Respiratory involvement in inherited primary muscle conditions

Recent Advances in Pre-Clinical Development of Adiponectin Receptor Agonist Therapies for Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD) is caused by genetic mutations in the cytoskeletal-sarcolemmal anchor protein dystrophin. Repeated cycles of sarcolemmal tearing and repair lead to a variety of secondary cellular and physiological stressors that are thought to contribute to weakness, atrophy, and fibrosis. Collectively, these stressors can contribute to a pro-inflammatory milieu in locomotor, cardiac, and respiratory muscles. Given the many unwanted side effects that accompany current anti-inflammatory steroid-based approaches for treating DMD (e.g., glucocorticoids), there is a need to develop new therapies that address inflammation and other cellular dysfunctions. Adiponectin receptor (AdipoR) agonists, which stimulate AdipoR1 and R2 isoforms on various cell types, have emerged as therapeutic candidates for DMD due to their anti-inflammatory, anti-fibrotic, and pro-myogenic properties in pre-clinical human and rodent DMD models. Although these molecules represent a new direction for therapeutic intervention, the mechanisms through which they elicit their beneficial effects are not yet fully understood, and DMD-specific data is limited. The overarching goal of this review is to investigate how adiponectin signaling may ameliorate pathology associated with dystrophin deficiency through inflammatory-dependent and -independent mechanisms and to determine if current data supports their future progression to clinical trials.

Exome sequencing in extreme altitude mountaineers identifies pathogenic variants in RTEL1 and COL6A1 previously associated with respiratory failure

Adaptation of humans to challenging environmental conditions, such as extreme temperature, malnutrition, or hypoxia, is an interesting phenomenon for both basic and applied research. Identification of the genetic factors contributing to human adaptation to these conditions enhances our understanding of the underlying molecular and physiological mechanisms. In our study, we analyzed the exomes of 22 high altitude mountaineers to uncover genetic variants contributing to hypoxic adaptation. To our surprise, we identified two putative loss-of-function variants, rs1385101139 in RTEL1 and rs1002726737 in COL6A1 in two extremely high altitude (personal record of more than 8500 m) professional climbers. Both variants can be interpreted as pathogenic according to medical geneticists' guidelines, and are linked to inherited conditions involving respiratory failure (late-onset pulmonary fibrosis and severe Ullrich muscular dystrophy for rs1385101139 and rs1002726737, respectively). Our results suggest that a loss of gene function may act as an important factor of human adaptation, which is corroborated by previous reports in other human subjects.

Disruption of cardio-pulmonary coupling in myopathies: Pathophysiological and mechanistic characterization with special emphasis on nemaline myopathy

The heart and lung are in continuous reciprocal interaction that creates a functional and anatomical reserve referred to as cardiopulmonary coupling (CPC). Disruption of CPC can occur due to various cardiac or pulmonary pathologies but also can occur in patients with myopathies. Nemaline myopathy (NM) is a skeletal muscle heterogeneous disorder due to contractile proteins' gene mutations that impact lung and cardiac mechanics and thus is expected to adversely affect CPC in a complex manner. We present a case of NM and we review the literature on cardiac and pulmonary effects of myopathy-related disruption of CPC.

Three-dimensional assessment of pharyngeal airway in individuals with myotonic dystrophy type 1

Background/aim

The objectives of this study were to assess pharyngeal airway volume (PAV) in patients with myotonic dystrophy type 1 (DM1) by cone-beam computerized tomography (CBCT) and to evaluate the impact of diaphragm thickness and pulmonary function tests on PAV.

Materials and methods

Thirty DM1 patients (10 female and 20 male; mean age 42.40 ± 12.07) were included in the study. Age and sex-matched thirty patients were participated as control group. In DM1 group pulmonary function tests (PFT) were performed. Independent t-test was used to compare PAV values of patients with DM1 and control group. The Mann–Whitney U test was used to compare the parameters according to sex ( p < 0.05). Pearson and Spearman correlation tests were used to evaluate the relationships between parameters of DM1 patients (p < 0.05). A multiple linear regression analysis was performed to explain the PAV with parameters that showed positive correlation with PAV.

Results

Age of onset and disease duration were 22.37 ± 8.45 and 20.03 ± 12.08, respectively, in patients with DM1. PAV values of control group were significantly lower than DM1 group ( p < 0.001). Forced expiratory volume in 1 s and forced volume vital capacity values were higher in males than females in DM1 group according to sex ( p < 0.001). PAV values were greater in male patients than females of the DM1 group ( p = 0.022). Diaphragm thickness in DM1 group after inspiration and expiration were 2.60 ± 0.65 and 1.94 ± 0.40, respectively. According to the regression analysis, DTai and FVC were significantly explained the PAV.

Conclusion

PAV was higher in DM1 group. There was a significant positive correlation between diaphragm thickness, pulmonary functions, and PAVs of DM1 patients. The amount of the PAV was mostly influenced by DTai and FVC. It is recommended to evaluate the PAV in patients with DM1 because of impaired respiratory functions and pharyngeal muscle involvement.

