glycogenolysis and glycolysis, which prevent both carbohydrate entering the tricarboxylic acid cycle and the production of lactic acid; (2) mitochondrial myopathies where the defect is usually within the electron transport chain, reducing the rate of oxidative phosphorylation; and (3) disorders of lipid metabolism. Gas exchange measurements derived from exhaled gas analysis during cardiopulmonary exercise testing can identify defects in muscle metabolism because [Formula: see text]o₂ and [Formula: see text]co₂ are abnormal at the level of the muscle. Cardiopulmonary exercise testing may thus suggest a likely diagnosis and guide additional investigation. Defects in glycogenolysis and glycolysis are identified by a low peak [Formula: see text]o₂ and absence of excess [Formula: see text]co₂ from buffering of lactic acid by bicarbonate. Defects in the electron transport chain also result in low peak [Formula: see text]o₂, but because there is an overreliance on anaerobic processes, lactic acid accumulation and excess carbon dioxide from buffering occur early during exercise. Defects in lipid metabolism result in only minor abnormalities during cardiopulmonary exercise testing. In defects of glycogenolysis and glycolysis and in mitochondrial myopathies, other features may include an exaggerated cardiovascular response to exercise, a low oxygen-pulse, and excessive ammonia release.

Nuclear genes involved in mitochondrial diseases caused by instability of mitochondrial DNA

Mitochondrial diseases are defined by a respiratory chain dysfunction and in most of the cases manifest as multisystem disorders with predominant expression in muscles and nerves and may be caused by mutations in mitochondrial (mtDNA) or nuclear (nDNA) genomes. Most of the proteins involved in respiratory chain function are nuclear encoded, although 13 subunits of respiratory chain complexes (together with 2 rRNAs and 22 tRNAs necessary for their translation) encoded by mtDNA are essential for cell function. nDNA encodes not only respiratory chain subunits but also all the proteins responsible for mtDNA maintenance, especially those involved in replication, as well as other proteins necessary for the transcription and copy number control of this multicopy genome. Mutations in these genes can cause secondary instability of the mitochondrial genome in the form of depletion (decreased number of mtDNA molecules in the cell), vast multiple deletions or accumulation of point mutations which in turn leads to mitochondrial diseases inherited in a Mendelian fashion. The list of genes involved in mitochondrial DNA maintenance is long, and still incomplete.

Is 2D speckle tracking echocardiography useful for detecting and monitoring myocardial dysfunction in adult m.3243A>G carriers? - a retrospective pilot study

OBJECTIVES

Cardiomyopathy is a common complication of mitochondrial disorders, associated with increased mortality. Two dimensional speckle tracking echocardiography (2DSTE) can be used to quantify myocardial deformation. Here, we aimed to determine the usefulness of 2DSTE in detecting and monitoring subtle changes in myocardial dysfunction in carriers of the 3243A>G mutation in mitochondrial DNA.

METHODS

In this retrospective pilot study, 30 symptomatic and asymptomatic carriers of the mitochondrial 3243A>G mutation of whom two subsequent echocardiograms were available were included. We measured longitudinal, circumferential and radial strain using 2DSTE. Results were compared to published reference values.

RESULTS

Speckle tracking was feasible in 90 % of the patients for longitudinal strain. Circumferential and radial strain showed low face validity (low number of images with sufficient quality; suboptimal tracking) and were therefore rejected for further analysis. Global longitudinal strain showed good face validity, and was abnormal in 56-70 % (depending on reference values used) of the carriers (n = 27). Reproducibility was good (mean difference of 0.83 for inter- and 0.40 for intra-rater reproducibility; ICC 0.78 and 0.89, respectively). The difference between the first and the second measurement exceeded the measurement variance in 39 % of the cases (n = 23; feasibility of follow-up 77 %).

DISCUSSION

Even in data collected as part of clinical care, two-dimensional strain echocardiography seems a feasible method to detect and monitor subtle changes in longitudinal myocardial deformation in adult carriers of the mitochondrial 3243A>G mutation. Based on our data and the reported accuracy of global longitudinal strain in other studies, we suggest the use of global longitudinal strain in a prospective follow-up or intervention study.

A population-based epidemiologic study of adult neuromuscular disease in the Republic of Ireland

Objective:

To estimate the prevalence rates (PRs) of acquired and inherited neuromuscular diseases (NMD) in the adult Irish population, reflecting the burden of these conditions in a single country.

Methods:

This population-based study was performed in the Republic of Ireland (RoI), with a PR estimated for December 2013. Multiple case ascertainment sources were utilized. Demographic and clinical information and relevant diagnostic results were registered.

