Comparison of magnetic and electrical phrenic nerve stimulation in assessment of phrenic nerve conduction time

Abstract

Cervical magnetic stimulation (CMS), a nonvolitional test of diaphragm function, is an easy means for measuring the latency of the diaphragm motor response to phrenic nerve stimulation, namely, phrenic nerve conduction time (PNCT). In this application, CMS has some practical advantages over electrical stimulation of the phrenic nerve in the neck (ES). Although normal ES-PNCTs have been consistently reported between 7 and 8 ms, data are less homogeneous for CMS-PNCTs, with some reports suggesting lower values. This study systematically compares ES- and CMS-PNCTs for the same subjects. Surface recordings of diaphragmatic electromyographic activity were obtained for seven healthy volunteers during ES and CMS of varying intensities. On average, ES-PNCTs amounted to 6.41 +/- 0.84 ms and were little influenced by stimulation intensity. With CMS, PNCTs were significantly lower (average difference 1.05 ms), showing a marked increase as CMS intensity lessened. ES and CMS values became comparable for a CMS intensity 65% of the maximal possible intensity of 2.5 Tesla. These findings may be the result of phrenic nerve depolarization occurring more distally than expected with CMS, which may have clinical implications regarding the diagnosis and follow-up of phrenic nerve lesions.

Validation of improved recording site to measure phrenic conduction from surface electrodes in humans

“…We had reasons to believe that, in our setting, surface electrodes could provide an uncontaminated diaphragm signal with CMS as well as with ES. Among the strongest of these reasons was the fact that surface electrodes could be silent in response to CMS in patients with phrenic paralysis (1,6,35). In our view, the most likely explanation for the difference between our findings and those of others is our use of a variant placement of the electrodes over the chest wall.…”

“…As in a previous study (35), the technique was slightly modified compared with its initial description (33,34) in that the subjects kept the neck in a neutral position instead of bending it forward. The handle of the coil was directed caudally and held either parallel to the vertebral column or at a 45°angle.…”

“…niques are now available, including unilateral magnetic stimulation (UMS) (24), bilateral anterior magnetic phrenic stimulation (23), and anterior magnetic stimulation (27,28,35). The mechanical response of the diaphragm to those stimulations is well described (15,27,36), and there have been various pathophysiological and clinical applications.…”

“…The PNCT derived from electrical stimulation in the neck (ES-PNCT) is known from numerous studies (see Table 1 in Ref. 35) and revolves around a 7-ms figure. Measuring ES-PNCT is necessary to make the diagnosis of many types of phrenic nerve damage and is instrumental to their follow-up.…”

Are the neurophysiological techniques useful for the diagnosis of diaphragmatic impairment in multiple sclerosis (MS)?

“…Recently this technique has been successfully proposed for the non-invasive study of the diaphragm (Dia) (Zifko et al, 1996, Similowski et al, 1996 and phrenic roots (Chokroverty et al, 1995;Similowski et al, 1997). There are only few studies which addressed the diaphragmatic impairment in MS by means of transcranial magnetic stimulation (TMS) (Garland et al, 1996;Lagueny et al, 1998).…”