Reliability of diaphragmatic motor evoked potentials induced by transcranial magnetic stimulation.

The diaphragmatic motor evoked potential (MEP) induced by transcranial magnetic stimulation (TMS) permits electrophysiological assessment of the cortico-diaphragmatic pathway. Despite the value of TMS for investigating diaphragm motor integrity in health and disease, reliability of the technique has not been established. The study aim was to determine within- and between-session reproducibility of surface electromyogram recordings of TMS-evoked diaphragm potentials. Fifteen healthy young adults participated (6 females, age=29±7 years). Diaphragm activation was determined by gradually increasing the stimulus intensity from 60-100% of maximal stimulator output (MSO). A minimum of seven stimulations were performed at each intensity. A second block of stimuli was delivered 30 minutes later for within-day comparisons, and a third block performed on a separate day for between-day comparisons. Reliability of diaphragm MEPs was assessed at 100% MSO using intraclass correlation coefficients (ICC) and 95% limits of agreement (LOA). MEPlatency (ICC=0.984, p<0.001), duration (ICC=0.958, p<0.001), amplitude (ICC=0.950, p<0.001) and area (ICC=0.956, p<0.001) were highly reproducible within-day. Between-day reproducibility was good to excellent for all MEP characteristics (latency ICC=0.953, p<0.001; duration ICC=0.879, p=0.002; amplitude ICC=0.789, p=0.019; area ICC=0.815, p=0.012). Data revealed less precision between-day versus within-day, as evidenced by wider LOA for all MEP characteristics. Large within- and between-subject variability in MEP amplitude and area was observed. In conclusion, TMS is a reliable means of inducing diaphragm potentials in most healthy individuals.