Localization of diaphragm motor cortical representation and determination of corticodiaphragmatic latencies by using magnetic stimulation in normal adult human subjects.
The aims of this study were to identify the motor cortical representation of the diaphragm and to assess the corticodiaphragmatic pathway from both hemispheres. Specially designed bipolar surface electrodes were used to record the ipsilateral and contralateral compound motor evoked potentials (CMEPs) of the diaphragm after transcranial magnetic stimulation (TMS) of the motor cortex. In addition, the response to cervical magnetic stimulation of the phrenic nerve roots, effected using a figure-of-eight magnetic coil, was also recorded. The study involved 30 normal adult male volunteers. The average point of optimal excitability (POE) was determined to be 3.7 cm lateral to the mid-sagittal plane and 0.89 cm anterior to the preauricular plane. The largest response was obtained at a stimulus coil orientation of 0-90 degrees. The TMS of either hemisphere produced CMEPs in the contralateral and ipsilateral diaphragm muscles. TMS of either hemisphere elicited CMEPs that had significantly greater amplitudes and shorter latencies from the contralateral muscles compared with the ipsilateral response (P<0.0001). The central motor conduction time of the crossed tract (8.8 ms) was significantly shorter than that of the uncrossed tract (12.2 ms). No significant interhemispheric differences were recorded. The recorded CMEPs recorded in response to TMS were facilitated during volitional inspiration. Phrenic nerve latency was 5.7 ms and 5.6 ms for the right and left phrenic nerves, respectively, with no significant difference between these values. Both bilateral crossed and uncrossed corticospinal connections to the diaphragm were usually present, with the crossed tract predominating. The technique used in this study may be useful for investigations into the function and integrity of central and peripheral pathway of the diaphragm muscles in various neurological disorders.