Diaphragmatic Compound Muscle Action Potentials Correlate With the Clinical Severity of Myasthenia Gravis.

Introduction/aims: Patients with myasthenia gravis (MG) often experience impairment of respiratory function. Some case reports have explored the diaphragmatic compound muscle action potential (DCMAP) amplitudes in patients with MG using single-stimulus phrenic nerve conduction studies. Herein, we aimed to investigate whether the DCMAP amplitude correlated with the clinical severity of MG and whether this improved with treatment.

Methods: We retrospectively evaluated patients with MG who underwent right phrenic nerve conduction studies. We evaluated the patients' clinical characteristics and the clinical severity of MG, as assessed by compound muscle action potential (CMAP) amplitudes, and compared the CMAP amplitudes before and after treatment in patients with generalized MG (gMG).

Results: This study included 76 patients with MG (30 with ocular MG [OMG] and 46 with gMG). The median DCMAP amplitudes were significantly higher in OMG than in gMG (364 vs. 212 μV, p < 0.01). Receiver operating characteristic analysis showed that DCMAP amplitude < 256 μV distinguished gMG from OMG with 71.7% sensitivity and 93.3% specificity. The DCMAP amplitudes were moderately correlated with the MG activities of daily living score (p < 0.01, r = -0.52) and quantitative myasthenia gravis score (p < 0.01, r = -0.60). Median DCMAP amplitudes significantly increased in patients with gMG after treatment (from 184 to 336 μV, p < 0.01).

Discussion: DCMAP amplitudes may correlate with the clinical severity of MG, differentiate gMG from OMG, and reflect the treatment response. We propose that DCMAP measurement be included in the evaluation of patients with MG.