Hypoxia and Hypoxia-Inducible Factor Signaling in Muscular Dystrophies: Cause and Consequences

Muscular dystrophies (MDs) are a group of inherited degenerative muscle disorders characterized by a progressive skeletal muscle wasting. Respiratory impairments and subsequent hypoxemia are encountered in a significant subgroup of patients in almost all MD forms. In response to hypoxic stress, compensatory mechanisms are activated especially through Hypoxia-Inducible Factor 1 α (HIF-1α). In healthy muscle, hypoxia and HIF-1α activation are known to affect oxidative stress balance and metabolism. Recent evidence has also highlighted HIF-1α as a regulator of myogenesis and satellite cell function. However, the impact of HIF-1α pathway modifications in MDs remains to be investigated. Multifactorial pathological mechanisms could lead to HIF-1α activation in patient skeletal muscles. In addition to the genetic defect per se, respiratory failure or blood vessel alterations could modify hypoxia response pathways. Here, we will discuss the current knowledge about the hypoxia response pathway alterations in MDs and address whether such changes could influence MD pathophysiology.

Clinicopathologic Features of Oculopharyngodistal Myopathy With LRP12 CGG Repeat Expansions Compared With Other Oculopharyngodistal Myopathy Subtypes

Importance

Repeat expansion of CGG in LRP12 has been identified as the causative variation of oculopharyngodistal myopathy (OPDM). However, to our knowledge, the clinicopathologic features of OPDM with CGG repeat expansion in LRP12 (hereafter referred to as OPDM_LRP12) remain unknown.

Objective

To identify and characterize the clinicopathologic features of patients with OPDM_LRP12.

Design, Setting, and Participants

This case series included 208 patients with a clinical or clinicopathologic diagnosis of oculopharyngeal muscular dystrophy (OPDM) from January 1, 1978, to December 31, 2020. Patients with GCN repeat expansions in PABPN1 were excluded from the study. Repeat expansions of CGG in LRP12 were screened by repeat primed polymerase chain reaction and/or Southern blot.

Main Outcomes and Measures

Clinical information, muscle imaging data obtained by either computed tomography or magnetic resonance imaging, and muscle pathologic characteristics.

Results

Sixty-five Japanese patients with OPDM (40 men [62%]; mean [SD] age at onset, 41.0 [10.1] years) from 59 families with CGG repeat expansions in LRP12 were identified. This represents the most common OPDM subtype among all patients in Japan with genetically diagnosed OPDM. The expansions ranged from 85 to 289 repeats. A negative correlation was observed between the repeat size and the age at onset (r2 = 0.188, P = .001). The most common initial symptoms were ptosis and muscle weakness, present in 24 patients (37%). Limb muscle weakness was predominantly distal in 53 of 64 patients (83%), but 2 of 64 patients (3%) had predominantly proximal muscle weakness. Ptosis was observed in 62 of 64 patients (97%), and dysphagia or dysarthria was observed in 63 of 64 patients (98%). A total of 21 of 64 patients (33%) had asymmetric muscle weakness. Aspiration pneumonia was seen in 11 of 64 patients (17%), and 5 of 64 patients (8%) required mechanical ventilation. Seven of 64 patients (11%) developed cardiac abnormalities, and 5 of 64 patients (8%) developed neurologic abnormalities. Asymmetric muscle involvement was detected on computed tomography scans in 6 of 27 patients (22%) and on magnetic resonance imaging scans in 4 of 15 patients (27%), with the soleus and the medial head of the gastrocnemius being the worst affected. All 42 muscle biopsy samples showed rimmed vacuoles. Intranuclear tubulofilamentous inclusions were observed in only 1 of 5 patients.

Conclusions and Relevance

This study suggests that OPDM_LRP12 is the most frequent OPDM subtype in Japan and is characterized by oculopharyngeal weakness, distal myopathy that especially affects the soleus and gastrocnemius muscles, and rimmed vacuoles in muscle biopsy.

Acute and chronic tirasemtiv treatment improves in vivo and in vitro muscle performance in actin-based nemaline myopathy mice

Nemaline myopathy, a disease of the actin-based thin filament, is one of the most frequent congenital myopathies. To date, no specific therapy is available to treat muscle weakness in nemaline myopathy. We tested the ability of tirasemtiv, a fast skeletal troponin activator that targets the thin filament, to augment muscle force-both in vivo and in vitro-in a nemaline myopathy mouse model with a mutation (H40Y) in Acta1. In Acta1H40Y mice, treatment with tirasemtiv increased the force response of muscles to submaximal stimulation frequencies. This resulted in a reduced energetic cost of force generation, which increases the force production during a fatigue protocol. The inotropic effects of tirasemtiv were present in locomotor muscles and, albeit to a lesser extent, in respiratory muscles, and they persisted during chronic treatment, an important finding as respiratory failure is the main cause of death in patients with congenital myopathy. Finally, translational studies on permeabilized muscle fibers isolated from a biopsy of a patient with the ACTA1H40Y mutation revealed that at physiological Ca2+ concentrations, tirasemtiv increased force generation to values that were close to those generated in muscle fibers of healthy subjects. These findings indicate the therapeutic potential of fast skeletal muscle troponin activators to improve muscle function in nemaline myopathy due to the ACTA1H40Y mutation, and future studies should assess their merit for other forms of nemaline myopathy and for other congenital myopathies.

