Peak expiratory flow and the single-breath counting test as markers of respiratory function in patients with myasthenia gravis

Introduction: Myasthenia gravis (MG) is an antibody-mediated autoimmune disease characterised by fluctuating, fatigable muscle weakness, frequently involving bulbar and respiratory muscles. Considering the severity of respiratory involvement in MG, routine evaluation of respiratory function is essential. The aim of this study was to identify a useful clinical marker of respiratory involvement in patients with MG.

Methods: We performed an observational study of patients with MG. All cases were evaluated with the single-breath counting test, peak expiratory flow (PEF), a modified Medical Research Council dyspnoea scale (mMRC), and a neck strength assessment. The results of these parameters were correlated with forced vital capacity (FVC), maximal inspiratory pressure (MIP), and maximal expiratory pressure (MEP).

Results: The study included 45 patients with MG: 2 patients classified as grade I on the Myasthenia Gravis Foundation of America classification at the time of evaluation, 35 classified as grade II, 7 classified as grade III, and one classified as grade IV. Positive correlations were found between single-breath counting test scores and FVC values (r = 0.57, p = .000), and between PEF and FVC values (r = 0.76, p = .000). Severity of dyspnoea according to the mMRC scale showed a negative correlation with FVC values (r = -0.31, p = .03). PEF also showed a significant correlation with MEP (r = 0.51, p = .002).

Conclusions: PEF, the single-breath counting test, and the mMRC scale are useful measures for evaluating respiratory function in patients with MG.