INFLUENCE OF NECK MUSCLES ON MOUTH PRESSURE RESPONSE TO CERVICAL MAGNETIC STIMULATION

Measurement of mouth pressure (Pm) in response to electrical phrenic n erve stimulation (E-S) provides a simple noninvasive means to assess d iaphragm function. An even simpler measure would be to use the Pm twit ch response (Pm,t) to cervical magnetic stimulation (CMS) rather than to E-S. Because CMS coactivates the diaphragm and inspiratory neck mus cles (INM), CMS-Pm,t accurately reflects diaphragm function only if th e corresponding INM contraction does not produce inspiratory pressures by itself. In patients with recent-onset bilateral diaphragm paralysi s, it has been demonstrated that CMS-Pm,t was indeed zero; however, IN M hypertrophy could change this situation and lead CMS-Pm,t to overest imate the performance of the diaphragm. To address this issue, we stud ied nine patients with amyotrophic lateral sclerosis (ALS) who had evi dence of diaphragmatic paralysis and compensatory hypertrophy and hype ractivity of inspiratory neck muscles. The response to CMS was describ ed in terms of diaphragm electromyogram (EMG), Pm, and abdominal (AB) and rib cage (RC) motion. No EMG response to CMS could be observed in most cases, and CMS was always associated with AB paradox. Nevertheles s, a negative Pm,t swing was recorded with an amplitude of -2.6 +/- 1. 0 cm H2O (mean +/- SD). We conclude that inspiratory neck muscle hyper trophy can significantly influence the Pm response to CMS. This should be taken into account when using the CMS-Pm combination in patients w ith possible chronic diaphragm dysfunction.