Respiratory Failure Secondary to Diaphragmatic Paralysis from Acute Exacerbation of Dermatomyositis

Dermatomyositis (DM) is an idiopathic inflammatory disorder that presents with proximal muscle weakness and typical DM skin changes. DM can involve other organs such as the lung, esophagus, and heart. Diaphragmatic muscle paralysis is an unrecognized clinical presentation of acute DM exacerbation. A 58-year-old man with a history of DM presented to the hospital after sustaining a cardiorespiratory arrest. Before arrest, he had been suffering from progressive dyspnea and muscle weakness. Immunosuppressive therapy of tacrolimus for DM was recently discontinued due to renal toxicity. Bedside ultrasound of the diaphragm while intubated revealed evidence of bilateral diaphragmatic paralysis. After extubation, supine and upright pulmonary function tests (PFT) and sniff test results strengthened the diagnosis of diaphragmatic paralysis. The patient was worked up for an acute DM exacerbation as the likely etiology of the severe diaphragmatic muscle weakness (diaphragmatic paralysis) and ventilatory failure. Skin and muscle biopsy confirmed the diagnosis of active DM. The patient was treated with high dose steroids and mycophenolate mofetil, following which he soon recovered.

ERS International Congress, Madrid, 2019: highlights from the Sleep and Clinical Physiology Assembly

The 2019 European Respiratory Society (ERS) International Congress took place in Madrid, Spain, and served as a platform to find out the latest advances in respiratory diseases research. The research aims are to understand the physiology and consequences of those diseases, as well as the improvement in their diagnoses, treatments and patient care. In particular, the scientific sessions arranged by ERS Assembly 4 provided novel insights into sleep-disordered breathing and new knowledge in respiratory physiology. This article, divided by session, will summarise the most relevant studies presented at the ERS International Congress. Each section has been written by Early Career Members specialising in the different fields of this interdisciplinary assembly.

Congenital Myotonic Dystrophy and Brugada Syndrome: A Report of Two Cases

BACKGROUND Congenital myotonic dystrophy is a subtype of type 1 myotonic dystrophy presenting in the neonatal period. Cardiac involvement is commonly seen in patients with type 1 myotonic dystrophy beyond the neonatal period. Brugada syndrome is a conduction abnormality associated with a mutation in the sodium voltage-gated channel alpha subunit 5 (SCN5A) gene and has been described in adult patients with type 1 myotonic dystrophy. Two cases are presented of type 1 myotonic dystrophy in neonates, one who had family members with a confirmed diagnosis of Brugada syndrome. CASE REPORT Case 1: A female infant at 40 weeks gestational age, birth weight of 3,395 grams was born to a 40-year-old gravida 4, para 3 (G4P3) mother. The mother had previously been diagnosed with Brugada syndrome. Multiple family members were identified and diagnosed with type 1 myotonic dystrophy and Brugada syndrome. The infant is being monitored closely with a plan to perform genetic testing for Brugada syndrome if she develops cardiac conduction abnormalities. Case 2: A male infant at 37 weeks gestational age, with a birth weight of 2,900 grams, was born to a 24-year-old gravida 2, para 1 (G2P1) mother. He was admitted to the neonatal intensive care unit (NICU) secondary to poor respiratory effort and generalized hypotonia. Severe polyhydramnios was diagnosed during pregnancy. The mother had previously been diagnosed with type 1 myotonic dystrophy. CONCLUSIONS Infants with congenital myotonic dystrophy should be carefully monitored for both structural and conduction abnormalities of the heart, supported by genetic testing.

Noninvasive Ventilation and Mechanical Insufflator-Exsufflator for Acute Respiratory Failure in Children With Neuromuscular Disorders

Children with neuromuscular disorder (NMD) usually have pulmonary involvement characterized by weakened respiratory muscles, insufficient coughing, and inability to clear airway secretions. When suffering from community-acquired pneumonia, these patients are more likely to develop acute respiratory failure (ARF). Therefore, recurrent pneumonias leading to acute on chronic respiratory failure accounts for a common cause of mortality in children with NMD. For many years, noninvasive ventilation (NIV) has been regarded as a life-prolonging tool and has been used as the preferred intervention for treating chronic hypoventilation in patients with advanced NMD. However, an increasing number of studies have proposed the utility of NIV as first-line management for acute on chronic respiratory failure in NMD patients. The benefits of NIV support in acute settings include avoiding invasive mechanical ventilation, shorter intensive care unit or hospital stays, facilitation of extubation, and improved overall survival. As the difficulty in clearing respiratory secretions is considered a significant risk factor attributing to NIV failure, combined coughing assistance of mechanical insufflator-exsufflator (MI-E) with NIV has been recommended the treatment of acute neuromuscular respiratory failure. Several recent studies have demonstrated the feasibility and effectiveness of combined NIV and MI-E in treating ARF of children with NMD in acute care settings. However, to date, only one randomized controlled study has investigated the efficacy of NIV in childhood ARF, but subjects with underlying NMD were excluded. It reflects the need for more studies to elaborate evidence-based practice, especially the combined NIV and MI-E use in children with acute neuromuscular respiratory failure. In this article, we will review the feasibility, effectiveness, predictors of outcome, and perspectives of novel applications of combined NIV and MI-E in the treatment of ARF in NMD children.