Results:

A total of 2,641 adults were identified, giving a PR of 62.6/100,000 (95% confidence interval [CI] 59.95–65.24) for all NMD in RoI. Disease-specific PR include chronic inflammatory demyelinating polyradiculoneuropathy 5.87/100,000 (95% CI 5.06–6.68), Charcot-Marie-Tooth 10.52/100,000 (95% CI 9.44–11.61), hereditary neuropathy with liability to pressure palsies 0.84/100,000 (95% CI 0.54–1.15), myotonic dystrophy type I 6.75/100,000 (95% CI 5.88–7.61), Duchenne muscular dystrophy 3.0/100,000 (95% CI 2.33–3.70), Becker muscular dystrophy 2.2/100,000 (95% CI 1.64–2.88), facioscapulohumeral dystrophy 2.59/100,000 (95% CI 2.05–3.13), limb-girdle muscular dystrophy 2.88/100,000 (95% CI 2.31–3.45), periodic paralysis 1.72/100,000 (95% CI 1.28–2.15), myotonia congenita 0.32/100,000 (95% CI 0.18–0.56), paramyotonia congenita 0.15/100,000 (95% CI 0.06–0.34), Kennedy disease 0.83/100,000 (95% CI 0.40–1.27), Lambert-Eaton myasthenic syndrome 0.29/100,000 (95% CI 0.11–0.47), myasthenia gravis 15.12/100,000 (95% CI 13.82–16.42), and sporadic inclusion body myositis 11.7/100,000 (95% CI 9.82–13.58). PR for amyotrophic lateral sclerosis was established from an existing Register as 7.20/100,000 (95% CI 6.34–8.15).

Conclusions:

The PR of all adult NMD in RoI is relatively high when compared with other chronic neurologic disorders, although some figures may be an underestimate of the true prevalence. The data provide a framework for international comparison and service planning.

Stable nuclear expression of ATP8 and ATP6 genes rescues a mtDNA Complex V null mutant

We explore the possibility of re-engineering mitochondrial genes and expressing them from the nucleus as an approach to rescue defects arising from mitochondrial DNA mutations. We have used a patient cybrid cell line with a single point mutation in the overlap region of the ATP8 and ATP6 genes of the human mitochondrial genome. These cells are null for the ATP8 protein, have significantly lowered ATP6 protein levels and no Complex V function. Nuclear expression of only the ATP8 gene with the ATP5G1 mitochondrial targeting sequence appended restored viability on Krebs cycle substrates and ATP synthesis capabilities but, failed to restore ATP hydrolysis and was insensitive to various inhibitors of oxidative phosphorylation. Co-expressing both ATP8 and ATP6 genes under similar conditions resulted in stable protein expression leading to successful integration into Complex V of the oxidative phosphorylation machinery. Tests for ATP hydrolysis / synthesis, oxygen consumption, glycolytic metabolism and viability all indicate a significant functional rescue of the mutant phenotype (including re-assembly of Complex V) following stable co-expression of ATP8 and ATP6 Thus, we report the stable allotopic expression, import and function of two mitochondria encoded genes, ATP8 and ATP6, resulting in simultaneous rescue of the loss of both mitochondrial proteins.

Mutation in cytochrome b gene of mitochondrial DNA in a family with fibromyalgia is associated with NLRP3-inflammasome activation

BACKGROUND

Fibromyalgia (FM) is a worldwide diffuse musculoskeletal chronic pain condition that affects up to 5% of the general population. Many symptoms associated with mitochondrial diseases are reported in patients with FM such as exercise intolerance, fatigue, myopathy and mitochondrial dysfunction. In this study, we report a mutation in cytochrome b gene of mitochondrial DNA (mtDNA) in a family with FM with inflammasome complex activation.

METHODS

mtDNA from blood cells of five patients with FM were sequenced. We clinically and genetically characterised a patient with FM and family with a new mutation in mtCYB. Mitochondrial mutation phenotypes were determined in skin fibroblasts and transmitochondrial cybrids.

RESULTS

After mtDNA sequence in patients with FM, we found a mitochondrial homoplasmic mutation m.15804T>C in the mtCYB gene in a patient and family, which was maternally transmitted. Mutation was observed in several tissues and skin fibroblasts showed a very significant mitochondrial dysfunction and oxidative stress. Increased NLRP3-inflammasome complex activation was observed in blood cells from patient and family.

CONCLUSIONS

We propose further studies on mtDNA sequence analysis in patients with FM with evidences for maternal inheritance. The presence of similar symptoms in mitochondrial myopathies could unmask mitochondrial diseases among patients with FM. On the other hand, the inflammasome complex activation by mitochondrial dysfunction could be implicated in the pathophysiology of mitochondrial diseases.

Genetic differentials of child abuse: Is your case rare or real?

