Diurnal hypercapnia in patients with neuromuscular disease

Neuromuscular Disorders in Pediatric Respiratory Disease

Respiratory sequelae are a frequent cause of morbidity and mortality in children with NMD. Impaired cough strength and resulting airway clearance as well as sleep disordered breathing are the two main categories of respiratory sequelae. Routine clinical evaluation and diagnostic testing by pulmonologists is an important pillar of the multidisciplinary care required for children with NMD. Regular surveillance for respiratory disease and timely implementation of treatment including pulmonary clearance techniques as well as ventilation can prevent respiratory related morbidity including hospital admissions and improve survival. Additionally, novel disease modifying therapies for some NMDs are now available which has significantly improved the clinical trajectories of patients resulting in a paradigm shift in clinical care. Pulmonologists are 'learning' the new natural history for these diseases and adjusting clinical management accordingly.

Sleep Disordered Breathing in Children with Neuromuscular Disease

Sleep disordered breathing (SDB) in children with neuromuscular disease (NMD) is more prevalent compared to the general population, and often manifests as sleep-related hypoventilation, sleep-related hypoxemia, obstructive sleep apnea, central sleep apnea, and/or disordered control of breathing. Other sleep problems include, sleep fragmentation, abnormal sleep architecture, and nocturnal seizures in certain neuromuscular diseases. The manifestation of sleep disordered breathing in children depends on the extent, type, and progression of neuromuscular weakness, and in some instances, may be the first sign of a neuromuscular weakness leading to diagnosis of an NMD. In-lab diagnostic polysomnography (PSG) remains the gold standard for the diagnosis of sleep disordered breathing in children, but poses several challenges, including access to many children with neuromuscular disease who are non-ambulatory. If SDB is untreated, it can result in significant morbidity and mortality. Hence, we aimed to perform a comprehensive review of the literature of SDB in children with NMD. This review includes pathophysiological changes during sleep, clinical evaluation, diagnosis, challenges in interpreting PSG data using American Academy of Sleep (AASM) diagnostic criteria, management of SDB, and suggests areas for future research.

ATS Core Curriculum 2022. Pediatric Pulmonary Medicine: Updates in pediatric neuromuscular disease

The American Thoracic Society Core Curriculum updates clinicians annually in pediatric pulmonary disease. This is a concise review of the Pediatric Pulmonary Medicine Core Curriculum presented at the 2022 American Thoracic Society International Conference. Neuromuscular diseases (NMD) comprise a variety of conditions that commonly affect the respiratory system and cause significant morbidity including dysphagia, chronic respiratory failure, and sleep disordered breathing. Respiratory failure is the most common cause of mortality in this population. Substantial progress has been made in diagnosis, monitoring and treatment for NMD over the last decade. Pulmonary function testing (PFT) is utilized to objectively measure respiratory pump function and PFT milestones are utilized in NMD-specific pulmonary care guidelines. New disease modifying therapies are approved for the treatment of patients with Duchenne muscular dystrophy and spinal muscular atrophy (SMA), including the first ever approved systemic gene therapy, in the case of SMA. Despite extraordinary progress in the medical management of NMD, little is known regarding the respiratory implications and long-term outcomes for patients in the era of advanced therapeutics and precision medicine. The combination of technological and biomedical advancements has increased the complexity of the medical decision-making process for patients and families, thus emphasizing the importance of balancing respect for autonomy with the other foundational principles of medical ethics. This review features an overview of PFT, noninvasive ventilation strategies, novel and developing therapies, as well as the ethical considerations specific to the management of patients with pediatric NMD.

Pulmonological issues

Pediatric leukodystrophies are rare neurodegenerative diseases involving multiple systems. Each form has unique neurologic features but are characterized by encephalopathy with accompanying impairments evidenced in reflexes, muscle tone and movement control. Weakness of expiratory, inspiratory, and upper airway muscles may lead to impaired airway secretion clearance resulting in recurrent respiratory infections, dysphagia, sleep-disordered breathing, restrictive lung disease, and ultimately chronic respiratory insufficiency.