Functional Characterization of the Intact Diaphragm in a Nebulin-Based Nemaline Myopathy (NM) Model-Effects of the Fast Skeletal Muscle Troponin Activator tirasemtiv

Respiratory failure due to diaphragm dysfunction is considered a main cause of death in nemaline myopathy (NM) and we studied both isometric force and isotonic shortening of diaphragm muscle in a mouse model of nebulin-based NM (Neb cKO). A large contractile deficit was found in nebulin-deficient intact muscle that is frequency dependent, with the largest deficits at low-intermediate stimulation frequencies (e.g., a deficit of 72% at a stimulation frequency of 20 Hz). The effect of the fast skeletal muscle troponin activator (FSTA) tirasemtiv on force was examined. Tirasemtiv had a negligible effect at maximal stimulation frequencies, but greatly reduced the force deficit of the diaphragm at sub-maximal stimulation levels with an effect that was largest in Neb cKO diaphragm. As a result, the force deficit of Neb cKO diaphragm fell (from 72% to 29% at 20 Hz). Similar effects were found in in vivo experiments on the nerve-stimulated gastrocnemius muscle complex. Load-clamp experiments on diaphragm muscle showed that tirasemtiv increased the shortening velocity, and reduced the deficit in mechanical power by 33%. Thus, tirasemtiv significantly improves muscle function in a mouse model of nebulin-based nemaline myopathy.

Acute non-invasive ventilation - getting it right on the acute medical take

Non-invasive ventilation (NIV) given to the right patient, in the right setting, in the right way and at the right time improves outcomes. However, national audits reveal poor practice in patient selection, clinical judgement, treatment initiation and availability of trained staff. NIV is indicated for persistent acute hypercapnic respiratory failure (AHRF) with acidosis after usual medical management in chronic obstructive pulmonary disease (COPD) exacerbation and even without acidosis in neuromuscular disorders or other restrictive conditions eg obesity hypoventilation or kyphoscoliosis. Having trained staff in a suitable environment with adequate equipment are keys to its success, along with close monitoring. A plan should be put in place at the time of initiating NIV about the ceiling of care, eg escalation to intubation or palliation, if the patient is not improving with NIV. Early NIV failure is most likely due to technical issues, such as inadequate pressures or mask leak, while late failure is usually the consequence of advanced disease. Any presentation with AHRF is a poor prognostic indicator and outpatient respiratory follow-up is indicated following discharge. For selected patients with COPD who remain hypercapnic 2 weeks after an exacerbation, domiciliary NIV can reduce admissions and improve survival. For patients with neuromuscular disorders or kyphoscoliosis a presentation with AHRF almost always indicates the need for domiciliary NIV.

A model to predict ventilator requirement in myotonic dystrophy type 1

INTRODUCTION

Respiratory failure is one of the most common causes of mortality in myotonic dystrophy type 1 (DM1). The variation in the DM1 phenotype causes difficulty in clinically predicting the severity of respiratory involvement, and variables such as daytime somnolence are insensitive for identifying patients who require continuous or bilevel nocturnal positive airway pressure (NPAP).

METHODS

We retrospectively analyzed a cohort of 126 adult onset patients with DM1 at the point of their first respiratory assessment to identify significant factors in predicting ventilator requirement.

RESULTS

Triplet repeat years score and Muscle Impairment Rating Scale were significantly linearly related to NPAP and, thus, formed the model.

DISCUSSION

We devised a simple model to aid clinicians in predicting at first visit those patients with DM1 who are likely to require NPAP. We also describe the causes of failure to tolerate NPAP in DM1. Muscle Nerve 59:683-687, 2019.

Metabolic Myopathies and the Respiratory System

Metabolic myopathies are a heterogeneous group of disorders characterized by inherited defects of enzymatic pathways involved in muscle cellular energetics and adenosine triphosphate synthesis. Skeletal and respiratory muscles are most affected. There are multiple mechanisms of disease. The age of onset and prognosis vary. Metabolic myopathies cause exercise intolerance, myalgia, and increase in muscle breakdown products during exercise. Some affect smooth muscle like the diaphragm and cause respiratory failure. The pathophysiology is complex and the evidence in literature to guide diagnosis and management is sparse. Treatment is limited. This review discusses the pathophysiology and diagnostic evaluation of these disorders.

Making sense of the clinical spectrum of limb girdle muscular dystrophies

The expansion of the spectrum of limb girdle muscular dystrophies (LGMDs) in recent years means that neurologists need to be familiar with the clinical clues that can help with their diagnosis. The LGMDs comprise a group of genetic myopathies that manifest as chronic progressive weakness of hip and shoulder girdles. Their inheritance is either autosomal dominant (LGMD1) or autosomal recessive (LGMD2). Their prevalence varies in different regions of the world; certain ethnic groups have documented founder mutations and this knowledge can facilitate the diagnosis. The clinical approach to LGMDs uses the age at onset, genetic transmission and clinical patterns of muscular weakness. Helpful clinical features that help to differentiate the various subtypes include: predominant upper girdle weakness, disproportionate respiratory muscle involvement, distal weakness, hip adductor weakness, ‘biceps lump’ and ‘diamond on quadriceps’ sign, calf hypertrophy, contractures and cardiac involvement. Almost half of patients with LGMD have such clinical clues. Investigations such as serum creatine kinase, electrophysiology, muscle biopsy and genetic studies can complement the clinical examination. In this review, we discuss diagnostic clinical pointers and comment on the differential diagnosis and relevant investigations, using illustrative case studies.

