Respiratory Muscle Training May Help Breathing Function in SMA

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by Steve Bryson PhD |

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Respiratory muscle training in spinal muscular atrophy (SMA) has the potential to stabilize or improve breathing function, a study has found, although the findings need to be substantiated by further research.

In the study, adults and children with SMA demonstrated a dose-dependent increase in breathing muscle fatigue using a respiratory testing device with a large individual variation in fatigue response. These findings were seen without an increase in exercise-induced muscle weakness or perceived fatigue.

Although respiratory muscle endurance seemed adequate in these patients, reduced endurance before testing suggested a therapeutic window to improve respiratory muscle strength.

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The study, “Respiratory Muscle Fatigability in Patients with Spinal muscular atrophy,” was published in the journal Pediatric Pulmonology.

SMA is marked by progressive muscle weakness and atrophy (shrinkage), mainly affecting body movements, but often causes symptoms such as speaking, swallowing, and breathing problems.

Progressive weakness of the chest muscles can cause severe breathing and coughing difficulties, leading to poor clearance of lower airway mucus and increasing the risk of lung infections and carbon dioxide buildup.

What is respiratory muscle fatigue (RMF)?

Breathing impairment may also be caused by a lack of endurance of chest muscles, referred to as respiratory muscle fatigue (RMF) — the inability to maintain a task at the same intensity, resulting in a decline in physical performance.

Previously, a team of researchers at the University Medical Center Utrecht, in the Netherlands, showed that 85% of those with SMA had worse leg-, arm- and hand-function fatigability. Because of a lack of data, the team tested the fatigability of the respiratory muscles in SMA patients.

“More insight into respiratory muscle weakness and especially RMF in SMA will facilitate clinical management, with the aim of reducing respiratory failure in patients with SMA,” the team wrote.

To this end, researchers recruited 55 people with SMA, including 19 children (seven girls) and 36 adults (20 women). Among them, 29 had SMA type 2, and 26 had SMA type 3.

To assess respiratory muscle fatigue, participants underwent a respiratory endurance test (RET), which determines the maximal inspiratory mouth pressure (PImax), reflecting inspiratory (breathing in) muscle strength. Respiratory endurance was measured using the POWERbreathe K5, a device that applies a gradual increase in breathing resistance and provides immediate test feedback.

Here, RMF was defined as the inability to continue for 60 consecutive breaths, and the number of breaths until exhaustion (task failure) was recorded.

Each participant performed tests at two visits. At visit one, the individual PImax was measured while breathing through the mouthpiece with the nose blocked. The test was repeated at least five times with 30 seconds of rest in between. There were three different threshold loads — 20%, 35%, and 55% — of patients’ PImax.

At 55% of PImax, task failure occurred in 75% of the RETs. At 35% of PImax, task failure was seen in 31% of the tests, while at 20% PImax, task failure occurred in 19% of the tests. Data analysis showed that for every 10% increase in the threshold load (percentage of PImax), the likelihood of task failure increased by 69%.

At the second visit, five threshold loads — 20%, 35%, 45%, 55%, and 70% — of PImax were assessed. Task failure was observed in 50% of RETs executed at 70% PImax, and in 100% at 55% PImax. Task failure occurred in 36% at 45% of PImax; 29% at 35% of PImax; and 7% at 20% PImax. Here, with every 10% increase in threshold load, the probability of failure increased by 59%.

“The probability of experiencing RMF in our study was the highest at an inspiratory threshold load of 55% of their individual PImax, which is similar to the probability in healthy individuals,” the researchers noted.

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How did patients compare in exercise-induced muscle weakness or perceived fatigue?

To determine breathing fatigability, the team compared patients with and without respiratory muscle fatigue based on second-visit RETs. Before RET (baseline), there were no statistically significant differences between the characteristics of the two groups in age, SMA subtype, motor function, and lung function.

There were, however, lower median baseline values of PImax % predicted in patients with respiratory muscle fatigue compared to those without RMF. Among the individuals with RMF, 56% were classified as having inspiratory (breathing in) weakness versus 33% in those without RMF.

Between the two groups, no statistically significant differences were observed in change in PImax, calculated as the difference between PImax immediately after and before the RET.

“Our findings do confirm that fatigability in the respiratory muscles seems less prominent than fatigability of the muscles in arms and legs,” the researchers added.

Data showed large variability in individual responses, with both those with and without RMF showing increases and decreases in maximal inspiratory mouth pressure.

In both groups, there was a small significant increase in a change in perceived fatigue, assessed before and after RET and evaluated with the OMNI scale of perceived exertion. No differences in change in perceived fatigue were seen between the two groups.

“Patients with SMA types 2 and 3 demonstrate a dose-dependent increase in RMF without severe increase in exercise-induced muscle weakness or perceived fatigue,” the researchers concluded. “Inspiratory loading appears feasible in patients with SMA and they are capable of breathing against a high inspiratory threshold load.”

“Respiratory muscle endurance seems adequate in patients with SMA, but the low baseline levels of PImax suggest a therapeutic window for respiratory muscle strength training,” the researchers added.

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