The clinical geneticist can be called upon to play a role in the medical evaluation of children with clinical findings concerning for child abuse. This Introduction describes a case of suspected child abuse in an 8-month-old baby referred to clinical genetics to exclude osteogenesis imperfecta. The experience from this case raised medical and ethical considerations and prompted consideration of the role of the clinical geneticist in distinguishing rare mimics of child abuse from real cases. From this single case, and a discussion regarding similar cases, arose the idea of this issue in Seminars in Medical Genetics, Genetic Differentials of Child Abuse: Is Your Case Rare or Real? In thinking about child abuse from a clinical genetics perspective, we categorize clinical presentations into fractures, skin lesions, hemorrhage, growth disturbances, and concern for caregiver-fabricated illness (previously known as Munchausen syndrome by proxy). In this Introduction, we also discuss recent questions regarding Ehlers-Danlos syndrome and infantile fractures and concerns about caregiver-fabricated illness in the context of mitochondrial or other rare diseases. The goal is that this issue on child abuse and genetics will serve as a resource to help distinguish the rare causes from the real cases of child abuse, and those critical distinctions and correct diagnoses may be life-saving for some infants and children.

Causes of Death in Adults with Mitochondrial Disease

INTRODUCTION

Mitochondrial diseases are a clinically, biochemically and genetically heterogeneous group of disorders with a variable age of onset and rate of disease progression. It might therefore be expected that this variation be reflected in the age and cause of death. However, to date, little has been reported regarding the 'end-of-life' period and causes of death in mitochondrial disease patients. For some specific syndromes, the associated clinical problems might predict the cause of death, but for many patients, it remains difficult to provide an accurate prognosis.

AIMS

To describe a retrospective cohort of adult mitochondrial disease patients who had attended the NHS Highly Specialised Services for Rare Mitochondrial Diseases in Newcastle upon Tyne (UK), evaluate life expectancy and causes of death and assess the consequences for daily patient care.

METHODS

All deceased adult patients cared for at this centre over a period of 10 years were included in the study. Patient history, data on laboratory findings, biochemical investigations and genetic studies were analysed retrospectively.

RESULTS

A total of 30 adult mitochondrial patients died within the time period of the study. The main mitochondrial disease-related causes of death in this patient cohort were respiratory failure, cardiac failure and acute cerebral incidents such as seizures and strokes. In almost half of the patients, the cause of death remained unknown. Based on our study, we present recommendations regarding the care of patients with mitochondrial disease.

Nature and frequency of respiratory involvement in chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) is a relatively common mitochondrial disorder. Weakness of the extra-ocular, limb girdle and laryngeal muscles are established clinical features. Respiratory muscle involvement however has never been studied systematically, even though respiratory complications are one of the main causes of death. We therefore determined the prevalence and nature of respiratory muscle involvement in 23 patients with genetically confirmed CPEO. The main finding was decreased respiratory muscle strength, both expiratory (76.8% of predicted, p = 0.002) and inspiratory (79.5% of predicted, p = 0.004). Although the inspiratory vital capacity (92.5% of predicted, p = 0.021) and the forced expiratory volume in 1 s (89.3% of predicted, p = 0.002) were below predicted values, both were still within the normal range in the majority of patients. Expiratory weakness was associated with a decreased vital capacity (ρ = 0.502, p = 0.015) and decreased peak expiratory flow (ρ = 0.422, p = 0.045). Moreover, expiratory muscle strength was lower in patients with limb girdle weakness (62.6 ± 26.1% of predicted vs. 98.9 ± 22.5% in patients with normal limb girdle strength, p = 0.003), but was not associated with other clinical features, subjective respiratory complaints, disease severity or disease duration. Since respiratory involvement in CPEO is associated with severe morbidity and mortality, the present data justify periodic assessment of respiratory functions in all CPEO patients.

Epidemiology of pediatric mitochondrial respiratory chain disorders in northwest Spain

Our knowledge of mitochondrial respiratory chain diseases has increased dramatically in recent years, but relatively little information is available about their prevalence and incidence, either in pediatric or adult patients. This study reports incidence and prevalence estimates, and summarizes the clinical, biochemical, histologic, and genetic characteristics of 51 patients age 0-16 years. The overall annual incidence of all mitochondrial respiratory chain diseases was estimated to be 1.43 cases per 10(5) in the population as a whole, and 2.85 cases per 10(5) in the under-6 population. The overall prevalence of all mitochondrial respiratory chain diseases was estimated as 7.5 cases per 10(5) in the under-19 population, and 8.7 cases per 10(5) in the under-16 population. These incidence and prevalence estimates are higher than in most previous studies of pediatric populations. Estimated prevalences of specific mitochondrial respiratory chain diseases were 2.05 cases per 10(5) for Leigh syndrome, 0.68 per 10(5) for mitochondrial deoxyribonucleic acid (mtDNA) deletions and deletions-duplications, 1.59 per 10(5) for mtDNA depletions, and 0.45 per 10(5) for mtDNA point mutations. Leigh syndrome was the most frequent clinical syndrome. The estimates of the prevalences of mtDNA deletions, deletions-duplications, and point mutations set forth here are lower than in similar previous studies, whereas the estimate of the prevalence of mtDNA depletions is rather higher. Sixteen of these patients manifested phenotypic syndromes that have not been previously reported in association with mitochondrial respiratory chain diseases.