The neurology of acutely failing respiratory mechanics

Forces involved in breathing-which effectively pull in air-are the diaphragmatic, intercostal, spine, and neck muscles. Equally important is the bulbar musculature maintaining the architecture of a patent airway conduit and abdominal wall and internal intercostal muscles providing cough. Acute injury along a neural trajectory from brainstem to muscle will impair the coordinated interaction between these muscle groups. Acutely failing respiratory mechanics can be caused by central and peripheral lesions. In central lesions, the key lesion is in the nucleus ambiguus innervating the dilator muscles of the soft palate, pharynx, and larynx, but abnormal respiratory mechanics rarely coincide with abnormalities of the respiratory pattern generator. In peripheral lesions, diaphragmatic weakness is a main element, but in many neuromuscular disorders mechanical upper airway obstruction from oropharyngeal weakness contributes equally to an increased respiratory load. The neurology of breathing involves changes in respiratory drive, rhythm, mechanics, and dynamics. This review focuses on the fundamentals of abnormal respiratory mechanics in acute neurologic conditions, bedside judgment, interpretation of additional laboratory tests, and initial stabilization, with practical solutions provided. Many of these respiratory signs are relevant to neurologists, who in acute situations may see these patients first. Ann Neurol 2017;81:485-494.

Comparison of sitting and supine forced vital capacity in collagen VI-related dystrophy and laminin α2-related dystrophy

BACKGROUND

Progressive, restrictive, respiratory insufficiency is the major cause of morbidity and mortality in Congenital Muscular Dystrophy (CMD). Nocturnal hypoventilation precedes daytime alveolar hypoventilation, and if untreated, may lead to respiratory failure and cor pulmonale. CMD consensus care guidelines recommend screening for respiratory insufficiency by conventional and dynamic (sitting to supine) pulmonary function testing (PFT) and evaluating for sleep disordered breathing if there is more than 20% relative reduction from sitting to supine FVC(L) (ΔFVC).

OBJECTIVE

The objective of this retrospective study was to explore and characterize dynamic FVC measures in 51 individuals with two common subtypes of CMD, COL6-RD, and LAMA2-RD.

METHODS

We compared sitting and supine FVC in patients with confirmed mutation(s) in either COL6 or LAMA2. We investigated influences of age, CMD subtype, gender, race, ambulatory status, and non-invasive positive pressure ventilation (NIPPV) status on FVC percent predicted (FVCpp) and ΔFVC.

RESULTS

COL6-RD participants exhibited a significant difference between sitting and supine mean FVCpp (sitting 66.1, supine 55.1; P < 0.0001) and were 5.4 times more likely to have -ΔFVC >20% than those with LAMA2-RD when controlling for ambulant status. FVCpp sitting correlated inversely with age in individuals ≤18 years.

CONCLUSION

FVCpp sitting decreases progressively in childhood in both CMD subtypes. However, our results point to a difference in diaphragmatic involvement, with COL6-RD individuals having more disproportionate diaphragmatic weakness than LAMA2-RD. A ΔFVC of greater than -20% should continue to be used to prompt evaluation of sleep-disordered breathing. Timely initiation of NIPPV may be indicated to treat nocturnal hypoventilation. Pediatr Pulmonol. 2017;52:524-532. © 2017 Wiley Periodicals, Inc.

Diaphragm Dysfunction: Diagnostic Approaches and Management Strategies

The diaphragm is the main inspiratory muscle, and its dysfunction can lead to significant adverse clinical consequences. The aim of this review is to provide clinicians with an overview of the main causes of uni- and bi-lateral diaphragm dysfunction, explore the clinical and physiological consequences of the disease on lung function, exercise physiology and sleep and review the available diagnostic tools used in the evaluation of diaphragm function. A particular emphasis is placed on the clinical significance of diaphragm weakness in the intensive care unit setting and the use of ultrasound to evaluate diaphragmatic action.

Physiology of respiratory disturbances in muscular dystrophies

Muscular dystrophy is a group of inherited myopathies characterised by progressive skeletal muscle wasting, including of the respiratory muscles. Respiratory failure, i.e. when the respiratory system fails in its gas exchange functions, is a common feature in muscular dystrophy, being the main cause of death, and it is a consequence of lung failure, pump failure or a combination of the two. The former is due to recurrent aspiration, the latter to progressive weakness of respiratory muscles and an increase in the load against which they must contract. In fact, both the resistive and elastic components of the work of breathing increase due to airway obstruction and chest wall and lung stiffening, respectively. The respiratory disturbances in muscular dystrophy are restrictive pulmonary function, hypoventilation, altered thoracoabdominal pattern, hypercapnia, dyspnoea, impaired regulation of breathing, inefficient cough and sleep disordered breathing. They can be present at different rates according to the type of muscular dystrophy and its progression, leading to different onset of each symptom, prognosis and degree of respiratory involvement.