Chronic moderate hypercapnia suppresses ventilatory responses to acute CO<sub>2</sub> challenges

Chronic hypercapnia (CH) is a hallmark of chronic lung disease, and CH increases the risk for acute-on-chronic exacerbations leading to greater hypoxemia/hypercapnia and poor health outcomes. However, the role of hypercapnia per se (duration and severity) in determining an individual's ability to tolerate further hypercapnic exacerbations is unknown. Our primary objective herein was to test the hypothesis that mild-to-moderate CH (arterial [Formula: see text] ∼50-70 mmHg) increases susceptibility to pathophysiological responses to severe acute CO₂ challenges. Three groups (GR) of adult female goats were studied during 14 days of exposure to room air (<i>GR 1</i>; control) or 6% inspired CO₂ (<i>GR 2</i>; mild CH), or 7 days of 6% inspired CO₂ followed by 7 days of 8% inspired CO₂ (<i>GR 3</i>; moderate CH). Consistent with previous reports, there were no changes in physiological parameters in <i>GR 1</i> (RA control), but mild CH (<i>GR 2</i>) increased steady-state ventilation and transiently suppressed CO₂/[H⁺] chemosensitivity. Further increasing InCO₂ from 6% to 8% (<i>GR 3</i>) transiently increased ventilation and arterial [H⁺]. Similar to mild CH, moderate CH increased ventilation to levels greater than predicted. However, in contrast to mild CH, acute ventilatory chemosensitivity was suppressed throughout the duration of moderate CH, and the arterial - mixed expired CO₂ gradient became negative. These data suggest that moderate CH limits physiological responses to acute severe exacerbations and provide evidence of recruitment of extrapulmonary systems (i.e., gastric CO₂ elimination) during times of moderate-severe hypercapnia.<b>NEW &amp; NOTEWORTHY</b> Moderate levels of chronic hypercapnia (CH; ∼70 mmHg) in healthy adult female goats elicited similar steady-state physiological adaptations compared with mild CH (∼55 mmHg). However, unlike mild CH, moderate CH chronically suppressed acute CO₂/[H⁺] chemosensitivity and reversed the arterial to mixed expired CO₂ gradient. These findings suggest that moderate CH suppresses vital mechanisms of ventilatory control and recruits additional physiological systems (i.e., gastric CO₂ release) to help buffer excess CO₂.

Neuromuscular disorders and chronic ventilation

Morbidity and mortality have decreased in patients with neuromuscular disease due to implementation of therapies to augment cough and improve ventilation. Infants with progressive neuromuscular disease will eventually develop respiratory complications as a result of muscle weakness and their inability to compensate during periods of increased respiratory loads. The finding of nocturnal hypercapnia is often the trigger for initiating non-invasive ventilation and studies have shown that its use not only may improve sleep-disordered breathing, but also that it may have an effect on daytime function, symptoms related to hypercapnia, and partial pressure of CO₂. It is important to understand the respiratory physiology of this population and to understand the benefits and limitations of assisted ventilation.

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.

Unusual Ventilatory Response to Exercise in Patient with Arnold-Chiari Type 1 Malformation after Posterior Fossa Decompression

