Single-Count Breath Test Can Help Assess Respiratory Function in MG

A simple breathing and counting exercise may be a meaningful tool to evaluate respiratory function in people with myasthenia gravis (MG), according to a new report.

The systematic review, published in Journal of Clinical Neuroscience, shows that the single-count breath test (SCBT) produces results that correlate with forced vital capacity (FVC) to a moderate degree. The findings therefore suggest that the SCBT can be used to help identify potential respiratory distress in people with MG.

One of the reasons MG can be life threatening is that the neuromuscular disorder can result in respiratory muscle weakness and respiratory failure. FVC testing is the standard tool used to evaluate respiratory function in patients with MG, but the method has limitations in this patient population, explained the study authors. They noted that access to spirometry equipment is limited for some patients and MG can cause weakness in facial muscles that can make it difficult for test administrators to get an effective seal on the mouthpiece used in the test.

The SCBT does not have those same limitations because it does not require equipment. Instead, it typically involves asking patients to sit upright, take a deep breath, and then start counting upward using their normal speaking voice. Patients are asked to use a pace of approximately 2 numbers per second and to count as high as they can without taking another breath.

The test has been favorably evaluated in other diseases, said the investigators, including Guillain-Barré syndrome and amyotrophic lateral sclerosis. But they said efficacy in those disease areas may not necessarily imply that the test is effective in MG, since fatigable bulbar weakness can also be a factor in MG.

The investigators decided to undertake a systematic review of studies that used counting tests to evaluate patients with MG. After screening studies and excluding those that did not meet the review’s requirements, they were left with 6 studies on which to base their review.

These studies showed that the SCBT has a moderate correlation with FVC, meaning that the test may be helpful in identifying cases where patients are experiencing an MG exacerbation. They added that such an evaluation can also be done over the phone, adding to the test’s flexibility.

Although different studies used different thresholds, the authors found that a cut-off value of 25 was a good indication of normal respiratory function. In other words, if a patient could count to 25 before needing another breath, their respiratory system was likely functioning normally.

The authors described their findings as a promising first step, but that more research should be done to validate the test. They added that the review is based on studies involving English and Spanish speakers or studies in which the speaker’s language was not indicated. They said the threshold of counting to 25 may not be appropriate for speakers of other languages.

In addition, since MG involves muscle weakness, some patients may not be able to sit upright to count, the authors said, a fact that might affect the usefulness of the SCBT for those patients.

In summary, the authors found that the SCBT appears to be a quick, inexpensive, and reasonably helpful tool, but they said its real-world utilization could be improved with additional, larger study.

Reference

Dishnica N, Vuong A, Xiong L, et al. Single count breath test for the evaluation of respiratory function in myasthenia gravis: a systematic review. J Clin Neurosci. Published online April 22, 2023. doi:10.1016/j.jocn.2023.04.011