KEY POINTS

A common feature of muscular dystrophy is respiratory failure, i.e. the inability of the respiratory system to provide proper oxygenation and carbon dioxide elimination.In the lung, respiratory failure is caused by recurrent aspiration, and leads to hypoxaemia and hypercarbia.Ventilatory failure in muscular dystrophy is caused by increased respiratory load and respiratory muscles weakness.Respiratory load increases in muscular dystrophy because scoliosis makes chest wall compliance decrease, atelectasis and fibrosis make lung compliance decrease, and airway obstruction makes airway resistance increase.The consequences of respiratory pump failure are restrictive pulmonary function, hypoventilation, altered thoracoabdominal pattern, hypercapnia, dyspnoea, impaired regulation of breathing, inefficient cough and sleep disordered breathing.

EDUCATIONAL AIMS

To understand the mechanisms leading to respiratory disturbances in patients with muscular dystrophy.To understand the impact of respiratory disturbances in patients with muscular dystrophy.To provide a brief description of the main forms of muscular dystrophy with their respiratory implications.

Utility Of diaphragm ultrasound in myopathy

INTRODUCTION

Ultrasound (US) evaluation of diaphragm thickness and contractility is an effective tool in neurogenic diaphragm dysfunction. There are limited data about the value of this technique in patients with myopathy.

METHODS

We performed a retrospective chart review of cases with electromyography (EMG) -confirmed myopathy and real-time US evaluation of the diaphragm. Diaphragm thickness and thickening ratio (maximal inspiration/expiration) were measured. Demographic, imaging, pathology, and genetic data were reviewed, and the clinical diagnosis was recorded.

RESULTS

There were 19 eligible cases, of which 14 (73.7%) had abnormal US findings. Mean diaphragm thickness was 0.12 cm (SD 0.10), and the mean thickening ratio was 1.29 (SD 0.35). In all cases with abnormal US evaluation, the thickening ratio was abnormal. There were no cases with abnormal thickness alone.

CONCLUSIONS

US examination can detect diaphragm dysfunction in myopathy. It is important to measure both the baseline thickness and thickening ratio to maximize sensitivity. Muscle Nerve 55: 427-429, 2017.

Muscle weakness in respiratory and peripheral skeletal muscles in a mouse model for nebulin-based nemaline myopathy

Nemaline myopathy is among the most common non-dystrophic congenital myopathies, and is characterized by the presence of nemaline rods in skeletal muscles fibers, general muscle weakness, and hypotonia. Although respiratory failure is the main cause of death in nemaline myopathy, only little is known regarding the contractile strength of the diaphragm, the main muscle of inspiration. To investigate diaphragm contractility, in the present study we took advantage of a mouse model for nebulin-based nemaline myopathy that we recently developed. In this mouse model, exon 55 of Neb is deleted (Neb^ΔExon55), a mutation frequently found in patients. Diaphragm contractility was determined in permeabilized muscle fibers and was compared to the contractility of permeabilized fibers from three peripheral skeletal muscles: soleus, extensor digitorum longus, and gastrocnemius. The force generating capacity of diaphragm muscle fibers of Neb^ΔExon55 mice was reduced to 25% of wildtype levels, indicating severe contractile weakness. The contractile weakness of diaphragm fibers was more pronounced than that observed in soleus muscle, but not more pronounced than that observed in extensor digitorum longus and gastrocnemius muscles. The reduced muscle contractility was at least partly caused by changes in cross-bridge cycling kinetics which reduced the number of bound cross-bridges. The severe diaphragm weakness likely contributes to the development of respiratory failure in Neb^ΔExon55 mice and might explain their early, postnatal death.

Emery-Dreifuss muscular dystrophy: a test case for precision medicine

Emery-Dreifuss muscular dystrophy (EDMD) is characterized by the clinical triad of scapulohumeroperoneal muscle weakness, joint contractures, and cardiac defects that include arrhythmias and dilated cardiomyopathy. Although there is a defining group of clinical findings, the proteins responsible and their underlying gene defects leading to EDMD are varied. A common aspect of the gene defects is their involvement in, or with, the nuclear envelope. Treatment approaches are largely based on clinical symptoms. The genetic diversity of EDMD predicts that a cure will ultimately depend upon the individual's defect at the gene level, making this an ideal candidate for a precision medicine approach.

Gene therapy in monogenic congenital myopathies

Current treatment options for patients with monogenetic congenital myopathies (MCM) ameliorate the symptoms of the disorder without resolving the underlying cause. However, gene therapies are being developed where the mutated or deficient gene target is replaced. Preclinical findings in animal models appear promising, as illustrated by gene replacement for X-linked myotubular myopathy (XLMTM) in canine and murine models. Prospective applications and approaches to gene replacement therapy, using these disorders as examples, are discussed in this review.

Muscle-Derived Proteins as Serum Biomarkers for Monitoring Disease Progression in Three Forms of Muscular Dystrophy

Background: Identifying translatable, non-invasive biomarkers of muscular dystrophy that better reflect the disease pathology than those currently available would aid the development of new therapies, the monitoring of disease progression and the response to therapy.

Objective: The goal of this study was to evaluate a panel of serum protein biomarkers with the potential to specifically detect skeletal muscle injury.

Method: Serum concentrations of skeletal troponin I (sTnI), myosin light chain 3 (Myl3), fatty acid binding protein 3 (FABP3) and muscle-type creatine kinase (CKM) proteins were measured in 74 Duchenne muscular dystrophy (DMD), 38 Becker muscular dystrophy (BMD) and 49 Limb-girdle muscular dystrophy type 2B (LGMD2B) patients and 32 healthy controls.