We present a case of a 17-year-old Hispanic male with Arnold-Chiari Type 1 [AC-Type 1] with syringomyelia, status post decompression, who complains of exercise intolerance, headaches, and fatigue with exertion. The patient was found to have diurnal hypercapnia and nocturnal alveolar hypoventilation. Cardiopulmonary testing revealed blunting of the ventilatory response to the rise in carbon dioxide (CO₂) resulting in failure of the parallel correlation between increased CO₂ levels and ventilation; the expected vertical relationship between PETCO₂ and minute ventilation during exercise was replaced with an almost horizontal relationship. No new pathology of the brainstem was discovered by MRI or neurological evaluation to explain this phenomenon. The patient was placed on continuous noninvasive open ventilation (NIOV) during the day and CPAP at night for a period of 6 months. His pCO₂ level decreased to normal limits and his symptoms improved; specifically, he experienced less headaches and fatigue during exercise. In this report, we describe the abnormal response to exercise that patients with AC-Type 1 could potentially experience, even after decompression, characterized by the impairment of ventilator response to hypercapnia during exertion, reflecting a complete loss of chemical influence on breathing with no evidence of abnormality in the corticospinal pathway.

Respiratory assessment in centronuclear myopathies

The centronuclear myopathies (CNMs) are a group of inherited neuromuscular disorders classified as congenital myopathies. While several causative genes have been identified, some patients do not harbor any of the currently known mutations. These diverse disorders have common histological features, which include a high proportion of centrally nucleated muscle fibers, and clinical attributes of muscle weakness and respiratory insufficiency. Respiratory problems in CNMs may manifest initially during sleep, but daytime symptoms, ineffective airway clearance, and hypoventilation predominate as more severe respiratory muscle dysfunction evolves. Respiratory muscle capacity can be evaluated using a variety of clinical tests selected with consideration for the age and baseline motor function of the patient. Similar clinical tests of respiratory function can also be incorporated into preclinical CNM canine models to offer insight for clinical trials. Because respiratory problems account for significant morbidity in patients, routine assessments of respiratory muscle function are discussed.

Natural history of pulmonary function in collagen VI-related myopathies

The spectrum of clinical phenotypes associated with a deficiency or dysfunction of collagen VI in the extracellular matrix of muscle are collectively termed 'collagen VI-related myopathies' and include Ullrich congenital muscular dystrophy, Bethlem myopathy and intermediate phenotypes. To further define the clinical course of these variants, we studied the natural history of pulmonary function in correlation to motor abilities in the collagen VI-related myopathies by analysing longitudinal forced vital capacity data in a large international cohort. Retrospective chart reviews of genetically and/or pathologically confirmed collagen VI-related myopathy patients were performed at 10 neuromuscular centres: USA (n = 2), UK (n = 2), Australia (n = 2), Italy (n = 2), France (n = 1) and Belgium (n = 1). A total of 486 forced vital capacity measurements obtained in 145 patients were available for analysis. Patients at the severe end of the clinical spectrum, conforming to the original description of Ullrich congenital muscular dystrophy were easily identified by severe muscle weakness either preventing ambulation or resulting in an early loss of ambulation, and demonstrated a cumulative decline in forced vital capacity of 2.6% per year (P < 0.0001). Patients with better functional abilities, in whom walking with/without assistance was achieved, were initially combined, containing both intermediate and Bethlem myopathy phenotypes in one group. However, one subset of patients demonstrated a continuous decline in pulmonary function whereas the other had stable pulmonary function. None of the patients with declining pulmonary function attained the ability to hop or run; these patients were categorized as intermediate collagen VI-related myopathy and the remaining patients as Bethlem myopathy. Intermediate patients had a cumulative decline in forced vital capacity of 2.3% per year (P < 0.0001) whereas the relationship between age and forced vital capacity in patients with Bethlem myopathy was not significant (P = 0.1432). Nocturnal non-invasive ventilation was initiated in patients with Ullrich congenital muscular dystrophy by 11.3 years (±4.0) and in patients with intermediate collagen VI-related myopathy by 20.7 years (±1.5). The relationship between maximal motor ability and forced vital capacity was highly significant (P < 0.0001). This study demonstrates that pulmonary function profiles can be used in combination with motor function profiles to stratify collagen VI-related myopathy patients phenotypically. These findings improve our knowledge of the natural history of the collagen VI-related myopathies, enabling proactive optimization of care and preparing this patient population for clinical trials.