Diaphragm fatigue may contribute to breathing issues in SMA: Study
Results could guide therapeutic strategies to help patients breathe easier
Fatigue in the diaphragm — the large muscle at the base of the chest that pulls air in and out of the lungs — may contribute to difficulty breathing in people with spinal muscular atrophy (SMA), a new study reports.
The study did not find evidence of fatigue in other muscles around the lungs that are involved in breathing. Scientists said the results may help inform strategies for therapy to support breathing ability in patients with SMA.
The study, “Respiratory muscle activity and fatigue response during respiratory endurance testing in patients with spinal muscular atrophy,” was published in Clinical Neurophysiology.
Breathing issues a common problem for SMA patients
SMA is a genetic disorder characterized by muscle weakness and wasting. This affects not only the skeletal muscles required for movement, but also the respiratory muscles necessary for breathing. Difficulty breathing is a common problem for people with SMA, with many relying on medical interventions such as ventilators to ensure they get enough oxygen.
“Respiratory muscle weakness is an important cause of morbidity and mortality in patients with SMA,” the scientists wrote, noting that “reduced cough strength and poor secretion clearance cause frequent respiratory tract infections” and other issues.
In addition to muscle weakness, fatigue is a commonly reported symptom of SMA. Physiologically, fatigue in muscles results in a decrease in muscle electrical activity when a muscle is used repetitively. This can be detected using a technique called surface electromyography (EMG), which measures muscle electrical activity using sensors placed on the skin.
The diaphragm showed the most significant [EMG-based] signs of fatigue, while accessory inspiratory muscles like the scalene muscles showed no clear [EMG-based] fatigue signs.
In this study, a team of scientists in the Netherlands sought to investigate how fatigue in respiratory muscles may contribute to breathing difficulties in people with SMA. To do this, they analyzed EMG data from 46 patients with SMA types 2 or 3 who underwent a test requiring them to breathe into a machine that offered resistance, thereby making the respiratory muscles work harder to pull air in and out of the lungs.
“More insight into the mechanisms underlying respiratory fatigue responses can help develop and evaluate respiratory function treatment strategies,” the researchers wrote.
Of the 46 participants, 16 were unable to complete 60 breaths during the breathing test. These individuals were categorized as having respiratory muscle fatigue. The researchers noted that these patients generally did not self-report breathing fatigue, which indicates that “perceived fatigue was not a reliable indicator of … actual muscle fatigue.”
In the 16 patients who exhibited signs of fatigue during the test, EMG revealed reduced electrical activity in the diaphragm. Other muscles that are involved in breathing — including the scalene muscles at the side of the neck and the intercostal muscles that run between the ribs — did not show physiological signs of fatigue based on EMG testing.
“The diaphragm showed the most significant [EMG-based] signs of fatigue, while accessory inspiratory muscles like the scalene muscles showed no clear [EMG-based] fatigue signs,” the researchers wrote. The scientists noted that diaphragm fatigue “is unlikely to be the sole explanation for why participants exhibit RMF [respiratory muscle fatigue].” Still, they suggested that the findings could help inform strategies for therapy aimed at improving breathing ability in SMA patients.
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