Results: All four proteins were significantly elevated in the serum of these three muscular dystrophy patient populations when compared to healthy controls, but, interestingly, displayed different profiles depending on the type of muscular dystrophy. Additionally, the effects of patient age, ambulatory status, cardiac function and treatment status on the serum concentrations of the proteins were investigated. Statistical analysis revealed correlations between the serum concentrations and certain clinical endpoints including forced vital capacity in DMD patients and the time to walk ten meters in LGMD2B patients. Serum concentrations of these proteins were also elevated in two preclinical models of muscular dystrophy, the mdx mouse and the golden-retriever muscular dystrophy dog.

Conclusions: These proteins, therefore, are potential muscular dystrophy biomarkers for monitoring disease progression and therapeutic response in both preclinical and clinical studies.

Results of a two-year pilot study of clinical outcome measures in collagen VI- and laminin alpha2-related congenital muscular dystrophies

Potential therapies are currently under development for two congenital muscular dystrophy (CMD) subtypes: collagen VI-related muscular dystrophy (COL6-RD) and laminin alpha 2-related dystrophy (LAMA2-RD). However, appropriate clinical outcome measures to be used in clinical trials have not been validated in CMDs. We conducted a two-year pilot study to evaluate feasibility, reliability, and validity of various outcome measures, particularly the Motor Function Measure 32, in 33 subjects with COL6-RD and LAMA2-RD. In the first year, outcome measures tested included: Motor Function Measure 32 (MFM32), forced vital capacity (FVC) percent predicted sitting, myometry, goniometry, 10-meter walk, Egen Klassification 2, and PedsQL(TM) Generic and Neuromuscular Cores. In the second year, we added the North Star Ambulatory Assessment (NSAA), Hammersmith Functional Motor Scale (HFMS), timed functional tests, Measure of Activity Limitations (ACTIVLIM), Quality of Upper Extremity Skills Test (QUEST), and Patient-Reported Outcomes Measurement Information System (PROMIS) fatigue subscale. The MFM32 showed strong inter-rater (0.92) and internal consistency (0.96) reliabilities. Concurrent validity for the MFM32 was supported by large correlations (range 0.623-0.936) with the following: FVC, NSAA, HFMS, timed functional tests, ACTIVLIM, and QUEST. Significant correlations of the MFM32 were also found with select myometry measurements, mainly of the proximal extremities and domains of the PedsQL(TM) scales focusing on physical health and neuromuscular disease. Goniometry measurements were less reliable. The Motor Function Measure is reliable and valid in the two specific subtypes of CMD evaluated, COL6-RD and LAMA2-RD. The NSAA is useful as a complementary outcome measure in ambulatory individuals. Preliminary concurrent validity of several other clinical outcome measures was also demonstrated for these subtypes.

Neuromuscular disease and anesthesia

Patients with neuromuscular disease pose many anesthetic challenges and are at greater risk for perioperative complications, including respiratory or cardiovascular dysfunction and pulmonary aspiration. Therefore, these patients require special precautions, including interdisciplinary communication between primary care physicians, neurologists, physiatrists, surgeons, and anesthesiologists. Preoperative evaluation and optimization of comorbid conditions is critical. These patients may have adverse response to neuromuscular blocking drugs and the reversal drugs (e.g., neostigmine). They should be used with caution and titrated based on objective neuromuscular monitoring. Drugs that potentiate neuromuscular blocking drugs should also be avoided or their doses limited if possible. The risk of malignant hyperthermia in certain neuromuscular diseases mandates avoidance of triggering agents such as succinylcholine and inhaled anesthetics. Patients with neuromuscular disease may also be sensitive to sedative-hypnotics and opioids, which should be used judiciously. Finally, the postoperative period requires close monitoring due to increased risk of postoperative cardiorespiratory dysfunction.

Respiratory care in neuromuscular disorders

In neuromuscular disorders beginning in childhood, the involvement of respiratory muscles is common. Deterioration of respiratory function occurs insidiously, contributes to significant morbidity, and is often responsible for mortality. Since symptoms of respiratory involvement are not obvious, especially in the presence of maintained ambulation, respiratory failure must be systematically searched for in order to allow early care. The key to care of respiratory problems in neuromuscular disorders is a preventive approach. Careful monitoring of symptoms, regular assessment of pulmonary function, appropriate presurgical management, and aggressive treatment of respiratory infections must be considered a standard of care.

Establishing clinical end points of respiratory function in large animals for clinical translation

Respiratory dysfunction due progressive weakness of the respiratory muscles, particularly the diaphragm, is a major cause of death in the neuromuscular disease (NMD) X-linked myotubular myopathy (XLMTM). Methods of respiratory assessment in patients are often difficult, especially in those who are mechanically ventilated. The naturally occuring XLMTM dog model exhibits a phenotype similar to that in patients and can be used to determine quantitative descriptions of dysfunction as clinical endpoints for treatment and the development of new therapies. In experiments using respiratory impedance plethysmography (RIP), XLMTM dogs challenged with the respiratory stimulant doxapram displayed significant changes indicative of diaphragmatic weakness.

