COVID-19 News: Post COVID Respiratory Muscle Weakness And Reduced Exercise Capacity Is Associated With Reduced Phrenic Nerve CMAP

COVID-19 News: The global repercussions of the COVID-19 pandemic extend beyond the acute phase, with emerging evidence suggesting lingering health issues for individuals, even those with mild symptoms during the initial infection.


The phrenic nerve sends signals that cause the diaphragm to contract (become thicker and flatter).
This movement gives the lungs room to expand and take in air (inhalation)

A recent comprehensive study conducted by Klinik Pirawarth in Wien-Austria, Medical University of Vienna-Austria, Klinik Floridsdorf, Vienna-Austria, Ludwig Boltzmann Institute for Rehabilitation Research-Austria, and University of Innsbruck-Austria has delved into the post-COVID scenario, focusing on patients experiencing persistent exertional dyspnea. This COVID-19 News report aims to provide an in-depth exploration of the study's findings, specifically examining the intricate relationship between respiratory muscle weakness, reduced exercise capacity, and the function of the phrenic nerve.
 
For those who do not know, the phrenic nerve controls one’s diaphragm (the large dome-shaped muscle between the abdominal and chest cavities). It's essential to breathing. The phrenic nerve sends signals that cause the diaphragm to contract (become thicker and flatter). This movement gives the lungs room to expand and take in air (inhalation).
 
Thailand Medical News had in past coverages featured studies that showed SARS-CoV-2 can cause damage to the phrenic nerves and the muscles of the diaphragm.
 
https://www.thailandmedical.news/news/covid-19-news-study-shows-that-sars-cov-2-infections-can-cause-acute-phrenic-neuropathy-and-diaphragmatic-dysfunction
 
https://www.thailandmedical.news/news/even-asymptomatic-and-mild-covid-19-causes-neuromuscular-damage-to-the-diaphragm-resulting-in-the-long-covid-manifestations-of-fatigue-and-dyspnea
 
The Study's Scope
The study set out to assess the function of the phrenic nerve, inspiratory respiratory muscle, and cardiopulmonary exercise capacity in individuals grappling with prolonged exertional dyspnea post-COVID-19 recovery. The research involved a multifaceted approach, including electrophysiological examinations of the phrenic nerve, evaluations of inspiratory muscle capacity, lung function tests, 6-minute walk distance (6MWD) assessments, and cardiopulmonary exercise tests in 22 post-COVID patients.
 
Detailed Results and Observations
The study's resu lts unveiled significant impacts on exercise capacity among post-COVID patients. Both peak workload and peak oxygen uptake were notably reduced, reflecting an impairment in the overall cardiopulmonary performance of these individuals. Moreover, maximum inspiratory pressure (MIP), a crucial indicator of inspiratory muscle strength, demonstrated a decrease, pointing towards respiratory muscle weakness in this patient cohort.
 
An intriguing aspect of the study was the electrophysiological assessment of the phrenic nerve in these individuals. Among the 22 patients, 6 exhibited a pathologically decreased compound muscle action potential (CMAP) of the phrenic nerve, suggesting compromised muscle fiber contraction capacity without concomitant signs of neuropathy. This finding brought attention to the potential role of muscle fiber pathology in the diaphragm as a contributing factor to the prolonged respiratory symptoms observed in post-COVID patients.
 
Discussion on Prolonged Effects of COVID-19
The lingering effects of COVID-19 have been a subject of increasing concern. Even individuals with mild symptoms during the acute phase can experience prolonged physical impairments, including dyspnea, fatigue, and muscle weakness. The study highlights that respiratory muscle weakness and reduced exercise capacity are associated with decreased phrenic nerve CMAP, indicating a potential link to muscle fiber pathology in the diaphragm.
 
The study explored the multifactorial nature of the muscle-related symptoms post-COVID, considering factors such as systemic inflammation, immune cell infiltration, hypoxia, adverse effects of medication, and muscle disuse. Understanding these contributing elements becomes crucial for developing effective rehabilitation strategies for individuals experiencing persistent symptoms after COVID-19 infection.
 
Exploration of Electrophysiological Findings
The study's electrophysiological examinations of the phrenic nerve revealed abnormalities in 27.1% of the patients. Reduced CMAP, indicative of pathological changes in the muscle itself, was observed without signs of axonal loss. The absence of axonal loss, as evidenced by normal phrenic nerve latency, led the researchers to propose that the reduced CMAP observed in these patients may result from muscle fiber pathology rather than nerve-related issues.
 
This emphasis on muscle fiber pathology as a potential contributor to respiratory symptoms after COVID-19 distinguishes the study's findings from previous investigations. The exploration of the diaphragm's role in prolonged exertional dyspnea provides valuable insights into the nuanced effects of the virus on the respiratory system.
 
Correlations and Interrelations
The study's findings also shed light on positive relationships between 6MWD and MIP, as well as between quality of life questionnaire (CRQ) scores and MIP, particularly in patients with reduced CMAP.
 
These interrelations suggest that the decreased CMAP of the phrenic nerve may contribute to lower inspiratory muscle force, diminished walking performance, and a reduced quality of life in post-COVID patients.
 
These correlations emphasize the interconnected nature of respiratory muscle function, exercise capacity, and overall well-being in individuals grappling with persistent respiratory symptoms. Understanding these interrelations is crucial for developing targeted interventions aimed at improving both respiratory and overall health outcomes in post-COVID patients.
 
Implications and Limitations
While the study provides valuable insights, it acknowledges certain limitations. The small sample size and explorative nature of the research limit the ability to draw definitive conclusions. The absence of significant associations between MIP, CRQ, and 6MWD in patients with unaffected CMAP poses questions about the varied causes of exercise limitation, such as deconditioning, co-morbidities, and medication.
 
The researchers emphasize the need for larger observational and longitudinal studies to unravel the underlying mechanisms and gain a comprehensive understanding of the prolonged effects of COVID-19. Such studies would allow for more robust statistical analyses and better correction for potential confounding variables.
 
Conclusion and Future Directions
In conclusion, this study provides a detailed exploration of the recent study's findings on respiratory muscle weakness, exercise capacity, and phrenic nerve function in post-COVID patients. The identified association between decreased phrenic nerve CMAP and respiratory symptoms without signs of neuropathy points towards muscle fiber pathology in the diaphragm as a potential pathophysiological factor.
 
As the world struggles with the ongoing challenges posed by the COVID-19 pandemic, understanding the complexities of post-infection symptoms becomes crucial for effective patient care and rehabilitation strategies. The study calls for further research, emphasizing the necessity of larger observational and longitudinal studies to unravel the intricacies of the prolonged effects of COVID-19 on respiratory health. Such insights will not only contribute to improved patient outcomes but also inform preventive measures and therapeutic interventions for individuals recovering from COVID-19.
 
The study findings were published in the peer reviewed journal: Frontiers in Neurology.
https://www.frontiersin.org/articles/10.3389/fneur.2023.1308443/full
 
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