Neurophysiological assessment of cortical motor function: A direct comparison of methodologies.

OBJECTIVE

Assessment of cortical function with threshold tracking transcranial magnetic stimulation (TT-TMS) has developed as a biomarker to inform disease pathophysiology, particularly in neurodegenerative disease and dementia. At present, a fully integrated testing system does not exist. To advance clinical utility, and to streamline software design to integrate with diagnostic approaches in an outpatient setting, the present series of studies assessed the effects of altering diagnostic paradigms to measure interstimulus interval (ISI) including serial ascending [T-SICIs] and parallel [T-SICIp] methodologies as measures of cortical motor function (the MagXite software).

METHODS

Cortical excitability was assessed in 30 healthy controls with a figure-of-eight coil, using an integrated approach compared to previously established experimental paradigms. Motor evoked responses were recorded over the contralateral abductor pollicis brevis muscle. Short interval intracortical inhibition (SICI) was recorded with each testing paradigm and validated in a healthy control cohort.

RESULTS

The integrated system determined a robust measure of T-SICIs between ISI 1-to-7 ms (16.6 ± 2.2 %) that was comparable to previously established testing paradigms (P = 0.34), but greater than T-SICIp (MagXite 10.7 ± 1.5 %, P = 0.016; Sydney TT-TMS 8.7 ± 1.4 %, P = 0.03). SICI peaks at ISI 1 and 2.5-to-3 ms were evident with both protocols. Significant correlations were evident between mean T-SICIs- and T-SICIp- (R = 0.599, P < 0.001).

CONCLUSION

The present series validates a fully integrated motor cortical functional assessment to provide reproducible measures of SICI, with data obtained for intracortical inhibition that is more prominent when assessed using the method of serial ascending order.

SIGNIFICANCE

An integrated system for transcranial magnetic stimulation of the human motor system has been validated for clinical practice, suitable for the assessment of cortical function in neurological disease in an outpatient clinic setting.