Consensus statement on standard of care for congenital muscular dystrophies

Congenital muscular dystrophies are a group of rare neuromuscular disorders with a wide spectrum of clinical phenotypes. Recent advances in understanding the molecular pathogenesis of congenital muscular dystrophy have enabled better diagnosis. However, medical care for patients with congenital muscular dystrophy remains very diverse. Advances in many areas of medical technology have not been adopted in clinical practice. The International Standard of Care Committee for Congenital Muscular Dystrophy was established to identify current care issues, review literature for evidence-based practice, and achieve consensus on care recommendations in 7 areas: diagnosis, neurology, pulmonology, orthopedics/rehabilitation, gastroenterology/ nutrition/speech/oral care, cardiology, and palliative care. To achieve consensus on the care recommendations, 2 separate online surveys were conducted to poll opinions from experts in the field and from congenital muscular dystrophy families. The final consensus was achieved in a 3-day workshop conducted in Brussels, Belgium, in November 2009. This consensus statement describes the care recommendations from this committee.

Phosphofructo-1-kinase deficiency leads to a severe cardiac and hematological disorder in addition to skeletal muscle glycogenosis

Mutations in the gene for muscle phosphofructo-1-kinase (PFKM), a key regulatory enzyme of glycolysis, cause Type VII glycogen storage disease (GSDVII). Clinical manifestations of the disease span from the severe infantile form, leading to death during childhood, to the classical form, which presents mainly with exercise intolerance. PFKM deficiency is considered as a skeletal muscle glycogenosis, but the relative contribution of altered glucose metabolism in other tissues to the pathogenesis of the disease is not fully understood. To elucidate this issue, we have generated mice deficient for PFKM (Pfkm^−/−). Here, we show that Pfkm^−/− mice had high lethality around weaning and reduced lifespan, because of the metabolic alterations. In skeletal muscle, including respiratory muscles, the lack of PFK activity blocked glycolysis and resulted in considerable glycogen storage and low ATP content. Although erythrocytes of Pfkm^−/− mice preserved 50% of PFK activity, they showed strong reduction of 2,3-biphosphoglycerate concentrations and hemolysis, which was associated with compensatory reticulocytosis and splenomegaly. As a consequence of these haematological alterations, and of reduced PFK activity in the heart, Pfkm^−/− mice developed cardiac hypertrophy with age. Taken together, these alterations resulted in muscle hypoxia and hypervascularization, impaired oxidative metabolism, fiber necrosis, and exercise intolerance. These results indicate that, in GSDVII, marked alterations in muscle bioenergetics and erythrocyte metabolism interact to produce a complex systemic disorder. Therefore, GSDVII is not simply a muscle glycogenosis, and Pfkm^−/− mice constitute a unique model of GSDVII which may be useful for the design and assessment of new therapies.

Type VII glycogen storage disease (GSDVII), or Tarui disease, is a rare genetic disorder characterized by glycogen accumulation in skeletal muscle. The molecular cause is loss of activity of the muscle isoform of phosphofructokinase (PFK), which phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate, commiting glucose to glycolysis. Entry of fructose-6-phosphate into glycolysis is thus blocked, increasing glycogen synthesis and accumulation. Clinical manifestations of the disease are heterogeneous, ranging from exercise intolerance to early childhood death. To further understand the human pathology, we generated mice lacking muscle PFK. As in human patients, these mice showed severe exercise intolerance, hemolysis, and most died young. Lack of glycolysis in skeletal muscle also causes alterations in bioenergetics and compensatory changes in key metabolic genes. Additionally, although erythrocytes retained 50% of normal PFK activity, their overall functionality was impaired, aggravating the muscle dysfunction. Moreover, marked metabolic alterations in the heart lead to chronic hypertrophy, suggesting that cardiac pathology in GSDVII may be underestimated or misdiagnosed. This study indicates that this disease is more complex than a muscle glycogenosis and that symptoms other than those classically described should be taken into consideration. Finally, this animal model will enable us to develop new therapeutic approaches and better diagnostic tools.

Abnormalities of respiratory control and the respiratory motor unit

BACKGROUND

Control of ventilation depends on a brainstem neuronal network that controls activity of the motor neurons innervating the respiratory muscles. This network includes the pontine respiratory group and the dorsal and ventral respiratory groups in the medulla. Neurologic disorders affecting these areas or the respiratory motor unit may lead to abnormal breathing.

REVIEW SUMMARY

The brainstem respiratory network contains neurons critical for respiratory rhythmogenesis; this network receives inputs from peripheral and central chemoreceptors sensitive to levels of carbon dioxide (PaCO2) and oxygen (PaO2) and from forebrain structures that control respiration as part of integrated behaviors such as speech or exercise. Manifestations associated with disorders of this network include sleep apnea and dysrhythmic breathing frequently associated with disturbances of cardiovagal and sympathetic vasomotor control. Common disorders associated with impaired cardiorespiratory control include brainstem stroke or compression, syringobulbia, Chiari malformation, high cervical spinal cord injuries, and multiple system atrophy. By far, neuromuscular disorders are the more common neurologic conditions leading to respiratory failure.

CONCLUSIONS

Respiratory dysfunction constitute an early and relatively major manifestation of several neurologic disorders and may be due to an abnormal breathing pattern generation due to involvement of the cardiorespiratory network or more frequently to respiratory muscle weakness.